Fuel line purge timer (L62P). Actually what it does? how it’s purging the liquid fuel line after change over from liquid fuel to gas fuel.

We are having naphtha, gas and diesel as intermediate fluid. I mean diesel fuel pumps are always running and recalculating the diesel in parallel with either naphtha or gas. Naphtha or gas changes over to diesel when it get appropriate problem. This fuel change over is accomplished by a three way valve with 2 limit switches(33TDD&TNC).

This L62P is originating from MANUAL_ST_POINT_LONG block. WHERE this logic is generated from a equal block where KFQLV1X[800 dm3](total naphtha fuel to purge) with FQLV1X. Whenever we are starting and running the unit with gas fuel, the message field shows "fuel line purge is in progress" where I found KFQLV1X not equal to FQLV1X so L62P is logic ZERO. KFQLV1X reads less than 200, and the same time I found in the other 2 units it was matching so the L62P is logic ONE no more "fuel line purge is in progress". In this particular engine we are forcing the L62P and running.

What I think is MANUAL_ST_POINT_LONG is generating some spurious output though control constants are matching with other machines. Plz explain this purging process and my system problem too.

 

Hello

Ours is Frame6 MKV GT. We are also using all these three fuels. In our case L62P is operated (i.e. its logic 0 or 1 with time delay of 240 sec.) by L33TDD limit switch located at 3way valve. This L62P is used in 1) Gas permissive and 2) Fired shutdown command.

1) There is no permissive to change over fuel from Naphtha to Gas as Naphtha has low flash point. So we are changing over Fuel from Naphtha to HSD but MKV is not allowing to change over to Gas immediately. It is allowing the change over after 240sec. or 4 minutes. Ensuring that Naphtha has purged properly.

2) Also in case of fired shutdown whenever we change fuel from Naphtha to HSD at lower load (10MW in our M/C) L33TDD picks up and after 4 minutes L62P picks up and as This L62P is used to pick up L94X there is time delay of 4 minutes to start fired shutdown command, To ensure proper purging of Naphtha. I think our logic is quiet simple and easy to understand. which M/C u r working on?

Thank you
CPPguy

 

Hi CPPGUY,

Can you eloberate on the point no. 2. I mean for changeover from one liquid fuel to another liquid fuel, why time delay is envisaged ?

 

Hello Mr. MOB,

As per the logic provided by GE and what I understood and experienced. Whenever Fired shutdown command is issued to frame6 Gas turbine It must suppose to be at HSD i.e. L33TDD contact is used indirectly with time delay of 240sec. i.e. L62P. Means If I am operating GT on Naphtha I have to change over the fuel to HSD. When I will issue changeover command to HSD, Three-way valve will line up the port to HSD side and L33TDD limit switch will actuate. and if I will issue fired shutdown command immediately, Fired shutdown will not start immediately, It will start 240sec after pick up of L33TDD as When L33TDD picks up after 240sec L62P will pick up as L62P is making the L94X logic through.

Once we had faced a problem. We had to stop the GT for MI. We brought the GT load to 10MW changed over fuel to HSD. Brought down the load to 5MW (we purge the fuel i.e. run the GT on HSD for min.10 minutes to ensure proper fuel purging) So at 5MW issued Fired Shutdown command. Generator breaker opened on Reverse power and GT came to FSNL but shutdown was not taking place Fuel started hunting, speed started hunting. At the time couldn't come to know what is happening. So we stopped HSD supply pump from MKV. But after checking in event logger found that L33TDD was toggling continuously. On checking its cable found faulty.

Anyway GE is ensuring that when running on Naphtha if you change over to HSD logic ensures that when L33TDD actuates then Fuel to be purged with HSD for 240sec.(They might have calculated the time required to replace the volume of Naphtha.(This is for our Frame6 GT may be different for other Frame's)

Thank you
CPPguy