In gas turbine, the purpose of SRV and GCV valves are understandable. Closed loop of GCV with setpoint given by FSR and feedback is from LVDT.

But, in SRV the valve setpoint is given by Turbine speed (converted to pressure) and feedback is taken from both Pressure transmitter and LVDT.

Doubts:

*set point is speed converted to pressure.this is understood that during start up from 0 to 100% Speed; to make fuel flow inside the pressurized combustion chamber inter valve pressure is increased.

But after synchronization, speed is constant, so setpoint will be constant. When Load is increased, fuel flow should be increased. So, inter-valve pressure should be increased. how this factor is included in the control logic?

*why two feedback for SRV.

-pressure Transmitter feedback
-LVDT feedback

why this two? and how its used in control logic.

 

Karthi,

1) When speed is stable, the P2 pressure is stable. When the GCV opens as the machine is loaded, the P2 pressure would tend to decrease--but the SRV opens to maintain the P2 pressure. As the machine is unloaded and the GCV closes the P2 pressure would tend to increase but the SRV closes to maintain the P2 pressure. The SRV is NOT stable when the machine is being loaded or unloaded; it moves as necessary to maintain the P2 pressure as the GCV opens/closes.

2) One of the reasons the P2 pressure is held stable when the machine is at rated speed is because flow through the GCV is proportional to stroke with a constant pressure upstream of the GCV. This makes flow-rate proportional to stroke, a nice linear relationship.

3) When the actual P2 pressure equals the P2 pressure reference the error between the two signals is zero--and the valve does not need to move from its current position. As the actual P2 pressure changes OR the P2 pressure reference changes, the error between the two signals changes which means the valve needs to move. When the actual P2 pressure is again equal to the P2 pressure reference, the error is zero and the valve stops at its current position.

The LVDT is used for additional stability; the position loop is an "outer" loop to the "inner" pressure loop--which is the primary control loop for the SRV.

Does this answer your questions?

(By the way, this topic--and many more like it--has (have) been covered before on control.com. There is a 'Search' field cleverly hidden in the far right corner of the Menu bar at the top of every control.com page. I suggest you use the 'Help' feature to learn how to enter words and terms into the search field.)

 

Hi sir,

got answer for the first question that how SRV and GCV is related.

can you please elaborate on the SRV loop using two feedback.

Just consider the case:

Load 10 MW, P2 is around 8 bar.FSR is 50%.

Now once load is increased, FSR will increase. So P2 is disturbed (<8 bar).

SRV loop controller setpoint will not change, since speed is constant. But since P2 is decreased error is generated.So, controller will increase the voltage to Servo valve to open more. Finally P2 is increased which is sensed by the transmitter and gives feedback to controller.

In this control loop, LVDT feedback how its included.

 

Karthi,

Let's say the load is 10 MW, P2 pressure reference is 8 barg, actual P2 pressure is 8 barg, SRV position is 42%, and FSR (GCV position) is 50%. At this condition, the actual P2 pressure is equal to the P2 pressure reference, so the SRV does not need to change position (42%).

Now the operator increases the load, to say 11 MW. This caused the FSR to increase (let's say, to 53%) which caused the GCV to open to 53% which caused the actual P2 pressure to decrease to say 7.8 barg because the SRV has not moved.

The P2 pressure reference is still 8 barg, and the difference in pressure (8 barg - 7.8 barg) means the SRV has to open more to achieve the P2 pressure reference, so the servo-output current changes to make the SRV open more to increase the actual P2 pressure. So the SRV starts moving open from 42%. (The error between the actual P2 pressure and the P2 pressure reference means the SRV has to change position.)

As the actual P2 pressure increases to 8 barg the error between the actual P2 pressure and the P2 pressure reference decreases which means the SRV doesn't have to keep opening. When the actual P2 pressure is again equal to the P2 pressure reference the SRV does not need to change position and stops--let's say at 45%.

Essentially, any error between actual P2 pressure and the P2 pressure reference is converted to an error in SRV position. When the actual P2 pressure is equal to the P2 pressure reference there is no need to move the SRV.

All of this takes places at very high speed and the above explanation is a very low-speed account of the action.

