MK IV Temp Control Curve Modification

from the mystery variables department...

MK IV Temp Control Curve Modification

Posted by Chris on 5 April, 2011 - 12:14 pm

We are near the end of a major inspection / maintenance period for one of our two GE frame 6 turbines. During this outage,

GE has upgraded the honeycomb seals at the third stage of the turbine.

This modification requires new temperature control curve setpoints. In the Mk IV, we have two curves: TTK0 and TTK1.

TTK0 is the Base curve, and TTK1 was added with a 'PEAK' operation modification sometime after comissioning, but far

before I arrived.

GE at first was unsure about what our TTK1 was for, I told them about the peak modification. I verified this and sent our

complete 1D list to GE. Here's what were our control curve constants:

TTK0_C 135 psi

TTK0_I 1055 F

TTK0_K 72 %

TTK0_M 2.49 F/FSR

TTK0_S 2.02 F/psi

TTK1_C 139.1 psi

TTk1_I 1070 F

TTK1_K 72.1 %

TTK1_M 2.37 F/FSR

TTK1_S 1.62 F/psi

So, my understanding is that these 2 sets of 5 constants make up our Isothermal (I), CPD BIAS (C and S), and FSR BIAS

(K and M) curves for both BASE and PEAK operation. Similar to what I have on Pages 1 and 6 of this pdf (

http://www.auto-ic.com/CURVES.pdf ).

I worked out the Y intercepts and came up with the following linear equations:

BASE

CPD BIAS -> Texh = -2.02 * CPD + 1327.7

FSR BIAS -> Texh = -2.49 * FSR + 1234.28

PEAK

CPD BIAS -> Texh = -1.62 * CPD + 1327.7

FSR BIAS -> Texh = -2.37 * FSR + 1234.28

This all seems pretty straight forward to me from a technician's perspective. But then I was handed GE's proposed new

constants (they even added an equation at the top of each set of constants... presumably to help 'clarify things'):

CONSTANT NAME Description Value Units

BASE Texh = -27.5676 * Xc + 1355.0956

TTK0_C Exhaust Temp Control curve #1 CPD Ref Corner 134.2619 psi

TTK0_I Exhaust Temp Control curve #1 Tx Ref isothermal 1055 deg F

TTK0_K Exhaust Temp Control curve #1 OFFSET DW bias 65.8827 MW

TTK0_M Exhaust Temp Control curve #1 GAIN DW bias 2.6605 F/MW

MK IV Temp Control Curve Modification http://www.control.com/thread/1302020042

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TTK0_S Exhaust Temp Control curve #1 CPD Ref Slope 2.0270 F/psi

CONSTANT NAME Description Value Units

PEAK Texh = -22.1296 * Xc + 1317.2271

TTK0_C Exhaust Temp Control curve #1 CPD Ref Corner 138.151 psi

TTK0_I Exhaust Temp Control curve #1 Tx Ref isothermal 1070 deg F

TTK0_K Exhaust Temp Control curve #1 OFFSET DW bias 79.9542 MW

TTK0_M Exhaust Temp Control curve #1 GAIN DW bias 1.7624 F/MW

TTK0_S Exhaust Temp Control curve #1 CPD Ref Slope 1.6272 F/psi

First things first, I questioned GE's units on the K and M curve. That was apparently an oversight; they meant to say FSR,

not MW or DW.

My first real question in all of this is, what is Xc in GE's equations above? I had to work out the CPD BIAS and FSR

BIAS curves again with these new numbers because their slope was so steep... and their Y-intercepts were so close...

Here's what I got:

BASE

CPD BIAS -> Texh = -2.027 * CPD + 1327.15

FSR BIAS -> Texh = -2.6605 * FSR + 1230.28

PEAK

CPD BIAS -> Texh = -1.6272 * CPD + 1294.80

FSR BIAS -> Texh = -1.7624 * FSR + 1205.62

So the curves I work out from their points dont correspond to the equations that they have given with either y intercept or

slope. I just can't wrap my head around what Xc represents. The intercept is *close* but not exact, and the slopes are

nowhere close.

The other thing that I can't work out is this. While the CPD curves do not change much with this modification, the FSR

curves do. The base load curves move in a more conservative direction. As FSR increases, we are more limited in

temperature. That makes sense with better 3rd stage seals; more work is being done --- for a constant exhaust temperature,

combustion temperature would have to be higher. Since we haven't done anything to allow higher combustion temps, the

exhaust limit would have to be lower to take this into account. SO...

Why would the slope be smaller than before, allowing for higher temps for a given FSR? (see pdf http://www.auto-ic.com

/CURVES.pdf page 4)

I'm looking for a general concurrence that this shift in curves makes sense or doesn't. And Xc.

Posted by CSA on 5 April, 2011 - 3:30 pm

There are several things that need correcting here.

