Servo Valve Null Bias Current

SERVO VALVE NULL Bias Current

2/14/2012 11:56:19 PMhttp://www.control.com/thread/1235491574

from the Maintenance department...

SERVO VALVE NULL Bias CurrentPosted by saha on 24 February, 2009 - 11:06 am

For GE Gas turbine Frame 5 controlled by Mark 5 TMR system, what can make the Null Bias current of a Servo Valve incorrect? In other words if the Null

Bias is far from -2.667mA, What should we check to make the null bias current close to -0.267mA?

Posted by CSA on 25 February, 2009 - 12:40 am

I'm going to split some hairs here (be very, very detailed) and make some corrections to what you're describing. This topic can't really be covered completelyin a forum like this; it's complicated. But, we're going to take a short swing at the high points.

In the Speedtronic turbine control panel, the error between a servo-valve output's regulator feedback and its reference is converted into servo current. When

the feedback of a regulator is equal to the reference the error is zero, so zero error would mean zero current. But, if zero current is applied to the servo-valve,the fail-safe spring in the servo-valve will make the hydraulic actuator move to shut off the flow of fuel or air or steam.

Null bias current is the value of current that is added to the servo-valve output current to overcome the tension of the fail-safe spring in the servo-valve. So,

some amount of current must be added to the output when the regulator error is zero (when the regulator feedback is equal to the reference) to providesufficient current to overcome the fail-safe spring to keep the device in position to maintain a steady flow of fuel or air or steam. In the Mark V, null bias

current is a *fixed* value of current, defined in the I/O Configurator, that is *added* to the output to overcome fail-safe spring tension.

In the Mark V, servo current is expressed as a percent of full-scale servo current. 100% servo current is equal to 10.0 mA, so 0.1 mA equals 1.0%.

The servo current values you see when the unit is running or when you are manually positioning a device are almost never the null bias currents. The servocurrent values you see when the unit is running are the total servo current being put out by the control processor, *including* the null bias current value.

You can't really see the null bias current portion of the total current that's being applied to the servo coils *unless the feedback is nearly exactly equal to thereference.* Then and only then is the servo current value being displayed equal to the the null bias current and only the null bias current.

And this is done by each control processor independent of the others in a TMR control panel. So, if one control processor thinks the feedback for some

device is different than the reference and different than another control processor's or processors', the total amount of current from the control processor willbe different than the current from the other control processor(s). Each control processor will have the *same amount* of null bias current added to its output,

but each control processor's output can be different if each control processor thinks its feedback is different than the others'.

When the regulator feedback is different from the reference, then the control processor will adjust its total current output *which includes the fixed null biascurrent value* to try to make its feedback value equal to the reference value. And each control processor is doing this for every servo-valve output. Again,

the null bias current is a fixed value, defined in the I/O Configurator, which is always added to the total current output of each control processor.

When something like what you are asking about happens, you need to find out what the feedback values are for all three control processors for the servooutput and you will likely find that one or two of them are very different from the other(s). If all three control processors don't think the feedback is the same

and equal to the reference (the reference should be the same for all three control processors), then each control processor will adjust its servo output currentto try to make its feedback equal to the reference.

For example, consider the GCV servo output. The GCV regulator feedback is the high-selected value of LVDT feedback from the two LVDTs on the GCV.

Let's say that <R> thought the GCV position was 57.8% and <S> thought the GCV position was 55.4% and <T> thought the GCV position was 54.8%, andthe reference position for the GCV was 55.2%, then the servo currents would likely be unbalanced. And probably by a fairly large amount. <R> might be

putting out -3.9% servo current, and <S> might be putting out - 1.9%, and <T> might be putting out -2.5%. Those values are *not* null bias currents, buteach one includes the fixed value of null bias current which is defined in the I/O Configurator.

In this example, the problem is *not* the fixed null bias current value. The problem is that the three processors have fairly different ideas about the position

of the GCV and each one is trying to move the valve to the reference position, and they all have to work together (and that means that one or two are tryingto overcome the other) to keep the valve at a steady state position. The bigger the discrepancy in what each control processor believes the feedback to be, the

bigger the discrepancy in the servo output currents (which include the fixed null bias current value).

Now, let's talk specifically about the null bias current value. Let's say that the value of null bias current defined in the I/O Configurator and that wasdownloaded to and being used by all three control processors was 2.667 % (the Mark V automatically inverts the value in the I/O Configurator!). Further,

let's say all the control processors were indicating a GCV position of 49.7%, the measured position was approximately 49.8%, and the reference was 50.0%and the three servo currrents were all indicating about -2.9% per control processor, or thereabouts.

If you changed the null bias current value in the I/O Configurator to approximately 3.0 (which would correspond to -3.0%; remember: the Mark V

automatically inverts the value from the I/O Configurator!), downloaded that value to all three control processors, and re-booted all three control processors,you'd probably find that the indicated valve position feedback for all three processors was nearly 50.0%, the measured position would be about 50.0%, and

the servo currents would be almost exactly -3.0% per control processor. In this case, the amount of current being displayed for each control processor wouldbe nearly equal to the null bias current amount, because each control processor thought the feedback was nearly identical to the reference *AND* because

the amount of null bias current was exactly equal to what was required to overcome the fail-safe spring tension.

But it should be clear that unless all three control processors believe their regulator feedback values to be nearly identical to each other, the servo currentsbeing put out by each control processor will not be the same. And it has nothing to do with the fixed value of null bias current being applied to the servo-

valve output. The value of current that is displayed when the unit is running is not just the null bias current unless all three control processors are using



valve output. The value of current that is displayed when the unit is running is not just the null bias current unless all three control processors are usingnearly the same value of feedback for the device and the feedback is very nearly identical to the reference.

The amount of null bias current required to overcome the fail-safe spring is actually a range: -0.267 mA, +/- 0.133 mA, or, -0.133 mA to -0.400 mA (-1.33%

to -4.00%). So, the actual amount of null bias current required for a particular servo may be anywhere between -0.133 mA to -0.400 mA and still be withinspec. The value of *null bias current* doesn't have to be exactly equal to -2.667%, but 2.67% is a fairly good value and works for the majority of servo-

valves in use on the majority of GE-design heavy duty gas turbines. About the only time that null bias servo currents need to be adjusted is for some DLNvalves, and even then, it's questionable whether or not it's really required.

The regulator feedback is compared to the reference 128 times per second, and the total servo current output is adjusted as necessary to try to make the

feedback equal to the reference. *BUT* the value that's shown on any display or in any VIEW tool capture or output is only updated four times per second.In other words, the value of servo current written into the control signal database is only updated 4 times per second, even though it could be changing at the

rate of 128 times per second. (I think that's different for Mark V LM panels, by the way.)

Lastly, the LFBV uses Liquid Fuel Flow Divider Feedback as its primary control feedback and the SRV uses P2 pressure feedback as its primary controlfeedback. So, feedback is not always position. Some LFBVs have LVDTs as another stabilizing element of the control loop.

Posted by Jeff Ladwig on 9 February, 2010 - 9:28 am

I am in the process of checking the calibration of PM1, 2, 3 and SRV control valves, and have run in to exactly the difficulty you describe in trying to

measure null current. Since we are currently offline, I tried disconnecting two of the three servo outputs and driving the servo to 50% stroke, to eliminatethe competition between the controlers. It seems that this would allow the one controller to match reference and feedback, and then go to zero output

(except for the null current).

For example, with one output active at a time, these are the VDC and calculated servo currents for PM1:

R active: 0.290 vdc, 0.280 mAS active: 0.280 vdc, 0.275 mA

T active: 0.300 vdc, 0.283 mA

This calculates to a null bias of 2.79%, the I/O configurator value is 2.67%. Should I put the new null bias value in the I/O configurator? Valve position iswithin 1% of reference when operating.

Posted by CSA on 9 February, 2010 - 12:23 pm

Did you record any as-found conditions, or are these the as-found conditions?

Did you, or based on the as-found conditions do you, feel it necessary to re-calibrate LVDT position feedback? Valve position feedback calibration

changes can impact emissions, since fuel splits are converted to valve positions. If you will be DLN tuning after the outage it might not be so critical. Butif you won't be DLN tuning after the outage it might be crucial not to change the LVDT feedback calibration.

When two of three controller outputs are disconnected, then two of the three null bias currents are not being applied to the servo-valve, and it will not be

able to make the actual position equal to the reference position. All this does (operating the servo-valve with a single controller output) is to verify thatthe polarity of the current being applied to the coil that is still connected is correct, and does not cause the device the servo is controlling to reduce the

flow of fuel (or air or steam) to the unit. Remember, the servo-valves used on GE-design heavy duty gas turbines are bipolar devices and the polarity of

the current applied to each individual coil is very, very important.

The three null bias currents are summed in the servo-valve to produce the total null bias current applied to the servo-valve. The summing is accomplishedby the magnetic force developed as the result of current flowing through the coils (amp-turns).

2.79% vs. 2.67% is a negligible difference, in my personal opinion, but changing it might make the actual position more closely match the reference

position; a 1% error is approaching the limit of tolerance (and that's a personal limit, not a GE-defined limit, since GE never defines one), especially for aunit with DLN combustors.

Posted by Jeff Ladwig on 10 February, 2010 - 9:25 am

The values given for PM1 are as found. In addition, the actual position value as shown on the valve calibration screen while at 50% stroke ranged from

a low of 51.15% to a high of 52.48%.

I followed the same procedure for PM2, and found that with only one core active, the valve would only achieve 44.5% to 45.5% of the 50.0% strokereference. This now makes sense, realizing that all three cores are needed to supply the required null current.

PM2 is also about 2% off from reference online.

At 50% stroke, the feedback percentages are:R - 48.17% (LVDT 1) 48.99% (LVDT 2)



R - 48.17% (LVDT 1) 48.99% (LVDT 2)S - 49.13% (LVDT 1) 48.87% (LVDT 2)

T - 48.79% (LVDT 1) 48.92% (LVDT 2)

If I replace a servo, is it recommended to change the null bias current value in the I/O configurator to the value supplied by MOOG in the attachedspecs, or is the procedure that GE gives in the control spec better? Thanks.


It would sound as if there's something amiss with the null bias current value(s) for PM1? What is the actual physical position of the valve with respect

to LVDT feedback?

I am not a fan of the null bias calculation procedure in the control Specification. I've seen it cause more trouble and lost time than it's worth. If the null

bias current doesn't fall within the range that GE specifies (-0.8 mA, +/-0.4 mA total, for a TMR) then there's something wrong with the servo-valve.And many people try to use the null bias current for something it was not intended for.

If the valve (or the IGVs) doesn't behave the way they think it should, then they start changing the null bias current(s) after they spent hours trying to

calculate one based on the Control Specification value.

And worse, most people don't have any idea how to change individual servo current values for a TMR Mark V and end up just making things worse inthe end.

If Moog is providing null bias currents with their servo-valves, it would seem to make sense to use them as starting points at a minimum--as long as

they are in the range specified by GE.

Posted by Jeff FLadwig on 12 February, 2010 - 4:12 pm

I have not measured the actual position.

I don't have the tool that attaches to the side of the cylinder. It rides the actuator spring bushing up & down, and provides a small platform to rest theend of the dial indicator on.

I measured PM1 servo currents at 50%, following the GE procedure, and came up with 1.01% calculated null bias, which is outside the range

allowed by the procedure. The calculated servo currents:R - 0.007 mA

S - 0.160 mAT - 0.136 mA


And what happens when you put the 2.67% value (negative) in the Speedtronic for all three processors and then tell PM1 to go to 50% stroke?

