We have GE frame 6B with rating 33MW and using Alstom's Turboline 3000 control system. The GT normally running on pre-selected load in between 20-25MW. Currently, we investigate our GT efficiency tend to decrease by observe some parameters such as CPD decreasing, FSR increasing, TTXM increasing and fuel consumption (flow meter installed outside GE skid). One of possible cause might be from gas fuel control system issue so i did look detail into it and found something strange. Pressure of gas fuel on PG1 is 21bar, PG2 is 10bar and PG3 is 9.5bar but the indication of FPG-1,2,3 (press. transducer 96FG-2A,B,C) read from PLC are 0.2-0.3 bar only. The process connection of 96FG-2A,B,C using 3-way valve with connected to venting line and downstream of SRV, its seem the 3-way position doesn't right, so maybe the pressure transducer doesn't connect to downstream of SRV instead it connect to venting line. All pressure transducer were calibrated during last outage.

I've read from document manual and PLC software that SRV command is ratio of speed (TNH) multiplied with constant SET_FPKGNG (our PLC value is 0.206 V/%N) then subtract with SET_FPKGNO (our PLC value is 1.625) the output of calculation become FPGROUT. To become SRV servo command output, SRVOUT, the FPGROUT will subtract with FPG (median value of 96FG-2A,B,C). Current condition with 25MW load, FPGROUT value is around 18.87, FPG value is 0.2, SRVOUT is 18.65% and LVDT of SRV (96SR-1,2) value is around 55% and servo current output of SRV is 0.7mA.

Regarding our conditions above, i have some questions that maybe did happens in other plant as well
1. During rated speed, how many pressure downstream of SRV or differential pressure with gas supply pressure? i guess the differential pressure between supply pressure and downstream SRV should be not big as per GT startup / shutdown.
2. If the problem was from 3-way valve and we can rectify it, the FPG value will be rapidly increase to 10bar as per PG2. From calculation, SRVOUT will be decrease from 18.65% into 8.65% and on steady-state it will balance with ratio from speed and SET_FPKGNG. What are the possible effects if we increase indication of FPG? I guess by the calculation, SRV will tend to more close than before, affect gas pressure to combustor and affecting FSR and TTXM in overall. Is possible that gas fuel consumption will be decreased after we rectify FPG indication?

Looking forward to you help, many thanks.

 

For a simple answer first, once shaft is at 100% speed P2 should be pretty well constant but, of course, SRV will keep opening as load increases to maintain P2, at Base Load P2 wants to 1-2 Bar above P3. Once at 100% speed it is P2 pressure feedback is what is controlling the SRV, something is wrong but you must be getting P2 feedback from somewhere.
Take a look at the attached document, although it is for MKVI Speedtronic the control principle is pretty much the same, the section on Gas Fuel Control is a very good description of how the system works.
I'm guessing but I'm suspicious of the 3-way valves