Competitive advantages of BTG Rotating Consistency transmitter type MEK-2300

P2007en © BTG 2003

Ingemar JanssonBTG Pulp & Paper Sensors

Competitive advantages of BTG Rotating Consistency transmitter type MEK-2300

Competitive advantages of BTG Rotating Consistency trans-mitter type MEK-2300This paper is based upon BTG Consistency seminar slide presentation available from BTG. It covers the MEK-2300 Rotating Consistency transmitter section of this presentation.

Fig 1: MEK-2300 Rotating shear force measurement.

Introduction:The MEK-2300 Rotating consistency transmitter uses the shear force principle. It's a complete family with different variants specified for most applications in the P&P industry. It can measure between 1-16% consistency.

Fig 2: Rotating shear force consistency measurement. Before discussing all competitive advantages and details for the transmitter it is very important to understand some basic facts. When studying the data sheets for different kinds of consistency transmitters, produced by BTG or some other company, you must understand what a data sheet really says. All data that are stated here are generally based upon practi-cal and theoretical studies obtained in a laboratory environ-ment. The data are probably fully correct and can be trusted, but are normally only valid during the specific test condi-tions. At such a laboratory test you try to keep all variables as controlled as possible to focus on the specific data you are looking for. I.e. if you are trying to find the data for "Sensi-tivity" you use one specific kind of fiber at a specific consist-ency and freeness level, controlling the temperature and the flow velocity etc. Then you extract a large number of lab samples in a very controlled way and evaluate the result in the lab also using very controlled conditions etc. The result

is then statistically evaluated and finally you obtain a figure like: "better then +/- 0.005% consistency" for the MEK-2300 rotating transmitter or "+/-0.075%" for the active blade trans-mitter MBT-2300. Looking on these data you see that there is no major difference, independent on what transmitter you study. The data stated in the data sheets are basically fully sufficient for all applications, for all transmitters!

The question is, how can you trust these data and use them in a real specific application? In the specific application/position you probably have a lot of variations in the pulp suspension. The fiber type, the flow velocity, the temperature, conductivity, air content, pressure etc.; all may change from time to time. Most probably you never have laboratory conditions in this application and you will see distinct differences between the different models, comparing them against each other.

Unrivalled transmitter for the most demanding

and difficult applications.

Provides the firm foundation for optimal process control.

Rotating shear force measurement

MEK-2300Consistency Consistency transmittertransmitter

This paper will discuss these differences, based upon theo-retical evaluations and practical experiences. It will show that MEK-2300 is an "Unrivalled transmitter for the most demanding and difficult applications. And why it Provides the firm foundation for optimal process control." The paper will also describe why MEK-2300 still provides the most accurate, safe and reliable consistency measure-ment on a general basis although new measuring principles have emerged over the years.

MEK-2300

Rotating shear force consistency measurement provides:

• High accuracy

• Safe operation

• Universal use

Presently still the safest and most accurate method to measure and control consistency

Consistency Consistency transmittertransmitter

• Background

• Product description

• Applications

MEK-2300

Rotating shear force measurement1-16 % Consistency


Fig 3: Background, Product description, Applications.

The Paper:The paper is divided in 1. Background for the transmitter, 2. A description of it including its competitive advantages compared to other transmitter types and 3. Applications; some typical applications where it shows specific advantages over other instruments.

P2007en © BTG 2003 2

Let´s start with 1. Background for the transmitter:See fig. 4 Rotating shear force.BTG is one of the pioneers making consistency transmitters. The first model was developed in the late 20´s by the former of the original Swedish company AB Källe-Regulatorer; Dr.Torsten Källe. At that time the transmitter was used together with the hydraulic controller, also developed by Dr. Källe, connected to a control valve for the dilution water. A complete consistency control loop was introduced.

The first model was quite simple and it was a kind of prede-cessor of the later static blade shear force transmitter. Already at that time, Dr. Källe understood that you have to avoid the impact of different flows and he implemented a pump in the system to create a stable flow. This is the princi-ple still used in the BTG rotating transmitters of today, although the transmitter is completely different in its design.

Many models have seen the light over the years. In the mid 50´s the K22 model came. It opened up the world market to the company, introducing a simple and reliable way to con-trol consistency. It was used together with the hydraulic con-troller tied to the dilution valve to form a simple easily maintained control loop. The K22 model became a large success and was produced in large numbers for 30 years, a record hard to beat. By time the mills closed their process, no open vessels anymore, and a new in-line measuring prin-ciple was required.

In the early 60´s the K30/K31 model was developed. It was an in-line transmitter using a rotating sensing element to achieve high sensitivity and accuracy and a propeller to avoid the impact of flow changes. Patents were granted.

Thousands of units were sold all over the world. It intro-duced the company, Eur-Control which was the name then, to the large American market and became an immediate suc-cess there. At that time DeZurik, an American company, was the dominant player there with their rotating transmitter.

The K31 model was economic and easy to find a place for in the piping system since it could be fitted close to the pump. It was recognized by its reliability and many units have been working in the field for 20-30 years. It was originally made in a version to fit the Källe hydraulic controller but later on modified to provide 3-15 psi pneumatic output signal and 4-20 mA electric signal. Pneumatic controllers were very pop-ular at that time so the most common transmitter type was the pneumatic one.

The measuring principle developed for the K30 model is the one still used for the BTG rotating transmitter, although the details have been refined by time.

In the mid 70´s the MEK and MPK-41 model came. The measuring principle from K31 was the same but the design of the transducer for the electrical model was considerably mod-ified to give a more reliable signal. The mechanical design of the drive part was simplified using a belt driven system instead of an oil-filled gearbox.

The weight was reduced from 60 to 40 kg and the target was to make the transmitter less costly to produce, resulting in a lower sales price.

Many targets with the project were reached, the transmitter was cheaper and the MEK-41, the electrical model gave a more accurate measurement. The model however suffered from maintenance problems mainly relating to the stuffing box for the rotating shaft. The model was produced between 1974 and 1984 and also here several thousands of transmitters were sold. A need to increase the reliability of the transmitter and decrease the maintenance cost at the same time as the measuring accuracy was maintained, or preferably improved, was the base of the project to develop an improved model; the MEK-2000 (electrical output signal) and the MPK-2000 (pneumatic output signal) models including their many vari-ants for specific applications.

In the 60´s

In the 50´sIn the 20 and

30´s

In the 70´s In the 80´s

K31

K22K3

MEK- 2000

Rotating shear force Consistency transmitters. Models over the years

Fig 4: Rotating shear force Consistency transmitters. Models over the years.

3 © BTG 2003 P2007en

P2007en © BTG 2003 4

The pneumatic controllers had become more and more out-dated and replaced by electrical controllers or even DCS, so the MEK-2000 became the most popular model,These models became a very large success for BTG. They are known for their very high reliability and good measuring properties. Close to 10 000 units have been produced between 1984 to 2001. The present model, MEK-2300, is mechani-cally very similar to the early produced units although numer-ous modifications have been implemented over the years.MEK-2000 had an analogue electronics, but to increase the measuring accuracy a digital electronics can bring considera-ble advantages. The output signal can be linearized, which result in an accurate measurement over the entire measuring span, and not only at the control point. With different measur-ing ranges you can calibrate the transmitter to read very accu-rate even if you change fiber grade or type etc. This resulted in a new model, the "intelligent" MEK-2200. The difference to the -2000 is a different electronics unit, the control box mounted on the wall. This model was produced, in parallel to the MEK-2000 model, from 1992-2002. Several thousand units have been sold.