Again, I see more and More and MORE people never bothering to measure the stroke or angle. Speedtronic panels are good, but, I don't know if

they're that good. But it does prove a major point: Gas turbine control IS NOT rocket science, no matter how hard people make it out to be. I've

seen people spend tens of hours, literally, trying to get the zero stroke reading for LVDTs to be exactly 0.700 VAC RMS, as if that improved the

accuracy of the operation of the turbine.

I've seen three-inch stroke dial indicators for about GBP50.00 and mag bases for around GBP10.00 recently. I've even used dial calipers in a pinchwith a couple of zip ties to hold one portion stationary and manually moving the other portion. Before GE started using Woodward valves the GE-

designed valves used to have scales on them which were very handy. (I still find it simply amazing that GE uses Woodward valves, and the likelymajor reason is cost. They're certainly not as good as Fisher valves, but they are less expensive, though they do require more maintenance and

repair when in service. Guess another old adage is proven true: You get what you pay for.)

Posted by A. OZTAS on 25 February, 2009 - 11:33 am

In general: Internal mechanical failure in the servo will lead to deviation of null bias current. Possible causes are, degraded lube oil (clogging of servo ports),wrong calibration of the null bias (maybe you have refurbished servo valves from none OEM supplier). Although, there is a way to calibrate the null bias,

my advice is to replace it with new one from OEM.

Good Luck...Tempus Fugit...




A. Oztas brings up a point I neglected to address. Usually, when I get this question it's related to one single coil that has a servo current that is out of

balance with the others. One cannot *really* fix the problem with a single control processor's servo current being greatly different from the other two bychanging the null bias current value.

There is only *ONE* fail-safe spring, so changing the null bias value and downloading it to all three control processors will only mask the problem with

the one control processor. Yes, it might lessen the differential between the three control processor's servo currents, but it's not addressing the problem ofwhy the one control processor's servo current is out of balance. That is usually related to the feedback for that control processor being out of balance with

the other two control processors.

If all three control processors have basically the same feedback and it's not very equal to to the reference, then all three control processors will likely betrying to overcome some problem with the servo-valve: clogging or varnishing of internal components leading to sticky or sluggish operation, worn internal

passages or o-rings.

Also, a single open-circuit in one of the three coils of a servo-valve will cause the output currents from the other two control processors to increase to try tosupply the "missing" torque which would have been produced by the lack of current in the one coil. (Some documentation refers to electro-hydraulic servo-

valves as "torque motors.") Usually, in this case, the difference between the reference and the feedback will also usually be a little greater than normal.

A. Oztas brings up another issue which has been reported by many sites which have tried to use rebuilt or refurbished servo-valves. That is, the the fail-safespring tension usually is not adjusted per GE-design specifications after refurbishment. Adjusting the spring tension is not an easy task even in a factory or

lab. One needs special equipment to monitor oil flow-rates and volumes in addition to the currents applied to the three coils. Every time I've seen people tryto adjust null bias spring tension on a servo-valve which is in service, it has resulted in having to replace the servo-valve (that has been said in previous

posts here on control.com, also).

And usually when they're trying to adjust the spring tension, it's an attempt to bring the servo current of one control processor into line with the others.And that simply can't be done with the adjustment on a single fail-safe spring.

As I've said before, I'm saving my pennies to buy Moog. These things are so misunderstood and people replace them so quickly without understanding how

they work or what they're capable of that Moog must be making a fortune because I know of sites that have replaced a single servo-valve several timesbefore fixing the real cause of the problem. In the process, the once perfectly good servos get dirty and are not handled very well, and are basically useless

after that.

Cha-CHING!

Posted by sd on 25 February, 2009 - 10:07 pm

Wonder why my earlier reply is not shown here.NVM.. let me rephrase it again now.

Well.. I think you observed is actually some optical illusion.. ;)In AutoCalib page, the value for null bias current is shown in percentage and not in amperes. Which mean that -2.667mA is actually shown as -0.267%.

Don't worry too much as your servo is still fine;)


>Well.. I think you observed is actually some optical illusion.. ;)In AutoCalib page, the value for null bias current is shown in percentage and not in amperes. Which mean that -2.667mA is actually shown as -0.267%.

From the fourth paragraph of the first response:

In the Mark V, servo current is expressed as a percent of full-scale servo current. 100% servo current is equal to 10.0 mA, so 0.1 mA equals 1.0%.

-0.267 mA would be represented as -2.67%.

I just looked through several ACALIB.DAT files, from early Mark Vs (with <I>s) and from very late Mark Vs (with GE Mark V HMIs). None of them hadlines to display null bias current.

ACALIB.DAT is an ASCII text file that ACALIB.EXE uses to configure the displays seen in AutoCalibrate is running. Would you please open that file on

you operator interface and copy one of the lines which carries the words "null bias" and paste it into a response?

My suspicion is that someone found the TCQA RAM address for the null bias current value that gets downloaded from the I/O Configurator, or someonehas re-labeled the servo current line to read "null bias".



Posted by Walter on 13 March, 2009 - 2:26 pm

In the Mark V, servo current is expressed as a percent of full-scale

servo current. 100% servo current is equal to 10.0 mA, so 0.1 mA equals 1.0%.

According to this explination we tried to compare the current servo values on the Mark V screen with the ones measured directly on the screw of theboard. For instance..

For the SRV (as seen on interface)

<R>Required Position 15,53

Actual Position 99,43Servo Current -2,50

<S>

Required Position 15,53Actual Position 99,39

Servo Current -2,21

<T>Required Position 15,53 %

Actual Position 99,37 %Servo Current -2,87 %

Then checking in the file TC2kReport and we found the screw number and board to measure the output tension (?) to the servo. That was <R> QTBA

screw number 27 and 29. We got -0,4v in <R> ( bearing in mind that the resistance is 1Kohm would be 0,4mA), is that what should be? That is fardifferent from the value we get on the screen (-0,25 mA), means that that on the screen is not taking account of the null bias?

or we are measuring in a wrong place?

How can i get in the demand desplay the signal's name of the requiered position since I can not find it in the Longname.dat.

Thanks very much in advance.

Posted by CSA on 15 March, 2009 - 1:36 am

The first thing I see wrong with this post is that if the required position is 15.53% and the actual position is 99%, then I would say that something isreally, really, really, REALLY wrong with the servo-valve output. Because, if there is that much difference between the setpoint and the actual, then the

control ain't working. Was the unit running when you were observing these values and taking these voltages?

QTBA-27 & -29 are for servo-valve output #1. There are eight servo-valve outputs from a Mark V. So, I think the second thing that's wrong with thispost is that we don't know if you were looking at the required position for SVO1 and the feedback for SVO1. In TC2KREPT.TXT, the column labeled

"Signal Name" is the value you need to look up in the CSP and work "backwards" from that to find the reference signal name.

SVO1 is usually assigned to the Stop/Ratio Valve of a gas fuel system. The reference for the SRV is usually signal name FPRG and the feedback forthe SRV is usually FPG2. You won't find this in LONGNAME.DAT. You will find it in the CSP.

And, the signal name for the SRV servo current is usually FAGR.

If you want to see what value of null bias current is being applied to a particular servo-valve output, you need to look in the I/O Configurator for that

particular servo-valve output. And, remember that the value in the I/O Configurator is *inverted* (negative) in the Mark V. (If you see 2.667, the actualvalue will be -2.667%, or -0.2667 mA.)

Whatever value of current you see or measure is the total current that's being applied to the servo coil and includes the value of null bias current that's

being applied to the output. If you've been reading this post, unless the reference and the actual values for the output are nearly identical the value ofcurrent you see on the display will not be the value of null bias current. From the data you provided, the reference and the actual are *far* from equal.

Tension is another name for voltage, and since this is a DC output, it can have positive and negative values of voltage (tension) and current. The typical

resistance of a servo coil used for a GE-design heavy duty gas turbine is approximately 1000 ohms, so -4.0 V DC would equate to a servo current ofapproximately -0.4 mA. But, any measurement you make would only be an assumption unless you know the exact value the coil resistance.

Posted by walter/roberto on 17 March, 2009 - 3:43 am

Hello! Here we go again.

First thing, at the moment we took all that data the turbine was runing at base load (MS 6001B MKV TMR). Since CSA warned us about the wide



First thing, at the moment we took all that data the turbine was runing at base load (MS 6001B MKV TMR). Since CSA warned us about the widedifference between SRV required and actual position we went into the old files ( I'd dare to say ever since commissioning) and found out that that

error was as wide as is currently.

Could be that problem related to the inadequate fuel pression supply? According to CSP FPRGOUT must be 248,1 psi at 99.85% TNH , and ouractual P2 is approx 223 psi at the same TNH. So, as FPRGOUT is not reached ( because of the lack of fuel) the valve goes practicaly to 99 % of its

position in order to provide 248,1 psi. Is that possible?Other doubt is why we get the signals FPRG and FPG2 in psi and not in % as is should be to set the position?

Stop Ratio Valve.

<R>

Required Position: 15,50 %Actual Position: 99,79 %

Servo Current: -1,87 %

<S>Required Position: 15,50 %

Actual Position: 99,57 %Servo Current: -2,28 %

<T>

Required Position: 15,50%Actual Position: 99,55%

Servo Current: -1,97%

As we understand in these posts, the servo current we see in the screen is the TOTAL output current to the servo, ( servo null bias + output current toadjust the error). So, here the output SVO1 servo current is - 1,87 % for <R> , then our maths don't work out, since the servo null bias current is

already -2,667%. And measuring on the QTBA SVO1 output currrent we get -0,4 mA ( 4%), which would be more logical ( null bias (-0,2667mA) +output current to the servo). Any clue where are we going wrong? Could be something wrong with the configuration of SRV Autocalibrate display?

Thanks very much again and sorry for our insistence.

Greetings from Argentina.

Posted by CSA on 17 March, 2009 - 3:29 pm

Hello to Argentina, one of the futbol powerhouses of the world!

"Here we go again." Do you mind explaining that comment, please?

You seem to have pieces of the puzzle and the answers, but aren't able to put them all together. If the gas fuel supply pressure is less than the P2pressure reference, then the SRV is going to go wide open to try to get the P2 to be equal to the reference. Since there's approximately a 0.7 barg

pressure drop across the SRV (typically) that would mean that, per the information you provided, the supply pressure would need to beapproximately 258 psig to keep the SRV from opening fully.

You did not provide the gas fuel supply pressure reading upstream of the SRV. What is the supply pressure? I have seen clogged y-strainers cause

high pressure drops, and most units have a y-strainer just upstream of the SRV. Has it been checked for cleanliness recently? There's not usually a d-p gauge across the y-strainer, and if you can read the pressure directly upstream of the y-strainer, what is it versus the gas fuel supply pressure

upstream of the SRV?

Some units have some kind of fuel filters upstream of the y-strainer, as well. Some times they are coalescing filters or just "sock" filters. If they arepresent, have they been checked? I was at one site where they said the d-p gauge had never registered any d-p. When the filter canister was opened,

the filter element had been so dirty at some point that it had ruptured and was effectively non-existent. Someone also commented that they had hadexhaust temperature spread problems a couple of years earlier and that the gas nozzle tips were found to be plugged with some kind of stringy

material, the source of which was never identified. Turns out the gas fuel filter was one of those sock-type filters made of wound stringy material.

I want to warn you: That is a bad condition to be operating the unit in if you ever experience gas fuel supply pressure spikes or sudden loaddecreases. When the Speedtronic panel is putting out excessive current to try to open the SRV it can go into what's deemed "wind-up". Wind-up can

take a split-second to recover from if there is a sudden supply pressure increase or a sudden load decrease ("load rejection") and the unit can trip onexhaust overtemperature if it's being operated at or near Base Load when the disturbance occurs because the current has to be reduced at a rate and it

might not reduce the current sufficiently to prevent a "burst" of fuel from being admitted to the combustors.

