servo valve calibration
Posted by ESKAY
on 19 May, 2013 - 4:16 pm
Dear CSA and All, I would like to calibrate the distillate fuel 65FP servo, thro the "calibrate" mode facilit y in Mk6 system. [GE Fr5 Mk6 control]. Unlike, the Gas servo valve cali bration, where we set the servo valve output values thro "manual" set point and then compare t he error with the LVDT feed back. How can we make use of this procedure for the 65FP servo, where we have NO LVDT for the feed back and there is NO stem ste m (scale marking) on the 65FP servo to see the movement of the servo? We have only Flow divider as the fe ed back and our turbine don't have the auxiliary pump and need to run the turbine in "crank" mode for the calibration within limited time. Please guide me for the proper and effective calibration of the dist. fuel system. Thanks.. Reply to this post...
Posted by Bob Johnston on 19 May, 2013 - 7:09 pm
It's difficult I know, really the only way to check is do a "dummy fire" with no spark plugs and quickly check the fuel flow at a couple of Manual FSR points. Run a trend of t he major criteria at the same time and you can then review. Give the machine a bit of a Crank after testing and before firing to get rid of the excess e xcess fuel. Reply to this post...
Posted by CSA
on 20 May, 2013 - 6:23 am
ESKAY, This idea of "calibrating servo-operated devices" on GE-design heavy duty gas turbines is a HUGE myth. It's simply not possible to calibrate anything other than LVDT feedback. Even when GE field personnel say they are "calibrating" the SRV or the GCV or the IGVs or the LFBV (Liquid Fuel Bypass Valve they are NOT. They are only calibrating the LVDT feedback from those devices. There is zero effect on the servo or the servo-valve output stability after "calibration"; the only thing that changes is the LVDT feedback calibration. Full stop. Period. It's simply impossible to send a position reference to a device that has no position feedback and
expect that device to go to some position, or what the Speedtronic believes that position to be if there is no position feedback. So, don't believe everything you hear people say who seem to be knowledgeable about GE Speedtronic control systems, because they quite often don't say what they mean, and they quite often don't know what they are saying. Especially when they are talki ng about electro-hydraulic servo-valves. If you are having problem(s) with control of liquid fuel, tell us what the problem(s) are, what you have done to troubleshoot the problem, and what the results of your troubleshooting were. Then we can try to help you with resolving your problem, or with understanding the liquid fuel system and its components. As Bob Johnston says, if you don't have an AC motor-driven Auxiliary Hydraulic Pump then you are going to have to crank the unit to establish hydraulic pressure--just as with any other hydraulically-operated device. There have been some very creative people who have used the DC motor-driven Hydraulic Ratchet Pump to provide hydraulic pressure to the hydraulic system, but it takes some temporary tubing re-work. And I don't believe the Hydraulic Ratchet Pump output is filtered very well so there can be contamination of servos and/or servo filters i f that's used for that purpose without proper filtration. Also, the pressure pressure of the Hydraulic Ratchet Pump is not adjustable adjustable and is likely not the same as the Main Hydraulic Pump, nor is the flow-rate output of the Hydraulic Ratchet Pump anywhere near that of the Main Hydraulic Pump. It works for a crude check of system s ystem operation, but for anything more than that it i s necessary to crank the unit to get pressure from the Accessory Gear-driven Main Hydraulic Pump if there is no AC motor-driven Auxiliary Hydraulic Pump. The liquid fuel control valves of most GE-design GE -design heavy duty gas turbines are designed to control flow-rate using feedback from the liquid fuel flow divider. The flow control valve is p ositioned at whatever opening/closing it needs to be at in order to make the scaled feedback from the speed pick-ups on the liquid fuel fuel flow divider equal to the liquid liquid fuel flow-rate reference (for a Mark VI, that signal name is usually FQR, and the flow-rate feedback is usually FQL or FQLM or FQLM1). Even for LFBVs with LVDTs, the LVDTs are just there for "stability"; the main control "loop" for the liquid fuel system is flow-rate using the liquid fuel flow divider feedback. To properly calibrate LVDT feedback it is necessary to physically measure the position of the device with the LVDTs--something most people never do on GE-design heavy duty gas turbines when "calibrating" LVDT feedback. Most of the LFBVs on newer GE Frame 5s do not have valve stems or plugs that are visible so it's not even possible to verify the position of the valve plug. Again, if you are having problems with liquid fuel tell us what they are, what you've done, and what the results of what you've done were. If you only run liquid fuel occasionally and you have troubles transferring from gas fuel to liquid fuel, or trouble starting on liquid fuel then the LFBV is not the problem--nor would the t he "calibration" of the LVDTs on the LFBV be the problem. Reply to this post...
Posted by CSA
on 20 May, 2013 - 1:41 pm
ESKAY, I thought we'd been over this before.... http://www.control.com/thread/1336557419
Nothing has changed since since then. Nothing. Reply to this post...
Posted by ESKAY
on 21 May, 2013 - 1:33 pm
Dear CSA , Thanks for directing me to the old thread , for the same issue of calibrating the 65FP servo vALVE. As you have further clarified that there is NO way to calibrate the functionality of 65FP servo,other than running the turbine in distillate mode and and view the trend and various signals inserted in to the watch window to analyse the problem ,if any. As such, we have NO problem in the distillate fuel mode operation. I would like to get the knowledge about calibrating the dist.fuel servo , incase,if it is required. Thanks for the advice and your acknowledgement/quick response. Wishing you.. Regards.. Reply to this post...
