Relationship Between GT Speed And Liquid Fuel Recirculation Line Pressure And Flow During Gas Fuel Start Up


Thread Starter


Dear Members,

I am working in GE 7FA.05 Dual fuel gas turbine and we are using as a fuels natural gas (NG) & Arabian super light crude oil(ASL).

One day GT started by natural gas with ASL crude oil recirculation condition, when GT reached FSNL and after 5 min GT speed reduced to 95% speed, that time responsible person select Liquid fuel page and ''LIQUID FUEL RECIRCULATION/CONTROL CONNECT'' mimic ENABLE button select. But GT speed not raised to above 95% of speed. So he raised speed by manually and synchronized.

My question what is the relationship between GT speed and Liquid fuel recirculation line pressure and flow during gas fuel start up?

The liquid fuel recirculation system, its components and its operating philosophy/sequencing is an emerging process and technology. As such, nearly every one that is sold is a one-off in that as new things are learned from the operating experience of previous systems new components and new logic/sequencing are added--which makes commenting on any system at this time extremely difficult without being able to see the application code running in the Mark VIe at the site experiencing suspected problems.

From a pure theoretical perspective it would seem that turbine speed and liquid fuel recirculation while operating on natural gas would be mutually exclusive--that is, that simply recirculating liquid fuel during a turbine being started on natural gas would have no interaction/effect on turbine operation. The purpose of liquid fuel recirculation is to make sure that the liquid fuel supply piping and components are free of air and full of liquid fuel and that the system is "pressurized" all the way to the liquid fuel pump inlet, and in some system designs, even beyond that. (Again, the design of the systems is ongoing and changes quite frequently.)

I would suggest that something other than liquid fuel recirculation caused the turbine speed reference to decrease to 95, which is typically the minimum operating speed reference allowable, called the "Low Speed Stop"; it is the minimum allowable turbine operating speed/speed reference, and if turbine speed ever drops below approximately 94.5% speed the Speedtronic automatically goes into a normal fired shutdown.

The reason that operation (or a speed reference and operation) below approximately 95% speed is not allowed is that it might be possible to inadvertently pause turbine speed at a critical speed which could quickly cause extensive damage to the turbine and/or axial compressor. So, operators, and automatic turbine control operation, are not allowed to set a speed reference below the Low Speed Stop.

I would also suggest that anything that resulted in the automatic decrease of TNR (the turbine speed reference) from 100% to 95% would be accompanied by a Process Alarm to alert the operator to the condition that was causing the speed reference to decrease. Unfortunately, most GE HMIs alarm text messages and alarm screens are poorly configured and most operators can't distinguish one alarm from the myriad of alarms that is requiring immediate operator action or is affecting turbine operation.

Again, without being able to see the exact application code running in the Mark VIe at your site it's really difficult to say anything for certain. It's not "logical" to think that liquid fuel recirculation has anything to do with turbine speed reference--especially when the turbine is off-line (the generator breaker is open)--but, without being able to see the application code from the .tcw file (the ToolboxST configuration file) for your site it's not possible to say anything with much certainty--I'm just theorizing at this point.

And, again, I would suspect that somewhere among the alarms which were being annunciated at the time this event occurred there is at least one which, when analyzed, will reveal the cause for the automatic decrease in TNR. (I would imagine that the "responsible person" probably had to initiate a MASTER RESET to be able to manually raise the turbine speed to synch speed, but, that's also just a presumption based on experience. Most operators, even if they don't know if the condition requires a MASTER RESET, will just click on MASTER RESET as a matter of course without understanding the implications of the action--which is poor operating procedure, but happens all too frequently.)

Hope this helps!