The need to decrease the sales price and to simplify for the user resulted in the BTG Smart family where all consistency transmitters and also other kind of transmitters were coordi-nated. All transmitters are then fed with 24 VDC from a junc-tion box with a common design to fit all models and using a handheld terminal for their setting. The user interface is very similar for all the instruments resulting in an easy training procedure and great user friendliness. The MEK-2300 model came in 1995 and in the beginning it used a simple junction box (type JCT-1000). The Junction box is mainly a box where the connections for all cables and handheld terminal are made and also a convenient place to fit the power supply. In year 2000 this junction box was replaced with type JCT-1100 where a display and pushbuttons for sim-ple settings was introduced together with a new surface mounted digital processor card in the transmitter. Totally from 1984 to 2002 close to 15 000 units in the series starting with MEK-2000 and ending with MEK-2300 has been sold. If you include the older rotating types over 30 000 units have been sold over a period of more than 80 years. This is one of the fundamental reasons what makes BTG the lead-ing consistency company.

MEK-2300

• Almost 15 000 units installed – MEK-2000 series

• Over 25 000 units installed – all models

• Installed in Most Mills – All over the World

Proven product – Proven benefits – Proven process savings - Proven profit $$/££…

ConsistencyConsistency transmittertransmitter

Let´s now continue with the Product description.

The rotating shear force principle is a unique principle. You should not equate this method with other shear force measure-ment principles used; it is very different from the measure-ment made by blade transmitters or even by other types of rotating transmitters. They are all inferior in all respects, which will be proven in this paper.

The product:The MEK-2300 model is distinguished by1. High measuring accuracy, i.e. its general design properties and ability to handle disturbances of the measurement. 2. Safe operation; i.e. why it is able to work well over a very long time of operation and still keeping its high accuracy. All limitations are known and you know that you can trust the measurenent. No worries, you can sleep well at night! 3. Universal use I.e. why you are able to use it almost every-where in the process with a good result and in what specific applications it brings an added value.

Fig 6: Very high measuring accuracy and repeatability.

High measuring accuracyLet´s discuss the High measuring accuracy properties. Then you first have to understand how the transmitter operates and why it's designed the way it is. We will not describe the func-tion of it in this context, it is already described in the data sheet, in the service manual etc. We will just touch the benefits of the design.See fig. 7 Measuring principle

Fig 5: Number of installed units over the years.

• Very high measuring accuracy andrepeatability

• Universal transmitter suitable for most applications

• Very high reliability minimizes need of maintenance

Consistency range: 1-16%BTG transmitter MEKBTG transmitter MEK--23002300

5 © BTG 2003 P2007en

Fig 7: Measuring principle.Representative and stable measurement: Looking on the part of the transmitter located into the pulp stream you find a rotating sensing element and a rotating propeller. They both rotate with the same speed. The pulp is mixed by the propeller and pushed to the sensing element. If you study fig. 8 you see a comparison of how large portion of the pulp volume in the pipe you are measuring on. We compare MEK to blade and optical transmitters. The figures are relative to each other and no absolute values. You can see that MEK measures on a five times larger volume than a blade transmitter does, i.e. the sample volume you measure on is five times larger. It is very important that you really measure on a representative sample because it will have a direct impact on the measuring result. The larger the sample is, the larger is the possibility for measuring on a representa-tive sample! With MEK it is less critical than with other trans-mitter types that the pump is a good mixer of the dilution water (which it seldom is). It means that the measurement will be more stable and accurate using MEK. It is commonly known that blade transmitters often show unstable signal resulting in a need to use a high damping (filter time), which results in a slow and inaccurate consistency control. This is partly a result of this fact.

MEK-2300Measuring principle

4-20 mA /HART

Force balance transducer with feed-back function

Difference in light opening

450 rpm


MEK-2300Measured relative volume

Rotating: 100 Optical: < 1Blade: 20


Fig 8: Measured relative volume

Flow independent, high sensitivity to fiber consistency, high signal-to-noise ratio and physical measurement: The sensing element is rotating by its own force independent of the flow velocity in the pipe. Its outer speed is typically around 5 m/s, i.e. double the normal pipe flow velocity. It is also fed by its own flow, supported by the propeller, independent of the pipe flow velocity. Provided that the transmitter is fitted into the pipe in a correct way, it will be virtually unaffected by flow changes in the pipe. No other transmitter type has this advantage and flow velocity problems are very common, particularly for static blade type transmitters, but also for rotating transmitters using different measuring principles and sometimes even for certain types of optical transmitters.Due to the rotating motion of the sensing element, the transmit-ter will also be very sensitive to consistency changes, but only to the fiber consistency! Compared to blade transmitters this fact is very important when measuring at lower consistency levels due to the higher shear force acting on the sensing ele-ment. Also type of fiber, fiber length, freeness, amount of fines etc. will have an impact. More of this discussion later. Gener-ally spoken you should not use a blade transmitter on a consist-ency lower than 2-2.5% depending on these factors, while the MEK can be used down to 1-1.5%. The signal-to-noise ratio shows the quality of the measurement signal. The higher this level is the higher the quality. You can say it reflects how much of the measurement signal consists of non-desired background noise and how much the actual con-sistency signal is. Compared to static blade transmitters it has been found in independent tests that the ratio for MEK is approximately three times higher. So, also in this respect MEK provides a more representative measurement.Compared to measurement methods like microwave, nuclear and certain optical types, the signal quality of MEK relies on direct physical measurement properties. It's the forces created when separating the fiber network because the sensing element is cutting through it (together with drag forces acting on the side of the sensing element). There are no complicated com-pensation algorithms needed for MEK. There are no additional sensors (for compensation) needed, which may need their own calibration, and no such components needed that may fail and disturb the consistency measurement. MEK provides a straight on, accurate and reliable measuring method. Protected sensor: Since the sensing element of MEK is located in the measuring chamber, inside the pipe wall, it will be protected from foreign objects like stones or pieces of metal or lumps of thickened pulp, flowing into the pipe at certain occasions (start-up or shut-down etc.). Static blade transmitters are often mechanically damaged while MEK very seldom is damaged due to these reasons. Accurate and long-time stable signal transducer: MEK uses the force-balance principle with a built-in electro-magnetic feedback function. As you can see from the description of the measuring principle, we measure the angular difference between the two shafts when a force created by the fibers in the pulp suspension acts on the rotating sensing element. Then you may ask yourself, why not just measure this angular deflection using a force-movement type of system; why use a complicated force-balance system? The reason is that we are looking of achieving very high accuracy and sensitivity and long-time stability of the transducer system.

P2007en © BTG 2003 6

Using a force-balance system you can work with very small movements of the mechanical parts and thus obtain a very high sensitivity, i.e. a small consistency change will give a large signal difference. The signal resolution will improve as well as the signal-to-noise ratio.You will also simultaneously obtain a compensation for aging of components (rubber parts etc.) and thus a very good long time signal stability of the system.Fig. 9 is a summary of all the specific features for MEK-2300 also including how sensitive it is for disturbing factors. This will be further discussed and proven in the next section: Universal transmitter for most applications.

MEK-2300Specific features MEK-2300• Representative measurement: Measure on a large sample, mixed

and separated from main flow: Very good

• Sensitivity for disturbance factors: Grade, Fiber type, Freeness, Kappa No, Flow velocity : Very small - virtually negligiblepH, Temperature: Normally very small - in most cases negligible (can be compensated if large changes)Pressure, Air, Conductivity, Fillers: None (If total consistency is required compensation can be made)

• Signal quality: Signal damping and filtering: Very low - high quality Response time and deadtime: Very short - good control abilitySignal-to-noise ratio: Very high - high quality

• Calibration: Single sample or preset calibration for quick start. Multi-point calibration for maximum accuracy

Gives best measurement and control result - at almost all applications


Fig 9: Specific features MEK-2300How disturbances will affect measurement.As we concluded earlier, MEK is virtually unaffected by flow changes. Fig 10 is one proof of that. The test shows the actual consistency deviation, shown in % consistency, at a typical consistency level: 3.3%. If you compare to a static blade trans-mitter (type A = common type), and imagine that you change the flow velocity from 1 to 3 m/s, you find that instead of showing 3.3%, like the lab sample does, the transmitter shows 3.75 %, an error of 0.45% consistency or approximately 14%. The MEK does not show any error!

