For GE Gas turbine Frame 5 controlled by Mark 5 TMR system, what can make the Null Bias current of a Servo Valve incorrect? In 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?

 

I'm going to split some hairs here (be very, very detailed) and make some corrections to what you're describing. This topic can't really be 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 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 is zero, so zero error would mean zero current. But, if zero current is applied to the servo-valve, the fail-safe spring in the servo-valve will make the hydraulic actuator move to shut off the flow of fuel or air or steam.

Null bias current is the value of current that is added to the servo-valve output current to overcome the tension of the fail-safe spring in the 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 the fail-safe spring to keep the device in position to maintain a steady flow of fuel or air or steam. In the Mark V, null bias current is a *fixed* value of current, defined in the I/O Configurator, that is *added* to the output to overcome fail-safe spring tension.

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%.

The servo current values you see when the unit is running or when you are manually positioning a device are almost never the null bias currents. The servo current values you see when the unit is running are the total servo current being put out by the control processor, *including* the null bias current value. You can't really see the null bias current portion of the total current that's being applied to the servo coils *unless the feedback is nearly 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 independent of the others in a TMR control panel. So, if one control processor thinks the feedback for some device is different than the reference 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 will have the *same amount* of null bias current added to its output, but each control 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 its feedback value equal to the reference value. And each control processor is doing this for every servo-valve output. Again, the null bias current is a fixed value, defined in the I/O Configurator, which is always added to the total current output of each control processor.

When something like what you are asking about happens, you need to find out what the feedback values are for all three control processors for the servo output and you will likely find that one or two of them are very different from the other(s). If all three control 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), then each control processor will adjust its servo output current to try to make its feedback equal to the 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 <R> thought the GCV position was 57.8% and <S> thought the GCV position was 55.4% and <T> thought the GCV position was 54.8%, and the reference position for the GCV was 55.2%, then the servo currents would likely be unbalanced. And probably by a fairly large amount. <R> might be putting out -3.9% servo current, and <S> might be putting out - 1.9%, and <T> might be putting 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 Configurator.

In this example, the problem is *not* the fixed null bias current value. The problem is that the three processors have fairly different ideas about the position of the GCV and each one is trying to move the valve to the reference position, and they all have to work together (and that means that one or two are trying to overcome the other) to keep the valve at a steady state position. The bigger the discrepancy in what each control 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 the null bias current value. Let's say that the value of null bias current defined in the I/O Configurator and that was downloaded to and being used by all three control processors was 2.667 % (the Mark V automatically inverts the value in the I/O 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 changed the null bias current value in the I/O Configurator to approximately 3.0 (which would correspond to -3.0%; remember: the Mark V automatically inverts the value from the I/O Configurator!), downloaded that value to all three control processors, and re-booted all three control processors, you'd probably find that the indicated valve position feedback for all three processors was nearly 50.0%, the measured position would be about 50.0%, and the servo currents would be almost exactly -3.0% per control processor. In this 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 nearly identical to the reference *AND* because the amount of null bias current was exactly equal to what was required to overcome the fail-safe spring tension.

But it should be clear that unless all three control processors believe their regulator feedback values to be nearly identical to each other, the servo currents being put out by each control processor will not be the same. And it has nothing to do with the fixed value of null bias current being applied to the servo-valve 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 are 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 required 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 particular servo may be 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 equal to -2.667%, but 2.67% is a fairly good value and works for the majority of servo-valves 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, it's questionable whether or not it's really required.

The regulator feedback is 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 any display or in any VIEW tool capture or output is only updated 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 at the rate of 128 times per second. (I think that's different for Mark V LM panels, by the way.)

Lastly, the LFBV uses Liquid Fuel Flow Divider Feedback as its primary control feedback and the SRV uses P2 pressure feedback as its primary control feedback. So, feedback is not always position. Some LFBVs have LVDTs as another stabilizing element of the control loop.

 

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 is a way to calibrate the null bias, my advice is to replace it with new one from OEM.

Good Luck...Tempus Fugit...

 

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 two 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 differential 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 internal components leading to sticky or sluggish operation, worn internal passages or o-rings.