Posted by ESKAY
on 22 May, 2013 - 12:21 pm
Dear CSA sir, I could not find the answer thro the "search" facility OR earlier threads regarding my clari fication on Mk4 servo calibration, which was rejected by one of your moderator. ******************************************************************* Moderator's Moderator' s note: The moderators for Control.com are not employed by CSA. The moderators work for Control.com. CSA is one of the many readers of this forum who freely gives his advice to others. He and the numerous other forum readers and participants are who and what makes this forum possible. *******************************************************************
I FOLLOWED THE PROCEDURE GIVEN IN THE MANUAL, but I have certain issues. On the calibration display for SRV AND GCV, while GDAJ is varied, with L4X1-L4X-L3adjL20FGX1-L3GCV (for GCV) -L3GRV (for the SRV)- forced to "1", with AUX. Hydraulic oil pump switched ON. I find the SRV SRV servo valve stroking from its datum (closed) (closed) position to its full
open position with GDAJ set at -0.30 and above and vice versa. Where as for GCV, the SERVO does not lift at all. al l. Is it the JDAJ J DAJ hexa-decimal value, automatically changes to either 13 OR 12 with the FORCE ENTER is activated OR JDAJ value change is NOT required. What must be the value for LOCATE LOW and LOCATE HIGH, normally set for the servo? (for GCV/SRV). Why it is NOT possible to hold the SRV at any fixed value of GDAJ? Where am I wrong? Please help solve the problem in servo calibration. I seek your kind help/advice at your earliest, si r. Thanks..With regards.. Reply to this post...
Posted by CSA
on 22 May, 2013 - 2:28 pm
ESKAY, I sense your frustration. I, too, for a couple of years as a new GE employee was under the mistaken belief that liquid fuel bypass valves--indeed, any servo-operated device on a GE -design heavy duty gas turbine--required "calibration." It wasn't until I really thought about and understood what calibration is, in a general sense, and what is--or can be--calibrated on a GEdesign heavy duty gas turbine with servo-operated devices that it really became clear to me. I was only referring you to a previous post in which you asked, essentially, the same question: How to "calibrate" a servo, specifically a servo-operated li quid fuel bypass valve which does NOT have LVDTs for position feedback. When using the "Calibrate" feature of a Speedtron Speedtronic ic turbine control system on a servooperated device, the >>ONLY<< "thing" that is calibrated is LVDT (position) feedback. IF THERE ARE NO LVDTs, THEN >>NOTHING<< CAN BE CALIBRATED.
The servo is not "calibrated." The valve (or IGVs) are not "calibrated." Only LVDT feedback from a device with an electro-hydraulic servo-valve is calibrated using the "Calibrate" feature of a Speedtronic turbine control system.
Regardless if the Speedtronic is a Mark IV, Mark V, Mark VI, or Mark VIe. If there are no LVDTs on the servo-operated device then nothing can be calibrated. "Calibration" of a servo-operated device with LVDTs for position feedback is the process of converting the voltage feedback from the LVDT(s) to position, in percent of rated stroke (travel) of the device (or, in the case of IGVs, degree of angle of opening). If the 0% stroke occurs at 0.700 VAC RMS voltage feedback, and the 100% stroke occurs at 3.500 VAC RMS (the values are for exemplary purposes ONLY), then the offset (or "zero") for the feedback voltage is 0.700 VAC RMS, and the gain (or "span") for the feedback is 0.028 VAC RMS per percent stroke. The Speedtronic calculates the offset and gain internally after determining the values during a calibration; there is no operator or technician action required for this part of the process.
HOWEVER, after an LVDT calibration is performed the operator or technician IS RESPONSIBLE RESPONSIBL E for verifying that t hat the displayed value of scaled ("calibrated") LVDT feedback is very close to or nearly identical to the ACTUAL, PHYSICAL POSITION OF THE DEVICE. Calibration is nothing more than scaling the voltage feedback from the LVDTs to percent stroke (or, in the case of IGVs, degree of angle of opening). You have informed us the liquid fuel bypass valve on the unit in question at your site sit e does NOT have LVDTs. Therefore, "it" cannot be calibrated. If there are no LVDTs, then there is nothing to calibrate. As Bob Johnston has said, if you want to t o confirm the liquid fuel bypass valve is properly controlling liquid fuel flow-rate, then you will need to disable the spark plugs on t he turbine; you will need to have the proper liquid fuel supply pressure at the inlet t o the Liquid Fuel Stop Valve (from the Liq. Fuel Forwarding Pump); you will need to put the unit in FIRE mode (again--with (again--wi th the spark plugs disabled); you will need to START the unit (since you have said the unit in question does not have an AC motor-driven Auxiliary Hydraulic Pump) to establish hydraulic pressure; and you will need to monitor/record/Trend the liquid fuel flow divider feedback feedback during the firing time (usually 60 seconds) versus the liquid fuel flow-rate reference and determine if during the majority of the firing time the actual liquid fuel flow-rate closely matches the reference. At the end of the firing timer, the Speedtronic will annunciate 'Failure to Ignite'--which will be a true condition since the spark plugs were disabled! The Speedtronic will close the liquid fuel stop valve and the unit should remain spinning ("cranking") until the operator takes some action. The operator, at this point, should do very little for a few minutes since raw liquid fuel was being admitted to the combustors during the test, and there wil l be liquid fuel in the combustors, the turbine section, and the exhaust. There should be liquid fuel coming out of the False Start Drain valves of the turbine and exhaust. The unit should be l eft spinning to help purge the combustors, turbine and exhaust of combustible vapors, and then it should be STOPped. Once the unit reaches zero speed, if the test was deemed successful, the spark plugs should be reenabled and the unit can then be returned to service. Again, to perform this test it i t is necessary to admit raw unburned liquid fuel into the combustors, turbine, and exhaust. It should NOT be done when the turbine is "hot" (shortly after i t has been shut down from loaded operation). If the unit has an HRSG ("boiler") on the exhaust, additional purging should be done done as necessary since some liquid fuel vapors may condense or be present present in some areas of the HRSG. It is, however, no different from any other failure to start--except that it is a test, and appropriate precautions should be taken. But, there is no "calibration" to be done on the liquid fuel bypass valve on the turbine in question. Full stop. Period. None. If there are no LVDTs, there is nothing to calibrate.
