Servo Valve Null Bias Current

SERVO VALVE NULL Bias Current

Page 1

rom the Maintenance departme nt...

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

For GE Gas turbine Frame 5 controlled by Mark 5 TMR system, what can make the Null Bias current of a Servo Valve Valve incorrect? In other other words if the Null Bias is far from -2 .667mA, What should we check to make the null bias current close to -0.267mA?

Posted Posted by CSA

on 25 Februa February ry,, 2009 2009 - 12:40 12:40 am

I'm going to split some hairs here (be very, very detailed) and make some corrections corrections to what you're you're describing. This topic can't really be covered covered completely in a forum like this; it's complicated. But, we're going to take a short swing at the high points. In the Speedtronic turbine control panel, the error between a servo-valve servo-valve output's output's regulator feedback and its reference is converted into servo current. When the feedback of a regulator is equal to the reference the error i s zero, so zero error would mean zero current. But, if zero current is applied to the servo-valve, servo-valve, the fail-safe spring in the servo-valve servo-valve will make the hydraulic actuator move to shut off the flow of of fuel or air or steam. Null bias current is t he value of current that is added to t he servo-valve servo-valve output current to overcome overcome the tension of the fail-safe spring in the servo-valve. servo-valve. So, some amount of current must be added to the output when the regulator error is zero (when the regulator feedback is equal to the reference) to provide sufficient current to overcome overcome the fail-safe spring to keep the device device in position to maintain a st eady flow of of fuel or air or steam. In the Mark V, null bias current is a *fixed* value of current, defined in the I/O I/O Configurator, that is *added* to t he output to overcome fail-safe fail-safe spring tension. In the Mark V, s ervo current is expressed as a percent of full-scale servo current. 100% servo current is equal to 10.0 mA, so 0.1 mA equals 1.0% . The servo current values you see when the unit is running or when you you are manually positioning positioning a device are almost never the null bias currents. The servo current values you you see when the unit is running are the total servo current being put out by the control processor, processor, *including* the null bias current value. You can't really see the null bias current portion of the total current that's that's being applied to the servo coils *unless the feedback is nearly exactly exactly equal to the reference.* Then and only then is the servo current value being displayed equal to the the null bias current and only the null bias current. And this is done by each control processor processor independent of the others in a TMR control panel. panel. So, i f one control processor processor thinks the feedback for for some device is different than the re ference and different than another control processor's or processors', the total amount of current from the control processor will be different than the current from the other control processor(s). Each control processor processor will have the *s ame amount* of null bias current added to its output, but each control processor's processor's output can be different if each control processor thinks its feedback is different than the others'. When the regulator feedback is different from the reference, then the control processor will adjust its total current output *which includes the fixed null bias current value* to try to make it s feedback value equal to the r eference value. And each control processor processor is doing this for every servo-valve servo-valve output. output. Again, the null bias current is a fixed value, defined in the I/O I/O Configurator, Configurator, which i s always added to the total current output of each control processor. When something like what you you are asking about happens, you need to find out what the feedback values values ar e for all three control processors processors for the servo output and you will likely find that one or two of of them are very different from the other(s). If all three control processors processors don't think the feedback is the same and equal to the reference (the reference should be the same for all three control processors), processors), then each control processor processor will adjust its s ervo output output current to try to make its feedback equal to the reference. reference. For example, consider the GCV servo output. The GCV regulator feedback is the high-selected value of LVDT feedback from the two LVDTs on the GCV. Let's say that thought thought the GCV position was 57.8% and thought thought the GCV position was 55.4% and thought thought the GCV position was 54.8 %, and the reference position for for the GCV was 55.2%, then the servo currents would likely be unbalanced. And And probably by a fairly large amount. amount. might be putting out -3.9% servo current, and might be putting out out - 1.9 %, and might be putting out out -2.5% . Those values are *not* null bias currents, but each one includes the fixed value of null bias current which is defined in the I/O I/O Configurator. In this example, the problem is *not* the fixed null bias current value. The problem is that the three pr ocessors have fairly different ideas about the position of the GCV and each one i s trying to move the valve to the reference position, and they all have to work together together ( and that means that one or two are trying to overcome overcome the other) to keep the valve at a steady state position. The bigger the discrepancy in what each control processor processor believes the feedback to be, the bigger the discrepancy in the servo output currents (which include the fixed null bias current value). Now, let's talk specifically about about the null bias current value. Let's say that the value of null bias current defined in the I/O Configurato Configuratorr and that was downloaded downloaded to and being used by all three control processors processors was 2.667 % (the Mark V automatically inverts the value in the I/O Configurator!). Configurator!). Further, let's say all the control processors were indicating a GCV position of 49.7%, the measured position was approximately 49.8%, and the reference was 50.0% and the three servo currrents were all indicating about -2.9% per control processor, or thereabouts. If you you changed the null bias current value in the I/O Configurator Configurator to approximately 3.0 (which would correspond to -3.0%; remember : the Mark V automatically inverts inverts t he value from the I/O Configurator!), downloaded downloaded that value to all three control processors, processors, and re- booted all three control processors, processors, you'd you'd probably find that the indicated valve position position feedback for all three pr ocessors was nearly 50.0%, the measured position would be about 50.0%, and the ser vo currents currents would be al most exactly -3.0% per control processor. processor. In t his case, the amount of current being displayed for each control processor would be nearly equal to the null bias current amount, because each control processor thought the feedback was was nearly identical to the reference *AND* because the amount of null bias current was exactly equal to what was required to overcome overcome the fail-safe spring tension. But it should be clear t hat unless all thr ee control processors believe their regulator feedback values to be nearly identical to each other, the servo currents being put out by each control processor processor will not be the same. And it has nothing to do with the fixed value of null bias current being applied to the s ervo-


Page 2

valve output. output. The value of current that is displayed when the unit is running is not just the null bias current unless all three control processors processors a re using nearly the same value of feedback for the device and the feedback is very nearly identical to the reference. The amount of null bias current r equired to overcome the fail-safe spring is actually a range: -0.267 mA, +/- 0.133 mA, or, -0.133 mA to -0.400 mA (-1.33% to -4.00%). So, the actual amount of null bias current required for a parti cular servo may be anywhere anywhere between -0.133 mA to -0.400 mA and still be within spec. The value of *null bias current* doesn't have to be exactly exactly equal to -2.667%, but 2.67% is a fair ly good good value and works for the majority of of servovalves in use on the majority of GE-design heavy duty gas turbines. About the only time that null bias servo currents need to be adjusted is for some DLN valves, and even then, then, it's questionable whether or not it's really required. The regulator feedback is compared compared to the reference 128 times per second, and the total servo current output is adjusted as necessary to try to make the feedback equal to the reference. *BUT* the value that's shown on on any display or in any VIEW VIEW tool tool capture or output is only updated four four times per second. In other words, the value of servo current written into the control signal database is only updated 4 times per second, even though it could be changing changing at the rate of 128 times per second. (I think that's different for Mark V LM LM panels, by the way.) way.) Lastly, the LFBV uses Liquid Fuel Flow Divider Divider Feedback as its primary control feedback feedback and the SRV uses P2 pressur e feedback as its pr imary control feedback. feedback. So, feedback is not always position. Some LFBVs have LVDTs LVDTs as another stabilizing element of the control loop. loop.

Posted Posted by Jeff Ladwig Ladwig

on 9 Febru Februar ary, y, 2010 - 9:28 9:28 am am

I am in the process of checking checking the calibration of PM1, 2 , 3 and SRV control valves, valves, and have run in to exactly the difficulty you you describe in trying to measure null current. Since we are currently offline, I tried disconnecting two of the three servo outputs and driving the servo to 50% 50% stroke, to eliminate the competition between the controlers. It seems that this would allow the one controller to match reference and feedback, and then go to zero output (except for the null current). For example, with one output active at a time, these are the VDC and calculated servo currents for PM1: R active: 0.290 vdc, 0.280 mA S active: 0.280 vdc, 0.275 mA T active: 0.300 vdc, 0.283 mA This calculates to a null bias of 2.79%, the I/O configurator configurator value is 2.67%. Should I put t he new null bias value in the I/O configurator? configurator? Valve position is within 1% of reference when operating.

Posted by C SA SA

on 9 February, 2010 - 12:23 pm

Did you record record any as-found conditions, conditions, or are these the as-found conditions? Did you, or based on the as-found conditions do you, feel it necessary to re-calibrate LVDT position feedback? feedback? Valve Valve position feedback calibration changes can impact emissions, since fuel splits ar e converted to valve positions. If you you will be DLN tuning after the outage it might not be so critical. But if you won't be DLN DLN tuning after the outage it might be crucial not to change the LVDT feedback calibration. When two of of three controller outputs ar e disconnected, then two of the three null bias currents are not being appli ed to the ser vo-valve vo-valve,, and it will not be able to make the actual position equal to the reference position. All this does (operating the servo-valve servo-valve with a single controller output) is to verify that the polarity of the current being applied to the coil that is stil l connected is correct, and does not cause the device the servo is controlling controlling to reduce the flow of fuel (or air or steam) to the unit. Remember, the s ervo-valves ervo-valves used on GE-design heavy duty gas turbines ar e bipolar devices and the polarity of the current applied to each individual coil is very, very important. The three null bias currents are summed in the servo-valve servo-valve to produce the total null bias current applied to the s ervo-valve ervo-valve.. The summing is accomplished accomplished by the magnetic force developed developed as the r esult of current flowing through through the coils (amp-turns). 2.79% vs. 2.67% is a negligible difference, in my personal opinion, but changing it might make the actual position position more closely match match the reference position; a 1% error is approaching the limit of tolerance (and that's a personal limit, not a GE-defined limit, since GE never defines one), especially for a unit with DLN combustors.

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

The values given for PM1 are as found. In addition, the actual position value as shown on the valve calibration screen screen while at 50% stroke ranged from a low of 51.15% to a high of 52.48%. I followed followed the same procedure for PM2, and found that with only one core active, the valve would only achieve achieve 44.5% to 45.5 % of the 50.0% stroke reference. This now makes sense, realizing that all three cores are needed to supply the required null current. PM2 is also about 2 % off from reference online. At 50% stroke, the feedback percentages percentages are: -


Page 3

R - 48.17% (LVDT 1) 48.99% (LVDT 2) S - 49.13% (LVDT 1) 48.87% (LVDT 2) T - 48.79% (LVDT 1) 48.92% (LVDT 2) If I replace a servo, is it recommended to change the null bias current value in the I/O configurator to the value supplied by MOOG in the attached specs, or is the procedure that GE gives in the control spec better? Thanks.

Posted by C SA

on 11 February, 2010 - 1:00 am

It would sound as if there's something amiss with the null bias current value(s) for PM1? What is the actual physical position of the valve with respect to LVDT feedback? I am not a fan of the null bias calculation procedure in the control Specification. I've seen it cause more tr ouble and lost time than it's worth. If the null bias current doesn't fall within the range that GE specifies (-0.8 mA, +/-0.4 mA total, for a TMR) then there's something wrong with the ser vo-valve. And many people try to use the null bias current for something it was not intended for. If the valve (or the IGVs) doesn't behave the way they think it should, then they start changing the null bias current(s) after they spent hours t rying to calculate one based on the Control Specification value. And worse, most people don't have any idea how to change individual servo current values for a TMR Mark V and end up just making things worse in the end. If Moog is providing null bias currents with their ser vo-valves, it would seem to make sense to use them as start ing points at a minimum--as long as they are in the range specified by GE.