Lastly, when a hydraulically-operated device is at it's desired position there is NO flow of hydraulic fluid to or from the hydraulic actuator. To stop the flow of hydraulic fluid to the actuator it's necessary for the servo-valve to get a "zero" signal from the Speedtronic. So, negating the null bias current, when the SRV (or the GCV or the IGVs or the LFBV) is at it's desired position the servo-valve output is zero milliamps. To move the device it's necessary to allow hydraulic fluid to move to or from the hydraulic actuator and to do this the magnitude and polarity of servo-valve output current from the Speedtronic is changed. Negative servo current increases the flow of fuel or air, while positive servo current decreases the flow of fuel or air, and the magnitude of the positive or negative servo-valve output current dictates how much hydraulic fluid will flow to or from the hydraulic actuator through the servo-valve--until such time as the actual position matches the required position, and then the the servo-valve output current is again at "zero."

This happens even with pneumatically-operated valves, or electrically-operated valves that get a 4-20 mA position that is proportional to position or pressure or flow. The difference is that the positioner on the valve controlling the air flow or the electrical actuator does the "zeroing." Servo-valve operated devices on GE-design heavy duty gas turbines do the "zeroing" in the Speedtronic.

So your statement about increasing the voltage signal to the SRV is not correct. To make the valve move to increase the actual P2 pressure the current being applied to the servo-valve is decreased, and as the actual P2 pressure approaches the P2 pressure reference the negative current is reduced until, when the actual equals the reference, the servo current is again at "zero" to keep the valve at the new position to maintain P2 pressure equal to the reference.

 

Thank you CSA.

My doubts reg Pressure loop is clear. The role of LVDT in this, till now i can understand is that it is used to give SRV position.
Is that playing role in closed loop?

During calibration of any servo valve

*voltage is increased and decreased. at that time, LVDT min and max pos value is taken as reference. Gain is also calculated as

gain = (Max V -Min V)/(Max Pos-Min Pos)

so the gain unit is V/%. This is seen in control constants list.

So, this made me to ask that, for increasing the valve position voltage is increased. Pls explain more about this. If only current is varying, what is the role of this gain? And

from your explanation, i got another question.
Regarding, polarity of the current. I know superficially the internal of servo coil. So this polarity change, changes the Magnetic polarity of the armature in servo?

Pls explain this also like , as you gave detailed explanation of SRV P2 pressure. The doubt is, When valve is in 42%, so if stroke increase demand is there to 45%, then current will be negative? if again 45% to 43 % immediately current will become positive?

Another question on Servo coil. In TMR there are 3 individual coil. # diff current. But when out of interest opened the servo valve, i could see only 4 wires inside. How the 3 coils are arranges/wound over the armature?

 

Karthi,

As was written previously, there is an "inner" control loop and an "outer" loop.

As was written previously, also, try this thread for more information:

http://www.control.com/thread/1351780247#1351831993

The feedback from an LVDT is voltage, which is converted to percent. The reference for most (but NOT all) servo-output loops of a Speedtronic turbine control system is percent--percent reference is compared to percent feedback. Therefore, when calibrating LVDT feedback (one <b>DOES NOT</b> calibrate a servo-valve!!!) the voltage has to be converted to percent, hence the gain, and offset.

Yes; the servo-valves used on most GE-design heavy duty gas turbine hydraulic actuators are bipolar--meaning they react differently to positive and negative currents.

Lastly, if you have a TMR control system, and if you only found four wires coming out of a servo-valve that is allegedly connected to a TMR control system, then someone has used a SIMPLEX (two-coil) servo-valve instead of a TMR (three-coil) servo-valve.

 

Karthi,

LVDT calibration of the SRV is NOT critical to turbine operation. If the gas fuel supply pressure were to change when the turbine was running stably at, say, 12 MW, and the SRV was at 48% stroke this would cause the actual P2 pressure to drop with respect to the reference. This would cause the difference between the P2 pressure reference and the actual P2 pressure to call for a change in SRV position. The Speedtronic would put out some negative current to cause the SRV to move to a little more open position (say 50%) to make the actual P2 pressure equal to the P2 pressure reference.

The SRV position is NOT proportional to P2 pressure, since the valve has to be able to move when either the GCV changes position <b>OR</b> the gas fuel supply pressure changes in order to keep the actual P2 pressure equal to the P2 pressure reference. Whether or not the SRV is actually at 51% stroke when the LVDT is feeding back a voltage that was calibrated to say it's at 51% is not critical--the only critical thing is that the actual P2 pressure is equal to the P2 pressure reference. If the SRV is actually at 54% or 42% when the feedback says 51% is immaterial; the only thing that matters is if the actual P2 pressure is equal to the P2 pressure reference. That doesn't always occur at a particular SRV position. What does matter is that the LVDT feedback is linear (volts with respect to stroke, from zero to maximum). The accuracy of the SRV LVDT calibration is not critical as the valve is going to be driven to whatever position it needs to be at in order to make the actual P2 pressure equal to the P2 pressure reference for the current operating condition.

