LFBV calibration with out LVDT


Thread Starter


We are having Liquid fuel by pass Valve (LFBV) Moog. As i am a new person for the MARK system. I want to know the correct procedure of calibration of the LFBV. as the system is not having LVDT for feedback, only flow divider magnetic pick up are used as fuel flow feedback.

Kindly help me to clear the doubts for the topic
brijendra81 [at] gmail.com
This is a <b>HUGE</b> misconception, that one is calibrating the "servo" or the control element (control valve; IGVs; etc.).

On a GE-design heavy duty gas turbine, <b><i>one only calibrates the position feedback device(s), </i>NOT<i> the servo or the control element or the hydraulic actuator.</b></i>

There is nothing to calibrate if there is no position feedback.
Is there any procedure to calculate the null Bias of Moog valve in LFBV which are not having LVDT as a feedback device ,only Mahnetic pick up as a flow feedback

Unless someone has tried adjusting the null bias spring tension of the servo-valve, or unless there is some reason to believe the null bias may need adjusting, then the standard -2.667% (-0.2667 mA) per processor should be just fine. (Remember, some Speedtronic turbine control systems invert the value in the Null Bias current field! Ain't this stuff fun!)

The valve stem of the LFBV of most GE-design heavy duty Frame 6B machines can't be seen; the only way to see the movement of the valve plug is to remove the discharge pipe from the LFBV. And, because there is no LVDT and the feedback is from the Liquid Fuel Flow Divider, if one tries to "move" the servo with a liquid fuel flow reference, the valve plug will just move to the full closed position (because there is no flow to offset the reference).

The most important thing to ensure is that the polarity of the current from each processor being applied to each coil of the servo is correct (remember, the servos used on GE-design heavy duty gas turbines are bipolar so they are polarity-sensitive). And the only way to do this is to monitor the servo currents when liquid fuel is flowing to determine if one or more coil has the wrong polarity applied (the servo currents will be unbalanced if even one or more coil has the wrong polarity; if all three have the wrong polarity there should be no fuel flow at all!).

If the null bias current needs adjusting, then there's likely something wrong with the servo. It's been said many times before on control.com:

The standard null bias value of -2.67% almost never needs adjustment. And most people don't know or ignore the limits of null bias current adjustment (-2.67%, +/-1.33%) and put some wildly inappropriate values in the null bias field.

<b>WORSE</b>, they never actually measure the real physical stroke (position) of the device (valve, IGVs) versus the reference and try to use the null bias current to make the LVDT feedback equal to the reference! Which may actually increase the error between the reference and the actual physical stroke (position).

<b>That's just asinine.</b>

Full stop. Period. End of discussion.

In fact, most sites in the world never even check the actual physical stroke (position) against the LVDT feedback; they just "AutoCalibrate" without ever understanding the objective is make the LVDT feedback equal to the actual physical stroke (position) of the devices the LVDTs is sensing. And they AutoCalibrate whenever there's suspicion of a problem.

Yes, the actual position should be equal to the reference. But the feedback must be equal to the actual physical position. The null bias can be used to help fine-tune that relationship, but every attempt should be made to make the LVDT calibration reflective of the actual physical stroke, and then, and only then, should the null bias current be adjusted. And if measurements are taken properly, and AutoCalibration is performed properly, then 99 times out of 100 the LVDT feedback is very nearly equal to the actual physical position. And using the null bias to make a small adjustment isn't even necessary.

A lot has been written on control.com about null bias current adjustment. Use the 'Search' field (hidden in the far right corner of the menu bar at the top of every control.com page) and put the terms '+null +bias' (without the quotes!) in the field, and you can read the relevant posts.

In my personal opinion, the only time that null bias current needs adjustment is for some gas fuel control valves of some GE-design heavy duty gas turbines with DLN combustion systems. And that's only because fuel splits are used to control emissions and those fuel splits are referenced to fuel control valve position and LVDT calibration can be quite critical. Some valves have non-linear flow characteristics, and so LVDT calibration is very critical.

But, back to your question: Unless you have some evidence that there is a serious problem with the servo, there are literally thousands of GE-design heavy duty gas turbines running around the world with no perceptible problems with the standard null bias current value.

It's really unfortunate that a lot of the GE documentation is copy-and-paste, and worse, that some of the documentation is lacking or incorrect, so that copying-and-pasting poor or incorrect information leads to so many problems. But, until someone forces GE to provide proper documentation, this is all there is.

If you have some issue or problem that you are trying to solve, then, by all means, tell us and we can try to help. If you're reading some documentation that's telling you to calculate the null bias current for the LFBV of the Frame 6B GE-design heavy duty gas turbine at your site, remember: Read GE documentation for <b>intent</b>, not <b>content</b>, unless you wrote the documentation or you know who wrote the documentation. Read GE documentation for the intent, and determine what it's trying to tell you to do and then determine if it's necessary or if it can be done using the written documentation.

And, again, remember: There are thousands of GE-design heavy duty gas turbines running around the world with poorly calibrated LVDTs and with the standard null bias current values. Heck, there are even lots of GE-design heavy duty gas turbines running around the world with incorrect null bias current values; they run and produce power, maybe not as much power as they could, or as efficiently as they could (and in a few cases, more power than they should, resulting in increased thermal stresses on hot gas path parts). But they all run and produce power.

Lastly, if the polarity of the current being applied to even one servo coil is incorrect (and adjusting the null bias current won't solve that problem!), if either of the other two processors is powered-down or fails while the unit is running, then the turbine will trip. So, the polarity of the current being applied to each coil is very important, and it should be verified any time a servo is replaced (or when a turbine is new, or a new turbine control system is installed). It's as much or more important than adjusting the null bias current, especially if it's being adjusted only to make the LVDT feedback (from the "AutoCalibration") equal to the reference, without verification that the LVDT feedback is equal or nearly equal to the reference.

Hope this helps! It's not a simple topic, mostly because there are so many myths and wives' tales and incorrect understandings of servos and LVDTs and calibration. I'm trying to help people understand what null bias current adjustment does, and when it might even be necessary. It's not something that's done before LVDT feedback calibration, and it's not done simply to make the LVDT feedback equal to the reference.