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

I have not measured the actual position. I don't have the tool that attaches to the side of the cylinder. It rides the actuator spring bushing up & down, and provides a small platform to rest the end of the dial indicator on. I measured PM1 servo currents at 50%, following the GE procedure, and came up with 1.01% calculated null bias, which is outside the range allowed by the procedure. The calculated servo currents: R - 0.007 mA S - 0.160 mA T - 0.136 mA

Posted by C SA


And what happens when you put the 2.67% value (negative) in the Speedtronic for all t hree processors and then tell PM1 to go to 50% stroke? Again, I see more and More and MORE people never bothering to measure the stroke or angle. Speedtronic panels are good, but, I don't know if they're that good. But i t does prove a major point: Gas tur bine control IS NOT rocket science, no matter how hard people make it out to be. I've seen people spend tens of hours, literally, trying to get the zero str oke reading for LVDTs to be exactly 0.700 VAC RMS, as if that improved the accuracy of the operation of the turbine. I've seen three- inch stroke dial indicators for about GBP50.00 and mag bases for ar ound GBP10.00 recently. I've even used dial calipers in a pinch with a couple of zip ties to hold one portion stationary and manually moving the other portion. Before GE s tarted using Woodward valves the GEdesigned valves used to have scales on them which were very handy. (I still find it simply amazing that GE uses Woodward valves, and the l ikely major reason is cost. They're certainly not as good as Fisher valves, but they are less expensive, though they do require more maintenance and repair when in service. Guess another old adage is proven true: You get what you pay for.)

Posted by A. OZTAS

on 25 February, 2009 - 11:33 am

In general: Internal mechanical failure in the servo will lead to deviation of null bias current. Possible causes are, degraded lube oil (clogging of servo ports), wrong calibration of the null bias (maybe you have refurbished servo valves from none OEM supplier) . Although, there i s a way to calibrate the null bias, my advice is to replace it with new one from OEM. Good Luck...Tempus Fugit...


Posted by CSA

Page 4


A. Oztas brings up a point I neglected to address. Usually, when I get this question it's related to one single coil that has a servo current that is out of balance with the others. One cannot *really* fix the problem with a single control processor's servo current being greatly different from the other t wo by changing the null bias current value. There is only *ONE* fail-safe spring, so changing the null bias value and downloading it to all three control processors will only mask the problem with the one control processor. Yes, it might lessen the differenti al between the three control processor's servo currents, but it 's not addressing the problem of why the one control processor's servo current is out of balance. That is usually related to the feedback for that control processor being out of balance with the other two control processors. If all three control processors have basically the same feedback and it's not very equal to to the reference, then all three control processors will likely be trying to overcome some problem with the servo-valve: clogging or varnishing of i nternal components leading to s ticky or sluggish operation, worn internal passages or o-rings. Also, a single open-circuit in one of the three coils of a ser vo-valve will cause t he output currents from the other two control processors to increase to try to supply the "missing" torque which would have been produced by the lack of current in the one coil. (Some documentation refers to electro-hydraulic servovalves as "torque motors.") Usually, in this case, the difference between the reference and the feedback will also usually be a littl e greater than normal. A. Oztas brings up another issue which has been reported by many sites which have tried to use rebuilt or refurbished servo-valves. That is, the the fail-safe spring tension usually is not adjusted per GE-design specifications after refurbishment. Adjusting the s pring tension is not an easy task even in a factory or lab. One needs special equipment to monitor oil flow-rates and volumes in additi on to the currents applied to the three coils. Every time I've seen people try to adjust null bias spring tensi on on a servo-valve which is in service, it has resulted i n having to replace the ser vo-valve (that has been said in previous posts here on control.com, also). And usually when they're trying to adjust the s pring tension, it's an attempt to bring the servo current of one control processor into line with the others. And that simply can't be done with the adjustment on a si ngle fail-safe spring. As I've said before, I'm saving my pennies to buy Moog. These things are so misunderstood and people replace them so quickly without understanding how they work or what t hey're capable of that Moog must be making a fortune because I know of sites that have replaced a s ingle servo-valve several times before fixing the real cause of the problem. In the process, the once perfectly good servos get dirty and are not handled very well, and are basically useless after that. Cha-CHING!

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

Wonder why my earlier reply is not shown here. NVM.. let me rephrase it again now. Well.. I think you observed is actually some optical il lusion.. ;) In AutoCalib page, the value for null bias current is shown in percentage and not in amperes. Which mean that -2.667mA is actually shown as -0.267%. Don't worry too much as your servo is still fine;)

Posted by CSA

on 26 February, 2009 - 11:15 am

>Well.. I think you observed is actually some optical illusi on.. ;) In AutoCalib page, the value for null bias current is shown in percentage and not in amperes. Which mean that -2.667mA is actually shown as -0.267%. From the fourth paragraph of the first response: In the Mark V, servo current is expressed as a percent of full-scale servo current. 100% servo current is equal to 10.0 mA, so 0.1 mA equals 1.0% . -0.267 mA would be represented as -2.67%. I just looked through several ACALIB.DAT files, from early Mark Vs (with s) and from very late Mark Vs (with GE Mar k V HMIs). None of them had lines to display null bias current. ACALIB.DAT is an ASCII text file that ACALIB.EXE uses to configure the displays seen in AutoCalibrate is running. Would you please open that file on you operator interface and copy one of the lines which carries the words "null bias" and paste it into a response? My suspicion is that someone found the TCQA RAM address for the null bias current value that gets downloaded from the I/O Configurator, or someone has re-labeled the servo current line to read "null bias".


Posted by Walter

Page 5

on 13 March, 2009 - 2:26 pm

In the Mark V, servo current is expressed as a percent of full-scale servo current. 100% servo current is equal to 10.0 mA, so 0. 1 mA equals 1.0% . According to this explination we tried to compare the current servo values on the Mark V screen with the ones measured directly on the screw of the board. For instance.. For the SRV (as seen on interface) Required Position 15,53 Actual Position 99,43 Servo Current -2,50 Required Position 15,53 Actual Position 99,39 Servo Current -2,21 Required Position 15,53 % Actual Position 99,37 % Servo Current -2,87 % Then checking in the file TC2kReport and we found the screw number and board to measure the output tension (?) to the servo. That was QTBA screw number 27 and 29. We got -0,4v in ( bearing in mind t hat the resis tance is 1Kohm would be 0,4 mA), is that what should be? That is far different from the value we get on the screen (-0,25 mA), means that that on the screen is not taking account of the null bias ? or we are measuring in a wrong place? How can i get in the demand desplay the signal's name of the requiered position si nce I can not find it in the Longname.dat. Thanks very much in advance.

Posted by CSA

on 15 March, 2009 - 1:36 am

The first thing I see wrong with this post is that if the required position is 15.53% and the actual position is 99%, then I would say that something is really, really, really, REALLY wrong with the servo-valve output. Because, if t here is t hat much difference between the setpoint and the actual, t hen the control ain't working. Was the unit running when you were observing these values and taking these voltages? QTBA-27 & -29 are for servo-valve output #1. There are eight servo-valve outputs from a Mark V. So, I think the second thing that's wrong with this post is that we don't know if you were looking at the requir ed position for SVO1 and the feedback for SVO1. In TC2KREPT.TXT, the column labeled "Signal Name" is the value you need to look up in the CSP and work "backwards" from that to find the reference signal name. SVO1 is usually assigned to the Stop/Ratio Valve of a gas fuel system. The reference for the SRV is usually signal name FPRG and the feedback for the SRV is usually FPG2. You won't find this in LONGNAME.DAT. You will find it in the CSP. And, the signal name for the SRV s ervo current is usually FAGR. If you want to see what value of null bias current is being applied to a particular servo-valve output, you need to look in the I/O Configurator for that particular ser vo-valve output. And, r emember that the value in the I/O Configurator is * inverted* (negative) in the Mark V. (If you see 2 .667, the actual value will be -2.667% , or -0.2667 mA.) Whatever value of current you see or measure is the total current that's being applied to the s ervo coil and includes the value of null bias current that's being applied to the output. If you've been reading this post, unless the reference and the actual values for the output are nearly identical the value of current you see on the display will not be the value of null bias current. From the data you provided, the reference and the actual are *far* from equal. Tension is another name for voltage, and since this is a DC output, it can have positive and negative values of voltage (tension) and current. The typical resistance of a servo coil used for a GE-design heavy duty gas turbine is approximately 1000 ohms, so -4.0 V DC would equate to a servo current of approximately -0.4 mA. But, any measurement you make would only be an assumption unless you know the exact value the coil resistance.

Posted bywalter/roberto

on 17 March, 2009 - 3:43 am

Hello! Here we go again. First thin

at the moment we took all that data the turbine was runin at base load MS 6001B MKV TMR . Since CSA warned us about the wide


Page 6

First thing, at the moment we took all that data the turbine was runing at base load (MS 6001B MKV TMR). Since CSA warned us about the wide difference between SRV required and actual position we went into the old files ( I'd dare to say ever since commissioning) and found out that that error was as wide as is currently. Could be that problem related to the inadequate fuel pression supply? According to CSP FPRGOUT must be 248,1 psi at 99.85% TNH , and our actual P2 is approx 223 psi at the same TNH. So, as FPRGOUT is not reached ( because of the lack of fuel) the valve goes practicaly to 99 % of its position in order to pr ovide 248,1 psi. Is that possible? Other doubt is why we get the signals FPRG and FPG2 in psi and not in % as is should be to set the position? Stop Ratio Valve. Required Position: 15,50 % Actual Position: 99,79 % Servo Current: -1,87 % Required Position: 15,50 % Actual Position: 99,57 % Servo Current: -2,28 % Required Position: 15,50% Actual Position: 99,55% Servo Current: -1,97% As we understand in these posts, the servo current we see in the screen is the TOTAL output current to the se rvo, ( s ervo null bias + output current to adjust the error). So, here the output SVO1 servo current is - 1,87 % for , then our maths don't work out, since the servo null bias current i s already -2,667%. And measuring on the QTBA SVO1 output currrent we get -0,4 mA ( 4%), which would be more logical ( null bias (-0,2667mA) + output current to the ser vo). Any clue where are we going wrong? Could be something wrong with the configuration of SRV Autocalibrate display? Thanks very much again and sorry for our insistence. Greetings from Argentina.

Posted by CSA

on17 March, 2009 - 3:29 pm

Hello to Argentina, one of the futbol powerhouses of the world! "Here we go again." Do you mind explaining that comment, please? You seem to have pieces of the puzzle and the answers, but aren't able to put them all together. If the gas fuel supply pressure is les s than the P2 pressure reference, then the SRV is going to go wide open to try to get the P2 to be equal to the reference. Since there's approximately a 0.7 barg pressure drop across the SRV (typically) that would mean that, per the information you provided, the supply pressure would need to be approximately 258 psig to keep the SRV from opening fully. You did not provide the gas fuel supply pressure reading upstream of the SRV. What is the supply pressure? I have seen clogged y-strainers cause high pressure drops, and most units have a y-strainer just upstream of the SRV. Has it been checked for cleanliness recently? There's not usually a dp gauge across the y-strainer, and if you can read the pressure directly upstream of the y-strainer, what is it versus the gas fuel supply pressure upstream of the SRV? Some units have some kind of fuel filters upstream of the y-strainer, as well. Some times they are coalescing filters or just "sock" filters. If they are present, have they been checked? I was at one site where they said the d -p gauge had never registered any d-p. When the filter canister was opened, the filter element had been so dirty at some point that it had ruptured and was effectively non-existent. Someone also commented that they had had exhaust temperature spr ead problems a couple of years earlier and that the gas nozzle tips were found to be plugged with some kind of stringy material, the source of which was never identified. Turns out the gas fuel filter was one of those sock-type filters made of wound stringy material. I want to warn you: That is a bad condition to be operating the unit in if you ever experience gas fuel supply pressure spikes or sudden load decreases. When the Speedtronic panel is putting out excessive current to try to open the SRV it can go into what's deemed "wind-up". Wind-up can take a split-second to recover from if there is a sudden supply pressure increase or a sudden load decrease ("load rejection") and the unit can trip on exhaust overtemperature if it 's being operated at or near Base Load when the disturbance occurs because the current has to be reduced at a rate and it might not reduce the current sufficiently to prevent a "burst" of fuel from being admitted to the combustors. If your gas fuel supply pressure can never be greater than approximately 260 psig, you may be able to ask GE or the OES to recalculate the P2 pressure reference curve to allow the SRV to operate in a controlling region and fashion. But, be prepared to supply them with a recent gas analysis and some details of the configuration of the gas fuel supply system. They may also ask for P/Ns (part numbers) of the gas fuel nozzle tips installed in the machine, and if they're not OES equipment, be prepared to supply the flow characteristics from the vendor or the sizes of the orifices in the nozzle tips. This i nformation will be necessary to be able t o determine if a lower P2 press ure might be possible, and if so, to calculate a lower P2 pressure reference curve. Also, be prepared to supply all the star t-up, warm-up, and acceleration FSR Control Constants.


