My SVO3 in TMR MKV GT has a constant feedback difference of 4-5% from a reference signal. Both LVDT's are pretty close at all channels but when trying to run Autocalib i get a message "Not reach 50% +sat. more then 5 sec". I was thinking about electronic Null Bias adjustment, but dropped the idea after reading topics here about it. Same with a mechanical Null Bias adj. Giving 50% s.p. i get around 54% actual position,giving 0% i get ~4% and so on. But giving -4% i can reach 0% position which is a physical fully closed position of the valve. What are my possible actions before i will replace the servo? And - where can i find an interpretation of those messages in Autocalib program along with possible troubleshooting?

Thanks for your time and knowledge, guys.

 

When did you first notice this differential?

Why is it perceived to be a problem? What other issues/symptoms/conditions are you trying to resolve by calibrating LFBV LVDT feedback?

It's presumed when you say "...constant feedback difference..." you mean a differential between the actual position (measured) versus the feedback position.

[LVDT calibration is NOT about making the LVDT feedback equal to the position reference. It's about making the actual, measured position equal to the reference.]

While it's not unusual for there to be some differential, this does seem to be a little excessive--again, presuming it is a differential between measured, actual position and LVDT feedback.

Have you verified the polarity of the servo current being applied to all three coils is correct?

What is the value of servo current for each of the three processors when the LFBV is being held at some midstroke position?

What is the value of the Regulator Gain being for SVO3 (from the I/O Configurator)?

What is the value of Null Bias Current in the I/O Configurator for SVO3?

The LVDT feedback for LFBVs is used for stability in control--<b>NOT</b> for controlling the liquid fuel flow-rate. The regulator used for LFBVs with LVDTs is primarily a flow-rate control loop, with position control for stability. So, even if the LVDT feedback is off by 10 mm from the reference, as long as the fuel flow-rate feedback from the Liq. Fuel Flow Divider matches the Liq. Fuel Flow-rate reference, the valve will stay at it's present position. The valve is moved not by a differential in position, but a differential in flow-rate--when the actual flow-rate differs from the flow-rate reference.

It may indeed be a servo-valve problem, but unless you are experiencing other problems (which you haven't told us about), and this is just something which was noted for the first time recently it may well have existing for some time.

Be very careful if you are considering trying to adjust the mechanical null bias spring tension of the servo-valve. Many a good servo-valve has been ruined by well-intentioned "adjustments." Be certain everything else is correct (servo-valve polarity; filters/strainers clean; null bias and regulator gain values within reasonable limits; etc.) before attempting to adjust the null bias spring tension. Without the proper equipment (gauges and meters and pressure sources and oil at the proper temperature, etc.) the servo-valve may well become unserviceable if it wasn't before the null bias spring tension was tampered with.

Again, you have not provided sufficient information to give any more of an answer.

And, having said that, sometimes servo-valves do get worn and need to be replaced. But, they're pretty expensive to just be replacing them because they are "suspect". Moog makes a LOT of money on that kind of thinking!

By the way, good on you if you are actually measuring LFBV position and comparing it to the LVDT feedback! That happens so rarely it's worth a mention.

If you're not measuring actual LFBV position versus LVDT feedback, well, ... you should be.

 

First i have to apologize for a mistake - its an SVO4 (liq.fuel splitter vlv), not an SVO3 (liq.fuel bypass vlv).

> When did you first notice this differential?
- After a Major Inspection which we've completed recently

> Why is it perceived to be a problem?
- We tripped because of difference between reference and feedback(QSL) signal

> What other issues/symptoms/conditions
- Everything else looks normal.We flushed the oil system and changed filters before first startup.

> are you trying to resolve by calibrating LFBV LVDT feedback?
- Actually,LVDT's are OK (0% is really fully closed valve position and 100% is a max open) so i did not touch them

> It's presumed when you say "...constant feedback difference..." you mean a
> differential between the actual position (measured) versus the feedback position.
- Yes,its a diff. between a Ref.pos vs Act.pos is valve calibration program.

> Have you verified the polarity of the servo current being applied to all three coils is correct?
- Yes,i have.

