HI all

After a HGPI (GE 6581 dual DLN 1.0 CL extender) and during the fuel tests at FSNL there is a difference in pressure between the combustors as follow:

All the combustors have a 13 bars except for combustor 4: 11 bars, combustor 7: 12 bars and combustors 9: 12 bars .

We checked the fuel circuit, no leaks detected between the flow divider and the fuel nozzle.

Suspected of being in default, we changed the check valve air 7, A large quantity of fuel was found upstream and downstream of the check valve! also We have interchanged between the fuel nozzle of the combustors 7 and 9.

The TG was started with a setpoint of 5 mw of load, the pressure of the combustors was 16 bars except for combustor 4: 12 bars , combustor 7: 13 bars and combustors 9: 13 bars .

Please someone can tell me the cause of the pressure difference readed in the flow divider and what can i do to fix it?

Sincerely

 

mirak,

It's rare that all ten combustor liquid fuel pressures are equal. The accepted difference is +/-10% of the average, and that's generous. Any pressure more or less than 10% of the average is considered suspect when troubleshooting high exhaust temperature spreads or combustion trouble when operating on liquid fuel.

Typically, high pressure differentials will be seen in high exhaust temperature spreads.

You are on the right track--typical causes of high liquid fuel combustor differential pressures are: 1) failed or failing liquid fuel check valves: 2) failed or failing liquid fuel purge air check valves (on gas/liquid dual fuel machines); and, 3) failed, failing or improperly assembled liquid fuel nozzles. Nozzle passages can erode over time and cause higher than normal flow-rates which leads to lower pressures. Some nozzle designs have internal components that require staking, which if not doe or not done properly, can cause Lowe pressures. And some designs use a crush gasket which frequently gets improperly placed or improperly crushed (crushed more than once during assembly) which can cause liquid fuel leaks which leads to low pressures. All of which lead to high exhaust temperature spreads.

There have been isolated cases of dirt/metal shavings/rocks getting into the liquid fuel lines between the flow divider outlet and the liquid fuel check valve, causing low pressures during firing and acceleration and high pressures at rated speed and load (due to the added restriction). But these were not typical, and were usually the result of poor flushing practices during construction/commissioning.

Finally, the machining tolerances are never exact for fuel nozzle components. The accepted flow-rate differential (per the OEM) for a set of liquid fuel nozzles is 10%--which will also contribute to pressure differences. And, even liquid fuel check valves have flow-rate differences, so all of these combine to contribute to slight differences in individual combustor liquid fuel pressures. Again, it's unusual to see them all be identical. (And those gauges used on the flow dividers don't have the best resolution, either.)

Hope this helps!

 

HI CSA

Thank you for your quick response.

According to what I understood from your answer, I can increase power up to base load and recheck again the pressures of the combustors. stop the turbine if I have a pressure difference more than 10%.

I need also a help me to know the origin of fuel downstream and upstream the air check valve.

Sincerely

 

mirak,

As long as the exhaust temperature spreads are within reason, the unit can be loaded to Base Load.

As for the source of liquid fuel "above" and "below" the purge air check valve, there will be liquid fuel on the downstream side of the check valve when the unit is burning liquid fuel. The purpose of the purge air check valve is to prevent liquid fuel from getting into the purge air system--when it is working correctly. When the unit is at rated speed the three-way valve upstream of the purge air check valve (VA19-1, I believe) opens to allow purge air to flow through the check valve to push the liquid fuel out of the fuel nozzle to prevent coking, and to prevent the flow of hot combustion gases backwards into the fuel nozzle and to provide some cooling to the fuel nozzle while the unit is burning gas fuel.

So, if you found liquid fuel upstream of the purge air check valve the most likely source is <b>a</b> leaking purge air check valve. (Like all devices, they do eventually fail. And the history of many of the liquid fuel- and purge air check valves used in GE-design heavy duty gas turbines is not very good. Springs break; o-rings dry and beak; the poppets wear through.)

Hope this helps. Please refer to the P&IDs for details of the various systems. Sometimes you need to re-draw portions of associated systems to get a clearer "picture" of the interconnections. The downstream connections of BOTH the liquid fuel- and purge air check valves are connected together. If there's any doubt, have a look at the physical piping/tubing.