If your gas fuel supply pressure can never be greater than approximately 260 psig, you may be able to ask GE or the OES to recalculate the P2pressure reference curve to allow the SRV to operate in a controlling region and fashion. But, be prepared to supply them with a recent gas analysis

and some details of the configuration of the gas fuel supply system. They may also ask for P/Ns (part numbers) of the gas fuel nozzle tips installed inthe machine, and if they're not OES equipment, be prepared to supply the flow characteristics from the vendor or the sizes of the orifices in the

nozzle tips. This information will be necessary to be able to determine if a lower P2 pressure might be possible, and if so, to calculate a lower P2pressure reference curve. Also, be prepared to supply all the start-up, warm-up, and acceleration FSR Control Constants.

Can you find the section of ACALIB.DAT for the SRV (SVO1 or SVO01) and post it to this thread? You have never told us where you were reading



Can you find the section of ACALIB.DAT for the SRV (SVO1 or SVO01) and post it to this thread? You have never told us where you were readingthe servo currents from, and if it's from the AutoCalibrate display then you might be right: There might be a problem with the configuration of

ACALIB.DAT.

Did you put the signals FPRG, FPRGOUT, and FAGR on a Demand Display or on the Logic Forcing Display and observe them versus the readingsyou are seeing on the AutoCalibrate Display?

I would submit that you aren't doing anything wrong with the measurements you're taking, but it's something with the display values or the display

that you're reading the values from. Please put the above signals on a Demand Display or the Logic Forcing Display and tell us what the differencesare between them and the values you are seeing on the AutoCalibrate display, if that's where you're observing the values from (which, again, you

haven't told us).

0.4 mA (or 4%) would be more likely what one would expect to see if the Mark V were driving the SRV to be more open than the valve couldphysically travel such as what you are describing.

If you will look at Section 7, I believe, of GEH-6195, of the Mark V Application Manual, you will see that the SRV regulator uses FPRGOUT as the

primary reference and FPG2 for the feedback of the regulator summing junction. If the two are equal, then the output of the summing junction iszero, which means the SRV position is equal to what it needs to be to make the P2 pressure equal to the P2 pressure reference. After the primary

regulator summing junction, the SRV LVDT feedback is compared to the output of the summing junction, and if the summing junction output iszero, then no change to valve position is to be made.

If there is an error between FPRGOUT and FPG2, then that difference will be treated as a requiring a change to the valve position, and that error

will be compared to the current LVDT position feedback and the servo current will be changed to make the necessary position change to make theP2 pressure feedback equal to the reference.

The GCV and IGV regulators are "straight" position regulators and the feedback is from the LVDTs mounted on the device actuators. The LFBV

(Liq. Fuel Bypass Valve) reference is a liq fuel flow rate and the feedback is from the magnetic speed pick-ups on the Liq. Fuel Flow Divider.

But I'd really like to understand the, "Here we go again!" comment.

Posted by walter/roberto on 19 March, 2009 - 5:07 am

Hello CSA!, again from the land of football (no soccer) no offence..;) , you are invited to see a football match whenever you want!

First of all let us explain to you that we did not mean to be impolite writing "here we go again", we just wanted to express that we are here tryingwith the same topic again, just that.

Talking about gas fuel supply pressure upstream the SRV we have 260 psi what would be pressure enough. After reading your explanation it

seems that is not a gas fuel supply problem. It is more likely to be a problem on the Autocalibrate display. The servo currents we provided youwere taken from the Autocalibrate Display. Here goes the ACALIB.DAT from the SRV.

PROC Q

SOCKET 1SVO 1

IOP 21CARD "TCQA"

TITLE "GAS STOP RATIO VALVE"PERMISSIVE L3ADJ

POSITION_NEG_SAT 100POSITION_POS_SAT -0,1

POSITION_SCALE (F2 256,0 0,0 2 '% ')MAN_SCALE (F2 128,0 0,0 2 '%')

LINE01 BTEXT 'DESCRIPTION <R> <S> <T> UNITS'line 02 data 'SVO number' <D0A 09> (C1)

line 03 data 'Regulator type' <D0A 19> (H1)

LINE 05 DATA "LVDT #1 0% cal. Ref." <D0A 60> (F2 6,667 0 3 "V rms")LINE 06 DATA "LVDT #1 100% cal. Ref." <D0A 62> (F2 6,667 0 3 "V rms")

LINE 07 DATA "LVDT #2 0% cal. Ref." <D0A 64> (F2 6,667 0 3 "V rms")LINE 08 DATA "LVDT #2 100% cal. Ref." <D0A 66> (F2 6,667 0 3 "V rms")

;LINE 09 DATA "LVDT #3 0% cal. Ref." <D0A 68> (F2 6,667 0 3 "V rms");LINE 10 DATA "LVDT #3 100% cal. Ref." <D0A 70> (F2 6,667 0 3 "V rms")

;LINE 11 DATA "LVDT #4 0% cal. Ref." <D0A 72> (F2 6,667 0 3 "V rms");LINE 12 DATA "LVDT #4 100% cal. Ref." <D0A 74> (F2 6,667 0 3 "V rms")

LINE 14 DATA "LVDT #1 Voltage" <D0A 20> (F2 6,667 0 3 "V rms")



LINE 14 DATA "LVDT #1 Voltage" <D0A 20> (F2 6,667 0 3 "V rms")LINE 15 DATA "LVDT #2 Voltage" <D0A 26> (F2 6,667 0 3 "V rms")

;LINE 16 DATA "LVDT #3 Voltage" <D0A 32> (F2 6,667 0 3 "V rms");LINE 17 DATA "LVDT #4 Voltage" <D0A 60> (F2 6,667 0 3 "V rms")

LINE 23 DATA "Position at POS Cur Sat." <D0A 10> (F2 256 0 1 "%")

LINE 24 DATA "Position at NEG Cur Sat." <D0A 12> (F2 256 0 1 "%")LINE 25 DATA "Manual control position" <D0A 14> (F2 128 0 1 "%")

LINE 27 DATA "Required Position" <D0A 50> (F2 128 0 2 "%")

LINE 28 DATA "Actual Position" <D0A 58> (F2 128 0 2 "%")LINE 29 DATA "Servo Current." <D0A 56> (F2 100 0 2 "%")

LINE 31 STATUS 1 R "R STATUS:" <D0A 16>

LINE 32 STATUS 1 S "S STATUS:" <D0A 16>LINE 33 STATUS 1 T "T STATUS:" <D0A 16>

LINE 34 DATA "Permissive: L3ADJ" L3ADJLINE 35 DATA "Permissive: SPEED <28% (TNH)" TNH

Then we compared the signals in the Logic Forcing Dispay to the same ones in the Autocalibrate Display, and this is what we get:

In Logic Forcing Display: FPRGOUT :248,5 psi; FPG2: 233,5 psi; FAGR: -2,95 %In Autocalibrate Display: FPRGOUT (required pos): 15,52 %; FPG2( actual pos): 99,65 %; FAGR:-1,99 % all values in <R>

I found a signal which it seems to be the reference for us, FSGR, which in Logic Forcing Display is 99,59 % and in the Autocalibrate is, as yousaw above 99,65%, more closed to the value. Is it maybe a wrong signal reference we are seeing in the Autocalibrate Display as FSGR? And last

but not least In our second turbine (TG 2) happens exactly the same with the SRV autocalibrate display.

Again thanks very much in advance.

Posted by Ore Rotundo on 19 March, 2009 - 2:23 pm

Hombre,

This is not football, it look like that we are ping ponging Now get back reality.Quick look to your ACALIB data shows that is standard configuration for a FR5. I wonder whether your system is HMI or I ? Or is it may be in

the past upgraded to HMI? How about the PROM revisions? If your SRV is operating at 99%, definitely the upstream FG pressure is below thespecifications as given by the OEM. However this should not give the problems that you describe. There are two options for your problems:

1) DONT use ACALIB for calibration and monitoring purposes. Use the logic forcing display and/or pre-vote data display.2) Verify the ACALIB data for your GT and the revisions of the PROMs. Cross check also the IO_CFG SVOx configuration (stroke 100 % or

128 %)

Remember that you dont necessarily AUTOCLIB display to perform calibration. Just use the basic calibration procedure.

Docendo Discumus


If you've asked this question before on control.com, can you please provide the URL link to the post?

My bad for making some assumptions (I really try not to do that, but I failed on this one!). I assumed that you have verified pressure transducerreadings against reasonably accurate gauges and didn't ask you to confirm that. I assumed the P2 pressure transducer(s) (by the way, how many

transducers does the unit have: 1 or 3?) are reasonably well calibrated and that the feedback (input) is properly scaled in the Mark V. I'veassumed the SRV LVDTs have been calibrated properly and that the valve is physically at or near full open (something we haven't asked, but

which we are asking you to visually confirm).

I'm going to try to explain this again: When the servo-valve regulator feedback is equal to the servo-valve regulator reference, the servo-valveregulator error is zero. When the error is zero, the servo-valve output current would be equal to zero mA. However, the servo-valve has a spring

which, in the absence of current (zero mA), will drive the device to shut off the flow of fuel or air or steam. To overcome the spring and to keepthe device in a steady-state position such that the feedback is equal to the reference, a small amount of current is continually added to the servo-

valve output (at all times!). The only time that the current being applied to the servo-valve's coils is equal to the null bias current value is whenthe feedback is exactly equal to the reference and no additional current is required to keep the device in a steady-state position to make the

feedback equal to the reference.



Remember: The Moog servo-valves used for GE-design heavy duty gas turbines are polarity-sensitive devices, meaning that the polarity of the

applied DC voltage affects the flow of hydraulic oil through the servo-valve. With zero current, there is no force (torque) developed by the torque'motor' in the servo-valve, and the null bias spring will act just like the application of a positive current, which would be to shut off the flow of

fuel or air or steam. When the regulator error is zero, the output is zero mA, so the null bias current value defined in the I/O Configurator iscontinually added to the output. That's what a bias value is: something that's continually added to something (or subtracted, depending on the

application). So, when the error between the reference and the feedback is zero, the only current being supplied is the null bias current. And, inmy experience with the Mark V, the displayed value of servo current is always the total amount of current being supplied, which includes the

null bias current. Under normal conditions, only when the error is zero will the amount of current being supplied to the servo be equal to the nullbias value.

Which brings up another question I haven't thought to ask: To your knowledge, has anyone tried to adjust the null bias spring tension of the SRV

servo-valve?

At this time, based on the information you have provided and what you have chosen to provide, I cannot explain why the SRV servo current isless than what the expected null bias current should be. I suspect that in an attempt to try to get the SRV into a controlling position that someone

has changed the null bias current value in the I/O Configurator, but, we don't yet (!) know what that value is. I might also suspect that someonehas done something with the TCQC card configuration jumpers or even something "unique" in the CSP to try to rectify this SRV situation. Not

being able to look at your CSP and card jumpers, we can'tell that.

I am hoping that by answering the GCV questions that we can establish that one servo output is operating as expected, but I'm asking a questionthat I don't know the answer to, and I just might get a great big surprise, but I'm willing to take that chance at this point.

You have *NOT* provided all the information asked of you. Please don't arbitrarily choose what information you are going to provide or what

information you deem to be relevant or necessary. We're not asking questions to be making you run around needlessly; we're asking because wearen't on site and can't get the information for ourselves. And because you wrote here asking for help with an issue, we presume that you are

interested in learning something and providing the information requested to help you resolve your issue. If you don't want to fully participate inthe exercise of solving your issue, then there's no point in continuing this thread.

