How to Force Analog signal in GE MarkV Control System?

vedprakash. rao @ gmail. com

 

This is easy!

It can't be done.

And, it's a pretty sure bet that the designers of the Mk VI wished they hadn't make forcing analog signals so easy. More than one site has forced analog signals that have resulted in severe damage to the unit. Much more than any site which forced logic signals on Mark IVs or Mark Vs.

 

It is GE-single cycle Frame V (MS5001) Gas turbine with the control of simplex MK-V. Before trip Occurrence Gt was Runing normal at (Preselected Mode), Load was steady (18MW) while (96FG) P2 pressure was 15 bar (Exhaust Temp was 450C) and (96CD) CPD was 9.0 bar) and GT tripped with the Alarm of "Loss of Flame" as instance action both Flame detector were checked and found good but were replaced with new one. All the power fuses and FD wiring connection were checked and tighten from up to (P) core.

Gt was started normal without any delay. After one month GT was tripped with the same FOut Alarm "Loss of Flame". Nothing abnormal was found also this time but following corrective action were taken to eliminate the trip occurrence in future, (96CD) CPD and (96FG) P2 were calibrated both Servo GCV (65GC) and SRV (90SR) were replaced SRV/GCV manual stroke calibration was done with the feedback response of both LVDT Hyd pressure switch (63HQ) was calibrated. Still I suspected the problem is somewhere else which has not yet been diagnosed. Can you advise on this issue please?

 

What do your Trip Log printouts indicate for the two events? (You did print the trip log immediately after each event, didn't you?)

Loss of flame trips are usually the result of sudden loss of fuel. The GE standard is that any condition that results in a turbine trip, or emergency shutdown, has a Process Alarm associated with it. Now, that programming practice is not adhered to as rigorously as it should be, but presuming it was in your case, then a low-l-Low L.O. Pressure or a high-high L.O. Temperature or an Exhaust Overtemperature condition will result in a Process Alarm message to that effect.

If, for some reason, the Stop-Ratio Valve suddenly closed (such as a could happen if 20FG-1 had an intermittent connection in its circuit), a Process Alarm would still be annunciated: Loss of Flame trip, because there was no other condition detected that resulted in a loss of flame (trip).

Most of the time the "every-trip-must-have-a-Process-Alarm" rule gets violated is when there is some special trip input(s) added to the Speedtronic, such as a high-high level in the gas scrubber or HRSG boiler drum level low. Sometimes, people put these directly in the E-Stop circuit (which is connected to the <P> core) and forget to add a contact input to generate a Process Alarm when the trip is initiated (this type of trip through the hard-wired E-stop trip circuit should have two contacts connected to the Speedtronic: one for the trip circuit, and one for the alarm annunciation!).

So, it doesn't sound like you've exhausted all the possibilities, like checking all the wiring in the hard-wired E-stop trip circuit, which in a SIMPLEX panel typically includes the <C> back-up exhaust over-temperature protection outputs and any and all external trip contacts/push-buttons. Also, the 20FG-1 circuit from the <P> core to the solenoid and back to the <P> core. It's even possible that the mechanical dump valve which 20FG-1 actuates is failing (it has happened).

If the turbine has a SIMPLEX it may even have a protective interface module if 20FG-1 isn't really a solenoid but rather a servo-valve (some early units used a special configuration like this), so there may be some interposing circuitry between the <P> core and the "solenoid" which should be checked carefully.

Also, it could be something in the hydraulic circuit, or the control- or trip oil circuit.

To sum up, Loss of Flame trips usually occur because either there's a sudden loss of fuel (not necessarily caused by the Speedtronic but some device connected to it or even a device which is not directly connected to it!), or some trip input to the panel wasn't properly configured to annunciate an alarm when the trip was initiated.

If you have data available in the Trip Log printouts from the events, you should be able to see exhaust temperature and/or load decreasing immediately before the Loss of Flame trip, which would indicate a sudden loss of fuel, for whatever reason. If the Loss of Flame trip was not preceded by a drop in exhaust temp/load, then it might have been caused by something connected to the <P> core which didn't have a Process Alarm associated with it.

It might just be a loose crimp on a wire, or loose termination in the stop valve "solenoid" circuit.

It's always a good idea during any annual maintenance outage to have people open every junction box between the Speedtronic and the devices on the unit and auxiliaries and check every termination for tightness, and not just the screws, but gently pull on each wire which is in a crimp connector. If your unit has a SIMPLEX Mark V panel, it may also use plug-connected cables between the panel and the junction boxes. Sometimes the pins and receptacles in the connectors develop corrosion and/or come loose and need periodic tightening, as well. And, don't forget the check the tightness of the screws on the Mark V terminal boards, too. Just don't overtighten any screw; the objective is to ensure the screws are tight, not as tight as they can possibly be without breaking the screws/terminal boards!

And lastly, it has even been known to be a mechanical dump valve relay which failed which caused the control system to get unduly blamed!

You've still got a lot of things to check--right after you review the Trip Log. And, if you don't already know every device connected to the hard-wired E-stop trip circuit on the <P> core, now's an excellent time to find out, write them all down, and ensure that each one has a second contact connected to the Mark V to generate a Process Alarm when the trip is initiated.

We'd like to know how your troubleshooting progresses!

 

Respond to "The GE standard is that any condition that results in a turbine trip, or emergency shutdown, has a Process Alarm associated with it". I have met turbine trip without direct process alarm two times. It is difficult to find the cause and the events log is a very helpful tool for troubleshooting.