High Exhaust Temperature Spread Trip

L

Thread Starter

LTW

Dear All,

Would like to share our recent experience and hope can get some advice/feedback from other users here.

Our unit is 9FA single-shaft combined cycle gas turbine, utilizing DLN 2.0+ combustion system. Type of control system is Mark V TMR, with HMI.

We encountered 2 incidents of unit trip due to High Exhaust Temperature Spread when deloading for overnight shutdown, during change-over firing mode from Premix to Piloted Premix.

From the trip log, prior to tripping, exhaust thermocouples #28 - #31 and #1 - #6 detected much lower temperature. From the SOE, no abnormality is observed for the GCVs. Only Flame Detector Trouble alarm and Combustion Trouble Alarm activated prior to High Exhaust Temperature Spread.

Problem occurs intermittently since 9th Oct 2012 and not every unit shutdown has got this problem, probably I can say 50% / 50%. Has anyone experienced similar problem before?
 
LTW,

What have you done to try to troubleshoot the problem?

What were the results of your efforts?

The Combustion Trouble alarm is telling you that a higher-than-allowable exhaust temperature spread (one of several different conditions) exists. Unfortunately, it only gives cognizant operators about nine seconds to take some kind of action (if possible) to lower the exhaust temperature spread before tripping the turbine.

Combined with a Flame Detector Trouble alarm, it's pretty clear from the information provided that diffusion flame has been lost in several combustors, or perhaps diffusion flame has not been established in several combustors.

Are you sure the IBH Control Valve and IGVs are working properly?

How long since the last maintenance outage (Combustion Inspection; Hot Gas Path Inspection; Major)?

Do you have gas fuel filters and/or a gas fuel strainer? Have you checked the filter and/or strainer drains for liquids and contaminants? Do the filter and/or strainer differential pressure gauges indicate a higher than normal differential pressure?

Is there a "knock-out" drum in the gas fuel supply piping to remove liquids? Have you checked the drain for type of liquid (hydrocarbon oils; etc.)? Many fuel nozzles can become plugged with oil/oil vapors entrained in gas fuel.

What is the dewpoint of the natural gas being burned in the unit? Is the gas heated for performance improvement, or for superheat protection?

Has there been a recent change in natural gas supplier or make-up?

Please write back with the answers to the above questions, and to let us know of the resolution.
 
Hi CSA,

Nothing much had been done on our side, except to stroke check the GCV 1, which was found to be in good working condition.

IBH Control Valve and IGVs are working properly, no alarm for these 2 equipments from the SOE during unit in normal operation.

Last Combustion Inspection was carried out somewhere in March 2012, and Major Inspection was performed in Jan 2010. Coming HGPI will be early of next year.

We do have a gas fuel strainer and its dP reading is within the normal operating limit and no dP high alarm detected.

Not sure on the fuel gas dewpoint, but we can say there is not much changes in the gas composition and it has been quite consistent all these while. Our performance gas heater is mainly for Gas Turbine performance, by pre-heat the fuel gas before entering combustion chamber.

Would like to add one more thing here. Unit does not encounter any problem during start-up and problem only occur during shutting down.

Thanks to all for the advice/feedback given!
 
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