Specifically:

1) What is the value of null bias current listed in the I/O configurator for SVO1? (Open the I/O Configurator, and click on the TCQA card, and

scroll to the screen for SVO1, and tell us what is listed in the null bias current field. Exit the card, exit the I/O Configurator, without savinganything, and you won't disturb any of the I/O Cfgr. settings.)

2) Precisely, where are you measuring this 260 psi supply pressure? At the SRV inlet/supply pressure gauge in the Gas Fuel Compartment, which

would be downstream of the y-strainer, or some place in the gas fuel supply piping upstream of the two units' y-strainers? From a pressuretransducer on the gas fuel supply? From a pressure transducer on a metering tube and orifice in the gas fuel supply piping upstream of the unit's

y-strainers?

3) Has anyone visually checked the y-strainer recently?

4) Are there any filters upstream of the y-strainers, and if so, what is the d-p across the filters and have they been visually checked recently?

Now, eight new questions:

1) Please confirm the actual, physical position of the SRV is at or near full open.

2) What is the GCV position indication (from the LVDT feedback, usually signal name FSG (Fuel Stroke-Gas))?

3) What is the value of FSR?

4) What is the value of servo current being applied to the GCV (usually signal name FAG)?

5) What Diagnostic Alarms are active when the unit is running? (Include any locked-out Diag. Alarms in the list)

6) Please tell us exactly where you're measuring this 260 psi, and is it psig or could it be psia?. Many GE-design heavy duty gas turbines use ametering tube and orifice flow-meter to measure gas fuel flow-rate, and the static pressure transducer is usually calibrated in psia, not psig. So, if

you're reading a static pressure transducer for the supply pressure (and this is usually located upstream of the gas fuel y-strainer), please confirmthe calibration and scaling of the input (the signal name is usually FPG1, sometimes, FPG3).

7) What are the pressures on the three gas fuel pressure gauges in the Gas Fuel Compartment? One should be SRV inlet ("supply" pressure; one

should be Gas Fuel Valve Intervalve Pressure (P2 pressure); and one should GCV discharge pressure, or gas fuel manifold pressure.

8) Sometimes the pressure drop across the SRV is a little higher; sometimes a little lower. But 0.8 bar to 1.3 par is a typical range. Also, can youdescribe the SRV? Is it in a combined casting with the GCV or is it a separate valve from the GCV? If it's a separate valve, is it a rotary valve or a

plug valve?

The fact remains, if the SRV is at 99.93% and the P2 pressure reference is 248.5 psig and the actual P2 pressure is 232.5 psig (and I presume youare reading this from the P2 pressure transducer feedback on the Mark V operator interface display; can you please tell us what the P2 pressure

gauge reading is?) then there is *not* sufficient flow capacity from your gas fuel supply to achieve required P2 pressure. It's that simple. Therewould generally be no other reason for the SRV to be open so high and the P2 pressure to be lower than the P2 pressure reference.



Now, is the P2 pressure transducer calibrated properly? Is the feedback scaled properly in the Mark V? If the gauges in the Gas Fuel

Compartment are relatively accurate, this would be a good indication of whether or not the transducer(s) is(are) calibrated properly and thefeedback is scaled properly.

If the P2 pressure gauge in the Gas Fuel Compartment is reasonably accurate and is indicating roughly the same pressure as the transducer

feedback, then there's just not enough supply pressure and flow capacity to allow the SRV to operate in a controlling range (which should besomething less than 99.93% and less than full open). Because, even if the SRV LVDTs are calibrated properly, the regulator for the SRV is a

pressure control loop and it will put the SRV at whatever position it needs to be at to make the actual P2 pressure equal to the P2 pressurereference, provided there is sufficient pressure and flow capacity upstream of the SRV to allow the SRV to control the P2 pressure without going

full open (and that just doesn't seem to be the case in this instance, for two turbines).

The reason we're asking about the GCV information is to try to establish that at least one valve is operating in a properly controlled fashion, andto see what the value of servo current is that is being applied to that valve when operating in a properly controlled fashion.

FSGR (Fuel Stroke-Gas Ratio) is the typical signal name for the SRV LVDT feedback; it's not a reference, is the actual feedback.

The only other thing I could think of to cause the SRV to behave as it's being described is that if the P2 pressure transducer was not properly

calibrated or the feedback (input) was not properly scaled, but I would expect that reasonably accurate gauges would have alerted you to thisissue much sooner.

FPG2 is not the SRV position, and it's not listed in the ACALIB.DAT section you posted. Lines 27, 28, and 29 in the section you copied displays

the valve's reference position, actual position, and servo current, respectively. I can't recall if the reference position is "active" when the unit isrunning, or if it's only "active" when the valve is being manually positioned using AutoCalib. (The answers to the GCV question may help with

that!)

FPG2 is the scaled feedback (in psi) from the P2 pressure transducer(s), which should be calibrated in psig (gauge pressure). If you have morethan one P2 pressure transducer, the you can look at the signal name FPG2 in the Prevote Data Display to see the individual, pre-voted values of

the three feedbacks, and then report them to us, please.

Lastly, the amount of null bias current being applied to the SRV as understood in this thread has nothing whatsoever to do with the fact that theSRV appears to be at or near full open in an attempt to control P2 pressure at 248.5 psig. But it can't because the upstream supply pressure and

flow capacity isn't sufficient to be able to do that. That's what the data you have provided to date tells us. Perhaps the answers you haven'tprovided and the answers to the new questions will lead us in a different direction, but it's not really likely. And, it has nothing to do with null

bias current.

I agree with Ore Rotundo; the section of ACALIB.DAT that you have provided seems to be fairly generic.

Please provide all the information requested, and we can try to get to the bottom of your problem, which, again, should not be related to the nullbias current value.

Posted by servo_2001 on 18 March, 2009 - 4:39 pm

I would like to clarify that the answer is not just the OEM as the solution to the problem. To null the servovalve is the easiest calibration you can perform to

a servovalve. Do your homework and you will see that there are reputable repair facilities that have been repairing these valves for some time. Our companyhas specialized in electro-hydraulic servovalves since 1969. We were schooled at Moog on the repair of their valves. Do not let the OEM make you think

they are the only game in town.

Posted by CSA on 19 March, 2009 - 12:19 am

What's missing from your reply? Contact info for your company.

What's also missing from your reply? The fact that adjusting servo null bias springs is best done in a "laboratory" environment, with controlled flow-ratesand measuring equipment. It's easy under those circumstances.

The problems being discussed here are not generally related to null bias spring adjustment, but rather the methods of applying a null bias current and why

they are necessary and how to "measure" them. And how the currents being applied (including the null bias current) can be out of balance.

Nowhere in this thread did anyone claim that the OEM was the only game in town. There have been other posts here on control.com which listed firmswhich refurbish Moog servo-valves. Can you add yours to the group?

Posted by Mehul on 21 March, 2009 - 6:50 am

Dear CSA,

Here I have concept of NULL Bias, You are requested to confirm the same.



Dear CSA,

Here I have concept of NULL Bias, You are requested to confirm the same.

There is good talk on the Null Bias Current for the servo. We can use the drawing of Servo along with schematic of IGV or GCV-SRV to understand theconcept.

For this talk reference, let us take IGV scheme, the IGV is stable at one position 57 deg. By viewing the scheme of IGV actuating Piston & Cylinder,

both side of Piston must be equal pressured, as CSRGVOUT is CSRGVBAK(for Mark_IV) to keep IGV at 57 deg STABLE.

Now look at the drawing of Servo, to maintain the both sides of piston equally pressurized, the spool should be at the middle of the bushing(such that it

neither allows any port{connected to one side of piston} to drain nor gives extra pressure to Port{Other side of Piston}). And thus flapper (FeedbackSleeve) should be at centre and so the armature should be at the centre. And to keep the armature at physical centre position, amount of current applied, is

null bias current.

Consider what would happen, if the spool is not coming back to centre position. Definitely! it will drive the IGV in either extreme position.

Now consider, the IGV is required to be open to maintain the Exhaust temperature. The Controller will change the current to servo such that the armaturewill forces spool to move. The movement of the spool will be such that one port will be connected to pressure source and at the same time, opens the

other port to the drain. This will create the pressure diff. on both side of piston. Ultimately Piston will move the IGV to open. At the end, whenCSRGVOUT=CSRGVBAK(command=feedback), once again armature is required to be at to null position i.e. centre position, which is done by providing

null bias current by controllers

Similarly the mechanical null bias key has been provided to adjust the armature at centre positon.

Thanks in advance


I will confirm all but the last statement about the "null bias key".

If by null bias key you mean a tool to adjust the null bias spring tension, then, absolutely no! No one should be adjusting the null bias spring tension of

a three-coil electro-hydraulic servo-valve, with the exception of a facility with the appropriate means to do so and verify proper results.

Mehul, I really appreciate your questions, and anyone else's who's reading this thread and has some interpretation of this concept they wish to confirm. Ifind this to be one of the most difficult concepts to try to explain to people, with or without the ability to use diagrams or pictures, which doesn't seem to

make much difference. I am almost desperately searching for the right words and the right means to explain this to people.

I think that one of the things that most people misunderstand about servo-valves as used on GE-design heavy duty gas turbines is that they are not likethe overwhelming majority of most actuator outputs on most other types of control systems. Instead of an output that's proportional to desired position

such as a 4-20 mA output being at 12 mA for a reference of 50%, this one is proportional to the error between the reference and the feedback, plus anull bias current. And if the error is zero then the output is zero, plus a null bias current. If the error is greater than zero, then the output is greater than

zero plus a null bias current.

It gets even more complicated when there's more than one coil in the servo-valve, such as in a TMR control panel, and each control processor can outputa different current and each servo-valve output includes a null bias current (the same amount of current) to overcome a single spring.

So, to anyone reading this thread, if you have some idea about how to make this any easier to understand, PLEASE write and let me know. If it wouldhelp to try to relate it to some other control system concept or input or output, that would be great. But, I've been trying for years to put this in simple,

understandable terms, and I've yet to find the right words, with or without pictures.

Posted by Mehul on 26 March, 2009 - 4:59 am

Thanks once again CSA.

Definetly , one MUST NOT adjust spring tension to set null bias current without proper FACILITY .

I mentioned it here to confirm the null bias adjustment device available with the servo.

Otherwise, one should prefer to replace servo & maitaining clean Pall filter other than service,claening, replacement of spares for servo.

Regarding Servo Valve Maintenance, one of my earlier Comment in "IGV TEMP CONTROL TRIP" topic.

"Another thing, cleaning the moog servo or replacing the filter, spares of moog servo requires the extream extream exteram(03 Times)care to be taken

for the cleanliness of the tool tackels, oils, Hands of personals and envoirement at which the servicing is being carried out. So if possible try to replcethe servo, then serviceing the servo."




Many electronic manufacturers used to give away small screwdrivers to use to adjust the potentiometers on printed circuit cards. There used to be a

joke that they gave away those "tweakers" (as they were affectionately called) so that technicians and supervisors would misadjust the pots and haveto call the manufacturer in to readjust the pots.

I often wonder if Moog doesn't include the little hex key/wrench for the same reason.

It's interesting to note that the originator of this thread hasn't replied to the questions asked to try to help with their issue (which likely isn't related to

null bias current, but may be related to a misadjustment of the null bias spring!).

Posted by Ore Rotundo on 19 March, 2009 - 1:25 pm

Dear Servo 2001

Maybe its good to highlight that, we are not fundamentally promoting the OEM's and one thing is sure that we are also not getting paid for it to do it! Ourintention is to give the control.com community our opinions and suggestions in order to help them out with their technical issues. The servo valve users are

plenty and indeed healthy competition is welcome, such as after market parts. Finally its all about money, in order to safeguard the reliability of theirasset, professional companies will purchase anyway OEM servos. There are plenty of other GT users who are purchasing repaired parts such as the servo

valves. I know some of these GT users buying reconditioned servos who are complaining after having the servo valve in service less then a year, believe ornot!