Page 7

Can you find the section of ACALIB.DAT for the SRV (SVO1 or SVO01) and post it to this threa d? You have never told us where you were r eading the ser vo currents from, and if it 's from the AutoCalibrate display then you might be right: There might be a problem with the configuration of ACALIB.DAT. Did you put the signals FPRG, FPRGOUT, and FAGR on a Demand Display or on the Logic Forcing Display and observe them versus the readings you are seeing on the AutoCalibrate Display? I would submit that you aren't doing anything wrong with the measurements you're taking, but it 's something with the display values or the display that you're reading the values from. Please put the above signals on a Demand Display or the Logic Forcing Display and tell us what the differences are between them and the values you are seeing on the AutoCalibrate display, if t hat's where you're observing the values from (which, again, you haven't told us ). 0.4 mA (or 4%) would be more likely what one would expect to see if the Mark V were driving the SRV to be more open than the valve could physically travel such as what you are describing. If you will look at Section 7, I believe, of GEH-6195, of the Mark V Application Manual, you will see that the SRV regulator uses FPRGOUT as the primary reference and FPG2 for the feedback of the r egulator summing junction. If the two are equal, then the output of the summing junction is zero, which means the SRV position is equal to what it needs to be to make the P2 pressure equal to the P2 pressure reference. After the primary regulator summing junction, the SRV LVDT feedback is compared to the output of the summing junction, and if the summing junction output is zero, then no change to valve position is to be made . If there is an error between FPRGOUT and FPG2, then that difference will be treate d as a requiring a change to the valve position, and that error will be compared to the current LVDT position feedback and the servo current will be changed to make the necessary position change to make the P2 pressure feedback equal to the reference. The GCV and IGV regulators are "st raight" position regulators and the feedback is from the LVDTs mounted on the device actuators. The LFBV (Liq. Fuel Bypass Valve) reference is a liq fuel flow rate and the feedback is from the magnetic speed pick-ups on the Liq. Fuel Flow Divider. But I'd really like to understand the, "Here we go again!" comment.

Posted bywalter/roberto

on 19 March, 2009 - 5:07 am

Hello CSA!, again from the land of football (no soccer) no offence..;) , you are invited to see a football match whenever you want! First of all let us explain to you that we did not mean to be impolite writing "here we go again", we just wanted to express that we are here trying with the same topic again, just that. Talking about gas fuel supply pressure upstream t he SRV we have 260 psi what would be pressure enough. After r eading your explanation it seems that is not a gas fuel supply problem. It is more li kely to be a problem on the Autocalibrate display. The ser vo currents we provided you were taken from the Autocalibrate Display. Here goes the ACALIB.DAT from the SRV. PROC Q SOCKET 1 SVO 1 IOP 21 CARD "TCQA" TITLE "GAS STOP RATIO VALVE" PERMISSIVE L3ADJ

POSITION_NEG_SAT 100 POSITION_POS_SAT -0,1 POSITION_SCALE (F2 256,0 0,0 2 '% ') MAN_SCALE (F2 128,0 0,0 2 '%')

LINE01 BTEXT 'DESCRIPTION UNITS' line 02 data 'SVO number' (C1) line 03 data 'Regulator type' (H1) LINE 05 DATA "LVDT #1 0 % cal. Ref ." (F2 6,667 0 3 "V r ms") LINE 06 DATA "LVDT #1 100% cal. Ref." (F2 6,667 0 3 "V rms") LINE 07 DATA "LVDT #2 0 % cal. Ref ." (F2 6,667 0 3 "V r ms") LINE 08 DATA "LVDT #2 100% cal. Ref." (F2 6,667 0 3 "V rms") ;LINE 09 DATA "LVDT #3 0% cal. Ref." (F2 6,667 0 3 "V r ms") ;LINE 10 DATA "LVDT #3 100% cal. Ref." (F2 6,667 0 3 "V rms ") ;LINE 11 DATA "LVDT #4 0% cal. Ref." (F2 6,667 0 3 "V r ms") ;LINE 12 DATA "LVDT #4 100% cal. Ref." (F2 6,667 0 3 "V rms ") "

"

"

"


Page 8

LINE 14 DATA "LVDT #1 Voltage" (F2 6,667 0 3 "V r ms") LINE 15 DATA "LVDT #2 Voltage" (F2 6,667 0 3 "V r ms") ;LINE 16 DATA "LVDT #3 Voltage" (F2 6,667 0 3 "V rms") ;LINE 17 DATA "LVDT #4 Voltage" (F2 6,667 0 3 "V rms") LINE 23 DATA "Position at POS Cur Sat. " (F2 256 0 1 "%") LINE 24 DATA "Position at NEG Cur Sat ." (F2 256 0 1 "%") LINE 25 DATA "Manual control position" (F2 128 0 1 "%") LINE 27 DATA "Required Position" (F2 128 0 2 "%") LINE 28 DATA "Actual Position" (F2 128 0 2 "%") LINE 29 DATA "Servo Current." (F2 100 0 2 "%") LINE 31 STATUS 1 R "R STATUS:" LINE 32 STATUS 1 S "S STATUS:" LINE 33 STATUS 1 T "T STATUS:" LINE 34 DATA "Permissive: L3ADJ" L3ADJ LINE 35 DATA "Permissive: SPEED <28% (TNH)" TNH

Then we compared the signals in the Logic Forcing Dispay to the same ones in the Autocalibrate Display, and this is what we get: In Logic Forcing Display: FPRGOUT :248,5 psi; FPG2: 233,5 psi ; FAGR: -2,95 % In Autocalibrate Display: FPRGOUT (required pos): 15,52 %; FPG2( actual pos): 99,65 %; FAGR:-1,99 % all values in I found a signal which it seems to be the r eference for us, FSGR, which in Logic Forcing Display is 99,59 % and in the Autocalibrate is, as you saw above 99,65%, more closed to the value. Is it maybe a wrong signal reference we are seeing in the Autocalibrate Display as FSGR? And last but not leastIn our second turbine (TG 2) happens exactly the same with the SRV autocalibrate display. Again thanks very much in advance.

Posted by Ore Rotundo

on 19 March, 2009 - 2:23 pm

Hombre, This is not football, it look like that we ar e ping ponging Now get back reality. Quick look to your ACALIB data shows that is standard configuration for a FR5. I wonder whether your system is HMI or I ? Or is i t may be in the past upgraded to HMI? How about the PROM revisions? If your SRV is operating at 99%, definitely the upstream FG pressure is below the specifications as given by the OEM. However this should not give the problems that you describe. There are two options for your problems: 1) DONT us e ACALIB for calibration and monitoring purposes. Use the logic forcing display and/or pre-vote data display. 2) Verify the ACALIB data for your GT and the revisions of the PROMs . Cross check also the IO_CFG SVOx configuration (stroke 100 % or 128 %) Remember that you dont necessari ly AUTOCLIB display to perform calibration. Just use the basic calibration procedure. Docendo Discumus

Posted by CSA

on 20 March, 2009 - 12:14 pm

If you've asked this question before on control.com, can you please provide the URL link t o the post? My bad for making some assumptions (I really try not to do that, but I failed on this one!). I assumed that you have verified pressure transducer readings against reasonably accurate gauges and didn't ask you to confirm that. I assumed the P2 pressure transducer(s) (by the way, how many transducers does the unit have: 1 or 3?) are reasonably well calibrated and that the feedback (input) is pr operly scaled in the Mark V. I've assumed the SRV LVDTs have been calibrated properly and that the valve is physically at or near full open (something we haven't asked, but which we ar e asking you to visually confirm). I'm going to tr y to explain this again: When the servo-valve regulator feedback is equal to the ser vo-valve regulator r eference, the servo-valve regulator error is zero. When the error is zero, the servo-valve output current would be equal to zero mA. However, the servo-valve has a spring which, in the absence of current (zero mA), will drive the device to shut off the flow of fuel or air or steam. To overcome the spring and to keep the device in a s teady-state position such that the feedback is equal to the reference, a small amount of current is continually added to the servovalve output (at all times!). The only time that the current being applied to the servo-valve's coils is equal to the null bias current value is when the feedback is exactly equal to the reference and no additional current is required to keep the device in a steady-state position to make the feedback equal to the reference.


Page 9

Remember: The Moog servo-valves used for GE-design heavy duty gas turbines ar e polarity-sensitive devices, meaning that the polarity of the applied DC voltage affects the flow of hydraulic oil through the servo-valve. With zero current, there is no force (torque) developed by the torque 'motor' in the se rvo-valve, and the null bias spri ng will act just like the application of a positive current, which would be to s hut off the flow of fuel or air or steam. When the regulator error is zero, the output is zero mA, so the null bias current value defined in the I/O Configurator is continually added to the output. That's what a bias value is: something that's continually added to something (or subtracted, depending on the application). So, when the error between the reference and the feedback is zero, the only current being supplied is the null bias current. And, in my experience with the Mark V, the displayed value of servo current is always the total amount of current being supplied, which includes the null bias current. Under normal conditions, only when the er ror is zero will the amount of current being supplied to the servo be equal to the null bias value. Which brings up another quest ion I haven't thought to ask: To your knowledge, has anyone tried to adjust the null bias spring tens ion of the SRV servo-valve? At this time, based on the information you have provided and what you have chosen to pr ovide, I cannot explain why the SRV servo current is less than what the expected null bias current should be. I suspect that in an attempt to try to get the SRV into a controlling position that someone has changed the null bias current value in the I/O Configurator, but , we don't yet (!) know what that value i s. I might also suspect that someone has done something with the TCQC card configuration jumpers or even something "unique" in the CSP to tr y to rectify this SRV situation. Not being able to look at your CSP and card jumpers, we can'tell that. I am hoping that by answering the GCV questions that we can establish that one servo output is operating as expected, but I'm asking a question that I don't know the answer to, and I just might get a great big s urprise, but I'm willing to take that chance at t his point. You have *NOT* provided all the information asked of you. Please don't arbitrarily choose what information you are going to provide or what information you deem to be relevant or necessary. We're not asking questi ons to be making you run around needlessly; we're asking because we aren't on site and can't get the information for ourselves. And because you wrote here asking for help with an issue, we presume that you are interested in learning something and providing the information requested to help you resolve your issue. If you don't want to fully participate in the exercise of solving your issue, then there's no point in continuing this thread. Specifically: 1) What is the value of null bias current lis ted in t he I/O configurator for SVO1? (Open the I/O Configurator, and click on the TCQA card, and scroll to the screen for SVO1, and tell us what is listed in the null bias current field. Exit the card, exit the I/O Configurator, without saving anything, and you won't disturb any of the I/O Cfgr. setti ngs.) 2) Precisely, where are you measuring this 260 psi supply pressure? At the SRV inlet/supply pressure gauge in the Gas Fuel Compartment, which would be downstream of the y-strainer, or some place in the gas fuel supply piping upstream of the two units' y-strainers? From a pressure transducer on the gas fuel supply? From a pressure transducer on a metering tube and orifice in the gas fuel supply piping upstream of the unit's y-strainers? 3) Has anyone visually checked the y-strainer r ecently? 4) Are there any filters upstream of the y-strainers, and i f so, what is the d-p across the filters and have they been visually checked recently? Now, eight new questions: 1) Pl ease confirm the actual, physical position of the SRV i s at or near full open. 2) What is the GCV position indication (from the LVDT feedback, usually signal name FSG (Fuel Stroke-Gas))? 3) What is the value of FSR? 4) What is the value of servo current being applied to the GCV (usually signal name FAG)? 5) What Diagnostic Alarms are active when the unit is running? (Include any locked-out Diag. Alarms in the list) 6) Please tell us exactly where you're measuring this 260 psi, and is it psig or could it be psia?. Many GE-design heavy duty gas turbines use a metering tube and orifice flow-meter to measure gas fuel flow-rate, and the static pressure transducer is usually calibrated in psia, not psig. So, if you're reading a static pressure transducer for the supply pressure (and this is usuall y located upstream of the gas fuel y-strainer), please confirm the calibration and scaling of the input (the signal name is usually FPG1, sometimes, FPG3). 7) What are the pressures on the three gas fuel pressure gauges in the Gas Fuel Compartment? One should be SRV inlet ("supply" pressure; one should be Gas Fuel Valve Intervalve Pressure (P2 pres sure); and one should GCV discharge pressure, or gas fuel manifold pressure. 8) Sometimes the pressure drop across the SRV is a little higher; sometimes a little lower. But 0.8 bar to 1.3 par is a typical range. Also, can you describe the SRV? Is it in a combined casting with the GCV or is it a separate valve from the GCV? If it's a separate valve, is it a rotary valve or a plug valve? The fact remains, if the SRV is at 99.93% and the P2 pressure refer ence is 248.5 psig and the actual P2 press ure is 232.5 psig (and I presume you are reading this from the P2 pressure transducer feedback on the Mark V operator interface display; can you please tell us what the P2 pressure gauge reading is?) then there is *not* sufficient flow capacity from your gas fuel supply to achieve required P2 pressure. It's that simple . There would generally be no other reason for the SRV to be open so high and the P2 press ure to be lower than the P2 press ure reference.


