Dip in flow divider rpm


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We are having Frame 9E European Gas turbine with Mark IV Speedtronic control system
Following Events occurs Could any one Help??

GT liquid fuel pressure low trip N-low Low
GT liquid fuel pressure low trip low N-Low

This alarms came couple of times and unit got transfered to HSD than exhaust gas temp alarm appears which persists even after resetting master reset
By observing curves in CVG one may find that there was dip in the Flow divider RPM and Load and rise in the rpm and MW
what could be the reason of dip in the flow divider RPM??
Could anyone help also
Could anyone tell when "exhaust gas temp high alarm" resets
You need to refer to the sequencing which annunciates the alarms to determine the cause for each alarm. Usually, pressure switches were used to generate alarms such as these; it appears that the turbine-generator packager used a switch to indicate a low pressure condition (warning) and another pressure switch to indicate a very low (low-low) pressure condition, and from what can be understood from your statement initiated a transfer to another fuel (this author is not familiar with "HSD"--can you explain?).

Heavy-duty gas turbines which operate on liquid fuel usually have a Liq. Fuel Forwarding System, which pumps the fuel from storage tank(s) to the Accessory Base of the turbine. On the Accesory Base there is a High-Pressure Liquid Fuel Pump which raises the pressure of the Liquid Fuel being sent to the Fuel Nozzles. You will have to look at your piping and instrumentation diagrams to determine which pressures are being detected as low and low-low and causing the alarms to be annunciated--either the forwarding system pressure or the high-pressure liquid fuel pump discharge pressure.

At any rate, the control system has apparently detected a sufficiently low liquid fuel pressure that it has initiated a transfer to another fuel. Usually this would be something like Natural Gas Fuel, or sometimes a different liquid fuel--some units could run on distillate fuel (such as diesel), naptha, and/or heavy fuel oil (such as crude oil or residual ("bunker") fuel oil). It's not clear from your statements which fuel the unit was operating on initially and which fuel the control system transferred to.

CVG is another unfamiliar term (can you explain?), but might be some kind of distributed control system (???) which has some trending capability (???) to monitor Mark IV control system parameters. If so, a low or low-low pressure condition would certainly result in a drop in liquid fuel flow-rate (as detected by the Liquid Fuel Flow Divider speed sensors).

There have been cases of the Liquid Fuel Forwarding System solenoid-operated Stop Valve operating intermittently; frequently these Stop Valves are located outdoors, exposed to weather (rain, sunlight). If the Stop Valve were to suddenly close/open or just fail and close this could certainly cause a low pressure condition.

Also, there are usually filters in the Liquid Fuel System, some were located in the low-pressure side of the system and some in the high-pressure side of the system. Were there any high filter differential pressure alarms annunciated? Could the filters be dirty and differential pressure switches not working properly? Some paper filters used in liquid fuel systems will swell when water is passed through them and restrict the flow (and downstream pressure). Is your liquid fuel relatively free from contamination by water?

There are also usually strainers in the suction side of the Liquid Fuel Forwarding System pumps, and if these strainers become plugged they can cause flow restriction and pressure loss. There are usually differential pressure switches on the strainers to warn of plugging.

Problems with the Liquid Fuel pump(s) could also be possible. Couplings on the Liquid Fuel Forwarding System pumps have been know to slip, as has the electric high-pressure Liquid Fuel Pump clutch. Again, it's important to know where in the liquid fuel system the low pressures were detected to be able to determine the cause of the low pressure.

If fuel flow is reduced by loss of pressure, load will drop. If the pressure loss was "temporary" or transient in nature, then load would dip and then recover if the pressure returned to normal or near normal.

As for the high exhaust gas temperature alarm, one needs to refer to the SpeedTronic Mk IV elementary drawing to determine the exact cause of the alarm and when it will be or can be reset. In general, though, when the actual exhaust temperature (TTXM) exceeds the Base Load Exhaust Temperature Control Reference (TTRX) by more than 25 deg F the condition is alarmed as 'Exhaust Temperature High-Alarm'. If TTXM were to continue to rise and exceed the reference by more than 40 deg F, the control system usually trips (emergency shutdown) the turbine-generator. Again, the Mk IV elementary drawing for your unit will provide the specific details.



The alarm description is confusing as this alarm text seems to be implamented by the supplier, in particular for this project. The exact details are not conveyed and This unit uses two liquid fuels which I presume are HSD (High Speed Diesel) and Naphtha (another liquid fuel, which is having a lower density and low lubricity, hence necesitating a fuel additive to be dosed continuously). I also presume that Naphtha is the fuel, the units are continuously being run on. HSD is used as a startup fule and also as a fall back fuel (in case there is any trouble in naphtha fuel system).

coming to the problem of flow divider speed hunting, when the unit is being run on naphtha, the amount of fuel required to generate, lets say, 100MW is 3 gallons per second. For the same load with HSD as the fuel, the fuel flow rate may probably be 2.5 gallons per second (all values are arbitary and used for explanation only, actual values will be totally different). As the unit is running on naphtha and there is a trouble in naphtha system (such as low naphtha fuel pressure), the sequencing in Mark-IV will initiate a fuel transfer to the HSD system (which is a fall back fuel system. During this process fuel transfer, as the naphtha fuel is already losing its pressure, the ammount of fuel available reduces and the FSr increases thereby, closing the liquid fuel bypass valve of trhe on-base fuel pump. At the same time the fuel oil transfer valve (located outside the GT base) is transferring the fuel oil to a healthy fuel system and this healthy fuel oil is also denser. And the fuel transfer is probably occuring fast. Hence, the control system will take some minumum amount of time to respond and stabilise to the fast changing fuel availability conditions. This phenomenon is quite common and thats the reason, it is not recommended to perform any fuel transfer at or near base load conditions AND at FSNL or near FSNL conditions.

Master reset is usually used to reset any trip conditions of the turbine.

As detailed extensively by the markvguy, please refer to the elementaries and control specs for exhaust temperature of trip and reset values.