Docendo Discumus

Posted by minister on 17 July, 2009 - 9:45 am

I would like to add some more interesting facts and kindly ask for explanation.

We have 4 Frame 5 non-dln turbines running in the island mode. I have found something odd troubleshooting the moog of one of the machines, that

randomly changes its bias causing 3% position error - temporary solution we use(unit online) is to recalculate new Null Bias measuring coils voltage andhaving coil resistance measured before, then downloading the VSVO. This way it's fine for some time, usually until next start-up. I have to mention, that

we have already tried three different Moogs with the same result.

Back to the moog story. The odd thing is that each unit SRV, GCV and IGV servo currents change at the same time from one stable value to another one(usually by the step of 3, 4 %). The same time error doesn't change. I have trended hydraulic pressure 96HQ but there is no sign of the event. Also no

sign on P125DC and N125DC. I'm running out of ideas what to trend.

Also polarity has been checked many times so far by confiming that MKVI is able to drive the valve by each servo coil separetely.

Although it happens on all four units, the event doesn't occure at the same time looking from unit to unit (see my trends).

Please check the data collected I uploded below:http://www.speedyshare.com/476835562.html

Do not be surpirsed looking at these trends. One of our machines (G4) also have out of spec, positive Null Bias, since commisioning by GE (MKVI config

value -12, changing moogs didn't help, anyway machine is controlled correctly.)

Any explanation on the moog servo current changes? Why all units affected?

Posted by CSA on 18 July, 2009 - 2:14 am

You say you have four turbines and you're only having a problem with one. Or at least that's what it sounds like. Actually, I can't really tell how manyunits are having the problem now that I re-read the message.

But, here goes anyway.

You've changed the servos; you say the problem happens with all the servos.

You haven't told us what Diagnostic Alarms are present when this happens, or before this happens.

Do these units have IS barriers?

If you've been reading this thread, you know that the null bias current is the current required to overcome the null bias spring tension. So, if the current



If you've been reading this thread, you know that the null bias current is the current required to overcome the null bias spring tension. So, if the currentrequired to overcome the spring tension changes, it would seem that either something is making the spring tension change or something is causing the

coil resistance to change.

Servo valves are about the most misunderstood device I've ever run across. They're nothing more that electromagnets which produce small amounts oftorque. They're just coils of wire through which DC current is applied; the polarity and magnitude of the current causes the direction and amount of

torque to change. How can they be so complicated? They're not.

The overwhelming cause of "failure" is oil cleanliness and oil temperature. There's just not much more to them than that. Very tiny, orifice-likepassages and clearances which are very susceptible to dirt and varnishing.

Is there possibly a problem with oil temperature?

I have seen units with the relief valves set improperly, used as the "pressure regulators" for the hydraulic pump. This causes excessive flow through the

hydraulic pump.

The hydraulic system of a GE-design heavy duty gas turbine is basically a static system, meaning that under steady-state operating conditions there isno flow. There is only flow when a valve or the IGVs are being commanded to move.

I've also seen units with the hydraulic accumulator not properly in service. This also causes the flow through the hydraulic pump to be excessive which

could be a cause of high oil temperature.

Have you compared all the hardware ("Berg") jumpers on the TSVOs? And all the configuration settings on the VSVOs? Of the unit with the problemvs. a unit without the problem? What is the dither set to on the unit with the problems vs. a unit which seems to be running well (not the one with the

whacko null bias value).

It just doesn't seem like the servos can be the problem here, especially if all of them on one unit are behaving similarly. There's something else that'scommon to the servos that the problem.

Another couple of months and I should be in a position to make my bid for Moog. The way these things get changed on a whim, they must be raking in

the cash! I just want to buy the GE-design heavy duty gas turbine line, and I'll be paying my business loan off very early--I'm sure of that.

The 125 VDC battery supply voltage is converted to voltages by the rack power supplies which are then used by the VSVOs to drive the servos. Itwouldn't seem likely that a problem with 125 VDC battery voltage would manifest itself in the servo outputs.

There's something different about the hydraulic system of that unit. Or, possibly even and, there is some configuration and/or jumper settings that are

not correct. But, for a Moog servo to be changing its null bias spring tension requiring on-line null bias current changes? That's just pretty not right.

Anyway, that's about all I can think of. To address what I thought the problem was. Now, I'm not so sure.

One thing's for sure though: When I own that piece of Moog, I' won't be contributing to threads like this on control.com. I know better than to kill thegoose that lays golden eggs.


Thank you CSA for your reply.

>You say you have four turbines and you're only having a problem with one. Or at least that's what it sounds like. <

Actually we have problem with two servos:

1. On one machine the SRV changes it's Null Bias mostly after restart (I have greased the stem already and stroke after many times, result the same, soit's probably not the friction). This Moog creates error >3%, to avoid trip exceeding 5% value we just downloaded recalculated null bias. This really

drops the error to zero. Because of production I can't just stop the machine to do it off-line. (I'm aware about on-line software download, but hardware,single VSVO I tried many times with no bad luck).

2. Another machine GCV has positive Null Bias (MKVI RegNullBias =-11.96), and it's not the polarity.

Investigating these two issues I trended all machines and found interesting fact that I shared on previous post.

>You haven't told us what Diagnostic Alarms are present when this happens, or before this happens. <

The only alarm we have is the increased position error that can lead to turbine trip. Again, it's not the machine with positive NullBias value.

>Do these units have IS barriers? <

Yes we have the barriers on SRV and GCV (some MTL-7765ac), none for IGV. I have it planned to run the machine bypassing them, what do you

think? Especially, that two other barriers for seismics on the same machine had failed.

>Is there possibly a problem with oil temperature? <



I'm going to trend all servo currents with oil temperature starting this afternoon. Right now (2:42PM) the header temperatures are 57 to 58 deg C,

outside ambient 46 deg C (air oil cooler).

>I have seen units with the relief valves set improperly, used as the "pressure regulators" for the hydraulic pump. This causes excessive flow throughthe hydraulic pump. <

That's another good hint you gave me. I found our technician resetting hydraulic pressure on the PSV instead of using pump PCV. I will check

remaining units.

>I've also seen units with the hydraulic accumulator not properly in service. <

No accumulators here.

>Have you compared all the hardware ("Berg") jumpers on the TSVOs? And all the configuration settings on the VSVOs? Of the unit with theproblem vs. a unit without the problem? <

All TSVO the same (jumpers), all MKVI hardware configs the same (to be sure, I have also compared m6b with the MKVI).

Dither Amplitude 2.0, Freq 100hz on all Moogs (including IGV), on all units.

The SRV/GCV we have is the old design, containing both valves in one body, don't think we have to disable the dither as someone posted dither

disabling is recommended for new types of valves.

Regards!

Posted by CSA on 18 July, 2009 - 4:39 pm

One more time: The SRV regulator is a pressure loop. Even if the LVDT calibration is WAY off, the pressure loop is going to put the valve at theposition required to make the actual P2 pressure equal to the P2 pressure reference. It doesn't make any difference if the valve thinks it's at 35.4%

and it thinks it should be at 28.4% or 44.4%, or if it's actually at 31.4% stroke (physical measurement) and it thinks it should be at 37.1% and the

LVDT feedback says it's at 35.9%. As long as the actual P2 pressure is equal to the P2 pressure reference, everything is perfectly okay. Whenit's not perfectly okay is when the unit is a TMR panel and the three servo currents are not fairly equal; THAT'S a problem.

I'll bet any amount of money the differential is related to the calibration method of that combined SRV/GCV assembly.

Would you list the servo gains and null biases for all of the servos? For all of the turbines? A simple chart:

GT1 GT2 GT3 GT4

Null Bias/Gain Null Bias/Gain Null Bias/Gain Null Bias/Gain

SRV

GCV

IGV

I don't believe you told us if these are TMR or SIMPLEX Mark VI panels; if so, please remind me.

As far as dither goes, my personal belief is that none is needed with most of these legacy-style actuators.

I have seen barriers cause all manner of strange problems over the years. They appear to be fine, but they're not. Also, some seem to be temperature

and current sensitive when they start "failing."


Dear CSA,

Thank you this time for your reply!

> One more time: The SRV regulator is a pressure loop.---- snip ----

As long as the actual P2 pressure is equal to the P2 pressure reference, everything is perfectly okay. When it's not perfectly okay is when the

unit is a TMR panel and the three servo currents are not fairly equal; THAT'S a problem. <

Thanks for reminding that. The above was also mentioned many times on this forum. Although it's pressure control loop our concern is not to betripped by position error.



>Would you list the servo gains and null biases for all of the servos? For all of the turbines? A simple chart:

GT1 GT2 GT3 GT4

Null Bias/Gain Null Bias/Gain Null Bias/Gain Null Bias/Gain

SRV 2.4/1.8 3.9/1.8 3.2/1.8 -11.69/1.8

GCV 3.3/1.8 3.1/1.8 3.3/1.8 2.65/1.8

IGV 2.61/6.8 2.67/6.8 2.82/6.8 3.1/6.8

The way we calibrate these valves is to lift the stem and insert and leave filler gauge to get rid of the gap between the actuator and valve stem.This way the valve is still closed and LVDT indicates 2.5% difference from its rest position.

Hope this is the way.

> I don't believe you told us if these are TMR or SIMPLEX Mark VI panels; if so, please remind me. <

TMR, sorry, should have mention that at the beginning.

>I have seen barriers cause all manner of strange problems over the years. They appear to be fine, but they're not. Also, some seem to betemperature and current sensitive when they start "failing." <

Can I bypass these barriers? I know it's for EEX zone but I do not recall (I'm not 100% sure) seeing them on 9E turbine in Europe for example.

After trending the servo currents jumps together with Oil Temperature no relation could be observed (during night when temperature decreases,

the frequency of jumps seems to be the same)

Can anyone trend servo currents for at least 12hours to observe if these jump like it happens here? I do not understand why three servos jump at thesame time and it happen on all machines but at different times. We have running hrs varying from 6000 hrs to 12000 hrs and we have never

change the lube oil. I wonder if these jumps happen anywhere else?

Regards!


You have a TMR control system. That means that each control processor has it's own gain and null bias for each coil of the servo valve.

Can you please list the null bias for each processor for each servo of each turbine?

One thing all manufacturers do is to "copy" functions to similar applications, like the LVDT position error check. Since the position isn't theproblem for the SRV (the inability to control P2 pressure is the problem), do you think it makes sense to use the same settings for the SRV as for,

say, the IGVs or the GCV?

I'm still confused about how the position error can start out at one value (even if it's zero) and then change when the unit is running. And I'mconfused when you say you download new null bias value(s). Do you download a new value to one processor? Or to all three processors, one at a

time?

As MIKEVI says, there might be a problem with one of the processors thinking that the P2 pressure isn't the same as the other processors, andthat would cause that processor to change its current output and the other processors would have to change theirs to counteract the other. This

happens a LOT, and if you have three P2 pressure transducers and one of them drifts or has a leak or a wiring problem, that can cause a problemlike this.

How many P2 pressure transducers does each turbine have? One or three or ????

Have you trended the P2 pressures for all three processors to see what they're doing before and after this problem?

When you put the feeler gauges into the gap prior to performing the calibration of the SRV and GCV, that's to PUT them in their true zero

position. When you remove the feeler gauges and the valve stems drop and the indication goes negative, that's what's SUPPOSED to happen.

The purpose for the gap is to ensure that the valves aren't held open by the actuator bottoming out. Some places it's called "closed end overtavel".