Fig 10: Flow dependence

Output (%)

Cons. (%)

MEK-2300Fiber and grade dependence


Fig 11: Fiber and grade dependence MEK-2300

Comparing to a static blade transmitter you can see that the deviation is much larger than for MEK, fig 12. The shear force is here shown in force N (Newton) acting on the sensing element, but please compare the relative difference between different fiber types. If you compare the two figures you see that for MEK the difference around 3% consistency is just above 0.5% consistency while it for the blade transmitter is 2.5% consistency. The fiber types are not identical but it still gives a clue.

Reference: Valmet Automation

Fiber and grade dependence

G = TMP, freeness > 200 ml

F = TMP, freeness < 200 ml

E = Groundwood

J = Recycled fiber (OCC)

K = Recycled fiber (newsprint)

StaticStatic bladeblade transmittertransmitter

Fig 12: Fiber and grade dependence Ref. Valmet AutomationComparing a static blade and MEK, at the same conditions, changing from a kraft pulp to a mechanical groundwood pulp and imaging you try to control consistency to 3% you can see the result in fig 13.

The next figure shows how different types of fiber qualities influences MEK. Here you have very different pulp qualities, from a CTMP pulp giving a very strong fiber network and thus very high shear force, to mechanical groundwood with short fibers and a low network strength. MEK will have certain sen-sitivity for different fiber types and different pulp qualities. Since MEK-2300 has four different measuring ranges it can however be individually calibrated for each pulp type and thus avoid this problem.

7 © BTG 2003 P2007en

MEK-2300Fiber and grade dependence

MEK-2300 is much less influenced of changes in furnishMEK-2300 vs. Common type Statical blade transmitter

Blade

Rotating

3-2.622.62 =

3-2.903

=

0.382.62 = 16.4%0.10

3 = 3.3%

KR = Sulfate Kraft

GW = Mechanical groundwood


MEK will show 0.1% difference, i.e. 2.9% instead of the cor-rect 3% while the blade shows 2.62%, i.e. an actual consist-ency error of 0.38% that is almost four times larger than for MEK. This is a fact that is not commonly known. Competition claims that their static blades are just as good as an MEK but this is one of the proofs that it is not!Please note that this error does not only apply when changing from one specific pulp type to another, also changing or hav-ing different blends or fiber type (raw material) or changing the freeness level will have the same relative impact, a four times larger error for a blade transmitter, static or active, it's the same in this respect.

Fig.14 "What does Total consistency mean"The next figure, fig. 15, comes from a test made at BTG R&D. The idea was to start with a 3% consistency pulp and then just add fillers and observe the consistency reading. To be able to compare the result between the different measuring principles all results were recalculated to show the absolute consistency deviation expressed in % consistency. Naturally if you just add fillers, the total consistency will increase which is illustrated by "lab. cons.". As can be seen, "optical reflection" (BTG OCT-2300) is overcompensating, while all shear force transmitters show a very little influence, they all mainly measure the fiber consistency only.

Fig. 15 Influence of fillers, different measuring principlesA later made test, fig. 16 conclude that MEK does not sense the fillers, only the fibers. MEK is a fiber consistency trans-mitter.

Fig. 16 Influence of fillers MEKThe most common applications where the question if the transmitter measures total consistency or not, applies for broke chest, mixing chest and machine chest. This is where perhaps 90% of all microwave transmitters have been installed and thus replaced or beaten MEK at a project. So, are these applications proper for and safe for microwave transmitters to be installed at?

Fig.13 Fiber and grade dependence MEKAnother influence on the signal accuracy is the content of fill-ers of fines. First you have to understand what "Total consist-ency" really means. Fig. 14 explains this. Typical filler components are china clay, calcium carbonate, titanium diox-ide etc.

Total Consistency [%] = Fiber Consistency [%] + FillerConsistency [%]

Example: Total consistency is 3%. Ash content (= filler consistency) is 30 %. This means Filler consistency is 30% of 3% = 0.9% consistency.Fiber consistency is 3 – 0.9 = 2.1% consistency.

What does What does Total Total Consistency meanConsistency mean?? MEK-2300

Influence of fillers

MEK-2300 does not sense the fillers, just the fiber consistency. For total consistency measurement a compenstion can be made.

Fiber consistency %

Filler content %

Sign

al %

Consistency transmitterConsistency transmitterFeedback %

Feedback %

Fiber Consistency %

B-sensor. 10% Hardwood Kraft + 50% Groundwood + 40% coating clay

B – sensor. Fiber composition: 83% mechanicalgroundwood, 17% Hardwood kraft. Increasing level of fillers (coating clay)

Cons (%) Groundwood 3.0% Flow 2.0 m/s Temp 300 C. Coating clay added

Optical reflection

Lab.cons/ Optical Peak method

Microwave

Rotating sensorActive bladeStatic blade

Filler (%)

MEK-2300Influence of fillersConsistency Consistency transmittertransmitter

The microwave transmitter (MIC-2300, Metso MCA or Toshiba) does not measure the total consistency, only part of it. Typically it measures around 60% on the filler, part but this varies. Metso claims that their MCA measures the total con-sistency. This is not correct! Different filler types give different result. Both MIC-2300 and MCA have a possibility to imple-ment a compensation for different filler types and thus partly avoid the problem, at least for many applications. If the filler type varies you cannot compensate for it unless also different measuring ranges are used. Generally you can however say that, correctly adapted, microwave transmitters measure close to total consistency. BTG TCT-2300, however, does not need compensation and is the only transmitter that really measures total consistency.

P2007en © BTG 2003 8

First you must be aware of the general features between the different principles in this context, fig. 17. Microwave trans-mitters are very sensitive for air in the pulp; this is their main disadvantage. We know that attempts have been made to compensate for it, but so far without success. Pulp always contain a certain amount of air, typically around 3%, but it can be much more, 10%, or even 20% can occur in practice if the process equipment is not arranged in a suitable way so that air can enter. Close to the paper machine the air content is typically low, and controlled. This may be the reason why microwave transmitters often operate well here.