> What is the value of servo current for each of the three processors when the
> LFBV is being held at some midstroke position?
- Have not measured it yet

> What is the value of the Regulator Gain being for SVO3 (from the I/O Configurator)?
- Current Gain: 3.0 (we're talking about SVO4,yes?)

> What is the value of Null Bias Current in the I/O Configurator for SVO3?
- Current Bias: 0.0

> The LVDT feedback for LFBVs is used for stability in control- - <b> NOT</b> for
> controlling the liquid fuel flow- rate. The regulator used for LFBVs with LVDTs
> is primarily a flow- rate control loop, with position control for stability. So,
> even if the LVDT feedback is off by 10 mm from the reference, as long as the
> fuel flow- rate feedback from the Liq. Fuel Flow Divider matches the Liq. Fuel
> Flow- rate reference, the valve will stay at it's present position. The valve is
> moved not by a differential in position, but a differential in flow- rate- - when
> the actual flow- rate differs from the flow- rate reference.
- Agreed.The reference signal is built from FQLP/FQLT which is flow

> Be very careful if you are considering trying to adjust the mechanical null
> bias spring tension of the servo- valve. Many a good servo- valve has been ruined
> by well- intentioned "adjustments." Be certain everything else is correct
> (servo- valve polarity; filters/strainers clean; null bias and regulator gain
> values within reasonable limits; etc.) before attempting to adjust the null
> bias spring tension. Without the proper equipment (gauges and meters and
> pressure sources and oil at the proper temperature, etc.) the servo- valve may
> well become unserviceable if it wasn't before the null bias spring tension was
> tampered with.
- In fact,I had no choice but to do it - we have no spare MOOG and we were must run the turbine .So, after it had no effect at all by changing Current Bias in IO.CFG to 2.67, I've turned that small Allen adjuster on the servo and - lucky me! - now the diff. between Ref and Act.pos is down to 0.7%.

> By the way, good on you if you are actually measuring LFBV position and
> comparing it to the LVDT feedback! That happens so rarely it's worth a mention.
- Yes,we can measure actual vlv position - we have original scale on the valve's body

But still, when trying Acalib i get a message "Wrong servo null bias" from the program. Where can I get an interpretation and some action guideline for all these messages?

Why SVO4 is the only servo with zero current bias configured in IO. CFG? It's the same in our other turbine.

I will be very grateful to receive a response from guru like CSA and/or other experienced people. It's amazing to have this kind of support from another corner of the Earth and to realize that you're not the only one fiddling with THIS in this very same moment! Thank you guys!

 

I have to make an admission, also. I completely misread your post being about a liquid fuel splitter valve. That's a rather unique application, and I also note that you said the application was a Frame 9 unit, and that you didn't specify whether it was a Frame 9<b>E</b> or a Frame 9<b>F</b>.

In any case, you are <b>indeed</b> a lucky person to have been able to make an adjustment to the servo-valve spring without rendering the servo-valve useless.

I have never encountered a servo-valve used on a GE-design heavy duty gas turbine that <b>did not</b> require a null bias value. A certain packager of Frame 9s (both E- and F-class units) uses some unusual configurations. You should try looking in the Control Specification document for the unit(s) in Section 05.01 (which is usually the Liquid Fuel-related section) for any possible information they might have seen fit to provide.

As for why the two units at your site didn't have any null bias configured that's probably attributable to either the packager feeling it wasn't necessary, or to the commissioning engineer. It is presumed the servo-valve is a bipolar device, as the other servo-valves are.

As for the error message you are encountering when trying to AutoCalibrate the LVDTs, I've only ever seen that before on DLN-I gas fuel splitter valves, and <b>some</b> (not all!) of those valves required a null bias of -2.67%. AutoCalibrate error messages were never documented very well, if at all. When I've seen that error message before it didn't prevent an AutoCalibration, it seemed more like a warning.

I suggest you take your queries to the GE Controls Connect site for more information and assistance. And, write back to let us know the results and your impressions of the site. Here is a link to a thread with some information about the site (note: SSO stands for Single Sign-On, which is a GE login/password function, but since GE employees think everyone can speak GE TLAs they never bother to explain them (TLA=Three-Letter Acronym)).

 

I may have forgotten to include the link to the thread with the GE Controls Connect information:

http://www.control.com/thread/1326644263#1326725980