The true zero stroke position for these valves is NOT with the valve stem fully down when the valve is closed; in that position the valve stem isnot touching the bottom of the valve plug.

The true zero stroke position is when the valve is closed and the valve stem is touching the bottom of the plug, and that's what the feeler gauges

do: They keep the valve stem in contact with the bottom of the valve plug when the valve is closed and ensure that at the end of the calibrationprocedure that the valve will return to the same position as when it started (which is one of the checks that AutoCalibrate performs when run).

The use of barriers is entirely a function of the code and requirements of the country or regulatory agency where the unit is installed, OR the

policy of the company which is operating the unit. There are thousands of units operating in all manner of applications around the world which



policy of the company which is operating the unit. There are thousands of units operating in all manner of applications around the world whichdo not and have never had IS barriers installed.

So, only you can decide if you can operate without the barriers. We don't know the application or the site conditions so we can't tell you to bypass

them. We don't know if the IS barriers are installed on the LVDTs or the servo-valve outputs.

When you're trending oil temperature, you're not trending the temperature of the hydraulic pump discharge. You're trending the oil temperature ofthe oil at the inlet to the hydraulic pump.

I really don't understand this whole thing; just when I think I'm getting a handle on it, you say "... I do not understand why three servos jump at

the same time and it happen on all machines but at different times...." By "three" servos do you mean all the coils of a single servo valve? Or doyou mean all the servos on all the machines? You say it's only on the SRV, but then you say it happens on three servos on all machines at

different time. Is this happening on the SRV of all machines at different times?

Again, I can't imagine why the null bias spring tension would change during operation, or why the coil resistance would change during operation.Heat would seem to me to be the most likely cause for either, but I've never heard of this problem, or a problem described like this.

And, a lot of times that is a part of the problem: Someone attributes an occurrence to this reason or that reason without any real evidence that this

reason or that reason is causing the problem.

You don't know if the coil resistance is changing or if the null bias spring tension is changing.

All you know is that a position error is changing (increasing) and that you can change (reduce) the position error by changing the null bias currentvalue downloaded to "the Mark VI" and we don't know if you're downloading the change to one VSVO or to all three VSVOs one at a time.

I would really like to help solve this problem, but, again, every time I think I'm understanding what's happening then I re-read the posts and I get

confused even more.

I really think MIKEVI has suggested a good course of action, and that you need to also trend the P2 pressures of all the processors to try tounderstand what's happening.

You should be trending the P2 pressure of each processor, the LVDT feedback from both LVDTs as read by each processor, and the servo current

outputs from each processor.

I'm also keenly interested to try to solve the problem with the outrageous null bias current. You say you've replaced this servo and the new onestill behaves the same way???? And you've tested the servo current polarity under the individual control of each processor for this device (GCV or

SRV or ???) and it will closely maintain the position with only one processor????

I think there's a typo in your chart, because you've previously said that the GCV of one unit is the one with the outrageous null bias. We reallyneed to know what the running servo currents for each processor are for the servo with the outrageous null bias value.

Are you sure there's not an oil leak somewhere in the actuator of the device which this servo is installed on?

I apologize if my response seems a little "random" but I'm really baffled by this and at the same time very interested to solve these problems.

You should know this: On every GE-design heavy duty gas turbine operating anywhere in the world, there is likely some error in position

feedback vs. position reference on one or more servo outputs. Some more than others. But, they are all running.

And we're only talking about position feedback vs. position reference. I have been to more sites that have incorrectly calibrated LVDT feedbackso that the actual physical position is way off from the indicated LVDT feedback. And the turbines still run. And run well.

I heard a former colleague say once a long time ago about GE-design heavy duty gas turbine control, "This ain't rocket science." And he was very

correct. If it were rocket science, GE would have been out of business a long time ago because if every servo output had to operate with zeroerror the units wouldn't run. But they do run, and they run very well.

Sure, in a perfect world we'd all like the position error to be zero, and it should be zero. But, it doesn't have to be zero. What counts is: Is the

position error increasing or decreasing?

This has been said many times before on control.com: The value of something today isn't really informative. It's the value today vs. the value lasthour or yesterday or two days ago or one week ago or one month ago or six weeks ago, and whether or not that value has changed in that time and

how fast it has been changing.

Sure, a L.O. header temperature value of 90 deg C isn't good, but if the unit has been running for four years with an indicated L.O. Headertemperature of 90 deg C, would you say it's been running incorrectly? Or would you go to find another way to verify that reading?

In your case, the position reference error seems to be changing relatively quickly and then remaining relatively constant. From what we can

understand. Which changes with every reading.

Sorry; I'm both perplexed and intrigued. And, I'm a very literal person (but you couldn't tell that could you?).





CSA,

thanks for your post. I appreciate your time spent at the computer.

> Can you please list the null bias for each processor for each servo of each turbine?

The NullBias values I gave are the same in each processor for each servo coil (RST). You confused me a little with this question. UsingToolbox I have (v11.02.09), when I select Download Configuration, it goes directly to all three VSVO cards, with no possibility of selecting

destination processor (R, S or T). It could be only possible if all three VSVOs are in Simplex config like for example Thermocouple VTCCcards. To set-up hardware config you only have one common field, not three separate fields for R/S/T.

To have different NullBias values in each R/S/T card, the only way that comes to my mind is to remove Ethernet cables during download. Have

you heard about MKVI having different NullBias values for coils on the same servo? What is your way to achieve that?Gains are of course the same for R/S/T as NullBias and the rest of the HW config.

More on that, I confirmed the config with TSM, gain and null bias values are the same in all three processors.

What I found using TSM checking regulators (command A*pplications -> R*egultors) and then servos (commands S1 for SRV, S3 for GCV andS4 for IGV) : G1/G2/G4 have parameter FIX 1 and FIX 2 equal to zero (for all three GCV/SRV/IGV servos) and MKVI LVDT Min/Max

(0/100% position) values at LIMIT HI VOLTS and LIMIT LO VOLTS. The G3, the one that must have had SRV Null Bias corrected after twolast restarts has MKVI values of SRV and GCV LVDT min/max in FIX 1/FIX 2 fields and LIMIT HI VOLTS and LIMIT LO VOLTS are

different by around +-0.100 (FIX 1 + 0.100, FIX 2 - 0.100). IGV is the same as G1/G2/G4. I'm not sure if you are familiar with this part ofTSM. I found it today looking for any unusual thing. It may be that someone here started the calibration and haven't fix Min/Max value - have

no idea. Can only check on stopped unit. Also TSM has a lot of stuff that can be checked.

> do you think it makes sense to use the same settings for the SRV as for, say, the IGVs or the GCV? <

For some reason we have it (5% position error trip), I can't think of the situation that this protection is needed, but as it is know that SRV isfuel gas pressure controlled valve, its LVDT is used as a feedback for VSVO card (SRV VSVO regulator type is 2_LVPosMax), so loosing the

LVDT the card probably doesn't know how to adjust servo current (what direction to go).Am I close?

> Do you download a new value to one processor? Or to all three processors, one at a time? <

OK. See the trend I have uploaded below:

http://www.speedyshare.com/470864238.html

The whole story started thereYou can see that one day we started having the error. Servo command had to be higher then the position required to drive SRV to desired

position. Suddenly either SRV position increased by overcoming the friction or the moog current increased for some reason and then MKVIreduced SRV command and so we got both value (position and command) at the same level. I'm not sure what was the reason I can only

speculate as above (as I said I greased the SRV/GCV stems with the same result after restart). Then I started my research and found such Moogcurrent step changes on all machines (but again, on other machines the do not cause an position error). Once again, on each machine. lets say

on GT1, all three moogs SRV/GCV/IGV indicates the step change at the same time.Please see the trend I included in my first post. I highlighted each GT set of moogs with different color.

Again, in my post I wanted to confirm if anyone has ever observed these step changes, I had mention GT3 servo problem as an origin.

So, If you help me with all my problems I will be very grateful.

>As MIKEVI says ----snip----

>How many P2 pressure transducers does each turbine have? One or three or ????

>Have you trended the P2 pressures for all three processors to see what they're doing before and after this problem? <

I have just trended P2 (R/S/T) and see nothing (flat lines during the step change that just happened on GT1 and GT3). Will include it in GT3trend at next start-up.

> When you remove the feeler gauges and the valve stems drop and the indication goes negative, that's what's SUPPOSED to happen. <

Yes, that's right (the value (negative) is from 3% to 2.5% depending on machine).

>The purpose for the gap is to ensure that the valves aren't held open by the actuator bottoming out. <

That is exactly how I understand the valve. I'm assuming the gap is for safety if the seat is worn and no gap the valve will leak. Also the

drawing say to check that gap is within desired limits and if no grind the piston rod to obtain it.

> The true zero stroke position is when the valve is closed and the valve stem is touching the bottom of the plug, and that's what the feelergauges do: They keep the valve stem in contact with the bottom of the valve plug when the valve is closed and ensure that at the end of the

calibration procedure that the valve will return to the same position as when it started (which is one of the checks that AutoCalibrate performs



calibration procedure that the valve will return to the same position as when it started (which is one of the checks that AutoCalibrate performswhen run). <

No doubt about it.

>The use of barriers is entirely a ----snip----.

> So, only you can decide if you can operate without the barriers. We don't know the application or the site conditions so we can't tell you to

bypass them. We don't know if the IS barriers are installed on the LVDTs or the servo-valve outputs. <

Thanks for advice. I will feel more confident doing it.

>When you're trending oil temperature, you're not trending the temperature of the hydraulic pump discharge. You're trending the oiltemperature of the oil at the inlet to the hydraulic pump. <

No other option for me right now.

> I really don't understand this whole thing; just when I think I'm getting a handle on it, you say "... I do not understand why three servos jump at

the same time and it happen on all machines but at different times...." By "three" servos do you mean all the coils of a single servo valve? Or doyou mean all the servos on all the machines? You say it's only on the SRV, but then you say it happens on three servos on all machines at

different time. Is this happening on the SRV of all machines at different times? <

Again, as stated the thing above (it's really difficult to me to explain it as English is not my native language).

What I can trend, is jump/step change up/down/up/down (and so on) of the total current of each Moog. But I observed looking at card pointsServoIOutxNVR/S/T that it happens also in the same direction on all three coils (second screenshot I attached, you can call it static observation)

. So, on one single turbine, the change happens at the same time on all 9 coils (3xSRV, 3xGCV, 3x IGV), and you can observe the same on allmachines but from machine to machine it's not synchronized. Please see again the trend and color marked GT's.

>Again, I can't imagine why the null bias spring tension would change during operation, or why the coil resistance would change during

operation. Heat would seem to me to be the most likely cause for either, but I've never heard of this problem, or a problem described like this.

>And, a lot of times that is a part of the problem: Someone attributes an occurrence to this reason or that reason without any real evidence thatthis reason or that reason is causing the problem.

>You don't know if the coil resistance is changing or if the null bias spring tension is changing. <

I agree, I don't want to risk and disconnect servo coils one by one online. What I found after stop that the current is different each time (I register

in Excel sheet all NullBias calculations), not resistance. It stays on all machines 1200- 1300ohm on all machines (barriers give additionalresistance, I think I checked that a year ago).