Air is present in the pulp in two forms, either tied to the fibers or in the form of air bubbles. The size of the air bubbles depends upon the line pressure; the higher the pressure, the smaller they become. They also become solved in the water with increasing pressure. Thus you need a certain min. pres-sure for a microwave transmitter to avoid influence from the air content to the signal accuracy. One problem, however, is that it is sometimes not possible, with reasonable efforts, to increase the pressure and thus avoiding the problem; the measuring principle cannot be used, the pulp contains too much air. According to information from all companies supplying microwave transmitters the general rquest is min. 1.5 bar pipe pressure. This is valid if the air content is not too high, but what is an acceptable level? Do you know how much it is for the appli-cation in question and are you sure it doesn't increase abowe maximum at certain occasions? Is this a safe and reliable measurement? Will it have an impact on the paper machine runnability?There are many uncertainties. The major user of microwave transmitters, a Swedish mill, claim they need 1.8 bar pressure. At the machine chest they have had problems and were forced to install a larger pump, but then how much does this add to the energy cost, see example fig. 20.Other customers have said they even need 2.2 bar line pres-sure to be able to use microwave transmitters. Of course, this all depends upon the air content they have.Conductivity is not usually a problem close to the paper machine, but sometimes acid is used for pH adjustment and sometimes different dyes can make an impact. Generally, microwave transmitters should not be used where there is a conductivity > 3 mS/cm. You can make compensation at higher levels but since the compensation also depends upon the media temperature, the compensation is difficult. MIC-2300 has here an advantage over competition since compen-sation possibilities are implemented. If the conductivity exceeds 10 mS/cm, the signal becomes too much dampened and it's impossible to measure anymore. Also here MIC-2300 has an advantage over traditional flow-through models; the signal damping increases with distance between the antennas. MIC has a relatively short distance and is thus less influ-enced. The larger the pipe size (using flow-through models), the larger the difference is.For blend, mixing and machine chest applications, micro-wave transmitters have an advantage over MEK in regard to their ability to closer measure "total consistency" as well as their ability to more closely measure consistency if fiber qual-ity changes. This is true.

MEK however on the other hand, is not dependent of the air content, pressure and conductivity. This is also true.So which transmitter type should be used here? The answer is naturally: you must investigate the actual measuring condi-tions; the choice is not obvious without doing that!

MEK-2300

Influence of air, pressure and conductivity

• Inapplicable at high air content

• Must have a min.pipe pressure 1.5 – 2.2 bar

• Can not be used at high conductivity levels. Difficult to compensate at lower levels.

MEK-2300:• Not influenced by air content

• Not influenced by pipe pressure

• Not influenced by conductivity changes

Microwave and Nuclear transmitters:

Fig 17: Influence of air, pressure and conductivity

Fig. 18 shows an MEK installation in Finland at machine chest where they use TMP pulp and add fillers. Naturally it has been proven that MEK doesn't see the fillers, but in this case the mill only wanted to measure the fiber consistency; they are not interested in measuring the total consistency with the consistency transmitter. They have a retention analyzer and thus they find out the filler content and can calculate the total consistency. Using a microwave transmitter here they wouldn't know how large the fiber and how large the filler content were, they would both have been included in one sig-nal. Knowing the fiber content in the paper is of course also an important issue.

MEK-2300 installed at mixing chest at leading paper mill in Finland• Accurate control

• Measures fiber consistency

• Not dependent upon air content and pipe pressure

• No by-pass, installed in-line

Fig 18: MEK-2300 installed at mixing chest at leading paper mill in Finland

9 © BTG 2003 P2007en

Fig. 19 shows competitive microwave transmitters installed at a German mill. As can be seen they were forced to install sep-arate pump loops just to obtain correct pressure for the trans-mitters, an expensive and maintenance-demanding solution. Still the transmitters don't operate fully satisfactorily. MEK was also later on tried here, but because of low shear forces due to bad fiber quality (recycled) and high filler level (30%), MEK was not stable enough at the worst operation cases (short fiber recycled combined with high, and changing, filler level and large flow changes). Also MEK has its limitations.Later on it was proven that TCT-2300 gave the best result at this particular application.In many other applications of this kind it has been proven that MEK works much better than microwave transmitters and many of them have been replaced by MEK. Low pressure and too high air content is the main reason. Recently we replaced two competitive microwave transmitters at the machine chest in Asia. Due to the stuff box design the pressure could not be increased to reach proper level and the transmitters thus per-formed badly. MEK performed very well. At another machine chest application in China we tried our microwave transmit-ter, but the result would have been equal to any other brand. The data looked very good, high pressure far above min. rec. level etc. The transmitter however failed badly, large consist-ency errors, 0.5% and more. The answer to the problem was that the pulp contained a huge amount of air. Naturally the MEK fixed the problem and worked perfectly.This case also showed another observation; when the operat-ing conditions are unsuitable for a microwave transmitter the result is often very large consistency deviations, often 0.5% or more. For MEK the deviation in unsuitable cases is mostly much smaller, in the German case described above it was around 0.2% consistency.

Microwave transmitter installations at mixing and machine chest.

Fig 19: Microwave transmitter installationsAs mentioned earlier you must sometimes increase the pres-sure to be able to use a microwave transmitter. Fig. 20 shows the economical impact on such a measure. It is for one typical case. Imagine the power costs SEK0.5 / UScent 6.54 per Kwh. If you then need to increase the pressure by 1 bar, you see that the yearly increased power cost is approximately USD 8,000. This cost should be added to the increased cost of the microwave (or nuclear) transmitter compared to MEK.

50000

00

10000

0,5 1 1,5 2

20000

30000

[USD] [SEK]

[bar]

Power cost increase/year

Pipe size: 300 mm (12")Flow: 1,8 m/s (0,55 fps)Operation: 8000 h/year

3,27 Cent/KW

0,25 SEK/Kw

6,54 Cent/KW

0,5 SEK/Kw

Pressure increase

100000

150000

20000013

,08 C

ent/K

W

1 SEK/KW

∆Ρ= ∆ρηm

S

Cost influence on pump pressure increase

MEK is not influenced by air / pulp pressure and conductivity like nuclear and microwave consistency transmitters = Higher accuracy and Reduced costs

MEK-2300


Fig 20: Cost influence on pump pressure increaseConclusion: Summarizing MEK features regarding its ability to measure accurately and reliably and its usage for most applications you find that MEK provides very small or no sen-sitivity to most disturbance factors that can occur. It is more expensive than a blade transmitter but superior to them in all other respects.Compared to microwave transmitters it cannot measure total consistency and is more sensitive for fiber property changes but it is not dependent of air, pressure and conductivity changes. Microwave transmitters can only be successfully used for a very limited amount of applications where the con-ditions are well controlled while MEK works mostly anywhere in the mill. Generally MEK is the safest, most accurate and reliable choice.

• Very high measuring accuracy andrepeatability

• Universal transmitter suitable for most applications

• Very high reliability minimizes need of maintenance

Consistency range: 1-16%

Fig 21: Reliability and MaintenanceTransmitter design and maintenance questionsThe MEK-2300 transmitter is a mechanical transmitter with many components. Competitors claim that the transmitter is unreliable and will cause high maintenance costs. Is this true or just an attempt to attack a competitor? Previously we con-cluded that the transmitters have been sold in nearly 15,000 units during a long period of time. Would you really be able to sell a transmitter during so many years and in so many exam-ples if it causes excessive maintenance? The answer is natu-rally not; it's a false statement! Since the transmitter is the 7th generation since the 60´s, all weaknesses have been found and the transmitter has been continuously refined ever since - see fig. 23. For those interested fig. 24 shows major product improvements since the MEK-2000 model was introduced 1984. All of these improvements are aimed to increase the accuracy, reliability and simplify for the user at the same time as the reliability increases and the maintenance demand decreases.

