Pressure transmitters are not relaying pressure values of the field to its mark vi panel. Value on hmi of gas intervalve pressure for a frame 5 machine remains static during start up process causing srv to tend to 100% actuation while gcv is at 6 % with the unit going down on loss of flame after attaining fsnl. Transmitters have been verified to be okay. Transmitters are voltage based. Need to start this machine but require assistance from more experienced control guys.

 

When did this problem begin? After a maintenance outage, when the instruments were "calibrated"? After some repair or refurbishment of the gas valve assembly?

Are there any Diagnostic Alarms associated with the cards the transmitters are connected to (VAIC, if I recall correctly)? If so, what are they?

Is the Speedtronic annunciating a 125 VDC Battery Ground alarm? If so, what are the DC power supply voltages with respect to ground (positive and negative)?

There are usually some kind of three-way isolation valves provided on the tubing between the intervalve and the transmitters. It's common for these valves to be found in the wrong position after a maintenance outage. Have you checked any isolation/vent valves in the transmitter sensing lines to be sure they are in the proper position(s)?

It's not clear if you are saying these are the four-wire 0-5 VDC output devices which were common until the late 1980s (usually made by Dyncisco or Honeywell). Some of these devices required a differential, +12/-12 VDC power supply, not the +24 VDC grounded power supplies common today. Many Mark VI panels were not capable of directly providing this power without some additional equipment (sorry, I can't recall the name of the printed circuit card that was used with limited success). Are you certain the power supply source for the transmitter(s) is providing adequate power (voltage and current)?

That's another thing about those four-wire 0-5 VDC transmitters: they usually required a lot more current that present-day two-wire transmitters. And the Mark VI outputs were barely capable of providing the power on a good day.

So, if the unit was running fine and some change was made to the transmitters (replaced with new or refurbished units, for example), then that could be part of the problem.

You say the transmitters were verified, but you didn't say how. Were the transmitters removed to an instrument shop and powered with a "bench" power supply? Have you tested the transmitters while in place, powered by the Mark VI to see how their are behaving?

Are there IS barrier diodes used in the transmitter circuits? Have they been checked?

I have seen a similar problem several times, usually the result of improper sensing line valve positions, but once it was found that the inputs had been forced to zero for some testing purposes, and that the force had not been removed.

 

As CSA pointed out, check is there any force logic? as during transmitter calibration vent valve (20VG) will be forced.

all the very best
G.Rajesh