To answer the next question, "Why did "They" choose to make negative servo current open the SRV and positive servo current close the SRV?" I have been searching for the answer to that question for nearly three decades. It just seems to have been a decision that someone, somewhere, at some point in time made. Whether it was the servo-valve manufacturer, or GE, it really doesn't matter. That's what's required in these applications--and we just need to know that's what's required in order to know if the right signal is getting to the servo.

Finally, it's all about electro-magnetism. Current flowing through a coil produces amp-turns of magnetic flux which produce torque which moves the servo valve jet tube one direction or the other (as current polarity changes). The magnitude of the servo current dictates how much torque is produced which dictates how much hydraulic oil is going to be ported through the servo to/from the hydraulic actuator. More current (positive or negative; negative or positive) causes the actuator, and the device being positioned by the actuator, to move faster.

If two coils are getting negative servo current from two of the processors in a TMR control panel and the third coil is getting positive current from the third processor in the TMR control panel that means there is some "fighting" going on in the servo, because two coils are producing torque in one direction and the third coil is producing torque in the other direction. This is how servo outputs are "voted"--by summing the forces produced by the currents in the three coils.

That should answer most of your <i>questions</i> about electro-hydraulic servo-valves on GE-design heavy duty gas turbines. It's VERY important to note that when one is "calibrating the SRV" or the GCV or the IGVs or any electro-hydraulic servo-controlled device one is <b>>>NOT<<</b> calibrating the servo. The only thing that can be calibrated is LVDT feedback, so if the device has no LVDTs then there's nothing to calibrate. (Some liquid fuel control devices on GE-design Frame 5 (and Frame 6B) heavy duty gas turbines do NOT have LVDTs and people are always asking how to "calibrate" them. Can't be done.)

If you're new to control.com (Welcome!) there is a LOT of information about GE-design heavy duty gas turbines and GE Speedtronic turbine control systems already written on control.com The 'Search' feature, while not intuitive, is VERY useful. MANY related questions have been asked many times and in many ways over the years here at control.com, so you will likely find very similar--if not identical--questions have already been asked and answered. If you need clarification or have <i>questions</i> we are happy to provide more information or answers. (But we're not really thrilled about addressing 'doubts'...)

If you haven't discovered it already, those threads (topics) which have feedback from the original posters about the usefulness of the information provided are the best indications of whether useful and helpful information has been provided (or not, as the case may be). We like to say here, "Feedback is the most important contribution!"(c) because it's one of the things that distinguishes this forum from many others on the World Wide Web. So, as you have done, it's good to provide feedback--positive or negative. And, thanks for the feedback!

 

Thank you CSA for explaining.

Pls provide link/Manuals for understanding of Gas turbine controls more.

Am from Instrumentation. Have worked in Major inspection of Frame 5 gas turbine. Now, 90%aware about field instruments. Now , eager in knowing it in more. Links/topics in internet are very general. So, pls provide useful links if you have for

*Radial vibration monitoring

*RELATIVE EXPANSION IN STEAM TURBINE(Very specific)

*Excitation and synchronization signals. How Generator communicates with MARK system during this period. and its purposes.

 

Karthi,

There is precious little in the way of links/manuals available on the World Wide Web for gas turbine controls. There are several reasons for that, mostly because the OEMs aren't producing very much in the way of documentation and what they are producing they are vigorously protecting via copyright and IP (Intellectual Property) laws and lawsuits. Most of what you are going to find is going to be in forums like this, specific answers to specific questions.

There are also companies which sell training, and they, too, are protective of the materials they produce. (Contrary to popular belief, if it can be scanned it's <b>NOT</b> legal to post it to the Internet.)

If you have specific questions, and even some general ones, we are happy to try to answer them here. (The ones that haven't already been asked and answered, that is.)