As was said before, the liquid l iquid fuel bypass valve is a flow-control flow -control loop (even if, as on some larger l arger units there is LVDT feedback). The valve will be moved by the Speedtronic to whatever position it needs to be in in order to make the liquid fuel flow divider feedback equal to the liquid fuel flow-rate reference. And, in the case of m ost Frame 5s and Frame 6Bs, there is/are no LVDTs on the liquid fuel bypass valve. So there is nothing to calibrate. It's not possible to calibrate (scale) the position feedback voltage from a device--even if the device has an electro-hydraulic servo-valve--if the device doesn't have LVDTs to provide position feedback to the Speedtronic turbine control panel. The servo isn't being calibrated. Only the LVDT feedback is bei ng calibrated. If there aren't any LVDTs, there's nothing to calibrate. If there is no position feedback from a device (such as the liquid fuel bypass valve on the turbine in question), sending a position reference to the device will only cause it to go to the end of travel (in this case, fully closed). And, since you can't see the valve stem of the valve plug, you can't even determine if the valve has moved or not or even how much it has moved. With no feedback, the Speedtronic will just keep providing current to t ry to move the device to make the t he position feedback equal to the reference. I'm really trying to understand how to help you understand that just because you can "calibrate" the SRV and the GCV and the IGVs that you can't "calibrate" the liquid fuel bypass valve on your turbine. In fact, I'm trying to help you understand that you are NEVER calibrating the SRV or the GCV or the IGVs--you are only calibrating t he LVDT feedback from those devices. Not the servo. Not the valve (or the t he IGVs). Only the LVDT feedback. And if you don't verify the accuracy ac curacy of the LVDT feedback calibration, then you haven't really properly calibrated the LVDT LVDT feedback. When someone initially "calibrates" the output from a pressure transmitter that is connected to a control system of any kind, for example, what that person is doing is scaling the output from the transmitter into something that a control system or operator or technician can understand and recognize (engineering units instead of voltage or current). Say the t ransmitter is a 4-20 mA device, and it is calibrated such that 4.00 mA corresponds to 0 barg, and 20.0 mA corresponds to 16 barg. The offset (or span) on the control system the device is connected to is adjusted such that at 4.0 mA the control system indicates 0 barg, and the gain (or span) on the control system that the device is connected to is set so that the control system indicates 16 barg at 20.0 mA output. That's "scaling" or "calibration" of the feedback (output) from the pressure transmitter on the control system. Yes the pressure transmitter needs to be calibrated so that at 4.0 mA corresponds to 0 barg and 20.0 mA corresponds to 16.0 barg, but that 4 -20 mA output that is connected to the control system needs to be "re-converted" back into engineering units in the control system. That is part and parcel of "calibration" and "scaling." For many control systems this "scaling" at at the control system end is done only once, during commissioning. In the case of LVDT feedback to the Speedtronic, there is only one "scaling" calibration that needs to be performed--since the Speedtronic is converting the voltage output of the LVDT (the input to the Speedtronic) into position.
And when a pressure transmitter is calibrated--either at the device or at the control system--it is customary to VERIFY the feedback/display scaling if any adjustment is made to either the offset (zero) or gain (span). And the SAME THING needs to be done for LVDT feedback. And if it 's not, then the "calibration" isn't complete and wasn't done correctly. When a pressure transmitter is calibrated, even initially, a form is usually completed that lists the as-found conditions, and the as-left conditions if any changes are m ade. The same should be done for any LVDT calibration--the as-found condition should be documented, and the as-left condition should be documented if any changes are made. During commissioning of any control system, these same conditions are documented--in some form--to confirm that the calibrated output of a pressure transmitter, in this example, example, is calibrated or scaled scaled properly on the control control system into engineering units from a current source. But, in this case, if there is no LVDT position feedback voltage, there is NOTHING to cal ibrate. Plain and simple. Full stop. Period. What's not clear about this? Because I'm failing to understand how to explain it any differently or any better. Again, if you normally run on gas fuel, and have problems when trying to run or star t on liquid fuel, the "calibration" of the liquid fuel bypass valves is ALMOST NEVER the problem. If your Manager or Supervisor wants you to "calibrate" the li quid fuel bypass valve, you need to convince him that there is nothing to calibrate. Even though it has a servo like lik e the SRV or the GCV or the IGVs, since it doesn't have LVDTs for f or position feedback there is NOTHING to calibrate. If this isn't clear--to anyone--please help me to find a way to explain it better. Despite all the myths and wives' tales and mystery about servos on GE-design heavy duty gas turbines, they are really very simple devices. That don't require calibration. No matter how one describes describes "calibrating" a servo-operated device--the device--the servo, or the device device doesn't get calibrated. Only the LVDT feedback can be calibrated. If you sense any frustration on my part, it's not directed personally at you or anyone. It's because there is so much misunderstanding about servo-operated devices on GE-design heavy duty gas turbines. It's so HARD to undo what people have been told and have come to believe, and so many people are working on and responsible for maintaining and troubleshooting GE-design heavy duty gas turbines that haven't been properly trained and don't take the time to think through what they are doing or have been asked to do. They just think that the "AutoCalibrate" or "Calibrate" feature is going to do everything to solve every problem (real or perceived) and it's one of the first things to do to try to resolve any real or perceived problem. I try very hard to communicate as best as possible, but when the basics of control systems and devices aren't really well understood to begin with, and people are str uggling with little or no proper training and little
or no documentation, and there is so much misin formation it's very frustrating to try to find the right way to educate and correct. Sorry. Speedtronic control systems are just purpose-built control systems, and as such t hey do have some unique features. But, they don't really handle any inputs or out puts any differently than any other control system. It's just that they can seem so complicated, and there is so little written to use to understand them by so many people they can seen overwhelming. Try to step back and take a deep breath sometimes. This is one of those times. t imes. (And I need to do that now!) Reply to this post...