P2007en © BTG 2003 10

Fig 22: Reliability

Fig.24: Major product improvements

Fig 23: Cutaway view

Reliable, well proven design

Consistency range: 1-16%BTG transmitter MEKBTG transmitter MEK--23002300

• New electronics /MEK-2200: 1991• New transmission: 1991• New fastening of sensing element: 1992• New series of measuring vessel: 1994• New electronics /smart model MEK-2300: 1995• New Hand-Held terminal SPC-1000: 1995• New material Avesta 254 SMO: 1998• Single-phase motor: 1999• New Junction box JCT-1100/ Surface mounted card in tx.: 2000• Adapted for Fieldbus and OPC: 2000• Preset Calibration curves: 2000• Windowsbased program, complement to hand-held terminal: 2000

Major Major product improvements product improvements MEKMEK(from (from analogue modelanalogue model MEKMEK--2000)2000)

- All parts and material used in the design are optimized for the very demanding applications in the P&P industry. It's designed to withstand high temperature, corrosive media and atmosphere, high moisture in the surroundings etc. - The most critical parts to prevent failures are seals against the media. Thus a metal bellows seal is used as the rotating shaft seal. This seal type is an exclusive type designed to be very flexible and sturdy. It´s designed to withstand pressure shocks, excessive shaft run-out, corrosive media, certain dry running operations and high temperatures. The wearing faces are made from silicone carbide, a material next to diamond in hardness, to be highly wear resistant; at the same time it can withstand large temperature shocks. A mechanical seal is a wear item, but we have found that the average life time is over five years. More in the paper mill and somewhat shorter in the pulp mill, but it depends very much upon the application.

The flexible sealing between the two shafts is in fact a dual seal; an O-ring made in specially developed highly resistant fluor-rubber to suit virtually all applications in P&P. Only for applications where the black-liquor content is very high another quality, EPDM, is required. As a back up, if the O-ring should leak after a long time in operation, a special rub-ber bushing prevents any further leakage. In all normal cases it is only required to replace the low-cost O-ring on a regular basis as a preventive measure. Typically this is only required after 5-10 years of service.- The motor is a special type. It has tropical insulation and is wired to accept very large voltage variations; it's a wide-volt-age motor. Standard option is a 3-ph motor that typically gives the highest reliability but as an option a 1-ph motor is availa-ble to reduce the installation cost.The shaft bearing arrangement is designed by SKF to obtain a lifetime exceeding 10 year´s operation during highest possible line pressure. The bearings are permanently greased, pre-stressed type angular contact bearings supported in a steel sleeve to obtain a very stiff shaft arrangement that is benefi-cial for the mechanical seal lifetime. To protect from any pos-sible leakage into the bearings they are protected with labyrinth seals. To our knowledge, water has never penetrated into the bearings; it's a very reliable arrangement.The transmission is a modern timing type rubber belt. It is oversized four times in relation to the motor power. Typical lifetime exceeds 10 years before a replacement is needed- The electronics is uncomplicated in its structure to obtain a high reliability. The processor card located in the transmitter is of "state-of-the-art" surface-mounted type. It withstands vibrations very well. This also applies to the transmitter as a whole; we know from all the installations as well as from tests made at certification institutes, that this is true. We also know that high media or ambient temperatures do not affect the transmitter. When testing the transmitter we first inserted the wet part of it into 95ºC water. Then we installed the complete unit into an oven and raised the ambient temperature up to 70ºC; conditions that you hardly see even in the worst possi-ble application. The transmitter still worked without prob-lems. High temperatures and vibrations are commonly known problems particularly for certain static blade transmitter types.

11 © BTG 2003 P2007en

Based upon our vast experience we know that average time before the first maintenance requirement is > 5 years in con-tinuous operation. This typically means a replacement of the mechanical seal, a simple and not too expensive action.We also know that the transmitter typically requires a major maintenance after 10-15 years in operation. Typical action then is to replace all bearings, seals and the drive belt. Still such a repair does not cost much more than to have a static blade repaired (not valid for BTG transmitters, they can be more easily repaired).These times given are average, individual units can operate for either longer or shorter periods, depending upon the appli-cation and other factors that may influence. Preventative maintenance is the key factor to minimize actual mainte-nance. For the consistency transmitter this mainly means that the water flushing system for the mechanical seal should be regularly controlled and cleaned if necessary.The MEK-2300 transmitter has three years warranty, a proof that it is a very reliable unit, which needs very little mainte-nance!Due to its design and the way it operates, certain aspects upon how the transmitter is installed must be explained. Competi-tors accuse the unit to be expensive and complicated to install. This may me true, but in many cases the actual cost for other types of consistency transmitters like blade or microwave types can be just as expensive, and sometimes even higher. This is particularly true if you need to make spe-cial piping arrangement to decrease the flow velocity by installing a larger pipe to be within acceptable level (blade transmitters) or increase the pressure by installing a separate pump loop or replace the pump (microwave transmitters). Sometimes also the static blade transmitter needs a separate pump loop to have a stable velocity. All of these actions are very expensive.

Selecting the 1-ph motor and using the mechanical seal with-out water flushing can decrease the installation cost for MEK. Both of these possibilities can be used for most applications and reduce the cost considerably.Considering the way MEK-2300 is installed, you must con-sider that it can be mounted close to the pump that is advanta-geous in two ways - fig. 25.

Fig. 25 Typical installation

From a user and maintenance demand point of view the loca-tion is important since the transmitter in most cases is easy to reach and handle. The most important point is however its ability to bring a good consistency control, a result partly depending upon its location close to the pump. This will be discussed later on. Fig. 26 show a typical installation of the transmitter, in this case complete with a sampling valve, an item highly recom-mended. In this photo you can also find that the pump is equipped with a variable speed motor. This is an installation that can cause measurement problem particularly when the pump operates at low speed since the pulp/dilution water mix-ing then can become less efficient. As we concluded earlier, MEK-2300 however measures on a very large sample volume and is thus less affected than other transmitter types. This par-ticular installation also proves this theory; the unit works very well also in practice!

Fig 26: MEK-2300 installation photo

Fig. 27 illustrates a typical MEK-2300 delivery content, com-plete with options.

Fig. 27 Typical delivery content

• Optimal Control

• User friedly

• Easy to access – located close to floor level

• All settings made from hand-held terminal or Junction box

MEKMEK--2300 2300 typical typical installationinstallation

• User manual and Service manual, Test documentation

• Options: SPCWin PC program, Back-up card, Printer, SLS-1000 lab sampler device, PC-documentationsoftware, various special service tools, etc.

• Consistency transmitter MEK-23XX

• Junction box JCT-1100

• Hand-held terminal SPC-1000

• Consistency transmitter MEK-23XX

• Junction box JCT-1100

• Hand-held terminal SPC-1000

• Sampling valve type MPS-1000 or MCS-1000 or FVS-1000

• Sampling valve type MPS-1000 or MCS-1000 or FVS-1000

• Measuring vessel or Weld-in stud • Measuring vessel or Weld-in stud

TypicalTypical deliverydelivery contentcontent

P2007en © BTG 2003 12

Transmitter variants and its installationThe MEK-2300 transmitter is available in a large number of variants, and materials. Fig. 28 is an overview of available models. - The basic model, named MEK-2300, is used in consisten-cies from 1-10% and at max. pressure class PN10. The transmitter can be used in a large range of applications. It can even be installed at the wall of a chest, an application where few other transmitters can work.- The medium consistency models for consistency 8-16% are available in combinations for screened and "unscreened" pulp and in pressure classes PN10 or PN25. Compared to the standard transmitter, 1/3 higher torque levels can be balanced to handle the high shear forces and these high consistency levels. With "unscreened" pulp we mean pulp that contains large knots or foreign material like stones or pieces of metal that may mechanically damage the unit due to an impact. To protect the transmitter, the rotating shafts and sensing element are quipped with a special protection, a "banana"-shaped steel bar. Transmitters in pressure class PN25 have a thicker flange to handle the pressure requirements. The MEK-2310 and -2312 models are often used in the bleach plant and then the wetted material must often be selected to handle the corrosive bleaching chemicals. Avesta 254SMO or Hastelloy C materials are then available. - Designed for specific applications, MEK-2308 is equipped to handle the tough conditions after a blow tank.MEK-2320 is designed to handle extremely troublesome applications after a recycled fiber pulper where large amounts or spinning materials like strings and plastic strips are present.MEK-2340 is adapted for open vessels like for a "stuff box" and has very long shafts, up to 1100 mm, and no shaft seal.