Also, a single open-circuit in one of the three coils of a servo-valve will cause the 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 servo-valves as "torque motors.") Usually, in this case, the difference between the reference and the feedback will also usually be a little 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 spring tension is not an easy task even in a factory or lab. One needs special equipment to monitor oil flow-rates and volumes in addition to the currents applied to the three coils. Every time I've seen people try to adjust null bias spring tension on a servo-valve which is in service, it has resulted in having to replace the servo-valve (that has been said in previous posts here on control.com, also).

And usually when they're trying to adjust the spring 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 single 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 they're capable of that Moog must be making a fortune because I know of sites that have replaced a single 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!

 

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 illusion..
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

 

>Well.. I think you observed is actually some optical illusion..
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 <I>s) and from very late Mark Vs (with GE Mark 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".

 

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)
<R>
Required Position 15,53
Actual Position 99,43
Servo Current -2,50

<S>
Required Position 15,53
Actual Position 99,39
Servo Current -2,21

<T>
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 <R> QTBA screw number 27 and 29. We got -0,4v in <R> ( bearing in mind that the resistance is 1Kohm would be 0,4mA), 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 since I can not find it in the Longname.dat.

Thanks very much in advance.

 

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 there is that much difference between the setpoint and the actual, then 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 required 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 servo 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 servo-valve output. And, remember 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 servo 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.

 

Hello! Here we go again.

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 provide 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.

<R>
Required Position: 15,50 %
Actual Position: 99,79 %
Servo Current: -1,87 %

<S>
Required Position: 15,50 %
Actual Position: 99,57 %
Servo Current: -2,28 %

<T>
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 servo, ( servo null bias + output current to adjust the error). So, here the output SVO1 servo current is - 1,87 % for <R> , then our maths don't work out, since the servo null bias current is 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 servo). 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.

 

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 less 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 d-p 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 spread 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 information will be necessary to be able to determine if a lower P2 pressure might be possible, and if so, to calculate a lower P2 pressure reference curve. Also, be prepared to supply all the start-up, warm-up, and acceleration FSR Control Constants.

Can you find the section of ACALIB.DAT for the SRV (SVO1 or SVO01) and post it to this thread? You have never told us where you were reading the servo 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 that'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 regulator 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 treated 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 "straight" 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.

 

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 easiest calibration you can perform to a servovalve. Do your homework and you will see that there are reputable repair facilities that have been repairing these valves for some time. Our company has specialized in electro-hydraulic servovalves since 1969. We were schooled at Moog on the repair of their valves. Do not let the OEM make you think they are the only game in town.

 

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 rather 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?

 

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 the SRV we have 260 psi what would be pressure enough. After reading your explanation it seems that is not a gas fuel supply problem. It is more likely to be a problem on the Autocalibrate display. The servo 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 <R> <S> <T> UNITS’
line 02 data ‘SVO number’ <D0A 09> (C1)
line 03 data ‘Regulator type’ <D0A 19> (H1)

LINE 05 DATA “LVDT #1 0% cal. Ref.” <D0A 60> (F2 6,667 0 3 “V rms”)
LINE 06 DATA “LVDT #1 100% cal. Ref.” <D0A 62> (F2 6,667 0 3 “V rms”)
LINE 07 DATA “LVDT #2 0% cal. Ref.” <D0A 64> (F2 6,667 0 3 “V rms”)
LINE 08 DATA “LVDT #2 100% cal. Ref.” <D0A 66> (F2 6,667 0 3 “V rms”)
;LINE 09 DATA “LVDT #3 0% cal. Ref.” <D0A 68> (F2 6,667 0 3 “V rms”)
;LINE 10 DATA “LVDT #3 100% cal. Ref.” <D0A 70> (F2 6,667 0 3 “V rms”)
;LINE 11 DATA “LVDT #4 0% cal. Ref.” <D0A 72> (F2 6,667 0 3 “V rms”)
;LINE 12 DATA “LVDT #4 100% cal. Ref.” <D0A 74> (F2 6,667 0 3 “V rms”)

LINE 14 DATA “LVDT #1 Voltage” <D0A 20> (F2 6,667 0 3 “V rms”)
LINE 15 DATA “LVDT #2 Voltage” <D0A 26> (F2 6,667 0 3 “V rms”)
;LINE 16 DATA “LVDT #3 Voltage” <D0A 32> (F2 6,667 0 3 “V rms”)
;LINE 17 DATA “LVDT #4 Voltage” <D0A 60> (F2 6,667 0 3 “V rms”)