> All you know is that a position error is changing (increasing) and that you can change (reduce) the position error by changing the null bias

current value downloaded to "the Mark VI" and we don't know if you're downloading the change to one VSVO or to all three VSVOs one at atime. <

All three at one time. Simply calculation, have all three resistance noted before the start last month, I measure the voltage for each coil

separately, divide V/R, summarize all three results and finally divide by three to get average (negative result must be placed as positive inMKVI config and so on). This is the GE calculation way given by the ControlSpec and it works (offline :), for me also online)

>I'm also keenly interested to try to solve the problem with the outrageous null bias current. You say you've replaced this servo and the new one

still behaves the same way???? <

Actually It has been replaced by GE TA, that left it saying it's fine. They just wanted to leave the place :))))

> I think there's a typo in your chart, because you've previously said that the GCV of one unit is the one with the outrageous null bias. We reallyneed to know what the running servo currents for each processor are for the servo with the outrageous null bias value. <

I wrote G4 - on the trend you will find G4\signal - that was the reason I used it instead of GT4. Affected valve is SRV.

> Are you sure there's not an oil leak somewhere in the actuator of the device which this servo is installed on? <

I will check, I have to re-read your posts guys and prepare a checklist)

Finalizing the story: you can see that we have two major problems - very high positive null bias on one machine running relatively good -

position error that disappear after start-up or we have to help it disappears changing NullBias value (we did it last time instead of waiting).

...And one minor that is servos current step changes on all machines that I accidentally discovered.

It took all day to think about answers also have a headache. Sorry if I messed something again.

Regards



Posted by CSA on 21 July, 2009 - 12:42 am

In the early days of Toolbox, it was possible to download to individual VSVOs. Maybe that's not the case any longer; I haven't seen a Mark VI

in a couple of years.

I have never seen trends like this. My initial reaction is that there is something about/with the IS barriers, but that's just because I don't have alot of experience with them and the experiences I have had have not been very good.

As for the friction comment, when I have seen friction on a gas valve (SRV or GCV) the effect has been that the servo current increases with

no change in position until the device "jumps" usually to a position past the reference and then the current changes to try to drive device backto the reference with no change in position until it "jumps" to a position past the reference and this continues and in some cases gets worse.

So, the valve behavior was jumpy and erratic. On either side of the sticky portion of the valve stems, the action was normal. But where theshaft was worn and scored (on a couple of turbines) or where the cylinder walls were scored (on a couple of turbines) the behavior was jumpy

and erratic.

From what you describe, it seems that the current just "jumps" and without seeing the valve position feedback at the time the current jumps,it's difficult to say what's happening.

I don't recognize the IS barriers you cited. Have you reviewed the manufacturer's manuals/documents to see if these are properly applied?

I would like to know how the *circuit* resistance changes when this phenomenon occurs. It would be very interesting to monitor the voltages

of each servo coil and see what happens. A change in voltage would be indicative of a change in circuit resistance. Would it be possible tomonitor the voltage across the IS barrier and the voltage across the servo coil?

I do NOT believe the servo coil resistance is changing, but I've been wrong before and I'll be wrong again. I have seen Moog provide some very

good troubleshooting assistance when asked to get involved. I know they are slow to respond, but when they do get involved they are prettyhelpful.

I don't believe the null bias spring tension is changing.

Have you had your Lube Oil tested for contaminants? And agglomeration (I think that's what it's called)? Some of the newer formulations of

turbine lube oils seem to be causing lots of problems for servos. I think BP has a formulation that has been used successfully by a lot of heavyduty gas turbine users.

When these jumps in current take place, does the turbine power output change? Does the unit continue to run normally or does an operator

have to make a change to keep the unit running as desired?

If the AHJ (Agency Having Jurisdiction (thanks for that Phil Corso!)) would permit a test with the servo IS barriers disconnected it would bevery interesting to see what happens.

I'd really like to know what happens to the position feedback at the time the current jumps.

That's about all I can think of.


The more I think about this the more I think if you're seeing it on all the servos on all the units at your site, the one thing that's common to the

gas valves is the IS barriers. I also wonder if they're also affecting the other servo outputs on the VSVO, such as the IGVs.

I did some checking on the MTL site, and it says the MTL7765ac is primarily for high frequency low voltage applications. Hmmmm..... I alsowonder about the resistance readings you mentioned, which seem much higher than those listed in the tables in Sect. 8 of the manual.


CSA,

I had opportunity to shutdown GT1 and found something interesting.

It seems that "jumps" are not caused by the hydraulic part of the system as these also exist during cool down and after when Auxiliaries areoff.

Trend with some description added (trip occured :) ).


Will try today to bypass barriers and trend again.



> It would be very interesting to monitor the voltages of each servo coil and see what happens. <

The voltage changes for sure, I had usual average value of around -0.320V after last calibration, then when I started the machine and found

the position error, I made another measurement of coil voltages and as described before recalculated the NullBias and downloaded newVSVO config (average voltage this time -0.490V).

> I do NOT believe the servo coil resistance is changing, but I've been wrong before and I'll be wrong again. <

I hope barriers is the reason. Could it be changing MKVI cabinet inside temperature affecting them? I wonder if it's sync'ed with the cabinet

fan switching on/off (I can simply check it by keeping all fans on for few hours.

> Have you had your Lube Oil tested for contaminants? <

Yes we have it tested, but we are trying to make it more often (monthly). Nothing unusual last time, let's see next time.

> When these jumps in current take place, does the turbine power output change? <

Nothing unusual I have trended DWATT as well.

> test with the servo IS barriers disconnected it would be very interesting to see what happens. <

Yep, this I will provide soon.I have to find some extra terminals for barriers bypassing.

Cheers!


This is troubleshooting: Understanding the possible causes of a problem and working to eliminate them one at a time or to narrow the field of

possibilities down to a very few.

I think it's very telling that when these "jumps" occur when the turbines are running that there is no change in power output. The Mark VI isfast enough to maintain steady and stable power output in the face of whatever is causing the servo current to have to change. And, even if the

position error increases (for whatever reason) the Mark VI is automatically "compensating" and keeping the turbine operation stable andsteady.

You have not provided any information on specifically what's happening to the LVDT feedback when these jumps are occurring, and if the

LVDT excitation and/or feedback also have IS barriers. If so, what is the manufacturer/part number?

And here's where I'm going to suggest that the problem is not the changing servo currents, but the changing LVDT feedback. You say that the

position error changes, and when that happens the servo current would change to try to maintain the error closer to zero. Right?

Please trend the LVDT feedback as well as the servo current, and the LVDT feedback from each LVDT would be the best to trend, not thehigh-selected value. Even when the unit is not running.

It would be unusual for the Mark VI enclosure temperature to be changing very much. It should be located in a temperature- and humidity

controlled environment, the emphasis is on control. Are you saying the temperature in the Mark VI panel, or wherever the IS barriers arelocated, changes? If so, by how much during the course of the day? Are there fans controlled by temperature? If so, that's unusual, not unheard

of, but not typical.

I think the barriers you listed are considered "passive" devices and don't require bus power, but the manufacturer instruction book is reallyunclear on these particular barriers. If they are powered, where do they receive their power from?

What IS barriers are used for the LVDTs? Do all the LVDTs have IS barriers?

Another possible test would be to take suitably sized 1000 ohm resistors (heat dissipation) and connecting them directly to the TSVO in place

of the barriers and then trending the servo current. (This would have to be done when the unit is not running.)

If the current still changes, then I would suggest removing the IS barriers from the LVDT circuits and monitoring the individual LVDTfeedbacks and the servo currents.

This also happens sometimes in troubleshooting: The focus gets placed on the effect rather than the "affect" (the cause). The whole servo

regulator loop needs to be considered: the reference, the feedback, the output.

Let's not drop the theory that the servo barriers may be the problem, but let's not lose sight of the other possible contributing factors. You haveaddressed the hydraulic possibility pretty conclusively. But, servos do get very warm in the environments they are placed in.

Sometimes, the gas valve servos are located at the top of the L.O. reservoir/tank, where the vapors collect. The L.O. in the tank is hotter than

the L.O. header (which is cooled).



the L.O. header (which is cooled).

The IGV servo is usually located in the turbine compartment, which experiences large temperature swings during starting, operation, andshutdown.

Posted by minister on 23 July, 2009 - 12:26 pm

Thanks CSA for you input:

Post update:

>You have not provided any information on specifically what's happening to the LVDT feedback when these jumps are occurring, and if theLVDT excitation and/or feedback also have IS barriers. If so, what is the manufacturer/part number? <

No barriers at LVDT at all. Summarizing we have only 6 barriers installed for SRV and GCV.

I have used terminal block to bypass the barriers and during last 12 hours "jumps" occurred again. We can exclude IS barriers from servo

current change phenomena, but still have to take them into account for two running units, (position error and high positive null bias).

Let's keep in mind, that the offline unit is the one controlled "perfectly" (only servo current jumps" are observed). So let's concentrate on servocurrent jumps.

> And here's where I'm going to suggest that the problem is not the changing servo currents, but the changing LVDT feedback. You say thatthe position error changes, and when that happens the servo current would change to try to maintain the error closer to zero. Right? <

Right. This gave in another suggestion as the error appears some time after restart. It can be possible that we loose one LVDT by vibrations

while running unit (e.g.. loose wiring) or we are getting the LVDT back by the same reason.

> Please trend the LVDT feedback as well as the servo current, and the LVDT feedback from each LVDT would be the best to trend, not thehigh-selected value. Even when the unit is not running. <

Trend set-up and running. A hint for people that want to trend LVDT's. You have to turn Monitors on (we have it unused, but I met the

feature used on some sites). Select 1_LVposition as monitor type, select LVDT you want to monitor (see your Regulator), fill in Mn andMxLVDT1_Vrms with data you have in your Regulator. Keep in mind that Regulators use usually two LVDT's, and you have to separate

them using two Monitors. After the config is downloaded you will see each LVDT value in VSVO Card Point section under MON1,MON2....MON12 (depending of your set-up)

> It would be unusual for the Mark VI enclosure temperature to be changing very much. It should be located in a temperature- and humidity

controlled environment, the emphasis is on control. Are you saying the temperature in the Mark VI panel, or wherever the IS barriers arelocated, changes? If so, by how much during the course of the day? Are there fans controlled by temperature? If so, that's unusual, not unheard

of, but not typical. <

We have HVAC unit for the MKVI room so we have quite good environment.And yes, we have fans controlled by thermostats, I was aware of this as very often we have to clean cabinet ventilation inlet filters and fan

outlet filters from accumulated fine desert sand. After trending servo currents yesterday afternoon with all of fans running the servo currentphenomena still exists. The average temperature measured using TBTC cold junction is 30 deg C (IS barriers are mounted on the back side of

the cabinet where TBTC are mounted, also fans are on MKVI terminal board side only)

> I think the barriers you listed are considered "passive" devices and don't require bus power, but the manufacturer instruction book is reallyunclear on these particular barriers. If they are powered, where do they receive their power from? <

Passive barriers.

> Another possible test would be to take suitably sized 1000 ohm resistors (heat dissipation) and connecting them directly to the TSVO in

place of the barriers and then trending the servo current. (This would have to be done when the unit is not running.) If the current stillchanges, then I would suggest removing the IS barriers from the LVDT circuits and monitoring the individual LVDT feedbacks and the servo

currents. <

Will add to the plan.

> Sometimes, the gas valve servos are located at the top of the L.O. reservoir/tank, where the vapours collect. The L.O. in the tank is hotterthan the L.O. header (which is cooled). The IGV servo is usually located in the turbine compartment, which experiences large temperature

swings during starting, operation, and shutdown. <

That's the case we have, valve servos are mounted inside the tank underneath the valve. IGV in turbine compartment. As the unit will staysome days, temperature should come down and we will see possible effect on the trend.

After all day trending:

Please find another trends (40ms sample time), this time also one toolbox .trn file.