Page 10

Now, is the P2 pressur e transducer calibrat ed properly? Is the feedback scaled properly in the Mark V? If the gauges in the Gas Fuel Compartment are relatively accurate, this would be a good indication of whether or not the transducer(s) is(are ) calibrated properly and the feedback is scaled properly. If the P2 pressure gauge in the Gas Fuel Compartment is reasonably accurate and is indicating roughly the same pressur e as the transducer feedback, then there's just not enough supply pressure and flow capacity to allow t he SRV to operate in a controlling range (which should be something less than 99.93% and less than full open). Because, even if the SRV LVDTs are calibrated properly, the regulator for the SRV is a pressure control loop and it will put the SRV at whatever position it needs to be at to make the actual P2 pressure equal to the P2 pressure reference, provided there is s ufficient pressure and flow capacity upstream of the S RV to allow the SRV to control the P2 press ure without going full open (and that just doesn't seem to be the case in this inst ance, for two turbines). The reason we're asking about the GCV information is to try to establish that at least one valve is operating in a properly controlled fashion, and to see what the value of servo current is that is bei ng applied to that valve when operating in a properly controlled fashion. FSGR (Fuel Stroke-Gas Ratio) is the typical signal name for the SRV LVDT feedback; it's not a reference, is the actual feedback. The only other thing I could think of to cause the SRV t o behave as it's being described is that if the P2 pres sure transducer was not properly calibrated or the feedback (input) was not properly scaled, but I would expect that reasonably accurate gauges would have alerted you to this issue much sooner. FPG2 is not the SRV position, and it's not listed in the ACALIB.DAT section you posted. Lines 27, 28, and 29 in the section you copied displays the valve's reference position, actual position, and servo current, respectively. I can't recall if the reference position is "active" when the unit is running, or if it's only "active" when the valve is being manually positioned using AutoCalib. (The answers to t he GCV question may help with that!) FPG2 is the scaled feedback (in psi) from the P2 pressure transducer(s ), which should be calibrated in psig (gauge pressure). If you have more than one P2 pressure transducer, the you can look at the signal name FPG2 in the Prevote Data Display to see the individual, pre-voted values of the three feedbacks, and then report them to us, please. Lastly, the amount of null bias current being applied to t he SRV as underst ood in this thread has nothing whatsoever to do with the fact that t he SRV appears to be at or near full open in an attempt to control P2 pressure at 248.5 psig. But it can't because the upstream s upply pressure and flow capacity isn't sufficient to be able to do that. That's what the data you have provided to date tel ls us. Perhaps the answers you haven't provided and the answers to the new questions will lead us in a different direction, but it's not really likely. And, it has nothing to do with null bias current. I agree with Ore Rotundo; the section of ACALIB.DAT that you have provided seems to be fairly generic. Please provide all the information requested, and we can try to get to the bottom of your problem, which, again, should not be related to the null bias current value.

Posted by servo_2001

on 18 March, 2009 - 4:39 pm

I would like to clarify that the answer is not just the OEM as the solution to the problem. To null the servovalve is the easi est calibration you can perform to a servovalve. Do your homework and you will see that there ar e reputable repair facilities that have been repairi ng these valves for some time. Our company has specialized in electro-hydraulic servovalves since 1969. We were schooled at Moog on the r epair of their valves. Do not let the OEM make you think they are the only game in town.

Posted by CSA

on 19 March, 2009 - 12:19 am

What's missing from your reply? Contact info for your company. What's also missing from your reply? The fact that adjusting servo null bias springs is best done in a "laboratory" environment, with controlled flow-rates and measuring equipment. It's easy under those circumstances. The problems being discussed here are not generally related to null bias spring adjustment, but r ather the methods of applying a null bias current and why they are necessary and how to "measure" them. And how the currents being applied (including the null bias current) can be out of balance. Nowhere in this thread did anyone claim that the OEM was the only game in town. There have been other posts here on control.com which listed firms which refurbish Moog servo-valves. Can you add yours to the group?

Posted by Mehulon 21 March, 2009 - 6:50 am

Dear CSA,


Page 11

Dear CSA, Here I have concept of NULL Bias, You are requested to confirm the same. There is good talk on the Null Bias Current for the servo. We can use the drawing of Servo along with schematic of IGV or GCV-SRV to understand the concept. For this talk reference, let us take IGV scheme, the IGV is stable at one position 57 deg. By viewing the scheme of IGV actuating Piston & Cylinder, both side of Pi ston must be equal pressured, as CSRGVOUT is CSRGVBAK(for Mark_IV) to keep IGV at 57 deg STABLE. Now look at the drawing of Servo, to maintain the both sides of piston equally pressurized, the spool should be at the middle of the bushing(such that it neither allows any port{connected to one side of piston} to drain nor gives extra pressure to Port{Other side of Piston}). And thus flapper (Feedback Sleeve) should be at centre and so the armature should be at the centre. And to keep the armature at physical centre position, amount of current applied, is null bias current. Consider what would happen, if the spool is not coming back to centre position. Definitely! it will drive the IGV in either extr eme position. Now consider, the IGV is required to be open to maintain the Exhaust temperature. The Controller will change the current to servo such that the armature will forces spool to move. The movement of the spool will be such that one port will be connected to pressure source and at the same time, opens the other port to the drain. This will create the pr essure diff. on both side of piston. Ultimately Piston will move the IGV to open. At the end, when CSRGVOUT=CSRGVBAK(command=feedback), once again armature is required to be at to null position i.e. centre position, which is done by providing null bias current by controllers Similarly the mechanical null bias key has been provided to adjust the armature at centre positon. Thanks in advance

Posted by CSA

on23 March, 2009 - 8:10 pm

I will confirm all but the last statement about the "null bias key". If by null bias key you mean a tool to adjust the null bias spring tension, then, absolutely no! No one should be adjusting the null bias spring tension of a three- coil electro-hydraulic servo-valve, with the exception of a facility with the appropriate means to do so and verify proper results. Mehul, I really appreciate your questions, and anyone else's who's reading this thread and has some interpreta tion of this concept they wish to confirm. I find this to be one of the most difficult concepts to try to explain to people, with or without the ability to use diagrams or pictures, which doesn't seem to make much difference. I am almost desperately searching for the right words and the right means to explain this to people. I think that one of the things that most people misunderstand about servo-valves as used on GE-design heavy duty gas turbines is that they are not like the overwhelming majority of most actuator outputs on most other types of control systems. Instead of an output that's proportional to desir ed position such as a 4-20 mA output being at 12 mA for a reference of 50%, this one is proportional to the err or between the reference and the feedback, plus a null bias current. And if the error is zero then the output is zero, plus a null bias current. If the error is greater than zero, then the output is greater than zero plus a null bias current. It gets even more complicated when there's more than one coil in the servo-valve, such as in a TMR control panel, and each control processor can output a different current and each servo-valve output includes a null bias current (the same amount of current) to overcome a single spring. So, to anyone reading this thread, if you have some idea about how to make this any easier to understand, PLEASE write and l et me know. If it would help to try to relate it to some other control system concept or input or output, that would be great. But, I've been trying for years to put t his in simple, understandable ter ms, and I've yet to find the r ight words, with or without pictures.

Posted by Mehulon 26 March, 2009 - 4:59 am

Thanks once again CSA. Definetly , one MUST NOT adjust spring tension to set null bias current without proper FACILITY . I mentioned it here to confirm the null bias adjustment device available with the servo. Otherwise, one s hould prefer to replace servo & maitaining clean Pall filter other than service,claening, replacement of spares for servo. Regarding Servo Valve Maintenance, one of my earlier Comment in "IGV TEMP CONTROL TRIP" topic. "Another thing, cleaning the moog servo or replacing the filter, s pares of moog servo requires the extr eam extream exteram(03 Times)care to be taken for the cleanliness of the tool tackels, oils, Hands of personals and envoirement at which the servicing is being carried out. So if possible try to replce the se rvo, then ser viceing the servo."


Posted by CSA

Page 12

on26 March, 2009 - 2:55 pm

Many electronic manufacturers used to give away small screwdrivers to use to adjust the potentiometers on printed circuit cards. There used to be a joke that they gave away those "tweakers" (as they were affectionately called) so that technicians and supervisors would misadjust t he pots and have to call the manufacturer in to readjust the pots. I often wonder if Moog doesn't include the little hex key/wrench for the same reason. It's interesting to note that the originator of this thread hasn't replied to the questions asked to try to help with their issue (which likely isn't related to null bias current, but may be related to a misadjustment of the null bias spring!).

