One of our MarkV’s is getting intermittent voter mismatch alarms for <T> FPRG_INT.

Has anyone had this before? What was the solution?

I can see FPRG_INT in the ladder logic rungs. It is described as an "orphan signal" with no clue where it comes from or how it is calculated.

It appears to be involved in causing the SRV to maintain a constant interstage (P2) pressure so the GCV opening will provide the correct volume of gas to the turbine.

Maybe someone out there can provide a discussion about it? Any troubleshooting advice?

 

There have been many posts about FPRG_INT. If the machine has three P2 pressure transmitters (96FG-2A, -2B & -2C) it's probably likely that 96FG-2C's calibration has drifted and is causing the voting mismatch. You can see this easiest in the Pre-Vote Data Display; look for FPG2 and--if the unit has three redundant P2 pressure transmitters--you will likely see that <T> is not indicating the same as <R> and <S> (which may be a little mismatched, but not as much as <T>). (Of course, it's also possible that both 96FG-2A and 96FG-2B have drifted more than 96FG-2C.... but that would be unusual, but not impossible.)

Calibrate 96FG-2C, or replace it with one that has been verified to produce the proper output range for the input range, as specified in the Device Summary and that should be the end of the voting mismatch Diag. Alarms for FPRG_INT. (Actually while you're checking the calibration of 96FG-2C and adjusting it if necessary, it wouldn't be a bad idea to do the same for the other P2 pressure transmitters. Just don't forget to put the manual three-way valves back in the proper positions when finished.)

The SRV controls P2 pressure so that flow through the GCV is proportional to stroke, which means it's linear, when the unit is at rated speed (100% TNH). So, yes, the way you described it is another way of putting it.

 

Thanks,

We have noticed on runtime Prevote Data;
<T> core FPG2 value 2-3% low
<T> core FPRG_INT a couple % higher

The <T> core FPG2 transmitter has had a "drifty" zero. We plan to replace it and check calibrations.