Servo calibration configuration in GE-MARK VIe

#GE #Gastubine

When we Calib for servo valve, sometime need to adjust reg gain and regnullbias. Can everyone tell me how to calculation for reg gain and regnullbias?

Can someone share with me the GEK 116364 document? If you have this document, please share me via email [email protected]

Thank you so much.
If you're working on a GE-design heavy duty gas turbine there should never be a need for adjusting the regulator gain--it's precalculated by "the factory" for the servo-valve flow-rate and actuator size to produce the optimum operation while still remaining stable. If you're working on a GE-design heavy duty gas turbine you will find the regulator gains listed in the Control Specification drawing, which was (may still be) provided with every Mark* turbine control system.

The regulator null bias for most GE-design turbine applications is very simple: -0.8 mA, +/-0.4 mA. This is for a SIMPLEX, DUAL REDUNDANT or TMR application. When replacing a servo-valve with a new one (and even with a remanufactured one) if the regulator null bias value is anything other than the standard value, it should be changed to the standard value and observed to determine if the servo-valve is working correctly. [NOTE: GE Mark* turbine control systems automatically inverts (changes the polarity of) the regulator null bias value; so it will usually be listed in the Control Specification AND appear on the servo-valve/regulator configuration page as a positive value, but the Mark* changes it to a negative value for proper operation.]

Finally--and MOST IMPORTANTLY: The ONLY thing being calibrated with the Mark* on a servo-operated device is LVDT feedback. Full stop. Period. Nothing more; nothing less. You're not calibrating the valve or the IGVs (on a GE-design heavy duty gas turbine) or the liquid fuel bypass valve (on a GE-design heavy duty gas turbine)--the ONLY thing being calibrated with the Mark* on a servo-operated device is LVDT feedback. Not the servo-valve; not the actuator; not the component being moved by the actuator (valve; IGVs). ONLY the LVDT feedback--which means if the servo-operated device doesn't have one or more LVDTs, IT DOESN'T NEED CALIBRATION!

Because GE does such a POOR job of documenting many things--and PARTICULARLY LVDT calibration--there are MANY myths and wives' tales and complete falsehoods about what is being calibrated and how to do it. Even the instructions for LVDT calibration in a GE Control Specification are usually incorrect (specifically the part about verifying/correcting servo current to each of the coils of a servo valve AND calculating regulator null bias values--they're just plain wrong, inaccurate and misleading). Saying you are calibrating ANYTHING OTHER THAN LVDT FEEDBACK just perpetuates the myths and wives' tales and falsehoods, and leads to all manner of problems (real and perceived) for the persons doing the work and for anyone assisting or observing the work.

The subject of regulator null bias has been covered MANY TIMES BEFORE on and can be found using the Search feature at the top of every webpage. There should be no doubts about it, just clarifications if necessary. There have been skeleton procedures for LVDT calibration and regulator null bias adjustment in several (many) threads. But, to sum up briefly and concisely, regulator null bias adjustment should not be necessary and any adjustments should not exceed the range specified by GE (plus-or-minus 0.4 mA), AND adjusting regulator null bias is mostly a trial-and-error process, which takes time and patience (most of which are usually in very short supply when replacing a servo-valve). If the regulator null bias has to be adjusted more than plus-or-minus 0.4 mA from -0.8 mA, something is wrong with the servo-valve or the procedure being used to test/verify the null bias value. (It's not uncommon for new, unused servo-valves taken out of the box and installed on an actuator to have the null bias spring adjustment be incorrect--ESPECIALLY if the servo-valve is remanufactured by anyone other than the original manufacturer or a shop/facility approved by the original manufacturer. AND, adjusting the null bias spring tension IS NOT something which can be done with any degree of accuracy with the proper equipment and conditions. (It's also not uncommon for new/remanufactured servo-valves to be mishandled during installation and for the conditions where the work is being performed to be dusty and/or dirty, and it doesn't take much dust or dirt to cause a servo-valve to not work correctly.)

Regulator null bias should ALWAYS be set to the normal "default" value of -0.8 mA whenever a servo-valve is being replaced. ONLY if during verification it is determined that the regulator null bias needs to be changed should it be changed in small increments and operation re-verified. Servo-valve polarity checks/adjustment should be done with the normal "default" value of regulator null bias current, AND if it needs adjustment it will be obvious during verification of the LVDT calibration.

