We are trying to calibrate GCV\SRV through GCV\SRV Calibration Menu given in Mark-V <I>. No problem occurs while calibrating GCV. It is working properly on all given values.

Some problems occur while calibrating SRV. It is not working properly on all given values. It just completely opens the valve while only 25% signal is given. Please briefly share your knowledge about SRV calibration. What is the procedure required to calibrate SRV?

 

Dear Sir,

You can change the servo valve used to operate the SRV & then take a trial.

Following points to be taken care of while changing the servo:

1) Check the Type of Servo
2) Port location must remain identical
3) Polarity must be checked, i.e. remain same (Note the cable color code)
4) Dust free tools, hands of technician and environment should be provided

 

It is presumed you are using a User Defined- or Demand Display for calibrating the LVDT (Linear Variable Diffrential Transducer) feedback from the GCV (Gas Control Valve) and SRV (Stop-Ratio Valve). This display is left-over from procedures which had to be used before AutoCalibrate was developed.

The GCV servo regulator is a position loop--the GCV is told to go to some position and the actual position (from the LVDT feedback) is compared to the reference and the servo output current is modulated to make the feedback equal to the reference.

The SRV regulator is a pressure loop--the SRV is commanded to go to whatever position is required to make the actual P2 pressure (intervalve pressure--the pressure between the SRV and GCV), equal to the P2 pressure reference which is calculated in the CSP (Control Sequence Program). The SRV does have LVDT feedback--look in Sect. 7 of the Mk V Application Manual, GEH-6195, for details on the regulators and servo-valve outputs of the Mk V.

The servo regulator used for all position loops in heavy-duty gas turbines with Mk V turbine control panels is a type 43 (for those devices with two LVDTs), position control using the higher of two LVDT feedbacks. The servo regulator used for all pressure loops with position feedback in heavy-duty gas turbine is uaually a type 77 (for those SRVs with two LVDTs) which used pressure feedback for the inner loop and the higher of two LVDT feedbacks for the outer loop.

Basically what happens with the SRV is that the Mk V calculates a P2 pressure reference (usually CDB (Control Signal Database) signal name FPRG, Fuel Pressure Reference-Gas) and that is compared to the P2 pressure transducer feedback(s) from transducers 96FG-2A, and -2B & -2C, if equipped with three P2 pressure transducers. The actual P2 pressure CDB signal name is usually FPG2, Fuel Pressure Gas 2 (early Speedtronic control systems had gas fuel pressure transmitters monitoring the supply pressure and the GCV discharge pressure, for a total of three pressures, and P2 pressure is the pressure between the supply pressure and the GCV discharge pressure). If there is no difference, then the valve is in the correct position and no change in position is required.

If there is a difference (error) between the P2 pressure reference and the actual P2 pressure reference, the SRV is commanded to change position in proportion to the error between the actual- and reference pressures. The position error is where the LVDT feedback comes in; as the valve changes position and the pressure differential decreases the amount of change of position decreases, until the actual P2 pressure equals the P2 pressure reference, and then the valve position is equal to the position reference and the valve is held in that position because the output of the regulator goes to zero.

So, first of all, the accuracy of the calibration of the SRV LVDT feedback is not all that important--as the valve will be moved to whatever position it needs to go to in order to make the actual P2 pressure equal to the P2 pressure reference. It doesn't make any difference if the valve is actually at 20% of stroke when the Mk V thinks it's at 24.7% of stroke--if the P2 pressure at that position is equal to the P2 pressure reference, then the valve is at the "right" position.

When you're using the User Defined- or Demand Display for calibrating SRV LVDT feedback, you are giving the SRV a P2 pressure reference--and because the supply pressure had better be isolated --there is zero P2 pressure, so the valve goes full open!

To make the User Defined- or Demand Display work when calibrating the SRV LVDT feedback, you must first go into the I/O Configurator and change the SRV regulator to a type 43 regulator to ignore the P2 pressure reference and -feedback, save the change, download the IOCFG EEPROM section to <Q>, reboot <Q>, then perform the SRV LVDT feedback calibration and verification (which involves more I/O Config. changes, and downloading and rebooting--are we having fun yet?), and when finished you MUST change the regulator type back to type 77 (or whatever it was originally), save the change, download to <Q>, and then reboot <Q>.

AutoCalibrate automatically changes the regulator type when working with the SRV--no manual changing the regulator type and having to remember to change it back when finished! Not every <I> or HMI was shipped with AutoCalibrate available on the Main Menu or as a mouse click on a menu or list somewhere, but most have the executable, G:\EXEC\ACALIB.EXE. For it to work, there must be a file in the unit-specific directory (e.g., F:\UNIT1), ACALIB.DAT. There are some manual editing of this file which must be done for AutoCalibrate to work properly, and that is not very well documented anywhere--unfortunately.

markvguy

 

Check the LVDTs.

The LVDTs have two lenear ranges.

If you start to move the core from one end to the other, the graph of the output voltage will be like 0V to 3.0V and back to 0V.

Meaning, when you adjusted 0.7 Volts and start calibration, and if the valve moves beyond 3.0V then the now open position is considered as close position and Mark V sends command to open again.

So choose the right 0.7Vs.

Regards...