Posted by Mike Borrelli
on 23 May, 2013 - 8:51 am
When I check the LVDTs, I run the valves from -25 to +125 and check the t he LVDT voltages as 0 and 100%. I will update the iocfg if necessary. I also run the valve to mid position and will 'tweek' the null bias value to get the valve to be closer to those mid rang positions. I run the autocal and use the verify current and position tool to see if any core is seeing a difference in position or current which I think would point to potential a servo issue. Does this t his sound ok? Reply to this post...
Posted by CSA
on 23 May, 2013 - 3:37 pm
Mike Borelli, I'm presuming you are referring to a Mark V Speedtronic turbine control panel since you refer to "iocfg." Regardless, LVDTs are no different than pressure switches or temperature transmitters. The as found condition should be checked using a known source, and adjusted if necessary. And since LVDTs are devices for sensing linear position ("stroke") the actual positions of the valves (or IGVs) should be checked when performing a "calibration." And the way to do that is to physically measure position (stroke) not just compare scaled feedback to position reference. Many people, as you described, just run the valves to (or the IGVs) the full closed position by giving a reference of -25% and ass-u-me the t he valve is at 0% stroke. And then they give the valve a reference of +125% and ass-u-me the device is at 100% stroke. Without ever actually measuring the physical position of the valve (stem) or the IGVs. And then they give the valve some intermediate position, and wit hout measuring that position, they just ass-u-me the scaled feedback is exactly equal to the physical position (which it may--or may not--be!) and adjust the null bias value to make the feedback equal to the reference. And if they're not physically measuring the actual position of the valve (or IGVs) and comparing
the actual physical position to the scaled feedback then they're not actually verifying the accuracy of the "calibration." When someone "calibrates" a pressure switch or a temperature switch, they apply a known pressure or temperature and and observe when the switch switch contacts change state. They They record these as the as-found conditions, and only make adjustments if necessary. If they just applied pressure or temperature without knowing what the actual applied pressure or temperature was, and observed a change of state of the contacts at some pressure or temperature and proclaimed the devices were calibrated, that would be akin to the way most people "calibrate" LVDT feedback by not ever knowing what the actual physical position of the valve (or IGVs) was versus the scale d feedback. The "calibrated" LVDT feedback value changes state as the device is stroked, and it stops at 0% (or thereabouts) and 100% (or thereabouts) and therefore it MUST be accurately calibrated. And, then they adjust null bias to make the "calibrated" feedback equal to the reference--without ever knowing if the actual position is equal to the reference. And when adjusting the null bias value, they also don't observe limits of adjustment and quite often then don't download to all three processors, trying to adjust a single processor's feedback (without knowing the actual physical position) to be closer t o the reference value--and in the process introduce more error error to the loop. Resulting in more null bias adjustment, adjustment, and on and on and on and .... When calibrating most SRVs and GCVs provided with GE machines (the combined gas valve assemblies manufactured by Young & Franklin for GE), 0% stroke does NOT A LWAYS occur at -25% reference. (You didn't say what kind of machine, and gas valves, you work on.) And, some LFBVs (Liquid Fuel Bypass Valves) are "short-stroked" meaning the valve will never actually reach 100% of maximum expected fuel flow-rate (the travel i s usually limited to about 1/8th inch less than what would be required to reach maximum expected liquid fuel flow-rate on t he coldest expected day on a new-and-clean machine) and an improper calibration can result in difficulties during firing and acceleration as well as during synchronization. In all fairness, what you described is "GE" --Good Enough--for most machines. But, it's technically incorrect if I understand what you described. Without physically measuring the stroke when "verifying" a mid-stroke position (and a straight line is always characterized by two points, not just a single point) then it's just like applying an unknown pressure or temperature to a switch or transmitter, observing a change, and pronouncing the switch or transmitter "calibrated." It might, or might not, be accurately calibrated--all you know for sure is the contacts or the output changed as the applied pressure or temperature changed. In the case of Mark V, if you change IOCFG Constants, or even if you don't, but you then run AutoCal in order to be able to t o run 'Verify Position' or 'Verify Current' then you are "overwritin g" the IOCFG values in RAM with newly calculated IOCFG values from the AutoCal. And if you don't re-boot the processors to get the values from EEPROM back into RAM then they will run on the newly calculated AutoCal values--not values- -not the EEPROM values. That's what AutoCal does when executed: It overwrites the RAM values (which were copied from EEPROM the last time the processor was re-booted) re -booted) for the calibration constants with those determined during the AutoCalibration. Since the Mark V runs off RAM values--not EEPROM values, you are using a different set of calibration constants in each processor (since EACH processor determines its own own values during an AutoCal) AutoCal) than what you you downloaded to EEPROM EEPROM and re-booted to get into RAM. If any processor subsequently gets re-booted, it's going to copy the EEPROM value into RAM and use the EEPROM value--which can be different from t he AutoCal-calculated value. Which can lead to differences in perceived position by the processor(s).