Posted by Ore Rotundo

on 19 March, 2009 - 1:25 pm

Dear Servo 2001 Maybe its good to highlight that, we are not fundamentally promoting the OEM's and one thing is sure that we are also not getting paid for it to do it! Our intention is to give the control.com community our opinions and suggestions in order to help t hem out with their technical is sues. The servo valve users are plenty and indeed healthy competition is welcome, such as after market parts. Finally its al l about money, in order to safeguard the reliability of their asset, professional companies will purchase anyway OEM servos. There are plenty of other GT users who are purchasing repaired parts such as the servo valves. I know some of these GT users buying reconditioned servos who are complaining after having the servo valve in service less then a year, believe or not! Docendo Discumus

Posted byminister

on 17 July, 2009 - 9:45 am

I would like to add some more interest ing facts and kindly ask for explanation. We have 4 Fr ame 5 non-dln turbines running in the island mode. I have found something odd troubleshooting the moog of one of the machines, that randomly changes its bias causing 3% position error - temporary solution we use(unit online) is to recalculate new Null Bias measuring coils voltage and having coil resistance measured before, then downloading the VSVO. This way it's fine for some time, usually until next start-up. I have to mention, that we have already tried three different Moogs with the same result. Back to the moog story. The odd thing is that each unit SRV, GCV and IGV servo currents change at the same time from one stable value to another one (usually by the step of 3, 4 %). The s ame time error doesn't change. I have trended hydraulic pressure 96HQ but there is no sign of the event. Also no sign on P125DC and N125DC. I'm running out of ideas what to trend. Also polarity has been checked many times so far by confiming that MKVI is able to drive the valve by each servo coil separetely. Although it happens on all four units, t he event doesn't occure at the s ame time looking from unit to unit (see my trends). Please check the data collected I uploded below: http://www.speedyshare.com/476835562.html Do not be surpirsed looking at these trends. One of our machines (G4) also have out of spec, positive Null Bias, since commisioning by GE (MKVI config value -12, changing moogs didn't help, anyway machine is controlled correctly.) Any explanation on the moog servo current changes? Why all unit s affected?

Posted by CSA

on18 July, 2009 - 2:14 am

You say you have four turbines and you're only having a problem with one. Or at least that's what it sounds like. Actually, I can't really tell how many units are having the problem now that I re-read the message. But, here goes anyway. You've changed the se rvos; you say the problem happens with all the servos. You haven't told us what Diagnostic Alarms are present when this happens, or before this happens. Do these units have IS barriers? '


Page 13

If you've been reading this thread, you know that the null bias current is the current required to overcome the null bias spring tension. So, if the current required to overcome the spring tensi on changes, it would seem that eit her something is making the spring tension change or something is causing the coil resistance to change. Servo valves are about the most misunderstood device I've ever run across. They're nothing more that electromagnets which produce small amounts of torque. They're just coils of wire through which DC current is applied; the polarity and magnitude of the current causes the direction and amount of torque to change. How can they be so complicated? They're not. The overwhelming cause of "failure" is oil cleanliness and oil temperatur e. There's just not much more to them than that. Very tiny, orifice-like passages and clearances which are very susceptible to dirt and varnishing. Is there possibly a problem with oil temperature? I have seen units with the relief valves set improperly, used as the "pressure regulators" for the hydraulic pump. This causes excessive flow through the hydraulic pump. The hydraulic system of a GE-design heavy duty gas turbine is basically a static system, meaning that under steady-state operating conditions there is no flow. There is only flow when a valve or the IGVs are being commanded to move. I've also seen units with the hydraulic accumulator not properly in service. This al so causes the flow through the hydraulic pump to be excessive which could be a cause of high oil temperature. Have you compared all t he hardware ( "Berg") jumpers on the TSVOs? And all the configuration settings on the VSVOs? Of the unit with the problem vs. a unit without the pr oblem? What is the dither set to on the unit with the problems vs. a unit which seems to be running well (not the one with the whacko null bias value). It just doesn't seem like the servos can be the problem here, especially if all of them on one unit are behaving similarly. There's something else that's common to the ser vos that the problem. Another couple of months and I should be in a position to make my bid for Moog. The way these things get changed on a whim, they must be raking in the cash! I just want to buy the GE-design heavy duty gas turbine line, and I'll be paying my business loan off very early--I'm sure of that. The 125 VDC battery supply voltage is converted to voltages by the rack power supplies which are then used by the VSVOs to drive the servos. It wouldn't seem likely that a problem with 125 VDC battery voltage would manifest itself in the servo outputs. There's something different about the hydraulic system of that unit. Or, possibly even and, there is some configuration and/or jumper settings that ar e not correct. But, for a Moog servo to be changing its null bias spr ing tension requiring on-line null bias current changes? That's just pretty not right. Anyway, that's about all I can think of. To address what I thought the problem was. Now, I'm not so sure. One thing's for sure though: When I own that piece of Moog, I' won't be contributing to threads l ike this on control.com. I know better than to kill t he goose that lays golden eggs.

Posted byminister

on 18 July, 2009 - 11:42 am

Thank you CSA for your reply. >You say you have four t urbines and you're only having a problem with one. Or at least that's what it sounds like. < Actually we have problem with two servos: 1. On one machine the SRV changes it's Null Bias mostly after restart (I have greased the stem already and stroke after many times, result the same, so it's probably not the friction). This Moog creates error >3%, to avoid trip exceeding 5% value we just downloaded recalculated null bias. This really drops the error t o zero. Because of production I can't just stop the machine to do it off-line. (I'm aware about on-line software download, but hardware, single VSVO I tried many times with no bad luck). 2. Another machine GCV has positive Null Bias (MKVI RegNullBias =-11.96), and it's not the polarity. Investigating these two issues I trended all machines and found interesting fact that I shared on previous post. >You haven't told us what Diagnostic Alarms are present when t his happens, or before this happens. < The only alarm we have is the increased position error that can lead to turbine trip. Again, it's not the machine with positive NullBias value. >Do these units have IS barriers? < Yes we have the barri ers on SRV and GCV (some MTL-7765ac), none for IGV. I have it planned to run the machine bypassing them, what do you think? Especially, that two other barriers for seismics on the same machine had failed. >Is there possibly a problem with oil tempera ture? <


Page 14

I'm going to trend all ser vo currents with oil temperature starting this afternoon. Right now (2:42PM) the header temperatures are 57 to 58 deg C, outside ambient 46 deg C (air oil cooler). >I have seen units with the relief valves set improperly, used as the "pr essure regulators" for the hydraulic pump. This causes excessive flow through the hydraulic pump. < That's another good hint you gave me. I found our technician resetting hydraulic pressure on the PSV instead of using pump PCV. I will check remaining units. >I've also seen units with the hydraulic accumulator not properly in service. < No accumulators here. >Have you compared all the hardware ("Berg") jumpers on the TSVOs? And all the configuration settings on the VSVOs? Of the unit with the problem vs. a unit without the problem? < All TSVO the same (jumpers), all MKVI hardware configs the same (to be sure , I have also compared m6b with the MKVI). Dither Amplitude 2.0, Freq 100hz on all Moogs (including IGV), on all units. The SRV/GCV we have is the old design, containing both valves in one body, don't think we have to disable the dit her as someone posted dither disabling is recommended for new types of valves. Regards!

Posted by CSA

on18 July, 2009 - 4:39 pm

One more time: The SRV regulator is a pr essure loop. Even if the LVDT calibration is WAY off, the press ure loop is going to put the valve at the position required to make the actual P2 pressure equal to the P2 pressure reference. It doesn't make any difference if the valve thinks it's at 35.4% and it thinks it should be at 28.4% or 44.4%, or if it's actually at 31.4% stroke (physical measurement) and it thinks it should be at 37.1% and the LVDT feedback says it's at 35.9%. As long as the actual P2 pressure is equal to the P2 pressure reference, everything is perfectly okay. When it's not perfectly okay is when the unit is a TMR panel and the thr ee servo currents are not fairly equal; THAT'S a problem. I'll bet any amount of money the differential is related to the calibration method of that combined SRV/GCV assembly. Would you list t he servo gains and null biases for all of the ser vos? For all of the turbines? A simple chart:

GT1 Null Bias/Gain

GT2 Null Bias/Gain

GT3 Null Bias/Gain

GT4 Null Bias/Gain

SRV GCV IGV

I don't believe you told us if these are TMR or SIMPLEX Mark VI panels; if so, please remind me. As far as dither goes, my personal belief is that none is needed with most of these legacy-style actuators. I have seen barriers cause all manner of strange problems over the years. They appear to be fine, but t hey're not. Also, some seem to be temperature and current sensitive when they start "failing."

Posted byminister

on 19 July, 2009 - 10:59 am

Dear CSA, Thank you this time for your reply! > One more time: The SRV regulator is a pres sure loop. ---- snip ---As long as the actual P2 pressure is equal to the P2 pressure reference, everything is perfectly okay. When it's not perfectly okay is when the unit is a TMR panel and the three servo currents are not fairly equal; THAT'S a problem. < Thanks for reminding that. The above was also mentioned many times on this forum. Although it's pressure control loop our concern is not to be tripped by position error.


Page 15

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

SRV GCV IGV

GT1 GT2 GT3 GT4 Null Bias /Gain Null Bias /Gain Null Bias /Gain Null Bias /Gain 2.4/1.8 3.9/1.8 3.2/1.8 -11.69/1.8 3.3/1.8 3.1/1.8 3.3/1.8 2.65/1.8 2.61/6.8 2.67/6.8 2.82/6.8 3.1/6.8

The way we calibrate these valves is to lift the stem and insert and leave filler gauge to get rid of the gap between the actuator and valve stem. This way the valve is still closed and LVDT indicates 2.5% difference from its rest position. Hope this is the way. > I don't believe you told us if these ar e TMR or SIMPLEX Mark VI panels; if so, please remind me. < TMR, sorry, should have mention that at the beginning. >I have seen barriers cause all manner of strange problems over the years. They appear to be fine, but they're not. Also, some seem to be temperature and current sensitive when they start "failing." < Can I bypass these barriers? I know it's for EEX zone but I do not recall (I'm not 100% sure ) seeing them on 9E tur bine in Europe for example. After trending the ser vo currents jumps together with Oil Temperature no relation could be observed (during night when temperature decreases, the frequency of jumps seems to be the same) Can anyone trend s ervo currents for at least 12hours to observe if these jump like it happens here? I do not understand why three ser vos jump at the same time and it happen on all machines but at different times. We have running hrs varying from 6000 hrs to 12000 hrs and we have never change the lube oil. I wonder if these jumps happen anywhere else? Regards!

Posted by CSA

on19 July, 2009 - 8:09 pm

You have a TMR control system. That means that each control processor has it's own gain and null bias for each coil of the servo valve. Can you please list the null bias for each processor for each servo of each turbine? One thing all manufacturers do is to "copy" functions to similar applications, like the LVDT position error check. Since the position isn't the problem for the SRV (the inability to control P2 pressure is the problem), do you think it makes sense to use t he same settings for the SRV as for, say, the IGVs or the GCV? I'm still confused about how the position error can star t out at one value (even if it's zero) and t hen change when the unit i s running. And I'm confused when you say you download new null bias value(s). Do you download a new value t o one processor? Or to all three processors, one at a time? As MIKEVI says, ther e might be a problem with one of the processors thinking that the P2 pressure isn't the s ame as the other processors, and that would cause that processor to change its current output and the other pr ocessors would have to change theirs to counteract the other. This happens a LOT, and if you have three P2 press ure transducers and one of them drifts or has a leak or a wiring problem, that can cause a problem like this. How many P2 pressure transducers does each turbine have? One or three or ???? Have you trended t he P2 press ures for all t hree processors to see what they're doing before and after this problem? When you put the feeler gauges into the gap prior to per forming the calibration of the SRV and GCV, that's to PUT them in their true zero position. When you remove the feeler gauges and the valve stems drop and the indication goes negative, that's what's SUPPOSED to happen. The purpose for the gap i s to ensure that the valves aren't held open by the actuator bottoming out. Some places it's called "closed end overtavel". The true zero stroke position for these valves is NOT with the valve stem fully down when the valve is closed; in that position the valve stem is not touching the bottom of the valve plug. The true zero stroke position is when the valve is closed and the valve stem is touching the bottom of the plug, and that's what the feeler gauges do: They keep the valve stem in contact with the bottom of the valve plug when the valve is closed and ensure that at the end of the calibrat ion procedure that the valve will return to the same position as when it star ted (which is one of the checks that AutoCalibrate performs when run). The use of barriers is entirely a function of the code and requirements of the country or regulatory agency where the unit is installed, OR the