First, the exhaust temperature control "curves" represents constant FIRING temperature, **NOT** constant exhaust


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temperature. The temperature of the combustion gases leaving the first stage nozzles is the firing temperature, not the

exhaust temperature. The negative slopes of the curves mean that as CPD (or FSR) increases that for the same,

constant, firing temperature the exhaust temperature will decrease. This is counter-intuitive, but it is the way it is.

The rating of the gas turbine is based on the firing temperature, **NOT** the exhaust temperature.

'Exhaust temperature control' doesn't mean they are trying to maintain a constant exhaust temperature. It means they are

measuring CPD (or FSR) and exhaust temperature, and they know that there is a relationship between these parameters

and firing temperature and that firing temperature can be held constant by varying exhaust temperature in response to

changes in CPD (or FSR).

Second, the unit is only supposed to operate on EITHER primary (CPD-biased) OR secondary (FSR, in your case)

exhaust temperature control. A primary- and secondary exhaust temperature reference is being calculated at all times,

but only one exhaust temperature control reference (Primary or Secondary) should be active at any time.

Without being able to examine the sequencing in the Speedtronic at your site it very difficult to say when secondary

exhaust temperature control becomes active. Some older Mark IV Speedtronic panels would not shift to secondary

exhaust temperature control unless there was a failure of the CPD transducer(s). Most did a low-select of either the

primary- or the secondary exhaust temperature control reference (usually signal names TTRXP and TTRXS,

respectively) to decide which value was going to be TTRX (the exhaust temperature control reference).

The intent of the back-up, or secondary, exhaust temperature control is that it would never come into control unless

there was a failure of the CPD transducers. AND, that if there was a failure of the CPD transducers that the unit could

continue to run (while the CPD transducer problem was being quickly resolved!) until control could be reverted to

CPD-biased (Primary) exhaust temperature control.

Third, for the primary exhaust temperature control curve (CPD-biased), the x-axis is CPD. For the secondary exhaust

temperature control curve, the x-axis is FSR. It's very difficult to super-impose the two with any degree of accuracy, so

it's very difficult to compare the two unless one knows precisely the correlation between CPD and FSR.

The intent is that the back-up exhaust temperature control curve should "mirror" the primary exhaust temperature control

curve and be slightly offset from it (to the right of the primary exhaust temperature control curve) by a small amount, so

that if the CPD transducer(s) failed causing the primary exhaust temperature control reference to be greater than the

secondary exhaust temperature control reference that the unit would be able to continue to operate, albeit with a small

increase in power output (which also slightly increases firing temperature, which negatively impacts hot gas path parts

life--slightly, but it's implied that the CPD transducer problem will be quickly resolved).

The basic equation for exhaust temperature control is:

TTRXx = ISOTHERMAL -((BIAS - CORNER) * SLOPE)

where the lower-case 'x' is either P or S (Primary or Secondary).

TTKn_C = CORNER (PRIMARY). in PSIG

TTKn_I = ISOTHERMAL, in DEG F

TTKn_K = CORNER (SECONDARY), in % FSR

TTKN_M = SLOPE (SECONDARY), in DEG F/% FSR

TTKn_S = SLOPE (PRIMARY), in DEG F/PSIG

So, at all times, there are two calculations being made:

TTRXP = TTK0_I * ((CPD - TTK0_C) * TTK0_S))

TTRXS = TTK0_I * ((FSR - TTK0_K) * TTK0_M))

(The above is true when Base Load, the "0" array, is active at your site. When the Peak Load, the "1" array, is active at


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your site.)

I would say that the change in slope of the FSR curves represents a more modest attempt to prevent "overfiring" if the

unit ever operated on secondary exhaust temperature control (which it never should!). Overfiring occurs because the

FSR-biased, secondary exhaust temperature control curve is supposed to be to the right of the CPD-biased, primary

exhaust temperature control curve. As the sloped line is shifted to the right, the firing temperature increases; as it is

shifted to the left, the firing temperature decreases.

I hope it's clear that it's very difficult to have primary and secondary exhaust temperature control curves based on two

different variables (CPD and FSR) be exactly equal to each other at all times. So, that's why the two are slightly offset

from each other, with the secondary being slightly higher (to the right) of the primary. Again, the unit should never

operate on the secondary exhaust temperature control curve, and if it does, it should only do so for a short period of

time (while the CPD transducer problem is being resolved).

As for the Xc value, I've only recently seen this formula and notation, and I've not been curious enough to investigate.

I'm not even going to hazard a guess, because I have no idea what it means, nor how these values are determined, nor

how they are used to derive the TTKn_x array values. I would suggest you ask your GE field service person to ask

someone in GE Engineering what Xc means, and how the values in the formulae are determined. (My guess is that this

formula isn't supposed to be provided "to the field" and that it is considered proprietary and so it will be difficult to get

information about its terms and usage. But, that's just my personal guess; I hope I'm wrong, and that you can find out

what Xc means, and then explain it to all of us!)