Finally, contrary to popular--and false--belief that the servo-valve is being calibrated by the Mark*, IT IS NOT NECESSARY TO DO ANY CALIBRATION WHEN INSTALLING A NEW (or remanufactured) SERVO-VALVE. The ONLY times it's necessary to calibrate LVDT feedback is when one or LVDTs have been replaced, or the device (actuator; valve; IGVs) has been disassembled which might change, even slightly, the physical stroke of the device (how far it travels from stop to stop). Those are THE ONLY TIMES when it's necessary to perform a calibration of LVDT feedback.

I can hear the whining now, "We have to calibrate LVDT feedback once per year!!!" WRONG. GE chose to use LVDTs because the output RARELY drifts over time, and because LVDTs are commonly used on rocket motors and aircraft flaps and ailerons--and you don't see those being calibrated once per year. The LVDTs GE uses and specifies are robust devices, designed for high temperature- and high vibration environments.

LVDTs are just like any other device on a turbine or driven device--they don't really "get calibrated" on a regular basis, what the technicians do is check the existing calibration, note the as-found condition on a calibration data sheet, make any necessary adjustments and that's that. LVDTs are NO DIFFERENT--the accuracy of the feedback should be checked regularly (once a year or so), but that DOESN'T mean calibrating the LVDT feedback. It means verifying the accuracy of the feedback, nothing the as-found condition, and only if necessary performing a calibration of LVDT feedback. Why calibrate something when it's found to be properly calibrated? (Any technician will tell you that if the testing/verification of a pressure switch or a temperature switch or a limit switch or a level switch is found to be within tolerance, they disconnect all of their equipment and move on to the next device, and do the same--only making adjustments if necessary. LVDT feedback should be verified and only if it's found out of tolerance should any adjustments/calibrations be made. Simply calibrating LVDT feedback when it may not even be necessary is a waste of time, and if it's not done correctly, it can negatively affect turbine operation and performance.

More than you asked for, and, no, I don't have the GEK you asked for (which makes me think you are working on a steam turbine application of some sort), but the basics STILL apply for LVDT calibration whether it's a gas turbine or a steam turbine.

I believe the steam turbine engineers at GE did produce some document about calculating regulator gains (they don't usually calculate them like the gas turbine engineers do--which is confusing, isn't it?), but I haven't seen that document since the Mark* V days more than 20 years ago and I can't remember what the GE publication number was.

Again, regulator null bias adjustment shouldn't normally be necessary. Regulator null bias adjustment can't be done with a calculation; it's strictly a trial-and-error process. And, it has limits (plus-or-minus 0.4 mA). Regulator gains, at least for GE-design heavy duty gas turbines, are listed in the unit Control Specification and should be followed for stable, proper machine operation. And, calibration of any servo-operated device with any Mark* ONLY CALIBRATES LVDT FEEDBACK. Full stop. Period. Nothing more. Nothing less. Not the servo-valve; not the actuator; not the valve (or IGVs)--ONLY LVDT feedback.
#GE #Gastubine

When we Calib for servo valve, sometime need to adjust reg gain and regnullbias. Can everyone tell me how to calculation for reg gain and regnullbias?

Can someone share with me the GEK 116364 document? If you have this document, please share me via email [email protected]

Thank you so much.

WTF? Thank you for your feedback. We are using GE gas turbine frame 6FA and MARK VIe controller.

Normally we will do stroke test or calibration (LVDT calibration) for LFBV, GCV, SRV, IGV, but sometime the valve got hunting when operation. That time we need tuning by configuration reg null bias & reg gain for valve based on actual conditions. The reg gain do not provided in control specification so we don't know how to calculate it. I know it is mentioned in GEK 116364, but I don't have this document.


The IGV regulator gain value should be in Sect. 06.nn of the Control Specification; I don't remember the exact sub-section but it shouldn't be difficult to find in Sect. 06.

For a GE-design heavy duty gas turbine with hydraulically-operated valves and IGVs it SHOULD NOT be necessary to ever adjust or modify the regulator gain--unless the servo-valve is replaced with one that does not match the originally-supplied servo-valve, OR the hydraulic actuator is replaced with one that is smaller or larger than the originally-supplied hydraulic actuator. GE calculates the regulator gain based on the flow-rate of the servo-valve and the size of the hydraulic actuator and the hydraulic oil pressure and as long as none of those things change (which they shouldn't) there should NEVER be a condition which requires changing the regulator gain. Full stop. Period.

Oil cleanliness is the single most important thing for hydraulic actuators and electro-hydraulic servo-valves. Full stop. Period. If the oil isn't clean, the servos aren't going to work correctly AND it's also likely that the hydraulic actuators and/or their seals are going to experience abnormal wear and degradation.