Page 16

policy of the company which is operating the unit. Ther e are thousands of units operating in all manner of applications around the world which do not and have never had IS barriers inst alled. So, only you can decide if you can operate without the barriers. We don't know the application or the site conditions so we can't tell you to bypass them. We don't know if the IS barriers ar e instal led on the LVDTs or t he servo-valve outputs. When you're trending oil temperature, you're not trending the temperature of the hydraulic pump discharge. You're trending the oil temperature of the oil at the inlet to the hydraulic pump. I really don't understand this whole thing; just when I think I'm getting a handle on it, you say "... I do not understand why three ser vos jump at the same time a nd it happen on all machines but at different times...." By "three" servos do you mean all the coils of a single servo valve? Or do you mean a ll the servos on all the machines? You say it's only on the SRV, but then you say it happens on three ser vos on all machines at different time. Is this happening on the SRV of all machines at different times? Again, I can't imagine why the null bias spring tension would change during operation, or why the coil resistance would change during operation. Heat would seem to me to be the most likely cause for either, but I've never heard of this problem, or a problem described like this. And, a lot of times that is a part of the problem: Someone attr ibutes an occurrence to this reason or that reason without any real evidence that this reason or that reason is causing the problem. You don't know if the coil resist ance is changing or if the null bias spring tension is changing. All you know is that a position error is changing (increasing) and that you can change (reduce) the position error by changing the null bias current value downloaded to "the M ark VI" and we don't know if you're downloading the change to one VSVO or to all three VSVOs one at a time. I would really like to help solve this pr oblem, but, again, every time I think I'm understanding what's happening then I re-read the posts and I get confused even more. I really think MIKEVI has suggested a good course of action, and that you need to also trend the P2 pressur es of all the processors to try to understand what's happening. You should be trending the P2 pressure of each processor, the LVDT feedback from both LVDTs as read by each processor, and the servo current outputs from each processor. I'm also keenly interested to tr y to solve the pr oblem with the outrageous null bias current. You say you've replaced this servo and the new one still behaves the same way???? And you've tested the servo current polarity under the individual control of each processor for this device (GCV or SRV or ???) and it will closely maintain the position with only one processor???? I think there's a typo in your chart, because you've previously said that the GCV of one unit is the one with the outrageous null bias. We r eally need to know what the running servo currents for each processor are for the servo with the outrageous null bias value. Are you sure there's not an oil leak somewhere in t he actuator of the device which this servo is installed on? I apologize if my response seems a little "random" but I'm really baffled by this and at the same time very interested to solve these problems. You should know this: On every GE-design heavy duty gas turbine operating anywhere in the world, there is likely some error in position feedback vs. position reference on one or more servo outputs. Some more than others. But, they are al l running. And we're only talking about position feedback vs. position reference. I have been to more s ites that have incorrectly calibrated LVDT feedback so that the actual physical position is way off from the indicated LVDT feedback. And the turbines sti ll run. And r un well. I heard a former colleague say once a long time ago about GE-design heavy duty gas turbine control, "This ain't rocket science." And he was very correct. If it were rocket science, GE would have been out of business a long time ago because if every servo output had to operate with zero error the units wouldn't run. But they do run, and they run very well. Sure, in a perfect world we'd all like the position error to be zero, and it should be zero. But, it doesn't have to be zero. What counts is: Is the position error increasing or decreasing? This has been said many times before on control.com: The value of something today isn't really informative. It's the value t oday vs. the value last hour or yesterday or two days ago or one week ago or one month ago or six weeks ago, and whether or not that value has changed in that t ime and how fast it has been changing. Sure, a L.O. header temperature value of 90 deg C isn't good, but if the unit has been running for four years with an indicated L.O. Header temperature of 90 deg C, would you say it's been running incorrectly? Or would you go to find another way to verify that reading? In your case, the position reference error seems to be changing relatively quickly and then remaining relatively constant. From what we can understand. Which changes with every reading. Sorry; I'm both perplexed and intrigued. And, I'm a very literal person (but you couldn't tell that could you?).

Posted b minister

on 20 ul 200 - 11:4 am


Posted byminister

Page 17

on 20 July, 2009 - 11:49 am

CSA, thanks for your post. I appreciate your time spent at the computer. > Can you please list t he null bias for each processor for each servo of each turbine? The NullBias values I gave are the same in each processor for each ser vo coil (RST). You confused me a little with this questi on. Using Toolbox I have (v11.02.09), when I select Download Configuration, it goes directly to all three VSVO cards, with no possibility of selecting destination processor (R, S or T). It could be only possible if all three VSVOs are in Simplex config like for example Thermocouple VTCC cards. To set-up hardware config you only have one common field, not three separate fields for R/S/T. To have different NullBias values in each R/S/T card, the only way that comes to my mind is to remove Ethernet cables during download. Have you heard about MKVI having different NullBias values for coils on the same servo? What is your way to achieve that? Gains are of course the s ame for R/S/T as NullBias and the rest of the HW config. More on that, I confirmed the config with TSM, gain and null bias values are the same in all three processors. What I found using TSM checking regulators (command A*pplications -> R*egultors) and then servos (commands S1 for SRV, S3 for GCV and S4 for IGV) : G1/G2/G4 have parameter FIX 1 and FIX 2 equal to zero (for all three GCV/SRV/IGV servos) and MKVI LVDT Min/Max (0/100% position) values at LIMIT HI VOLTS and LIMIT LO VOLTS. The G3, the one that must have had SRV Null Bias corrected after two last restarts has MKVI values of SRV and GCV LVDT min /max in FIX 1/FIX 2 fields and LIMIT HI VOLTS and LIMIT LO VOLTS are different by around +-0.100 (FIX 1 + 0.100, FIX 2 - 0. 100). IGV is the same as G1/G2/G4. I'm not sure if you are familiar with this part of TSM. I found it today looking for any unusual thing. It may be that someone here started the calibration and haven't fix Min/Max value - have no idea. Can only check on stopped unit. Also TSM has a l ot of stuff that can be checked. > do you think it makes sense to use the same settings for the SRV as for, say, the IGVs or the GCV? < For some reason we have it (5% position error trip), I can't think of the situation that this protection is needed, but as it is know that SRV is fuel gas pr essure controlled valve, its LVDT is used as a feedback for VSVO card (SRV VSVO regulator type is 2_LVPosMax), so loosing the LVDT the card probably doesn't know how to adjust servo current (what direction to go). Am I close? > Do you download a new value to one processor? Or to all thr ee processors, one a t a time? < OK. See the trend I have uploaded below: http://www.speedyshare.com/470864238.html The whole story started there You can see t hat one day we started having the error . Servo command had to be higher then the position requir ed to drive SRV to desired position. Suddenly either SRV position increased by overcoming the friction or the moog current increased for some reason and then MKVI reduced SRV command and so we got both value (position and command) at the same level. I'm not sure what was the reason I can only speculate as above (as I said I greased the SRV/GCV stems with the same result after r estart). Then I started my research and found such Moog current step changes on all machines (but again, on other machines the do not cause an position err or). Once again, on each machine. let s say on GT1, al l thr ee moogs SRV/GCV/IGV indicates the step change at the s ame time. Please see the trend I included in my first post. I highlighted each GT set of moogs with different color. Again, in my post I wanted to confirm if anyone has ever observed these step changes, I had mention GT3 servo problem as an origin. So, If you help me with all my problems I will be very grateful. >As MIKEVI says ----s nip--->How many P2 pressur e transducers does each tur bine have? One or three or ???? >Have you trended the P2 pressures for all three processors to s ee what they're doing before and after this problem? < I have just trended P2 (R/S/T) and see nothing (flat lines during the step change that just happened on GT1 and GT3). Will include it in GT3 trend at next start-up. > When you remove the feeler gauges and the valve stems drop and the indication goes negative, that's what's SUPPOSED to happen. < Yes, that's right (the value (negative) is from 3% to 2.5% depending on machine). >The purpose for the gap is to ensure that the valves aren't held open by the actuator bottoming out. < That is exactly how I understand the valve. I'm assuming the gap is for safety if the seat i s worn and no gap the valve will leak. Also the drawing say to check that gap is within desired limits and if no grind the piston rod to obtain it. > The true zero stroke position is when the valve is closed and the valve stem is touching the bottom of the plug, and that's what the feeler gauges do: They keep the valve stem in contact with the bottom of the valve plug when the valve is closed and ensure that at the end of the calibration rocedure that the valve will return to the same osition as when it started (which is one of the checks that AutoCalibrate erforms


Page 18

calibration procedure that the valve will return to the same position as when it started (which is one of the checks that AutoCalibrate performs when run). < No doubt about it. >The use of barriers is entirely a ----snip----. > So, only you can decide if you can operate without the barrier s. We don't know the application or the site conditions so we can't tell you to bypass them. We don't know if the IS barriers are installed on the LVDTs or the servo-valve outputs. < Thanks for advice. I will feel more confident doing it. >When you're trending oil temperature, you're not trending the temperature of the hydraulic pump discharge. You're trending the oil temperature of the oil at the inlet to the hydraulic pump. < No other option for me right now. > I really don't understand this whole thing; just when I think I'm getting a handle on it, you say "... I do not understand why three ser vos jump at the same time and it happen on all machines but at different times...." By "three" servos do you mean all the coils of a single servo valve? Or do you mean all the servos on all the machines? You say it's only on the SRV, but then you say it happens on three ser vos on all machines at different time. Is this happening on the SRV of all machines at different times? < Again, as st ated the thing above (it's really difficult to me to explain it as English is not my native language). What I can tr end, is jump/step change up/down/up/down (and so on) of the total current of ea ch Moog. But I observed looking at card points ServoIOutxNVR/S/T that it happens also in the same direction on all three coils (second screenshot I attached, you can call it s tatic observation) . So, on one single turbine, the change happens at the same time on all 9 coils (3xSRV, 3xGCV, 3x IGV), and you can observe the same on all machines but from machine to machine it's not synchronized. Please see again the trend and color marked GT's. >Again, I can't imagine why the null bias spring tension would change during operation, or why the coil resist ance would change during operation. Heat would seem to me to be the most likely cause for either, but I've never heard of this problem, or a problem described like this. >And, a lot of times that is a par t of the problem: Someone attributes an occurrence to this r eason or that reason without any real evidence that this reason or that r eason is causing the problem. >You don't know if the coil resistance is changing or if the null bias s pring tension is changing. < I agree, I don't want to risk and disconnect servo coils one by one online. What I found after stop that the current is different each time (I register in Excel sheet all NullBias calculations), not resistance. It st ays on all machines 1200- 1300ohm on all machines (barrier s give additional resistance, I think I checked that a year ago). > All you know is that a position err or is changing (increasing) and that you can change (reduce) the position error by changing the null bias current value downloaded to "the Mark VI" and we don't know if you're downloading the change to one VSVO or to all three VSVOs one at a time. < All three at one time. Simply calculation, have all three r esistance noted before the start last month, I measure the voltage for each coil separately, divide V/R, summarize all three results and finally divide by three to get average (negative result must be placed as positive in MKVI config and so on). This is the GE calculation way given by the ControlSpec and it works (offline :), for me also online) >I'm also keenly interested to try to solve the problem with the outrageous null bias current. You say you've replaced this servo and the new one still behaves the same way???? < Actually It has been replaced by GE TA, that left it saying it's fine. They just wanted to leave the place :)))) > I think there's a typo in your chart, because you've previously said that the GCV of one unit is the one with the outrageous null bias. We r eally need to know what the running servo currents for each processor are for the servo with the outrageous null bias value. < I wrote G4 - on the trend you will find G4\signal - that was the r eason I used it inst ead of GT4. Affected valve is SRV. > Are you sure there's not an oil leak somewhere in the actuator of the device which this servo is installed on? < I will check, I have to re-read your posts guys and prepare a checklist) Finalizing the story: you can see that we have two major problems - very high positive null bi as on one machine running relatively good position error that disappear after start-up or we have to help it disappears changing NullBias value (we did it last time instead of waiting). ...And one minor that is se rvos current step changes on all machines that I accidentally discovered. It took all day to think about answers also have a headache. Sorry if I messed something again. Regards