(Again, I'm referring to Mark V procedures and processes in this description since it seems Mike Borelli is referring to Mark V turbine control panels.) One of the 'Verify' functions is useful for determining if there is a problem with LVDT feedback, and the other is useful for determining if the actuator or device is sticking and/or the servo may be a problem. I just can't remember which right now without being able to look at a Mark V manual, and I don't have access to one at this writing. So, if one doesn't check actual, physical position versus scaled ("calibrated") LVDT feedback then how does really know if the LVDT feedback is calibrated (scaled) correctly? Making feedback (which may or may not be accurately reflecting actual position) equal to reference isn't improving the calibration (scaling) either. There are really only two devices on a typical, conventional combustor-equipped GE-design heavy duty gas turbine that need to be accurately calibrated: GCV (at low positions--for firing and acceleration control), and the IGVs (at full open position--for the optimum and best heat rate and performance). Of the two, the IGVs are most important because because if they are closed below below what the indicated angle is (the indicated angle is greater than actual position), then the power output will be low and heat rate will be poor poor (the unit will be "under-fired"). If, If, on the other hand, the IGVs IGVs are open more than the indicated angle (the indicated angle is less than actual position) then the unit may be "over-fired" at Base Load which can lead t o decreased hot gas path parts life. SRV and LFBV LVDT feedback accuracy are not very critical, since they are not position loops. DLN-equipped machines are different, in that the "splitter" valve LVDT calibration can affect emissions if changed between DLN tuning operations. So, a lot of sites actually cause selfinflicted problems by incorrectly "calibrating" splitter valves between DLN tuning operations. (In other words, if the splitter valve LVDT calibration changes--the values necessary to scale the feedback change since the last "calibration"--then it may be necessary to re -tune the unit for emissions. Or, it may cause the emissions to be out of compliance requiring a re -tune.) That's why it's so important to verify the t he actual position versus the scaled (calibrated) LVDT feedback during any "calibration", and only change the calibration constants if necessary. Simply running AutoCal, or changing IOCFG values without knowing if the indicated position is equal to the actual position is not really "calibrating" LVDT feedback. Reply to this post...
Posted by Mike Borrelli
on 24 May, 2013 - 10:02 am
CSA, Thanks for all the detail. It does help give a better understanding to all. We are Mark 5 on 7FA DLN2.6, 2x1 D11. Thanks for your clear description of what a calibration is. We are really only checking the stroke to see if the LVDT feedback matches the command signal and that travel is smooth and servo currents are even. I will only make changes when we are scheduled to tune, as you say adjustments will affect the tune (been there done that) and I will recommend a tune if there is a significant error that should be corrected. I have mic'd the LVDTs on 2 separate occasions (with some difficulty due to access) but have found very little error. We mic (protractor) the IGVs every time and calculate the 0-100% per the t he Device Summary.
I have found LVDTs to be quite stable over time, but my experience is limited. Have you seen drift or change over time that would suggest more frequent mic'ing? Since getting the mic in there is difficult it is often put off. I did think that -25 to +125 was hitting stops, so your explanation does change my view on that process.. I thought I heard the valves thud into max max positions. If they were hitting stops then I would (did) feel comfortable with matching the LVDT feedback accordingly. I will have to review that process now. Can you help me understand what the Mark5 is doing when it commands a valve to a set point? Does it take the feedback and adjust the output to make the valve go to position, similar to other control systems (or PID)? Or does it send a specific servo current that should drive to that position and then does not refer to the t he feedback for correction? Yes, the Autocal does affect the RAM so you must reboot to return back or update, download, and reboot if some changes were made. Autocal on the IGV will cause significant error as Auto cal is assuming 0-100% but the IGV has hard stops t hat are not 0-100%. We Autocal the IGV occasionally so I can run the view position/current to see if the servo currents are similar to each other and the positioning is smooth. A download is necessary to correct the values put in place by the Autotune. Again thanks for sharing your experience and your explanations on this subject. Mike Reply to this post...
Posted by CSA
on 24 May, 2013 - 1:31 pm
Mike Borelli, You are most welcome for any insights I have provided. AutoCal on the Mark V uses an ASCII text file (ACALIB.DAT, if I recall correctly--but I'm having a memory lapse at the moment) which lists the ends of travel for the various servo-valve outputs. For the IGVs, one needs to physically measure the fully closed (up against the mechanical stop) and fully open (up against the mechanical stop) positions, and then put them in the file in the appropriate location--then save the changes, exit the file, then run AutoCal for the IGVs. It will then t hen calculate the proper values for calibration of IGV LVDT feedback. The usual values in the file are 34 and 84, which are just "default" values for t he typical operating ranges of many GE-design heavy duty gas turbines, but do NOT reflect t he mechanical stop positions of any GE-design heavy duty gas turbine. One needs to measure those positions and put the actual values in that file and then run AutoCal--and be surprised at how close the verification of calibration will be. Presuming one measures the IGVs properly--which means in the same place on the same blades and holding the blade in the same position every time a measurement is made. Too often, people just take measurements on any blade in any position without holding the blade in the normal position it will be in when air is flowing through the IGVs. The Speedtronic is
actually trying to close the IGVs for most machines (not all!) when the t he axial compressor is at rated speed because the air flow is trying t rying to open the IGVs. (Some newer F-class machines have the pivot points/axes of the IGV blades such that air flow is trying to close the blades when air is flowing through them.) Also, when taking a measurement there is always about a 1 DGA hysteresis in the blade angle because of the spacing of the gear and rack teeth. teeth. So, I recommend holding holding the blade being measured against the stop in the open direction (for units which air is trying to open the IGVs when at rated speed) when taking measurements. But, in reality, as long as the t he same procedure is used and the measurements are taken in the same place on the same blades blades with the blade in the same position position the measurements will will be fairly accurate. When verifying IGV LVDT calibration I always recommend opening the IGVs to the full open mechanical stop, then closing them to some angle, say 84 DGA, making t he measurement, then closing them to another angle, say 57 DGA, and taking the next measurement. For those machines that air flow is trying to open the IGVs when running at rated speed this represents the actual position the LVDTs will