LINE 23 DATA “Position at POS Cur Sat.” <D0A 10> (F2 256 0 1 “%”)
LINE 24 DATA “Position at NEG Cur Sat.” <D0A 12> (F2 256 0 1 “%”)
LINE 25 DATA “Manual control position” <D0A 14> (F2 128 0 1 “%”)

LINE 27 DATA “Required Position” <D0A 50> (F2 128 0 2 “%”)
LINE 28 DATA “Actual Position” <D0A 58> (F2 128 0 2 “%”)
LINE 29 DATA “Servo Current.” <D0A 56> (F2 100 0 2 “%”)

LINE 31 STATUS 1 R “R STATUS:” <D0A 16>
LINE 32 STATUS 1 S “S STATUS:” <D0A 16>
LINE 33 STATUS 1 T “T STATUS:” <D0A 16>
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 <R>
I found a signal which it seems to be the reference 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 least…In our second turbine (TG 2) happens exactly the same with the SRV autocalibrate display.

Again thanks very much in advance.

 

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 them out with their technical issues. The servo valve users are plenty and indeed healthy competition is welcome, such as after market parts. Finally its all 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

 

Hombre,

This is not football, it look like that we are 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 it 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 use 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 necessarily AUTOCLIB display to perform calibration. Just use the basic calibration procedure.

Docendo Discumus

 

If you've asked this question before on control.com, can you please provide the URL link to 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 properly 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 are asking you to visually confirm).

I'm going to try to explain this again: When the servo-valve regulator feedback is equal to the servo-valve regulator reference, 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 steady-state position such that the feedback is equal to the reference, a small amount of current is continually added to the servo-valve 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.

Remember: The Moog servo-valves used for GE-design heavy duty gas turbines are 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 servo-valve, and the null bias spring will act just like the application of a positive current, which would be to shut 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 error is zero will the amount of current being supplied to the servo be equal to the null bias value.

Which brings up another question I haven't thought to ask: To your knowledge, has anyone tried to adjust the null bias spring tension of the SRV servo-valve?

At this time, based on the information you have provided and what you have chosen to provide, 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 is. I might also suspect that someone has done something with the TCQC card configuration jumpers or even something "unique" in the CSP to try 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 surprise, but I'm willing to take that chance at this 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 questions 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 listed in the 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. settings.)

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 recently?

4) Are there any filters upstream of the y-strainers, and if so, what is the d-p across the filters and have they been visually checked recently?

Now, eight new questions:

1) Please confirm the actual, physical position of the SRV is 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 usually 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 pressure); 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 reference is 248.5 psig and the actual P2 pressure 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 pressure to be lower than the P2 pressure reference.

Now, is the P2 pressure transducer calibrated 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 pressure as the transducer feedback, then there's just not enough supply pressure and flow capacity to allow the 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 sufficient pressure and flow capacity upstream of the SRV to allow the SRV to control the P2 pressure without going full open (and that just doesn't seem to be the case in this instance, 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 being 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 to behave as it's being described is that if the P2 pressure 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 the 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 the SRV as understood in this thread has nothing whatsoever to do with the fact that the 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 supply pressure and flow capacity isn't sufficient to be able to do that. That's what the data you have provided to date tells 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.

 

Dear CSA,
Here I have concept of NULL Bias, You are requested to <b>confirm</b> 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 Piston 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 <b>neither</b> allows any port{connected to one side of piston} to drain <b>nor</b> 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 extreme 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 pressure 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

 

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 <b>no</b>! 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 interpretation 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 desired position such as a 4-20 mA output being at 12 mA for a reference of 50%, this one is proportional to the error 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, <b>PLEASE</b> write and let 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 this in simple, understandable terms, and I've yet to find the right words, with or without pictures.

 

Thanks once again CSA.

Definetly , one <b> MUST NOT</b> adjust spring tension to set null bias current without proper <b> FACILITY </b>.

I mentioned it here to confirm the null bias adjustment device available with the servo.

Otherwise, one should 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, spares of moog servo requires the extream 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 servo, then serviceing the servo."

 

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 the 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!).