Another term for "relative expansion" is 'differential expansion.' Several manufacturers (SKF and GE Bently-Nevada and Metrix) have all published various papers on their corporate websites about various methods of measuring relative- or differential expansion. The thing one needs to remember is: a turbine shell/casing and the turbine rotor do not expand at the same rate as they are heated. They are not made of the same materials and have different coefficients of expansion, and this must be taken into account when starting-up a steam turbine, whether the turbine be warm or cold or hot. Too much heat too quickly can cause rubs (the axial and radial clearances in steam turbines are very critical to performance and vibration) and over time these can cause serious loss of performance as well as damage.

A generator is really a pretty "dumb" device. It converts torque into amperes using electro-magnetic principles--in the same way that motors convert amperes into torque using electro-magnetic principles. (MANY people have a HUGE problem believing that (that generators convert torque into amperes), and it prevents them from really understanding power generation.) Electricity is all about producing torque and converting that torque to amperes and transmitting the amperes using wires to many remote locations where the amperes are turned back into torque (including "virtual torque" in computers!).

Try to turn a synchronous generator faster than it should be spinning and the extra torque (over what's required to keep it spinning at synchronous speed) is converted into amperes by the generator and those amperes are used to power motors and other devices that convert the amperes back into torque which performs work we would have to manually or couldn't do at all.

As far as "communications" between the generator and the Speedtronic there really isn't any. The Speedtronic monitors and converts turbine speed into generator frequency (in percent) since turbine speed and generator frequency are directly proportional. The Speedtronic also monitors the generator terminal voltage and the bus voltage when synchronizing, and when the turbine/generator speed (frequency) is "matched to the bus frequency AND the generator terminal voltage is "matched" to the bus voltage, and the electromagnetic field of the generator rotor is aligned with the electromagnetic field of the stator the Speedtronic closes the generator breaker.

So, the only "communication" is the monitoring of speed and voltage. Sometimes, the Speedtronic can control VArs or Power Factor (with special instrumentation) and will then send signals to the generator excitation system the increase or decrease generator terminal voltage as required to maintain the VAr- or Power Factor setpoint. But, other than that, there's not much more to tell.

The Speedtronic does generally tell the exciter regulator (the "AVR") to start or stop, and can send RAISE/LOWER VOLTAGE signals to the exciter, and can get some signals from the exciter about field voltage and -current, and stator currents. But those are just inputs to the Speedtronic for display on the HMI (in some cases the signals go directly to the HMI and don't even pass "through" the Speedtronic).

Hope this helps! Again, we like to answer questions here (we're not thrilled about addressing doubts!) and if the question has already been asked and answered but if you still need clarification we can provide that. Or, if it sparks further questions we are here to help with that, too.

GE-design heavy duty gas turbines are great machines to learn. If you really want to be a good GE-design heavy duty gas turbine technician you need to find and study the Piping Schematic drawings (every other company in the world calls them P&IDs). Then you need to go out to the turbine and auxiliaries and find each one of the devices (pressure switches; temperature switches; RTDs; T/Cs; transmitters; servo-valves; solenoids; limit switches; etc.).

You also need to find and keep together with your copy of the P&IDs the Device Summary document--which contains the settings for most of the pressure switches and temperature switches and calibration data for most of the transmitters supplied with the turbine and auxiliaries. These two sets of documents will help you immensely in your quest to understand--and troubleshoot--GE-design heavy duty gas turbines. (And the great news is: If you do this for your present site/turbine, if you go to any other site with a GE-design heavy duty gas turbine, most of the concepts and systems are exactly the same so it won't take you very long to get your copies of the same documents for the new site and understand the differences! And, there are LOTS of sites around the world with GE-design heavy duty gas turbines.)

Hope this helps!

 

Karthi,

One other document provided by GE that you might find helpful is the Control Specification. This may actually consist of more than one document, but the top level document is GE MLI (Model List Item) No. A010. It will have a drawing number, unique to each gas turbine project.

 

Here at work our GAS turbine suddenly tripped on Inter-valve pressure. we function checked the pressure transmitter and also calibrated and function checked SRV and GCV. The turbine do not want reach "READY TO LOAD" FSH stuck on 76% than it trips on "SPEED RATIO COMAND".

Any idea why?

 

leon,

It would seem the GAS turbine at your site is a two-shaft machine, which doesn't change the P2 pressure control but it is kind of nice to know.

When you say it tripped on inter-valve pressure--high pressure or low pressure?

What is the gas fuel supply pressure upstream of the SRV?

What is the actual P2 pressure?

What is the LP speed?

What are the values of the two Control Constants, FPKGNG and FPKGNO?