1) change observed only on GCV/SRV (I was measuring coil resistance a while before it happened. Resistance stays always the same):



1) change observed only on GCV/SRV (I was measuring coil resistance a while before it happened. Resistance stays always the same):http://www.speedyshare.com/727988916.html


2) GCV/SRV/IGV jump (see signal description in first file, scale for some signals is different, color the same)"Jump" UP

http://www.speedyshare.com/810365505.html"Jump" DOWN:


I'm trying try do the same with dither off. (As dither is an AC on the DC command signal, can it induct something in the circuit?):

After switching dither off, I observed that all current signals (SRV R/S/T, GCV R/S/T, IGV R/R/T) have moved in the middle of the "jumphigh" and "jump low" servo current values. It's moved down by 2% from the high value or 2% up from the low value.

Is it coincidence, when the dither amplitude is also 2%?

Please see dither off trend: http://www.speedyshare.com/860219617.html

These 4% current switches seems odd when I have checked servo currents loading the machine by 1MW (around +2% GCV and SRV) andcouldn't even indicate the difference on the trend. If it's not the coil resistance that changes, is it whole circuit loop resistance I mean inside

MKVI hardware? The dither seems to be the one not explored :)))

Another experiment: On running unit I have gradually changed dither amplitude for GCV from 2.0 to 0.2% (by 0.2 % step) , and observedservo current travelling from -7 % to -5% without load or position change.

Again, not to confuse you guys, these are trend taken on the offline machine, the machine that has no problems with position error. I will

troubleshoot affected unit ASAP (read when available after shutdown - software modification needed to trend LVDT's and servo outputsseparately).

At the end of the post, after 4 hrs of trending servos with removed dither I can confirm no jumps on offline GT so far (have to finish my shift).

Tomorrow morning I will updated whether jump occurred or not.

CSA, please share your thoughts and suggestions for next checks.

Cheers!


I was thinking in my bed about this changes of the servo current. It's not the resistance that changes, but the voltage and then current had tocompensate the change for the servo to do the same work (Power = U*I = const).

(I recall having sometime unusual higher voltage across coils during NullBias verification, I think I wrote the value before).

Ok have to go to sleep


Update:

After switching the dither off I haven't observed any more jumps last night.

I'm going to do another experiment, put all three servos on different dither frequency and amplitude trend. As MKVI fastest sample time is40ms I should see oscillation at least on 12.5Hz and 25Hz dither trend. I want to check if there is a relation between jumps and dither

frequency.

I suspect that these jumps are just recorded extrema of the oscillation signal when its frequency is higher than the sample rate. The trendingtool gets periodically unsynchronized with the servo current oscillation generator.

Catch you later.


Update 2:



Update 2:

I can observe full curve oscillation for servo currents with 12.5 Hz and 33Hz frequency dither.

The 25Hz (40ms) dither as being the same as trend sample could be catched only as shown on attached the screenshot:


Well, now I will focus on the other two issues I have here. I will write more when I have a chance to make more tests on these machines

Regards


This explanation would make more sense, especially because there is no upset in the turbine output when these "jumps" occur.

But what isn't explained is the position error that you reduce by changing the null bias.

I'm not a fan of dither for GE-design heavy duty gas turbine applications. The nature of operation doesn't really cause the control valves torequire dither from my experience, though I've recently been made aware of some "lore" (because it's not documented anywhere) that there

was non-configurable dither built into the Mark V. But there wasn't any dither in the Mark IV or earlier Speedtronic panels because the servooutputs were all analog circuits and there wasn't any dither that I was aware of.

GE Trending tools can be misleading, as can any manufacturer's. Technology is great, but, digital technology has its limits, and this is one of

them. An old-fashioned pen-and-ink chart recorder had its limits as well. In general, and this case is probably a great exception because of thevariable frequency rates for the dither option, the GE Trending tools are pretty good because the execution rate is not generally higher than the

fastest trending rate. But, again, this could be an exception because of the rate issue. Very highspeed toggling of discrete inputs is also hard tocatch on GE Trending tools.

This explanation also makes more sense because of the repeatable nature of the "jumps". They never seem to increase or decrease by more

than the same amount, at least from the trends you've sent.

But, the position error, which I haven't really seen a good trend of, is still puzzling.


> But, the position error, which I haven't really seen a good trend of, is still puzzling. <

Please see another trend I found in my collection for the position error. http://www.speedyshare.com/900781673.html

The error wasn't so high that time and went back to normal by itself. (I only changed NullBias having the alarm above 3%).Odd is, the step change is recorded at the same moment, it could be that what caused the error created also a delay (momentary desync) for

trending tool (???)

(what happens during that moment can be seen on this zoomed trend: http://www.speedyshare.com/585332895.html)

Looking at the trend and step changes, the overall picture is very confusing - that's why I wanted to raised the step changes issue.The dither on this machine has been disabled yesterday (machine online).

What I need for my tests is to set-up servo currents and LVDT's to be trended all separately.

Regards

Posted by minister on 27 November, 2009 - 6:25 am

An update (sorry for not updating for so long)

1) The turbine with reversed (positive null bias, negative as a value for MKVI config).

I managed to do some tests:

- barrier bypass - no change

- swapping the MKVI hardware, SRV and its LVDT's connected in place of GCV equipment - no change



This gives me a hint to look at the field hardware, as I haven't witnessed the previous MOOG replacement I'm not sure if it's true there was a

replacement.

I noticed that error on SRV position (positive one) of the value +1.4% is may cause a fired trip although the turbine is running fine on load asthe position is not considered in the process control loop (as we know the SRV reference will try to maintain the interstage pressure

proportional to the turbine speed regardless the LVDT feedback, at least on my machine where it's only used by VSVO hardware and HMI).

What has happened here was the valve being open during shutdown by the given above 1.4% while the command was 0% from the MKVI.This resulted in gas leak through SRV (Exhaust went up to 500 deg C!) with GCV being still open at about 6% and the speed at about 20%.

Then the protection tripped the turbine (shutdown exceeded the given period of time)

Funny, next day the position and the feedbackafter the restart were correct.

I will have to work on this valve later.

This is just an information (useful I think) what may happened with an error on the valve.

2) The other machine with jumping error that could be corrected adjusting the Null Bias online.

I have the LVDT's and moog currents setup in the MKVI to be able to trend as separate signals for each coil and LVDT, but the turbine isrunning fine since. Nothing unusual trending LVDT's or the moog servo current.

I will update the thread with any news I find.

Regards!

Posted by MIKEVI on 18 July, 2009 - 3:04 pm

Dear Minister,

I offer this suggestion as something to trend, although from your post it is difficult to tell if this is happening on one gas control valve only or multiple

valves. And if the problem is on one unit or multiple units.

Anyway I had an issue recently with a frame 7ea machine. We were intermittently getting an alarm for low P2 interstage pressure, at the same time wenoted the SRV valve position was erratic. Trending the position of the SRV in the MKVI confirmed that the SRV valve was not holding a steady

position, under constant load and constant main inlet fuel gas pressure. I trended the SRV servo current known as "fagr" and found it was erratic duringthe event. I then trended the individual servo currents from each MKVI core, in my unit these are called FAGR_R, FAGR_S, and FAGR_T. What I

found by running a high speed trend, 40ms, was that intermittently the "T" core would decide to fight the other 2 cores, it would try to close the valve.The other 2 cores, "S" and "T" would respond by opening the valve. Then "T" would decide to go back to normal, and "R" and "S" would have to catch

up.

I don't know if this will help, but it would be something else for you to check. Please also note that replacing the VSVO card repaired the problem. Alsonote that I have seen some sites where the signals were never added at the card level to be able to trend the servo currents individually. Look at your

.m6b file, under the VSVO card points, for the given regulator you are having trouble with look at the "ServOut#NVR, ServoOut#NVS, and ServoOut#NVT. If there is no point associated with it, it will say "not used", if that is the case you will need to create a point for each card point and download

before you can trend the currents individually.

Again this may not be the problem, but it would be something else you can check. Lastly this assumes that your MKVI is a TMR system, and not asimplex.


Dear MIKEVI

Thank your for reply.

I have read your post about the servo current trend 40ms before. I have this it in my plans but have to wait for scheduled shutdown to create new signalPINs.

Good idea with testing another VSVO. I realized I have one unused VSVO (R/S/T) and two terminal boards.

minister

Posted by MIKEVI on 20 July, 2009 - 10:55 am



Posted by MIKEVI on 20 July, 2009 - 10:55 am

Minister,

the great thing about GE's control systems are that they have lots of great tools to use for diagnosing problems. The downside is that all the sametools are not given to all the customers (some .m6b have pins, some don't etc.) I am not sure where the ball gets dropped, either during

commissioning or straight from Salem. But no matter, we have this forum full of knowledgeable people to take advantage of. And best of all wehave a participant such as yourself that provides the information we need to try and help solve a problem, and hopefully we all learn something in

the process. I wish you continued luck with your issue, and look forward to you resolving the problem.


MIKEVI

The forum is great thing, I agree with you.I will update my post if I get any new information.

Posted by RVT on 6 June, 2010 - 9:52 am

Great reading, was looking for this information for a very long time. thanks guys.

Posted by minister on 25 November, 2010 - 10:38 am

Latest (very late, sorry) update:

1) The turbine with reversed (positive null bias, negative as a value for the MKVI config):

Should have done that test before: I decided to check the behavior of the SRV (I think I made a mistake stating it was the GCV in previous

posts) having all three coils disconnected from the MKVI. The valve went fully open instead close. So, either the Moog is installed incorrectlysupplying the oil from the opposite port or the hydraulic block between the Moog and the actuator is installed incorrectly (if possible), or

something else inside the Moog. Unfortunately I have no detail drawing for the system and have to compare to another machine when possible.

Surprisingly GE left us the machine with servo working in such an unsafe setup.

2) The other machine with jumping error that could be corrected adjusting the Null Bias online.

I think I have finally solved this one. One day we couldn't restart the machine, SRV didn't open. We found that 20HD solenoid didn't workcorrectly - stuck. The WD40 wash helped. I think, before the trip solenoid valve was closing but being sticky was not providing enough oil to

the valve actuator (still draining some amount) when energized. Then after like two hours running on-line the situation changed, the tripsolenoid valve fully closed providing all the oil to the actuator (pressure changed) and we had to change the NullBias to correct the new

(described above in previous posts) situation.I suspect, that before the cleaning of the trip solenoid valve, after a shutdown when we were recalibrating the SRV, the sticky trip solenoid

made us the same condition (draining some oil to the tank) so we got the wrong null bias again.

Regardsminister

Posted by CSA on 25 November, 2010 - 7:40 pm

Thanks for the feedback!

Have you checked the servo valve part number against the appropriate parts list for the gas valve assembly? Are you sure the servo is the

correct one for the application?

A lot of people think that all the servos on some GE-design heavy duty gas turbines are interchangeable, and while some are, many are not.

I've seen incorrect servos been used in dire circumstances, forcing some rash things to be done to make them work. I've also seen them left inservice for so long because, well, they're working! And, then when they finally get around to replacing the servo (either because it fails or

because of valve refurbishment) everyone associated with the initial effort develops amnesia and a lot of blame gets misdirected. I'm *NOT*

saying that's what happened at your site, but I've seen similar things happen during commissioning as well. There is a popular sentimentamongst some TAs, especially when a job is over schedule and/or over budget to do whatever it takes to get finished and get off site. The best



amongst some TAs, especially when a job is over schedule and/or over budget to do whatever it takes to get finished and get off site. The bestintentions are to send the right part back to site, but sometimes others in the decision-making chain have this same mentality, "It's working! If

it's not broke, don't fix it!"

Anyway, thanks for the feedback!

Posted by minister on 7 December, 2010 - 9:35 am

Thanks CSA,

I haven't checked the part number as I believed the TA during commissioning, that he replaced it with another one taken from the spares,having similar result in operation after. I will try to schedule the check and inform you.

Regards to all of you guys and Merry Christmass!

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