For most GE-design heavy duty gas turbines the condition of the hydraulic accumulator is also VERY important--as when the hydraulically-operated device is commanded to move quickly and fast it's the hydraulic accumulator which provides the "oomph" (the flow at pressure) for that to happen stably and reliably. If the hydraulic accumulator charge (pressure) isn't correct or isn't maintained, or the bladder is broken, or the hydraulic accumulator valves aren't in the proper positions (the block valve should always be OPEN, and the bleed (drain) valve should always be CLOSED) then the hydraulically-operated valves aren't' going to work properly which is going to lead to problems like hunting/instability and improper operation of the hydraulic actuators--for which the servo-valves ARE ALWAYS BLAMED even though it's not their fault.

And, I'm presuming the hydraulic system pressure is as per the machine Device Summary and the Hydraulic System P&ID Notes, AND that the hydraulic system relief valves are properly adjusted and NOT being used to set hydraulic system pressure. It's important to know and understand that the hydraulic system of GE-design heavy duty gas turbines is basically a static system--there isn't much hydraulic oil flowing under steady-state operating conditions; it's only when hydraulically-operated devices are commanded to move that flow in the hydraulic system increases--and recognizing this GE doesn't over-size the hydraulic pumps AND uses hydraulic accumulators to provide instant flow to support system pressure when flow increases quickly.

If the polarity of the servo currents being applied to each servo-valve coil aren't properly verified that, too, can contribute to hunting/instability. It's been written MANY times on servo-valves removed from manufacturer packaging and installed with the exact same color coding of the coil leads DO OFTEN have one or more coils that ARE NOT electrically connected as the servo-valve being replaced, which leads to problems such as hunting/instability under certain conditions. Servo current polarity is critical to proper, reliable and stable operation--including LVDT calibration. And, if the regulator gain isn't per factory specification then incorrect servo current polarity is going to cause problems and, again, the servo is going to be blamed for that. (One contributor has written that six rebuilt servos were pulled out of the warehouse before a new servo was installed and it worked. Rebuilt servos, which significantly less expensive than new servos, aren't always subject to the same quality checks and tests as new servos. I would imagine the problems with those servos were that the null bias springs weren't adjusted correctly as per GE-specification and that one or more servo coils weren't working.)

But, again--oil cleanliness is critical to proper operation. If the oil isn't clean, and maintained properly (changing filters isn't the only thing which maintains oil cleanliness) the hydraulic system isn't going to work correctly. And since servos are the main "users" of hydraulic oil AND since they have very small internal passages if the oil isn't clean they aren't going to work properlty and will require excessive replacement. (It's also been written on that since about the year 2000 oil refiners have changed turbine oil formulations to improve lubricity, but those same changes have not been good for hydraulic systems that used turbine oil as their fluid (such as GE-design heavy duty gas turbines). One site in the UK was constantly replacing servos--with the lost generation that causes--and commissioned a study which proved that the changes in oil formulation were causing the excessive servo failures. The oil supplier (a major multi-national oil refiner) made a "special" batch of turbine oil as a test, and the number of servo replacements decreased immediately and continued to stay low for months afterwards, and hasn't really changed since the test results were analyzed. And, I'm pretty sure the "special" batch of turbine oil was pretty similar to, if not the same as, turbine oils produced prior to the year 2000...!)

Regulator gains and regulator null bias values for GE-design heavy duty gas turbines should never require modifications. Sometimes regulator null bias values do require a small change, but it's rare and the required change CAN'T be calculated. A small change is made, and the effect of the change is observed to see if it has the desired effect. Often, multiple small changes are necessary to achieve the desired effect. And WHENEVER a servo is replaced with a new one, the null bias value should be returned to the specified value and only changed if necessary. And, if the changes required to get proper operation fall outside the specified range, then the servo should be replaced--and the null bias value returned to the default value before checking the servo current polarity. (And--remember!!! LVDT calibration IS NOT required NOR recommended when replacing a servo, because replacing a servo DOES NOTHING to change the physical stroke (travel) of the hydraulically-operated device OR the adjustment of the LVDT!!!!!)

That's it. If you still can't find the default value of regulator gain in the Control Specification you should contact GE for assistance. If you know of another site with GE-design Frame 6FA units similar to yours you can contact them and ask for a copy of the Control Specification page(s) with the IGV settings. OR, maybe someone reading this that operates or maintains a GE-design Frame 6FA machine can post a copy of Section 06 of the Control Specification from their machine, and describe the gas valves (manufacturer) to be sure the two machines are similar. Over the production life of the 6FA machines GE has changed gas valve suppliers and configurations several times....