STD: 1-10% MC: 8-16% Special: 1-10%

MEK-230010 bar

MEK-231010 bar

MEK-231110 bar/Unscreened

MEK-231225 bar



MEK-231425 bar

MEK-232010 bar/Anti-spinning

MEK-2340Open flow box

Consistency: 1-16%

Materials: 316SS (std), 254SMO, Hastelloy C

Pressure class: 10 bar (std -150 psi) or 25 bar (360 psi)

MEK family. Variants available in MEK-2300 series

MEK-2300ConsistencyConsistency transmittertransmitter

Fig 28: Variants in the MEK-2300 series

The sensing elements are specifically adapted for different applications. Basically the sensing elements are streamlined and shaped like a star for consistency levels up to 10%. The idea is to create a representative measurement and avoid any problems from hang-up. The higher the consistency, the smaller the size is of the sensing element.

The size is selected to maintain a suitable working range for the feedback balancing system and to secure a stable signal combined with high sensitivity for consistency changes.The sensing elements for MC are shaped like an S and devel-oped to obtain good sensitivity and to minimize the flow sen-sitivity.

Basic type Fiber type Lower limit Upper limit Basic type Fiber type Lower limit Upper limit

Normal consistency Medium consistency

AA

BB

CC

GG

HH

II

JJ

210m

m8.3

in

185m

m

7.3in

120mm4.7in

LongShortSludge(2220-2320)

Long

Short

Long

Short

0.8 %0.8 %

1.5-2 %

1.0 %

1.5 %

1.5 %

2.0 %

2.5 %2.5 %

10-15 %

5.0 %6.5 %

8.5 %

10.0 %

150mm6in

125mm5in

100mm4in

80mm3.1in

Long

Short

LongShort

Long

Short

LongShort

5 %

6 %

7 %8 %

9 %10 %

11 %12 %

10 %

11 %

12 %13 %

14 %15 %

18 %18 %

MEK-2300Sensing elements

Specially adapted sensing elements for all applications


Fig 29: Sensing elements

When connecting the transmitter to the pipe, a special mount-ing arrangement is required - fig. 30. Basically an expansion of the pipe to 300 mm is needed when the pipe is between 100-250 mm. This pipe expansion is called "measuring ves-sel". Depending upon pressure, PN10 or PN25, the design is different, a streamlined model for PN 10 and a globe form for PN25. The low-pressure vessel is specially designed to avoid any air being trapped at horizontal installations and also to avoid the risk of plugging at higher consistencies. It is availa-ble in a range of variants; welded or flanged to the pipe, dif-ferent materials, left- or right-hand versions etc.For larger pipe dimensions, 300 mm and larger, a weld-in stud can be welded directly to the main pipe.

Measuring vessel• Pipe size 100-250 mm (4”-10”)

weld fitted (std) flange fitted (option)

• Sampling valve included (option)

MEK-2300

Mounting parts. Low pressure PN10 (150 psi)

Weld-in stud• Pipe size ≥300 mm (12”)

• Inspection cover (option)B


Fig 30: Mounting parts Low pressure

13 © BTG 2003 P2007en

Measuring vessel• Pipe size 150-250 mm (6”-10”)

weld fitted (std)Flange fitted, PN25/ANSI 300 (option)

Weld-in stud• Pipe size ≥ 250 mm (10”),

200 mm (8”) at suitable conditions

MEK-2300Mounting parts

High pressure version. Max. 25 bar at 226 °C (363 psi/439 °F)Available in 316SS (std), 254SMO and Hastelloy C


Fig 31: Mounting parts High pressureConsistency controlThe next topic to discuss is "Optimal Control"; why does the selection of transmitter type compared to another one make an impact on the control result? This discussion may seem theoretical to persons used to the day-to-day problems but still it is of greatest importance to the result; the well working consistency control loop. Still the matter is explained in a very simple way compared to what you find in general textbooks available. The consistency control loop itself is not discussed here. It is discussed in the general part of "BTG Consistency Control Seminar" and a specific paper is available. This paper has been presented at Tappi 99 and is called: "Consistency Con-trol; designing the Control loop". It is also discussed in the book "Accurate Consistency", mentioned above.

Fig 32: Typical installation First you may ask yourself: Why is it important to have an accurate control and what are then the implications?Un uncontrolled process shows large variations, in this case the consistency varies. By introducing a control (control loop) you may reduce the magnitude of the variations, i.e. you tighten the distribution. When making a step change, i.e. changing the controller set point to achieve a lower consist-ency, you should reach the new level as soon as possible avoiding oscillations. Reaching the new level faster also means a tighter distribution! A tighter distribution means "money"; reduced costs or better product quality!

Tightened Distribution

Setpoint Response and Regulation

WhyWhy is is accurateaccurate Control Control importantimportant ??

Fig 33: Why is accurate control important?

A good process control can provide many advantages. Fig. 34 shows what a good process control can mean at a bleaching stage in a pulp mill. By a tighter distribution (better and more accurate control) you can often be able to shift the set target. You can perhaps use less amount of expensive fibers and replace with less costly or reducing the amount of expensive chemicals. Also reduction of the energy consumption, widen-ing a production bottleneck to increase production rate etc., can be the result. There are numerous examples why a better control is important.

The most important control in a pulp or paper mill is consistency! If you know consistency (and flow) you know how much fibers are present in the different process stages. Thus consistency control is the basic parameter to control and a tighter distribution here is of greatest importance for the result!

Shift Target = $$$

Increased Production

Chemical Cost Reductionreduction in variability

AOX Reduction

Environmental Compliance

Easier Bleach Plant Operation - Recovery from Upsets

Improved Pulp Quality

Operating Flexibility

Variability Reduction Example: Bleachplant Control

Process Control Process Control ProvidesProvides…...…...

Fig 34: Variability reduction

Comparing Control ability for different consistency trans-mitter types: Even if you are only somewhat familiar with control issues you are probably aware of the "dead time issue" which is of central importance if you are aiming at designing a control loop.In an automation dictionary dead time is explained as follows:

• Optimal Control

• Reliable

• User friedly

MEK-2300Typical installation


P2007en © BTG 2003 14

"Time interval between the instant when a variation of an input variable is produced and the instant when the conse-quent variation of the output variable starts". Popularly explained, dead time is the time it takes from making an input change until the change starts to influence the process (or equipment). In a consistency control loop there are several dead times involved. You have built-in dead times in the equipment involved (in the consistency transmitter, in the dilution water valve positioner/actuator etc.). You may have a signal filtering damping the signal etc. All these factors mean certain dead times, in practice all added together to make an influence to the process to be controlled. The largest dead time however mostly is the dead time between where the dilution water is introduced to the pump suction and the consistency transmitter. This dead time is the time it takes for the pulp to flow between the dilution point and the measurement point.To achieve the best consistency control it's very important that this dead time becomes as short as possible since it will directly influence the control loop´s ability to reduce distur-bances. Factors that will influence dead time are:- The ability for the transmitter itself to be installed on a tur-bulent flow. A turbulence is created in a pump, bend or at a half-open valve. The fibers in the pipe will have different velocities and the conditions in the pipe are generally unsta-ble. At a certain distance after these disturbance sources the turbulence evens out and the flow seen across the pipe diame-ter is uniform, all the fibers will flow with the same velocity in the same direction. This stage is called plug flow. The location where plug flow is formed is not only depending on the distance in itself, but also depending on consistency level, fiber type and flow velocity. Generally lower consist-ency, shorter fibers and higher flow velocity all increase the distance to where the plug flow is formed - see "Accurate Consistency", chapter 2. Some types of consistency transmitters can be installed at more or less turbulent flow conditions. Some optical and microwave transmitter types can be installed on very turbu-lent flow conditions directly after the pump outlet, while MEK requires a relatively straight flow, even if it is turbulent. Thus a short calming length is needed after the pump, bend or valve. Blade consistency transmitters and other types of rotat-ing transmitters (not MEK) need fully developed plugflow conditions. - Practical possibilities to install a transmitter at a recom-mended point. Normally there is space enough directly after a pump, on the vertical section, to install an MEK since it can be installed on a turbulent flow (but also works on a plug flow). Thus the dead time in most cases can become short and it's also easy to reach the transmitter from the floor. A blade transmitter always need plug flow conditions and often there is not enough space on the vertical section after the pump to obtain this condition and follow the calming length recommendations. You often have to install it after a bend (one or several) on the horizontal section. The piping is in most cases already existing being designed and built with-out considering the demands for a consistency transmitter.Thus the distance between the dilution water inlet and the consistency transmitter very often becomes very long result-ing in a very long dead time.- The recommendation is to multiply the calming length cal-

