what causes the turbine fail to fire at startup on gas fuel?

GIVEN:
1- turbine frame: 5

2- Control system: MK II
3- 14HM>> 1

4- Spark Plugs work properly.

5- Gas Pressure(P1)>>17 bar.

6- At fire instant>> GCV responds and the piston opens about 50% on scale.

7- P2>> about 4 bar and the turbine couldn't fire.

8- Made a bypass gas line across the gas valve with a manual valve.

9-the turbine fires only by opening that pilot line and by closing it again after firing (30% speed).

the turbine continue to startup up to FSNL normally. what is the reason?

 

MA,

First of all, you didn't say when this problem started. I have heard of this problem before--and I was just as shocked then as I am now. And it was several years ago when I first heard of this problem.

Is the a single-shaft- or two-shaft machine?

Unless the gas fuel being burned has a significantly lower heating value than the machine gas valves and fuel nozzles were designed for, or the fuel nozzles currently installed in the machine are significantly different from design, or the gas valve internals are significantly different or damaged from design, it's extremely hard to imagine why it would be necessary to have 4 barg (approximately 60 psig!) and a GCV opening of 50% of stroke/position during firing--and still not be able to establish flame when the spark plugs are known to be in good working order. It's presumed that the unit is purging/cranking at normal speed (if the speed was higher than normal then the air flow through the machine would be higher than normal and it might be harder to establish flame). It's also presumed the IGVs (Inlet Guide Vanes) are at the proper angle for starting/firing (if the IGVs are variable and not fixed), and that the compressor bleed valves are open during firing (if the unit has compressor bleed valves).

Starting any internal combustion requires the same three things:

Fuel
Air
Spark

Of course the fuel and air must be in the proper ratio, and unless the gas fuel being burned has changed significantly from that being previously burned or the fuel nozzle orifices are significantly different from design it's pretty hard to fathom this issue--and the workaround being used (as it's extremely dangerous). Again, it's presumed turbine shaft speed is at rated during purging/firing, and the IGV angle is per specification, and if equipped with compressor bleed valves that they are open during purging/firing. It's just hard to imagine that with 4 barg P2 Pressure and 50% GCV stroke/position that there's insufficient fuel flowing into the machine to achieve the proper fuel/air ratio to establish flame with known working spark plugs. Something is definitely amiss.

This is one of the things that most people are extremely surprised--sometimes shocked--to learn about Speedtronic heavy duty gas turbine control systems (especially the new digital systems): The control system doesn't monitor or control fuel/air mixture. The design of the components (gas valve internals; fuel nozzle orifices) and the operating parameters for the expected fuel to be burned are chosen such that the proper stoichiometric ratio will be achieved during starting, accelerating, and loaded operation. If the fuel changes significantly, or the machine conditions change (purging/firing speed; IGV angle; etc.) or the components have been significantly changed then starting can be adversely affected.

I had considered that there was some problem with the gas fuel supply being restricted (the normal y-strainer was blocked/choked; or if the unit was equipped with filters of some sort that they might be choked/dirty), but if the unit runs that doesn't seem to be a problem. It would be extremely interesting to know that the P2 Pressure is at, say, 75% load or Base Load, as well as what the GCV position was at 75% load or Base Load.

The last time I heard about this problem the site was considering a turbine control system upgrade (from Mark II to Mark VIe) and was demanding an automatic bypass valve be installed around the SRV/GCV to permit starting as has been described with automatic control of the required flow-rate to achieve flame during starting (?!?!?!). I don't know if the root cause of the problem was ever found and resolved.... Or, if the control system upgrade was ever performed.