We are facing hunting in 77fd-1, 77fd-2 while the gas turbine (fr 9 -Mk Vie) is running on gas fuel. i.e the signal (fdbk-77fd1,2) has a non zero value and varying up to certain values, approximately 10HZ. this caused alarm of (liquid fuel bypass trouble) and inhibit transferring the fuel to liquid fuel if this value increased above a certain constant (3) for 10 sec.

Some how it's being a cause of tripping of LIQUID FUEL TROUBLE ALARM L3FLT. Below are the mentioned step that we have investigated

we have checked the gap voltages of probe & found normal as per device summary which is 0.26mm.

We also have check the complete loop of the sensor till BCCA card found normal.

We also have check the coil resistance as well which were found quite normal 525 Ohm.

 

Mustafa,

The Mark VI and Mark VIe speed pick-up inputs are MUCH more sensitive than earlier Speedtronic turbine control systems' which required a smaller gap setting to read lower liquid fuel flow-rates during firing and acceleration. So, mechanical vibration of the flow divider toothed wheel(s) can be interpreted by the Mark VI/VIe as liquid fuel flow. Especially if the flow divider is older and the bearings/clearances are worn. Also, some flow divider teeth are more prone to accentuate the problem.

The solution is to increase the gap setting, but the amount of the final gap has never been specified by GE.... I would not recommend increasing the gap above 0.035-0.040 inches (0.91-1.04 mm). (This problem usually shows up soon after a Mark VIe control system upgrade from an earlier Speedtronic control system on an older turbine, but can also happen to newer turbines as the flow divider wears, especially if the liquid fuel is dirty (like heavy fuel oils/crude oils) or corrosive (like naphtha). If the flow divider is older and/or worn increasing the gap may not solve the problem and may make operation on liquid fuel difficult if not impossible, so the age and condition of the flow divider as well as the liquid fuel being used should be considered--especially when considering increasing the gap setting to or above the limits recommended above.)

That's the most common problem, but there is another possibility which should NOT be overlooked or ignored--ESPECIALLY if the problem occurs most frequently at 100% speed and mid- to high load. Leaking liquid fuel check valves can result in hot combustion gases pushing liquid fuel back through the flow divider and even causing the liquid fuel stop valve to open very slightly. This can cause the flow divider feedback to increase (the Mark VI/VIe can't distinguish which way fuel is flowing) and result in the situation you are describing.

This is a serious problem which can cause coking of liquid fuel in the fuel lines and the flow divider. Changing the gap setting will not solve this problem and may mask coking and result in damage to fuel nozzles, the flow divider, and high exhaust temperature spreads.

Hope this helps--please write back to let us know how you resolve the issue.

 

We have machine of Frame9 & COD done in April 2014 so all the parts that you have mentioned like divider, LFBPV, check valves VCK-2 are replaced with new ones on all chambers almost new & in our observation there is no leakage or passing of LFBPV.

On other GT we have adjusted probe gap as per device summary 0.26mm but on the other GT where we are facing the issue already have been adjusted to 0.55mm.

Could there be any issue of flow divider vibration issue which is being cause of erratic reading of 77fd?

Sure I will inform you if we get fix it.

 

> have tried adjusting gain settings,

Or scan rate of BBL block?