will be in when trying to maintain a position position (again, because air will be trying to force the IGVs open and the Speedtronic will be trying to keep them closed at the desired angle). I think GE now has some super-duper IGV measuring tool for F-class turbines (probably at some super-duper price, too!). I've never seen or used one so I can't comment on their user-friendliness or accuracy. But, it's one of those tribal tri bal myths that AutoCal doesn't work on IGV LVDTs. If one properly configures the AutoCal reference file, and if one takes measurements in a consistent manner, one will get surprisingly good results with AutoCal when calibrating IGV LVDTs. As for the gas valves on F-class units, they are usually always 0-100%, stop-to-stop. So, nothing unusual there--pretty easy actually. No; I have seen very little LVDT feedback drift in my three decades decades of working on GE-design heavy duty gas turbines. Virtually none to speak of. These people who just "calibrate" whenever something happens to the turbine are just unthinking robots who don't understand what they are doing and are trying to look knowledgeable and competent. And, some times--too often, actually-they can really cause some pretty serious self-inflicted problems which always get blamed on the Speedtronic! Unless somehow the valve stroke changes (which it usually doesn't unless the valve is disassembled for repair/refurbishment) or the LVDT is disassembled and reassembled, there is usually very little difference in LVDT feedback over time. When LVDTs fail, they usually fail in the "open" condition, or have very jumpy, intermittent feedback at some mid -stroke position. Speedtronic servo-valve outputs are bipolar outputs, and if there were no null -bias spring in the servo the servo output current would be 0.00 mA when the feedback was equal to the reference. The only time the output is greater than null bias (approximately) is when the valve is being commanded to move. And, as the valve approaches the setpoint the current i s reduced, until when the error between the feedback and the reference is zero t he output current is equal to the null bias value (approximately). In a perfect world, the output would be exactly equal to the null bias value, but we don't live in a perfect perfect world, so it's a few hundredths hundredths of a mA high or low--which low--which is
nothing in a real world. This is the part about servo-valves that blows everyone's mind: they want a specific mA output to correspond to a specific position. Say, 8 mA for 25%, and 12 mA for 50%, etc. --like many other valve operators/positioners. But, inside the positioner for those other valves there is a the same thing--a device that senses actual position versus the reference and outputs zero air or hydraulic flow to hold the reference equal to the feedback. The Speedtronic just does that "positioning" control out there in plain sight--not hidden from sight as in a typical t ypical pneumatic or hydraulic valve positioner. When the reference calls for a change in valve position, the output current changes (positively to shut off the flow of fuel or air or steam on a GE application; negatively to increase the flow of fuel or air or steam in a GE application) and then returns to "zero" (null bias value-approximately, because we live in a non-perfect world) when the actual position equals the reference. Hope that helps! They are not rocket science, or magical, or even mysterious. One needs to remember that for any actuator to hold a device in a steady position the flow of air or hydraulic fluid to the actuator must stop for the device to remain in a stable, steady position. As long as air or hydraulic fluid is flowing the actuator is moving the device. So, something has to stop the flow when the device reaches the desired position. And, the Speedtronic does what many other I/P positioners do without the intermediate signal (the 4-20 mA, for example). example). It takes the position reference and compares it directly to the position feedback and when the two are equal (the error is zero) the servo output current is "zero" (null bias value, actually--approximately). Reply to this post...
Posted by CSA
on 22 May, 2013 - 4:45 pm
ESKAY, I now see you are asking new and different questions, and the information you are providing is different! I'm becoming even more confused--it that's even possible. You wrote about Mk4 servo calibration in t his newest post to a thread originally ori ginally about liquid fuel bypass valve calibration on a Mark VI!?!?!!!??? This newest information seems to be about a Mark IV with an Auxiliary Hydraulic Pump?? If you are using the Mark IV "autocalibration" feature for the SRV, you must first change some Berg jumpers on the appropriate HSAA card to t emporarily reconfigure the SRV regulator to a straight position loop. Otherwise, if you don't any slight change i n reference will cause the valve to just go wide open--since the reference will be for a pressure and not a position if the Berg jumpers are not temporarily moved. If you're using the Mark IV "calibration" " calibration" display you typically don't need to force L3ADJ (if I recall correctly). The "calibration" feature does all of that aut omatically if I recall correctly. As for why the GCV isn't i sn't moving, I can't even guess based on the i nformation provided. All I can say about the GCV is that you typically don't need to force anything to establish Trip Oil pressure to get high-pressure hydraulic oil to the GCV servo and actuator. You typically just need high
pressure hydraulic oil for for that operation, and to just use the Mark Mark IV "calibration" display. display. As for what "manual" you are using, I presume you are using the Control Specification-System Adjustments document/drawing. I think the information is in Sect. 05.02.nn for gas fuel valve LVDT feedback calibration. And, usually, that information was pretty good. But, without being able to understand what you're doing, how you're doing it, what control system you're working on in this post to t o this thread, and what instructions you're using I can't be of much more help. I can tell you this: If you are doing the first time without having read and understood the documents you are using first and having made some notes about what you are going to be doing and don't have some knowledgeable help there with you--and you are under pressure to do this during a maintenance outage--it's probably not going to turn out as well as it could. The preparation includes reviewing and understanding the relevant P&IDs (Trip Oil; Gas Fuel; Hydraulic Oil; Lube Oil; to name a few) to know what all is necessary to be able to establish Trip Oil pressure to get the SRV (at a minimum) to move. And, again--there's the little matter of temporarily t emporarily moving Berg jumpers on the appropriate HSAA card for the SRV regulator loop.... l oop.... Which I think is documented in the Control Specification-System Adjustments drawing. We would also need to know if any an y work has been done on the gas valve assembly and/or the gas valve LVDTs (SRV and/or GCV) to be able t o really provide more help. Sorry I can't be much more help without much more information--and clarity about the differences between the original post in this thread and this newest information. It's all so confusing for me! And, I'm sure if you were in my position, thousands of miles from the site, you would be just as confused. You do seem to have moved out of the control room, where there have been a goodly number of HMI issues, to the turbine(s)! To me, that would be a great improvement!!! Reply to this post...