culated for a vertical installation by a factor 1.5 if the trans-mitter is installed on a horizontal pipe after a bend.- Also multiple bends, pipe bends directly following each other, are common and in such a case the recommended calm-ing length is increased even further.

Consistency sensors are susceptible to …

_ _ _ _ _ _ _ _

Place them as close as possible to the dilution

DEADTIME

Sensor Sensor LocationLocation is is ImportantImportant!!

Fig 35: Sensor location is important

Fig 36: Effective Consistency controlImpact of dead time and controller tuning: The most com-mon method to tune a controller for consistency is the Lambda method (Lambda tuning). The main advantage of this method compared to traditional methods (Ziegler-Nichols, Chien, Hrones and Reswick etc.) is that you may decide how rapid the control should be, before optimizing it. In this method the variable is called λ (Lambda). To decide λ you have to make a step-response test of the proc-ess. The control loop must be open (i.e. it's not on control and the controller is set on "Manual"). Then you can find out the deadtime (Td) and open loop time constant (τ). The dead time (Td) is the time it takes from the change of the set point is made until the consistency transmitter start to pick up the actual consistency change.The open loop time constant (τ) is the time it takes for the signal to reach 63% of full value. According to Lambda tuning practice λ should be selected to 3 x τ or 3 x Td, whichever is the largest. For consistency con-trol tuning it is recommended to use Td as the denominating factor (reference: EnTech / Emerson Process Management 2002 - Consistency Control loop Dynamic Specification).

EffectiveEffective ConsistencyConsistency ControlControl

15 © BTG 2003 P2007en

In the example fig. 37 and fig. 38 a typical response for a blade type consistency transmitter is shown when a set point change on the controller is made to increase the consistency, i.e. the control valve is closing. In this example Td is approx-imately 20 s for the blade transmitter. (At a typical flow velocity 1 m/s it means that the distance between the dilution water injection point and the transmitter is 20 m, a very com-mon location in practice for a blade transmitter due to the practical implications mentioned above). Then λ becomes 3 x Td = 3 x 20 = 60 seconds. If you compare to a rotating transmitter type MEK (or micro-wave /optical), which can be located much closer to the pump, Td is 2 seconds and λ becomes 3 x 2 = 6 seconds.Based upon these values you can then calculate the parame-ters for the PID controller, the Controller gain Kc and the Integrating time Ti (Reset). The Derivate time (Rate) is set to 0. But what will this mean for the control result using the differ-ent types of transmitters? Then you have to study a factor called Cut-off frequency. The Cut-off frequency Tc is given by the formula Tc = 2π (λ + Td). Since we already know λ and Td we can easily calculate Tc. For the blade transmitter Tc = 2π (60 + 20) = 503 s ~ 8.5 min and for MEK Tc = 2π (6 + 2) = 50 s < 1 min.Since the Cut-off frequency Tc is the frequency where the consistency controller can begin to attenuate disturbances of a larger period, the value is critical. Frequencies faster than Tc will not be attenuated! Looking on the process, at different parts, what frequency variations can you expect? - Large tanks, storage chests etc. have long retention time, the cut-off frequency is low. Thus the dead time Td is not a very important issue. On the other hand, if the chest is equipped with a special compartment using an intense mixing and you have a double dilution, where the course dilution is fed into this mixing compartment and the fine dilution before the pump, the retention time can become short! Generally spoken, traditional large volume chests do not require transmitters giving very short cut-off frequency, a blade transmitter can work satisfactory in this respect. For modern type of chests using mixing compartment, they can be better controlled using transmitters like MEK (or optical/microwave). - Small volume tanks or chests, type mixing chests or couch pits or process equipment like washers, screens etc., means higher cut-off frequency. Here consistency transmitters giv-ing a short cut-off frequency can make a significant improve-ment for the control result, they are really able to treat the incoming variations so that the control loop can make the job: a smooth stable outgoing consistency without variations.

Td 4τ

Deadtime ~ 20 sec Open loop Time Constant (τ) ~ 30 sec

Blade consistency transmitter

B

Dilution valve opening

Output signal blade typetransmitter (not in control)

ExcessiveExcessive DeadtimeDeadtime and and FilteringFiltering

Control Control abilityability: : ComparisonComparison betweenbetween BTG BTG RotatingRotating(and (and MicrowaveMicrowave/ / inin--lineline opticaloptical) VS.) VS.

BladeBlade typetype consistencyconsistency transmitterstransmitters• Typical Dead time (Td ): Rotating 5 s. Blade 20 s (require plug flow

conditions)• Open loop time constant (τ): Rotating 10 s. Blade 30 s• Lambda (λ) method most common for consistency control tuning• Short period disturbances: λ =Closed loop time constant. λ = τ x 3 for

stability > Rotating 30 s, blade 90 s > Slower tuning for blade transmitter > Less response for incoming disturbances > Less accurate control!

• Larger period disturbances: Cut-off frequency Tc = 2π(λ +Td) is the frequency where the consistency controller can begin to attenuate disturbances of larger period. The cut-off frequency is ~ 4 min for rotating vs. 12 min for blade transmitter > A blade transmitter does not touch disturbances with periods less than 12 min > Less accurate control!

Fig 38: Control ability: Comparison between BTG rotating and blade transmitters

Applications and Proofs for MEKAs we discussed earlier, MEK can be successfully installed on a large variety of applications; in both pulp and paper mills, in all kinds of processes and in a very large consistency span, from 1 to 16%. The transmitter is universal in its wide field of applications.Some applications are particularly interesting, and often con-sidered as "difficult". The background for these applications and considerations for control and selection of type of consistency transmitter is not discussed in this paper, please see "Accurate Consistency" chapter 7 for details. Paper mill applications: Two typical examples of applica-tions in the paper mill are shown in figs. 39 to 43. Fig. 43 is particularly important since it includes two actual proofs that MEK provides an advantage over blade transmitters.

Fig. 37 Excessive dead time and filtering

P2007en © BTG 2003 16

IW

C ONS IS TEN C Y TR AN SMI TTER MEK-2300

DrainageRate Control

CRC

Specific RefiningEnergy Control

DRT-5200

MPS-1000MPS-1000MEK-2300

Cs=3.5%

OutputFT

Remotecontrol

Local setpoint

Power

CS

F

Refiner Control

Fig 39: Refiner Control

Fig 40: MEK for refiner control

Fig 41: Paper machine consistency control

Standard model MEK-2300

• Accurate Consistency Control - the foundation for successful Refiner Control

• Specific energy refining control – the most advancedcontrol system

• Improved and safer runnability of the paper machine - less web brakes, higher product quality

• Tighter tensile strength settings - reduces powerconsumption and product variations. Increased amountof low cost fibers can be used and risk for downgraded paper is reduced

• MEK provides safe accurate measurement, not depending on flow variations, furnish variations, line pressure etc.