The major take-away from this should be that GE, unlike MANY other manufacturers, does an EXCELLENT job of specifying regulator gains and null bias values (and the acceptable range of null bias values). Manufacturers that don't use the old "keep changing the gain until the device goes unstable and then back off one adjustment" method of adjusting gains. YOUR idea of stable operation and unstable operation WILL NOT THE SAME as anyone else's idea of stable and unstable operation. I guarantee it. That method of gain adjustment is not consistent or repeatable (except by the people who've practiced it for decades and insist their idea of stable operation is ALWAYS correct. GE has taken that subjective idea of stable and unstable operation and replaced it with tried and true values--no subjective observations necessary. So, set the values to match the Control Specification values and then find out why the devices are unstable--it's MOST PROBABLY NOT the servo or the servo gain or even the null bias value. It's things like oil condition, hydraulic accumulators condition, hydraulic system relief value adjustment, oil formulation, filter condition, servo current polarity, etc. Solve those issues--and you will find the devices operate stably AND don't require adjustments and the servos will also last longer.
Most everywhere I go (and went) the hydraulic system relief valves are (were) being used to set the hydraulic system pressure. BUT, there is a compensator on the hydraulic pump which should be used to set the hydraulic pump discharge pressure to be equal to the specified value of hydraulic system pressure. (If there are two hydraulic pumps, each pump's discharge pressure needs to be set individually.)

To check/set the hydraulic system relief pressure it's necessary to slowly increase the hydraulic pump discharge until the relief valve starts relieving, note that pressure, and then if necessary make any adjustment to the hydraulic relief valve, using the pump compensator as necessary, set the jam nut on the hydraulic relief valve and slowly reduce the hydraulic pump compensator until the desired hydraulic system pressure is reached, and then set the jam nut on the hydraulic pump compensator and reinstall the acorn nut firmly but not excessively. If there are two hydraulic relief valves, each valve must be set individually using its respective hydraulic pump.

Using the hydraulic relief valve to set the hydraulic system pressure increases the flow through the hydraulic system, which increases the load on the device driving the hydraulic pump (an induction electric motor, or the accessory gear). The pump and system IS NOT designed to have that much flow in the system continuously, and can result in the hydraulic system being unstable (cause hydraulically-operated devices to be unstable/hunt!) and not respond quickly when flow is needed to change device position quickly.

There usually is no written proceure for adjusting the hydraulic system pressure--or the hydraulic relief valve setting. (Surprise...!) But, that's the proper way to set the hydraulic system pressure--using the pump's compensator--and to check/set the hydraulic relief valve, which should ONLY have flow through it to protect the hydraulic system against over-pressure if the hydraulic pump compensator fails allowing pump discharge pressure to be excessively high.

I also often found the valves on the hydraulic accumulator to be set incorrectly--they are almost NEVER labeled/identified AND they are the same size. If you can find the hydraulic accumulator vendor documentation in the Operation & Maintenance Manuals it sometimes identifies which is the block (isolation) valve and which is the bleed (drain) valve. For normal operation, the block (isolation) valve should be OPEN and the bleed (drain) valve should be CLOSED. A special charging hose for use with the hydraulic accumulator was (or should have been) provided with the unit spares after commissioning was finished and the unit turned over to the operator/customer. This is necessary to properly check the system pressure and to add nitrogen or release nitrogen to achieve the proper accumulator charge pressure (usually about 50% of rated hydraulic system pressure). If the people using the charging hose don't know how to use it (there is often a label stuck to the top of the accumulator near the valve with some instructions for using the hose, but it quite often gets painted over and over time the adhesive fails and the label gets lost) the pressure in the accumulator can be lost very easily. Some of the vendor manuals also have some brief instructions for using the special hose for checking system pressure and adding or releasing nitrogen to achieve the proper pressure. (The block (isolation) valve needs to be CLOSED when checking/adjusting the pressure if a hydraulic pump is running, and the bleed (drain) valve needs to be open. If there is no pressure in the hydraulic system it's not require to change hydraulic accumulator valve positions. I ALWAYS recommend that the block (isolation) and bleed (drain) valves of the hydraulic accumulator be verified, and properly & permanently labeled for future reference and use. And, the site should ALSO write a SOP (Standard Operating Procedure) for checking/setting hydraulic accumulator pressure, with photos, also. (Yet ANOTHER procedure missing from most Operation & Maintenance Manuals.)

There are LOTS of devices in most of the systems of GE-design heavy duty gas turbines which are NOT monitored by the turbine control system AND are not controlled by the turbine control system. The proper operation--including valve positioning!!!--is very important and can be the source of problems and even turbine trips.