Posted by ESKAY on 23 May, 2013 - 4:10 pm
Dear CSA Sir, Thank you very much for the quick response.I did ask about the Fuel servo 65FP calibration with Mk6 in my previous post and it was understood that there is NO proper procedure to calibrate the servo alone with out a feed back signal,unlike calibrating the GAS servo valve.(SRV/GCV). I PUT FULL STOP TO MY DOUBTS. In my past post, my subject doubt is on the GAS servo calibration in i n Mk 4 speedtronic (EGT) system. As per the EGT manual, the following signals need to be Forced for the calibration: -L4L4X1-L20FGX1-L3ADJL4X1-L20FGX1-L 3ADJ- and L3GRV for SRV and L3GCV for GCV. As we have the Aux. Hyd. Pump, NO need to CRANK. I fully understand t he concept of the Mk4 system and do refer to GT manuals prior to doing any job. While calibrating the SRV, with the RAISE/LOWER of GSADJ, the SRV moves from the initial CLOSED position to FULLY OPEN and vice versa. SRV can not
be controlled in between. While stroking the GCV, It It failed to move. This could could be a problem of servo valve itself OR other problem, which I am yet to analyse. Thanks for the advice of changing certain berg jumper position in HSAA card to control the SRV during calibration,which I was NOT aware of. I shall look into the manual for l ocating the berg jumper information. Secondly, Secondly, how to set LOCATE LOCATE LOW and LOCATE LOCATE HIGH during the calibration, which will be in % of LVDT position OR in terms of LVDT RMS Voltage.Kindly guide me.. Sorry for creating the confusion between my previous post and the last post. AND thanks once again for responding to my clarification and reviewing my post. Regards... Reply to this post...
Posted by CSA
on 24 May, 2013 - 9:56 am
ESKAY, LOCATE LOW and LOCATE HIGH are used to tell the Mark IV what position (in percent of actual, physical stroke) the device is in. To do this you need to know what the t he total stroke (travel) of the device is. And, when working on combined SRV/GCV gas valve assemblies on a Frame 5 or -6B, you need to take the gap between the actuator rod and the valve stem into account, by using feeler gauges to "eliminate" the gap during calibration. I recommend the first step below be done without hydraulic pressure (i.e., the Aux. Hyd. Pump should be OFF). So, the first thing you need to do is to use a length of wood (a 2x4 or something similar) to push the valve stem down, which pushes the actuator rod down. I put the wood against the metal bar that is clamped to the valve stem and which the LVDT cores are attached. It shouldn't take a lot of force to do this, and you can pry against the bottom of the valve body when doing this. Keep the length of wood as close to the t he valve stem when doing this so as not to bend the metal bar. Then move the length of wood to the other side of the metal bar, and prying against the base of the valve assembly casting push the valve stem up. You should be able to create a small gap (0.030" to 0.050") between the bottom of the valve stem and the top of the actuator rod. Insert feeler gauges into the gap to "fill" " fill" the gap, and leave the feeler gauges there during the calibration and verification process. At this point the valve stem is at the true zero stroke condition. ZERO STROKE FOR THE SRV AND GCV IN THIS COMBINED GAS VALVE ASSEMBLY IS >>NOT<< WHEN THE VALVE STEM IS FULLY DOWN--BUT WHEN IT IS FULLY UP WHEN THERE IS NO HYDRAULIC PRESSURE ON THE ACTUATOR. At this point you need to use some kind of measuring device (a dial indicator; i ndicator; machinist calipers; the brass pointer on the metal bar clamped to the valve stem that points at the brass rule on the valve base) and record/note the zero stroke position. Then you can turn on the Aux. Hyd. Pump and start using the Mark IV to stroke the valve stem.