Posted by CSA

Page 19

on21 July, 2009 - 12:42 am

In the ear ly days of Toolbox, it was possible t o download to individual VSVOs. Maybe that's not the case any longer; I haven't seen a Mark VI in a couple of years. I have never seen trends li ke this. My initial reaction is that ther e is something about/with the IS barri ers, but that's just because I don't have a lot of experience with t hem and the experiences I have had have not been very good. As for the friction comment, when I have seen friction on a gas valve (SRV or GCV) the effect has been that the ser vo current increases with no change in position until the device "jumps" usually to a position past the reference and then the current changes to try to drive device back to the reference with no change in position until it "jumps" to a position past the reference and this continues and in some cases gets worse. So, the valve behavior was jumpy and erratic. On either side of the sticky portion of the valve stems, the action was normal. But where the shaft was worn and scored (on a couple of turbines) or where the cylinder walls were scored (on a couple of turbines) the behavior was jumpy and erratic. From what you describe, it seems that the current just "jumps" and without seeing the valve position feedback at the time the current jumps, it's difficult to say what's happening. I don't recognize the IS barrie rs you cited. Have you reviewed the manufacturer's manuals/documents to see if these are properly applied? I would like to know how the *circuit* re sistance changes when this phenomenon occurs. It would be very interesting to monitor the voltages of each servo coil and see what happens. A change in voltage would be indicative of a change in circuit resistance. Would it be possible to monitor the voltage across the IS barr ier and the voltage across the servo coil? I do NOT believe the s ervo coil resistance is changing, but I've been wrong before and I'll be wrong again. I have seen Moog provide some very good troubleshooting assistance when asked to get involved. I know they are s low to respond, but when they do get involved they are pretty helpful. I don't believe the null bias spring te nsion is changing. Have you had your Lube Oil tes ted for contaminants? And agglomeration (I think t hat's what it's called)? Some of the newer formulations of turbine lube oils s eem to be causing lots of problems for servos. I think BP has a formulation that has been used successfully by a lot of heavy duty gas turbine users. When these jumps in current take place, does the turbine power output change? Does the unit continue to run normally or does an operator have to make a change to keep the unit running as desired? If the AHJ (Agency Having Jurisdiction (thanks for that Phil Corso!)) would permit a test with t he servo IS barriers disconnected it would be very interesting to see what happens. I'd really like to know what happens to the position feedback at the time the current jumps. That's about all I can think of.

Posted by CSA

on21 July, 2009 - 10:48 pm

The more I think a bout this the more I think i f you're seeing it on all the servos on all the units at your site, the one thing that's common to the gas valves is the IS barriers. I also wonder if they're also affecting the other servo outputs on the VSVO, such as the IGVs. I did some checking on the MTL site, and it s ays the MTL7765ac is primarily for high frequency low voltage applications. Hmmmm..... I also wonder about the resistance readings you mentioned, which seem much higher than those listed i n the tables in Sect. 8 of the manual.

Posted byminister

on 22 July, 2009 - 5:27 am

CSA, I had opportunity to shutdown GT1 and found something interesting. It seems that "jumps" are not caused by the hydraulic part of the system as these also exist during cool down and after when Auxiliaries are off. Trend with some description added (trip occured :) ). http://www.speedyshare.com/748565606.html Will try today to bypass barriers and trend again.


Page 20

> It would be very interesting to monitor the voltages of each servo coil and see what happens. < The voltage changes for sure , I had usual average value of around -0.320V after last calibration, then when I started the machine and found the position error, I made another measurement of coil voltages and as described before recalculated the NullBias and downloaded new VSVO config (average voltage this ti me -0.490V). > I do NOT believe the servo coil resistance is changing, but I've been wrong before and I'll be wrong again. < I hope barriers is t he reason. Could it be changing MKVI cabinet inside temperature affecting them? I wonder if it's sync'ed with the cabinet fan switching on/off (I can simply check it by keeping all fans on for few hours. > Have you had your Lube Oil tes ted for contaminants? < Yes we have it tested, but we are trying to make it more often (monthly). Nothing unusual last time, let's see next time. > When these jumps in current t ake place, does t he turbine power output change? < Nothing unusual I have trended DWATT as well. > test with the servo IS barriers disconnected it would be very interesting to see what happens. < Yep, this I will pr ovide soon. I have to find some extra terminals for barriers bypassing. Cheers!

Posted by CSA

on22 July, 2009 - 2:17 pm

This is troubleshooting: Understanding the possible causes of a problem and working to eliminate them one at a time or to narrow the field of possibilities down to a very few. I think it's very telling that when these "jumps" occur when the tur bines are running that there is no change in power output. The Mark VI is fast enough to maintain steady and stable power output i n the face of whatever is causing the ser vo current to have to change. And, even if the position error increases (for whatever reason) the Mark VI is automatically "compensating" and keeping the turbine operation stable and steady. You have not provided any information on specifically what's happening to the LVDT feedback when these jumps are occurring, and if the LVDT excitation and/or feedback also have IS barri ers. If so, what i s the manufacturer/part number? And here's where I'm going to s uggest that the problem is not the changing servo currents, but t he changing LVDT feedback. You say t hat the position error changes, and when that happens the servo current would change to try to maintain the err or closer to zero. Right? Please trend the LVDT feedback as well as the servo current, and the LVDT feedback from each LVDT would be the best to tr end, not the high-selected value. Even when the unit is not running. It would be unusual for the Mark VI enclosure temperature to be changing very much. It should be located in a temperatur e- and humidity controlled environment, the emphasis is on control. Are you saying the temperature in the Mark VI panel, or wherever the IS barr iers are located, changes? If so, by how much during the course of the day? Are there fans controlled by temperature? If so, that's unusual, not unheard of, but not typical. I think the barriers you listed are considered "passive" devices and don't require bus power, but the manufacturer instruction book is really unclear on these particular barriers. If they are powered, where do they receive their power from? What IS barriers are used for the LVDTs? Do all the LVDTs have IS barriers? Another possible test would be to take suitably sized 1000 ohm resistors (heat dissipation) and connecting them directly to the TSVO in place of the barrier s and then trending the ser vo current. (This would have to be done when the unit is not running.) If the current still changes, then I would suggest removing the IS barriers from t he LVDT circuits and monitoring the individual LVDT feedbacks and the s ervo currents. This also happens sometimes in troubleshooting: The focus gets placed on the effect rather t han the "affect" (the cause). The whole servo regulator loop needs to be considered: the reference, the feedback, the output. Let's not drop the theory that the servo barriers may be the problem, but let 's not lose sight of the other possible contributing factors. You have addressed the hydraulic possibility pretty conclusively. But, servos do get very warm in the environments they are placed in. Sometimes, the gas valve servos are located at the top of the L.O. reservoir/tank, where the vapors collect. The L.O. in the tank is hotter t han


Page 21

the L.O. header (which is cooled). The IGV servo is usually located in the turbine compartment, which experiences large temperature swings during start ing, operation, and shutdown.

Posted byminister

on 23 July, 2009 - 12:26 pm

Thanks CSA for you input: Post update: >You have not provided any information on specifically what's happening to the LVDT feedback when these jumps ar e occurring, and if the LVDT excitation and/or feedback also have IS barri ers. If so, what i s the manufacturer/part number? < No barriers at LVDT at al l. Summarizing we have only 6 barriers instal led for SRV and GCV. I have used terminal block to bypass the barriers and during last 12 hours "jumps" occurred again. We can exclude IS barriers from servo current change phenomena, but still have to take them into account for two running units, (position error and high positive null bias ). Let's keep in mind, that the offline unit is the one controlled "perfectly" (only servo current jumps" are observed). So let's concentrate on servo current jumps. > And here's where I'm going to s uggest that the problem is not the changing servo currents, but t he changing LVDT feedback. You say t hat the position error changes, and when that happens the servo current would change to try to maintain the error closer to zero. Right? < Right. This gave in another suggestion as the err or appears some time after restart. It can be possible that we loose one LVDT by vibrations while running unit (e.g.. loose wiring) or we are getting the LVDT back by the same reason. > Please trend the LVDT feedback as well as the servo current, and the LVDT feedback from each LVDT would be the best to tr end, not the high-selected value. Even when the unit is not running. < Trend set-up and running. A hint for people that want to trend LVDT's. You have to turn Monitors on (we have it unused, but I met the feature used on some sit es). Select 1 _LVposition as monitor type, select LVDT you want to monitor (see your Regulator), fill in Mn and MxLVDT1_Vrms with data you have in your Regulator. Keep in mind that Regulators use usually two LVDT's, and you have to separate them using two Monitors. After the config is downloaded you will see each LVDT value in VSVO Card Point section under MON1, MON2....MON12 (depending of your set-up) > It would be unusual for the Mar k VI enclosure temperature to be changing very much. It should be located in a temperatur e- and humidity controlled environment, the emphasis is on control. Are you saying the temperature in the Mark VI panel, or wherever the IS barr iers are located, changes? If so, by how much during the course of the day? Are there fans controlled by temperature? If so, that's unusual, not unheard of, but not typical. < We have HVAC unit for the MKVI room so we have quite good environment. And yes, we have fans controlled by thermostats, I was aware of this as very often we have to clean cabinet ventilation inlet fil ters and fan outlet filters from accumulated fine desert sand. After tr ending servo currents yesterday afternoon with all of fans running the servo current phenomena still exists. The average temperature measured using TBTC cold junction is 30 deg C (IS barriers are mounted on the back side of the cabinet where TBTC are mounted, also fans are on MKVI terminal board side only) > I think the barri ers you listed are considered "passive" devices and don't require bus power, but the manufacturer instruction book is really unclear on these particular barriers. If they are powered, where do they receive their power from? < Passive barriers. > Another possible test would be to take s uitably sized 1000 ohm resistors (heat dissipation) and connecting them directly to the TSVO in place of the barrier s and then trending the s ervo current. (This would have to be done when the unit is not running.) If the current still changes, then I would suggest removing the IS barrier s from the LVDT circuits and monitoring the individual LVDT feedbacks and the servo currents. < Will add to the plan. > Sometimes, the gas valve servos are located at the top of the L.O. reservoir/tank, where the vapours collect. The L.O. in the tank is hotter than the L.O. header (which is cooled). The IGV servo is usually located in the turbine compartment, which experiences large temperature swings during starting, operation, and shutdown. < That's the case we have, valve servos are mounted inside the tank underneath the valve. IGV in turbine compartment. As the unit will stay some days, temperature should come down and we will s ee possible effect on the trend. After all day trending: Please find another tr ends (40ms sample ti me), this time also one toolbox .trn file.