At what speed is the HP shaft "stalling"? Or is the LP shaft that's not accelerating?

What is FSH?

Before you "calibrated" the SRV and GCV did you check the LVDT feedback calibration versus actual valve position? Because, that's the only thing that gets "calibrated"--not the servo, or anything else, just the LVDT feedback. And, unless it has drifted (which is rarely does--and would indicate other possible problems) there would be no reason to "calibrate" the LVDT feedback from the SRV or GCV to troubleshoot/resolve this problem.

Please provide the requested information, and we can probably provide some suggestions for troubleshooting and resolution.

 

Hi

thanx for the reply.

What we have found that everything is is OK but the GCV is not opening sufficiently. the GCV needs to open more so the Turbine can get more GAS. We proved this but when the machine is switch off the GCV do not go back to shut off position. It is open slightly so our dilemma is at the GCV.

 

How P2 reference pressure is generated?

> 1) When speed is stable, the P2 pressure is stable. When the GCV opens as the machine is loaded, the P2 pressure would
> tend to decrease--but the SRV opens to maintain the P2 pressure.

---- rest of lengthy post snipped by moderator ----

 

> How P2 reference pressure is generated?

FPRG = (FPKGNG * TNH) + FPKGNO

In words, the P2 pressure reference is equal to (the P2 pressure reference gain multiplied by the turbine shaft speed) plus the P2 pressure reference offset.

In other words, the P2 pressure reference is a function of turbine shaft speed. As the turbine is accelerating or decelerating the P2 pressure reference is changing, and when the turbine shaft speed is constant the P2 pressure reference is constant.

The P2 pressure reference gain and -offset values are parameters that are particular to each site, as natural gas characteristics and gas valve components can differ.

This topic has been covered many times on control.com, and can be found using the 'Search' feature cleverly hidden at the far right of the Menu bar at the top of every control.com page. Please use the Search 'Help' feature first.

 

In a gas turbine, the purpose of the SRV (Starter Relief Valve) is to regulate the pressure inside the compressor during startup, acceleration, and deceleration. The setpoint of the SRV valve is typically given by the turbine speed, which is converted to a corresponding pressure value based on the manufacturer's specifications. During startup, the valve is fully open, allowing the compressor to draw in air and build up pressure. As the turbine speed increases, the valve begins to close, regulating the pressure inside the compressor.

Regarding your first doubt, during steady-state operation when the turbine speed is constant, the setpoint for the SRV valve would also be constant. However, as you correctly pointed out, an increase in load would require an increase in fuel flow to maintain the turbine speed, which in turn would increase the pressure inside the compressor. This increase in pressure would cause the SRV valve to close slightly to maintain the setpoint pressure. In other words, the SRV valve would act as a pressure regulator, maintaining a constant pressure inside the compressor despite changes in load.

Regarding your second doubt, the feedback from both the pressure transmitter and LVDT (Linear Variable Differential Transformer) is used to provide redundant measurements of the compressor pressure and valve position. This redundancy ensures that the control system can accurately monitor and control the pressure inside the compressor and the position of the SRV valve. The pressure transmitter provides an electronic signal representing the actual pressure inside the compressor, while the LVDT provides a mechanical signal representing the position of the valve. Both signals are compared to the setpoint to determine the necessary valve position to maintain the desired pressure inside the compressor. The control logic uses both signals to ensure accurate and reliable control of the SRV valve.

 

I think in this context SRV refers to "Speed Ratio Valve", and its purpose it to maintain steady gas pressure (P2 pressure) upstream of the control valves to facilitate turbine speed control.

 

Er, ..., Um, ..., the MAIN purpose of the SRV (often called the Stop/Ratio Valve, or the Speed Ratio Valve, or the Stop/Speed Ratio Valve) is to act as the Gas Fuel Stop Valve, shutting off the flow of gas fuel to the machine either during an emergency trip or a normal, fired shutdown.

The secondary function of the SRV is to maintain a pressure upstream of the GCV (Gas Control Valve) based on the speed of the turbine shaft (the HP shaft, specifically). As the machine is started (at low speed) and accelerated to rated speed the intervalve gas pressure (the pressure between the SRV and GCV) increases with turbine speed. The intervalve pressure is also called the P2 pressure and the P2 pressure is, again, a function of the HP shaft speed. And, when the HP shaft is at rated speed the P2 pressure is steady--because the HP shaft speed is stable (steady).

And, that's all folks!