You will need to move the t he valve stem to the full open mechanical stop (up) using the Mark IV and record this position using your measuring method. The difference between the zero position and this position represents 100% stroke. Let's say it was 1.50 inches. (I think the brass scales on the valves are in inches.) Next you need to move move the valve using the Mark Mark IV to some position just above above zero stroke and measure that position and subtract the zero stroke position from this measurement and then calculate the difference as a percentage of stroke. So, for example let's say you moved the valve to about 20% of stroke, and you took a measurement and the result after subtracting the zero stroke measurement was 0.280 inches. 0.280 inches out of 1.50 inches equals 18.67% of total stroke (travel). You enter 18.67 into the field for LOCATE LOW and press the LOCATE LOW softswitch. Next you use the Mark IV IV to move the valve to some position near full stroke, stroke, say about 80% stroke. You measure that position, subtract the zero stroke measurement and calculate the position as a percentage of total stroke. If the difference after subtracting the zero stroke measurement was 1.24 inches, the valve is at (1.24/1.5) 82.67% of total stroke (travel). You enter 82.67 into the field for LOCATE HIGH and press the LOCATE HIGH softswitch. If I recall correctly (and it's been a LONG time since I've calibrated LVDTs with a Mark IV) the Mark IV will now calculate new offsets and gains for the LVDT feedback. And the indicated position for the valve should should go to approximately 82.67% (if the valve didn't move). Then you need to verify the accuracy of the calibration. Move the valve until the display says 25.0% (or thereabouts), and take a measurement, subtract the zero stroke measurement and you should come up with a value of approximately 0.375 inches, or very close to that. Then move the valve until the display says 75%(or thereabouts) t hereabouts) and take another measurement, subtract the zero stroke measurement from that and you should come up with a value of approximately 1.125 inches. If you're within a few thousandths t housandths of an inch, CONGRATULATIONS!!! CONGRATULATIONS!!! You've completed a proper calibration with a high degree of accuracy! Move the valve to the closed position, remove hydraulic pressure and all the forces, and then remove the feeler gauges. THE INDICATED POSITION FOR THE VALVE MAY GO TO SOME VALUE LESS THAN ZERO. >>THIS IS NORMAL!!!<< Since the zero stroke position is "above" the lowest point of travel of the valve stem this is normal!!! As long as the indication doesn't go to some value less than approximately -5.0%, (which it shouldn't if the gap is between 0.030" and 0.050") everything will be okay! Repeat the process for the other valve exactly as you did for the first valve--everything exactly the same. You need to establish the true zero stroke position, using the feeler gauges and leaving the feeler gauges in the gap during the entire process. You need to determine what the actual total travel (stroke) of the valve is, so you can calculate percentages of actual position. Then you can move the valve to intermediate positions (low and high) and use LOCATE LOW and LOCATE HIGH to tell the t he Mark IV what those intermediate positions are (after taking the measurements and calculating them as a percentage of actual, total t ravel (stroke)), and VOILA! And then you need to verify the accuracy of the calibration by measuring two positions and calculating them as a percentage of total travel (stroke) to t o see if the displayed value of scaled LVDT feedback is approximately equal to the actual position. And, that should be all that's necessary. Again, the displayed value for either or both of the valves will, at some point, go to a negative value. And that is to be expected!!! And normal. Unless the value is less than -5.0% and then
there is a problem--with the gap between the valve stem and the actuator rod. This is the proper and correct method of using the Mark IV to calibrate--and verify--LVDT feedback. It's not what most people do, but i t's the proper method for calibration. Hope this helps! As for why the GCV isn't i sn't moving, I can't even guess based on the i nformation provided. Usually, you do NOT need Trip Oil pressure to stroke the GCV, but that's USUALLY, not ALWAYS. You need to check the Gas Fuel P&ID to t o be sure. And, what you're seeing with the SRV, not being able to control any position other than full closed or full open is EXACTLY because the Berg jumpers are in the pressure loop setting, and they need to be temporarily moved to the position loop setting. I don't have access to Mark IV drawings at this writing, so I can't tell you what jumpers to t o move for the SRV calibration; sorry. sorr y. Write back to let us know how you fare! Reply to this post...
Posted by ESKAY
on 28 May, 2013 - 5:02 pm
>ESKAY, > >LOCATE LOW and LOCATE HIGH are used to >tell the Mark IV what position (in >percent of actual, physical stroke) the >device is in. Dear CSA, Thank you very much for the detailed explanation and your advice. I have calibrated both SRV and GCV servos in Mk4, and set the t he locate low and locate High as per your direction. The 100% stroke for SRV lift was 1.17 inch and GCV was 1.50 inch. I stroked t he valve for 20% and 80% value and measured the true lift after subtracting the initial 0% lift and set that true lift value in locate low and locate high respectively and verified the tr ue lift at 50-60-70 50 -60-70 and 90% lift and found to be accurate for both SRV and GCV servos. For SRV, to calibrate step by step at different lift value, that is to say at 20-40-60-80% stroke,I have changed the berg jumper position for REF1 from 2 to 1 at HSAA card at position 2E for R-S-T. and then revert back the berg jumper position to 1, on comletion of SRV calibration. The moog valve for the GCV was NOT Ok, and that was replaced with a new moog valve and then calibrated as said above. Later ,the turbine started and found both SRV and GCV worked very well and the turbine performance found to be OK with 15 MW load. Both reference and feed back (LVDT) voltages voltages were found to be with in limits for for both SRV and GCV. GCV. Thank you once again for the good advice and nice explanation about the calibration procedure by which, i could solve the problem and brought the turbine "on Line". Thanks & Regards... Reply to this post...
Posted by CSA
on 28 May, 2013 - 11:18 pm
ESKAY, I am very happy to hear you were successful with the LVDT calibrations, and that you were able to determine the correct jumper to move and what position to move it to for the SRV calibration. Most importantly, thanks for the feedback! "Feedback is the most important contribution!"(c) here at control.com. It really is what lets people know if the information was helpful--or not. And a lot of people around the world read and follow these posts, so you are helping a lot of people with your feedback. It's always a good thing when a turbine is returned to service with no problems. (Though we don't know what the issue were before the outage--if any.) Anyway, congratulations! Reply to this post...
Posted by CSA
on 29 May, 2013 - 3:57 pm
ESKAY, I would like to ask two questions, please. When you removed the feeler gauges after the LVDT calibration/verification of both the SRV and GCV, and you had operated the valves a couple of t imes, what was the indicated position when the valve stem was fully down? Was it negative or was it still zero? And, if it was negative, what were the values? Thanks very much! Reply to this post...
Posted by CSA
on 30 May, 2013 - 7:38 am
ESKAY, You wrote, twice: >I have calibrated both SRV and GCV servos.... You did NOT calibrate the servos; you calibrated the LVDT feedback from servo-operated devices. Or, you calibrated the LVDT feedback from devices with servos. But you did NOT calibrate the servo(s).
Please stop thinking--and writing--like this. It's what makes people think the servo-operated liquid fuel bypass valve without LVDTs can be or needs to be "calibrated." When people start speaking--and writing--and therefore thinking about what is actually being done (calibrating LVDT feedback--not calibrating "servos"), then people will start understanding. This is how misunderstandings start--by not speaking properly about what is being done.