Page 22

1) change observed only on GCV/SRV (I was measuring coil resistance a while before it happened. Resista nce stays always the same): http://www.speedyshare.com/727988916.html http://www.speedyshare.com/314203559.html 2) GCV/SRV/IGV jump (see signal description in fir st file, s cale for some signals is different, color the same) "Jump" UP http://www.speedyshare.com/810365505.html "Jump" DOWN: http://www.speedyshare.com/340410574.html I'm trying try do the same with dither off. (As dither is an AC on the DC command signal, can it induct something in the circuit?): After switching dither off, I observed that all current s ignals (SRV R/S/T, GCV R/S/T, IGV R/R/T) have moved in the middle of the "jump high" and "jump low" servo current values. It's moved down by 2% from the high value or 2% up from the low value. Is it coincidence, when the dither amplitude is also 2%? Please see dither off trend: http://www.speedyshare.com/860219617.html These 4% current switches seems odd when I have checked servo currents loading the machine by 1MW (around +2% GCV and SRV) and couldn't even indicate the difference on the trend. If it's not the coil resistance that changes, is it whole circuit loop resistance I mean inside MKVI hardware? The dither seems t o be the one not explored :))) Another experiment: On running unit I have gradually changed dither amplitude for GCV from 2.0 to 0 .2% (by 0.2 % step) , and observed servo current travelling from -7 % to -5% without load or position change. Again, not to confuse you guys, these are trend taken on the offline machine, the machine that has no problems with position error. I will troubleshoot affected unit ASAP (read when available after shutdown - software modification needed to trend LVDT's and se rvo outputs separately). At the end of the post, after 4 hr s of trending servos with removed dither I can confirm no jumps on offline GT so far (have to finish my shift). Tomorrow morning I will updated whether jump occurred or not. CSA, please share your thoughts and suggestions for next checks. Cheers!

Posted by minister on 23 July, 2009 - 6:12 pm

I was thinking in my bed about this changes of the servo current. It's not the resis tance that changes, but the voltage and then current had to compensate the change for the servo to do the same work (Power = U*I = const). (I recall having sometime unusual higher voltage across coils during NullBias verification, I think I wrote the value before). Ok have to go to sleep

Posted byminister

on 24 July, 2009 - 3:22 am

Update: After switching the dither off I haven't observed any more jumps last night. I'm going to do another experiment, put all three ser vos on different dither frequency and amplitude trend. As MKVI fastest sample time is 40ms I should see oscillation at least on 12.5Hz and 25Hz dither trend. I want to check if there is a relation between jumps and dither frequency. I suspect that these j umps are just recorded extr ema of the oscillation signal when its frequency is higher than the sample r ate. The trending tool gets periodically unsynchronized with the s ervo current oscillation generator. Catch you later .

Posted byminister

Update 2:

on 24 July, 2009 - 12:32 pm


Page 23

Update 2: I can observe full curve oscillation for servo currents with 12.5 Hz and 33Hz frequency dither. The 25Hz (40ms) dither as being the same as trend sample could be catched only as shown on attached the screenshot: http://www.speedyshare.com/660758165.html Well, now I will focus on the other two issues I have here. I will write more when I have a chance to make more tests on these machines Regards

Posted by CSA

on24 July, 2009 - 1:08 pm

This explanation would make more sense, es pecially because there is no upset in the turbine output when these "jumps" occur. But what isn't explained is the position error that you reduce by changing the null bias. I'm not a fan of dither for GE-design heavy duty gas turbine applications. The natur e of operation doesn't really cause the control valves to require dither from my experience, though I've recently been made aware of some "lore" (because it's not documented anywhere) that there was non-configurable dither built into the Mark V. But there wasn't any dither in the Mark IV or earlier Speedtronic panels because the servo outputs were al l analog circuits and there wasn't any dither that I was aware of. GE Trending tools can be misleading, as can any manufacturer's. Technology is great, but, digital technology has its limits, and this is one of them. An old-fashioned pen-and-ink chart recorder had its limits as well. In general, and this case is probably a great exception because of the variable frequency rates for the dither option, the GE Tr ending tools are pretty good because the execution rate is not generally higher than the fastest trending rate. But, again, this could be an exception because of the rate issue. Very highspeed toggling of discrete inputs is also hard to catch on GE Trending tools. This explanation also makes more sense because of the repeatable nature of the "jumps". They never seem to increase or decrease by more than the same amount, at least from the trends you've sent. But, the position error, which I haven't really seen a good trend of, is still puzzling.

Posted byminister

on 25 July, 2009 - 4:23 am

> But, the position error, which I haven't really seen a good trend of, is still puzzling. < Please see another trend I found in my collection for the position error. http://www.speedyshare.com/900781673.html The error wasn't so high that time and went back to normal by itself. (I only changed NullBias having the alarm above 3%). Odd is, the s tep change is recorded at the same moment, it could be that what caused the error created al so a delay (momentary desync) for trending tool (???) (what happens during that moment can be seen on this zoomed trend: http://www.speedyshare.com/585332895.html ) Looking at the trend and step changes, the overall picture is very confusing - that's why I wanted to raised the step changes issue. The dither on this machine has been disabled yesterday (machine online). What I need for my tests is to s et-up servo currents and LVDT's to be trended all separately. Regards

Posted byminister

on 27 November, 2009 - 6:25 am

An update (sorry for not updating for so long) 1) The turbine with reversed (positive null bias, negative as a value for MKVI config). I managed to do some test s: - barrier bypass - no change - swapping the MKVI hardware, SRV and its LVDT's connected in place of GCV equipment - no change


Page 24

This gives me a hint t o look at the field hardware, as I haven't witnessed the previous MOOG replacement I'm not sure if it's true there was a replacement. I noticed that error on SRV position (positive one) of the value +1.4% is may cause a fired tr ip although the turbine is r unning fine on load as the position is not considered in the process control loop (as we know the SRV reference will try to maintain the interstage pressure proportional to the turbine speed regardless the LVDT feedback, at least on my machine where it's only used by VSVO hardware and HMI). What has happened here was the valve being open during shutdown by the given above 1.4 % while the command was 0% from the MKVI. This resulted in gas leak through SRV (Exhaust went up to 500 deg C!) with GCV being still open at about 6% and the speed at about 20%. Then the protection tripped the turbine (s hutdown exceeded the given period of time) Funny, next day the position and the feedback after the restart were correct. I will have to work on this valve later. This is just an information (useful I think) what may happened with an error on the valve. 2) The other machine with jumping error that could be corrected adjusting the Null Bias online. I have the LVDT's and moog currents setup in the MKVI to be able to trend as separate signals for each coil and LVDT, but the turbine is running fine si nce. Nothing unusual trending LVDT's or the moog servo current. I will update the thread with any news I find. Regards!

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

Dear Minister, I offer this suggestion as something to tr end, although from your post it is difficult to tell if this is happening on one gas control valve only or multiple valves. And if the problem is on one unit or multiple units. Anyway I had an issue recently with a frame 7ea machine. We were inter mittently getting an alarm for low P2 interstage pressure, at the same time we noted the SRV valve position was erratic. Trending t he position of the SRV in the MKVI confirmed that the SRV valve was not holding a steady position, under constant load and constant main inlet fuel gas pressur e. I trended the SRV servo current known as "fagr" and found it was erratic during the event. I then trended the individual servo currents from each MKVI core, in my unit these ar e called FAGR_R, FAGR_S, and FAGR_T. What I found by running a high speed trend, 40ms, was that i ntermittently the "T" core would decide to fight the other 2 cores, it would try to close the valve. The other 2 cores, "S" and "T" would respond by opening the valve. Then "T" would decide to go back to normal, and "R" and "S" would have to catch up. I don't know if this will help, but it would be something else for you to check. Please also note that replacing the VSVO card r epaired the problem. Also note that I have seen some sites where the signals were never added at the card level to be able to t rend the servo currents individually. Look at your .m6b file, under the VSVO card points, for the given regulator you are having trouble with look at the "ServOut#NVR, ServoOut#NVS, and ServoOut# NVT. If there is no point associated with it, it will say "not used", if that is the case you will need to create a point for each card point and download before you can trend t he currents i ndividually. Again this may not be the problem, but it would be something else you can check. Lastly this assumes that your MKVI is a TMR system, and not a simplex.

Posted byminister

on 19 July, 2009 - 9:21 am

Dear MIKEVI Thank your for reply. I have read your post about the ser vo current trend 40ms before. I have this it in my plans but have to wait for scheduled shutdown to create new signal PINs. Good idea with testi ng another VSVO. I realized I have one unused VSVO (R/S/T) and two terminal boards. minister

Posted b MIKEVI

on 20 Jul , 2009 - 10:55 am


Page 25

Posted byMIKEVI

on 20 July, 2009 - 10:55 am

Minister, the great thing about GE's control systems are that they have lots of great tools to use for diagnosing problems. The downside is that all the same tools are not given to all the customers (some .m6b have pins, some don't etc.) I am not sure where the ball gets dropped, either during commissioning or straight from Salem. But no matter, we have this forum full of knowledgeable people to take advantage of. And best of all we have a participant such as yourself that provides the information we need to t ry and help solve a problem, and hopefully we all learn something in the process. I wish you continued luck with your issue, and look forward to you resolving the problem.

Posted byminister

on 21 July, 2009 - 3:37 am

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

Posted by RVT on6 June, 2010 - 9:52 am

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

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

Latest (very late, sorry) update: 1) The turbine with reversed (positive null bias, negative as a value for the MKVI config): Should have done that test before: I decided to check the behavior of the SRV (I think I made a mistake stating it was the GCV in pr evious posts) having all three coils disconnected from the MKVI. The valve went fully open instead close. So, either the Moog is installed incorrectly supplying the oil from the opposite port or the hydraulic block between the Moog and the actuator i s installed incorrectly (if possible), or something else inside the Moog. Unfortunately I have no detail drawing for the system and have to compare to another machine when possible. Surprisingly GE left us the machine with servo working in such an unsafe setup. 2) The other machine with jumping error that could be corrected adjusting the Null Bias online. I think I have finally solved this one. One day we couldn't restart t he machine, SRV didn't open. We found that 20HD solenoid didn't work correctly - stuck. The WD40 wash helped. I think, before the trip solenoid valve was closing but being sticky was not providing enough oil to the valve actuator (still draining some amount) when energized. Then after like two hours running on-line the si tuation changed, the trip solenoid valve fully closed providing all the oil to the actuator (pressure changed) and we had to change the NullBias to correct the new (described above in previous posts) sit uation. I suspect, that before the cleaning of the trip solenoid valve, after a s hutdown when we were recalibrat ing the SRV, the sticky trip solenoid made us the same condition (draining some oil to the tank) so we got the wrong null bias again. Regards minister

Posted by C SA

on 25 November, 2010 - 7:40 pm

Thanks for the feedback! Have you checked the servo valve part number against the appropriate parts list for the gas valve assembly? Are you sure the servo is the correct one for the application? A lot of people think that all the servos on some GE-design heavy duty gas turbines are inter changeable, and while some are, many are not. I've seen incorrect s ervos been used in dir e circumstances, forcing some r ash things to be done to make them work. I've also seen them left in service for so long because, well, they're working! And, then when t hey finally get around to replacing the servo (either because i t fails or because of valve refurbishment) everyone associated with the initial effort develops amnesia and a lot of blame gets misdirected. I'm *NOT* saying that's what happened at your site, but I've seen similar things happen during commissioning as well. There is a popular sentiment amon st some TAs, es eciall when a ob is over schedule and/or over bud et to do whatever it takes to et finished and et off site. The best


Page 26

amongst some TAs, especially when a job is over schedule and/or over budget to do whatever it takes to get finished and get off site. The best intentions are to send the right part back to site, but sometimes others in the decision-making chain have this same mentality, "It's working! If it's not broke, don't fix it!" Anyway, thanks for the feedback!

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

Thanks CSA, I haven't checked the part number as I believed the TA during commissioning, that he replaced it with another one taken from the s pares, having similar result in operation after. I will try to schedule the check and inform you. Regards to all of you guys and Merry Christmass! Your use of this site is subject t o the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expres sly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2012 Nerds in Control, LLC. All rights reserved. Users of this s ite ar e benefiting from open source technologies, including PHP, MySQL and Apache. Be happy. Fortune Machine-Independent, adj .: Does not run on any existing machine.

Servo Valve Null Bias Current

Recommend Documents