Hope this helps! Again, the unit should only operate on Primary (CPD-biased) exhaust temperature control. Secondary

(FSR-biased) exhaust temperature control should only be active in the event that there is some problem with the CPD

transducers, and even then it should only be operated on secondary exhaust temperature control while the CPD problem

is resolved.

And, finally, it's very difficult (at least for those of us without access to the data and computing horsepower that GE

has!) to closely correlate (cross-reference; super-impose) the two graphs (CPD-biased and FSR-biased) on top of each

other to see how they might interact. The best thing I can say about this is to monitor TTRXP and TTRXS when the unit

is operating. And that TTRXS should be slightly higher than TTRXP at all times when operating on Base Load or Peak

Load, so that the unit only operates on Primary exhaust temperature control.

Which really translates to firing temperature control!

Isn't this stuff fun?

Posted by MS5001Fan on 13 June, 2013 - 1:16 pm

This doesn't precisely relate to the issue of primary vs. secondary exhaust temperature control, but I have a few

questions on the general topic.

Background:

- 1x1 combined cycle with a Frame 5 with a 1991 vintage date.

- Control system is Mark IV and is original

- The unit has always operated at base load.

- The unit has always operated with a 100F increase in firing temperature. This is supported by an average GT

exhaust temperature of 1000F while the GT nameplate is 900F.

- Over the past few years, the unit has averaged 40hrs/start.


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A few questions:

- Does an average GT exhaust temp of 1000F corrolate to a 100F increased GT firing temperature? (I'm asking

becuase I'm trying to determine the impact to the hours-based maintenance factor)

- How easy is it to change the firing temperature? Would it require heat balance engineering analysis? Better phrased:

What would it require?

thanks in advance for your input.

Posted by CSA on 15 June, 2013 - 8:13 am

MS5001fan,

I don't understand this statement:

> - The unit has always operated with a 100F increase in firing temperature.

> This is supported by an average GT exhaust temperature of 1000F while the

> GT nameplate is 900F.

I don't know the correlation between exhaust temperature and firing temperature (the temperature of the combustion

gases leaving the first stage nozzle of a typical GE-design Frame 5 heavy duty gas turbine). I would tend not to think

it is 1:1, but that's just a guess; I may be completely wrong. At any rate, I don't believe it's the same ratio for all

GE-design heavy duty gas turbines based on some conversations I've had with Performance Test engineers over

time--but I've never directly asked or read anywhere what the relationship is between exhaust temperature and firing

temperature.

Every Speedtronic turbine control panel is provided with a Control Specification document which lists the

parameters and setpoints used for controlling and protecting the unit. If you have a unit packaged and provided by

GE of Mark IV vintage (presuming the unit was installed new with the Mark IV) you should find the Control

Specification in Vol. I or II of the Service Manuals, usually in the back of the manual in a tab labeled 'Control

Specification.'

Depending on the vintage of the Mark IV, there may be one or three documents comprising the Control Specification.

If there are three, you would be looking for the one called 'Control Specification - System Settings.' In Sect. 03 you

will find a listing of the exhaust temperature Control Constants (setpoint values) which were the original ones

provided with the unit. My guess is someone changed one or more of the Control Constant values to achieve a higher

firing temperature--and exhaust temperature. By comparing the original values to those running in the Mark IV you

should be able to see what was changed, and what it should be.

The Mark IV Maintenance Manual provides instructions for how to change Control Constants, and how to make the

change permanent (the latter depends on the vintage of the Mark IV). The Control Constants you are looking for are

contained in an array of Control Constants, and they usually are named TTKn_x, where 'n' is an integer from 0 to 7,

and 'x' is a letter like C, I, S, M, K, etc.

Hope this helps.

If the Control Constants in the Mark IV match those in the Control Specification, then you're going to need to contact

GE or some other firm for more help with determining what to change and how to change it. If all the Control

Constants are the same as those in the Control Specification then one of two things has happened: GE issues an

as-built Control Specification drawing, in which case they should have issued a new nameplate, or, the

logic/sequencing in the Mark IV was modified in a very unusual manner, and that will require someone

knowledgeable in reading relay ladder diagram logic such as is used in the Mark IV to review and understand in


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order to make any changes.

Please write back to let us know what you find. Hopefully some other contributor will write back with some idea

about the relationship between firing temperature and exhaust temperature. I've just always presumed it was not 1:1,

but I've never investigated it more fully.

Posted by Kevin on 23 November, 2011 - 11:26 pm

Hi Chris,

I think Xc means compressor pressure ratio (Xc=Pout/Pin). The Pin, if you reverse calcutae from the coefficient of Xc,

will arrive to 13.6 psi, which I think has taken into consideration of the total pressure drop due to intake and bell mouth.

The pressure ratio formula has been adopted when GE introduces MarkVI.

Regards,

Kevin

ITS-Industrial Turbine Services GmbH

info[at]turbineservices.asia

Posted by GE lover on 19 January, 2014 - 4:50 pm

Xc is the pressure ratio.

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MK IV Temp Control Curve Modification

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