Foundation for successful Refiner Control to optimize Paper Machine runnability and profitability

Low consistency 1-10%MEK for MEK for RefinerRefiner ControlControl

C O N S I S T E N C Y T R A N S MI T T E R C O N S IS T E N C Y T R A N S MIT T E R MEK-2300 MEK-2300

CRC

BWCS

MEK-2300 MEK-2300

VBW-10

RET-5300

Valve input

F

Cs SP

FT

Actual BW

Valv

e po

s

Paper machine Consistency Control

Mixing chest

Machine chest Wire Pit

Standard modelMEK-2300

• Accurate Consistency Control - The foundation forsuccessful Paper Machine operation

• Stock Proportioning Control- Accurate Control of each fiber flow is essential for optimal product quality and minimized consumption of high cost fiber

• Mixing Chest Control - Final Consistency Control, is essential for optimal Machine runnability and profitability

• After Machine chest - Essential part of the Basis weight Control system (QCS)

• MEK provides Accurate, Reliable control independent of flow velocity, conductivity, line pressure, fiber variations etc

Safe, accurate, reliable Control for the most critical applications in the Paper mill

Low consistency 1-10%MEK for MEK for PaperPaper machinemachine Control Control

Fig 42: MEK for paper machine control

Actual proof and savings fig. 43: The background for the first example, the illustration, is that the mill, located in Louisiana USA, is controlling the consist-ency after a mixing chest in the paper mill. The mill produces a paper grade where they use pine or a mixture of pine and/or blend kraft stock. The consistency control is very important since it decides the amount of kraft stock and also how much it needs to be refined. After replacing the static blade transmitter with an MEK they could see the standard deviation be reduced from 0.4343 to 0.0477, i.e. a reduction of almost 0.4% consistency. The good result was distributed in the mill´s internal newslet-ter. In the second example the situation was very similar, the mix-ing chest before the paper machine. The mill is located in Maine, USA. They mix mechanical groundwood and kraft stock. The produced paper quality specification states that to keep the required properties, the paper must contain min. 50% groundwood pulp. If it becomes lower the properties do not meet the standard, the paper must be degraded and sold to a lower price or repulped, which is not cost efficient. When using the static blade transmitter the control accuracy was found to be 3 +/- 0.15% consistency. This meant they had to increase the amount of kraft stock to 53% to meet the spec-ification. Also worth to mention was that the blade transmitter was installed using a separate pump loop to maintain a stable flow velocity. This pump loop did cost a lot of additional money for power and maintenance which they wanted to avoid. An MEK was installed directly in-line after the pump. The controlled consistency now was improved to 3 +/- 0.05%.

The result they got was a reduction of expensive kraft stock and they could replace it with the cheaper groundwood stock and thus save 2 MUSD / year! The payback time for the trans-mitter installation was less than a month!

17 © BTG 2003 P2007en

Why did the result in the two cases become so much better using MEK?- Much lower sensitivity for different fiber types. Chang-ing from mechanical groundwood to kraft pulp or from pine to blend kraft stock only meant a very small shift in transmit-ter signal. The shift was so small that it was difficult to detect using ordinary lab methods, including the statistical uncer-tainty. - Virtually no flow sensitivity. Installation directly in-line without expensive pump by-pass arrangements. - Better control ability. Short dead time between dilution and measurement results in considerably better ability to attenuate incoming disturbances and provide a smooth stable consistency control.

Fig 43: Consistency proof and savings

Pulp mill applications: Three examples from the chemical pulp mill; consistency con-trol after the continuous digester, after the blow tank and inlet to the bleach plant are described - fig. 44, fig. 46 and fig. 48. In the recycled fiber pulp mill the application after the pulper is shown - fig. 50. All of these applications are very difficult, but for various reasons, and other types of consistency trans-mitters can hardly be successfully used here. Why does MEK provide a superior result in the pulp mill?- Robust design. Withstands badly cooked woodchips, knots, stones and metal pieces, highly concentrated pulp jams, back-flows, heavy vibrations, high temperatures - in the pulp or ambient etc.- Really measures pulp consistency, small influence of dis-turbance factors. Virtually insensitive (or to a very small degree only) for flow variations, conductivity, air content, pressure, fiber type, freeness, etc. - Excellent controllability. Can control disturbances and changing conditions very well.

Continuous digester Consistency Control

Fig 44: Continuous digester consistency control

Fig 45: Comments to fig. 44

Fig 46: Blow tank consistency control

Special modelMEK-2315

• Enables early consistency control in process

• Improves digester stability (level control, retention time)

• Enables accurate yield and production rate calculations

• Stabilizes blow line Kappa no. and optimizes the internal washing

• Prevents hang-up in digester and plugging in the blow line, washer or refiner

• Transmitter really operates reliable in the very tough measuring conditions (MC, high pressure, high temp, high pH, varying flow, impurities)

• 30% higher torque

• Special MC-sensing element and mech. seal

• 25 bar (300 psi) flange with deflector

Enables better over-all control of the digester. Improves pulp mill economics

Medium consistency 8-16%ContinuosContinuos digester digester ConsistencyConsistency ControlControl

Blow tank consistency control(Or HD storage chest)

P2007en © BTG 2003 18


Fig 48: Bleach plant consistency control



Fig 50: Batch pulper consistency control


For special applications. Consistency 2-8%

• Deflector to protect from debris

• Extra sturdy mech. Seal

• No propeller

• Optimizes production capacity and eliminates risk for plugging in screens and cleaners

• Can withstand the very tough process conditions. Proven in hundreds of installations

• Insensitive for variations of air/pressure or conductivity in pulp.

• Is virtually insensitive for changes in flow velocity

The only reliable transmitter available for the application. Optimize screen and cleaner operation

BlowBlow tank tank consistencyconsistency controlcontrol

Bleach plant consistency control

Special modelMEK-2310/-2312

• For accurate consistency control and dry tonnage calculation

• Is insensitive for variations of air/pressure or conductivity in pulp.

• Is virtually insensitive for changes in flow velocity

• 30% higher torque

• Flange for 10 or 25 bar pressure

• Special MC sensing element

• Wetted parts in 316 SS, 254 SMO or Hastelloy C

For medium Consistency 8-16% (MC)

Brings the reliable foundation for correct chemical dosing. Minimizes chemical costs and improves pulp quality

BleachBleach plant plant consistencyconsistency controlcontrol

Batch pulper consistency control – recycled fiber


• Cone protects from spinning

• Special anti-spinning sensing element

• Clean-flushing with water

• For the most demanding pulper applications.Proven in many installations

• Enables early consistency control which stabilizes the subsequent steps in process

• Enables a high optimal consistency without risk for plugging in the screen and pump.

• Specially designed to prevent hang-up and spinning

• Standard MEK-2200/MEK-2300 for less demanding pulper applications

For special applications Consistency 1-10%

Optimizes pulper and cleaner production for maximum capacity and runnability

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Additional information: - Data sheets, manuals, papers, articles etc. available from BTG.- “Accurate Consistency”; A handbook on consistency measure-ment in pulp and paper processing. Available from BTG, TAPPI and Paptac.

Competitive advantages of BTG Rotating Consistency transmitter type MEK-2300

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Transcript of Competitive advantages of BTG Rotating Consistency transmitter type MEK-2300