One of our 6581B GE machine (Mark V) at 20% speed "liquid fuel control fault" alarm appeared while unit was running on Gas fuel selected. liquid flow was showing above 3%.

Liquid fuel control fault, L3LFLT, is indicated when flow feedback is above 3% with the stop valve closed or when the liquid fuel servo current is in negative saturation.

Liquid fuel accessory gear driven pump was rotating during the startup with 100-150 RPM, In other units also pump is rotation during startup on gas fuel, but alarm appeared in this units only.

We have checked all flow divider pickups, all check valves and stop solenoid valve leakages, all are found OK.

Anybody can suggest what to be checked further?
Appreciated for your valuable suggestions.

Thanks,
Joy

 

Electrical noise is the likely cause of the problem. There is an induced voltage/frequency causing the Mk V flow divider speed pick-up inputs to "see" liquid fuel flow.

Common causes are improperly shielded speed pick-up interconnecting cables (the shield ground wire should only be grounded at one place--usually at the Mk V shield ground bus bar). Also, multiple conductor cable overall shields should only be grounded in one place/at one end of the cable only--usually in the control panel. Armored cables grounded at both ends have been known to cause this problem.

Improperly run cables (low level signal wires run in close proximity and/or parallel to high level (120 VAC or higher) cables can also be the culprit. Poor cable routing under the turbine control panel and up into the turbine control panel can result in many nuisance Diagnostic Alarms (particularly "Contact Input Failure" and also erroneous liquid fuel flow divider feedback).

It's not uncommon for the liquid fuel pump to spin slowly when running on gas fuel because of "drag" between the clutch plates. Many times, the rotation while running on gas fuel can be stopped with a simple "brake" (piece of wood held lightly against the coupling).

Measure the voltage and frequency at the Mk V panel of the flow divider inputs when the unit is running on gas fuel--there should be none if the liquid fuel flow divider is not turning. If you have voltage and frequency, very carefully go over all the flow divider speed pick-up interconnecting wiring--from the Mk V all the way to the flow divider, paying particular attention to proper shield grounding practice (for the twisted, shielded pairs), the overall cable shield (if multi-pair cables were used), the cable armor shield grounding (if used), and the routing of the cables with respect to higher level voltages/signals.

 

On some Mark V systems this alarm is triggered by flow divider speed (as noted) or by the control valve lvdt feedback being negative (-7%) or positive (5%). We've seen this on some units (even if you're selected on gas). Check your LVDT feedback signal.

If you have any questions drop me an email at [email protected]. We develop unit specific alarm response manuals for these types of units and are always interested in solutions to "weird" problems.

Let us know what you find.
Lou

 

Generally, Frame 6Bs (and Frame 5s) did not have LVDTs on Liquid Fuel Bypass Valves. Newer Frame 7EAs do not have Liq. Fuel Bypass Valve LVDTs, either.

If these units are Frame 6s, and have diesel starting motots, does this erroneous Liq. Fuel Flow indication occur only during starting (on gas fuel) when the diesel is running? This has been reported in the past but never followed up on by the OEM. This would lend more credibility to the electrical "noise" theory, and provide a possible "source" fir the noise.

Most lilkely, the Liq. Fuel Flow Divider speed pick-ups and the diesel speed pick-ups pass through the same conduit/junction box and even through the same interconnecting cable at some point. Improper shield wire terminations could produce this false liq. fuel flow indication when the diesel was running.

Please let us know what you find.

 

65FP, solenoid valve should be checked.

 

We do not have LVDT feed back , feed back is from the flow divider pickups, 77FD1,77FD2 and 77FD3.

We did not get opportunity to check thoroughly in the affected unit, I have traced wires in other units, found all the three flow divider magnetic pickups wires are going through a conduit up to one junction, wires of staring Diesel Engine; PN (4-20mA) , 120VDC for the Diesel Engine control Panel, Digital I/O etc (there is no solenoid valves directly connected to mark V for Diesel Engine control) are coming to that junction and all these wires are going together about one meter and terminated in a JB. From that JB all magnetic pickups separately i.e. 77NH1/2/3 , 77HT1/2/3 and 77FD1/2/3 going together through plug and socket type connector cable up to Mark V panel . All shields are connected in the panel only.

All the above advices are very helpful and thanks a lot.

If any thing abnormal found in the affected units I will write again.

Best regards,
Joy

 

You are most welcome; it's very good to hear back from questioners whether or not they are satisfied with the responses.

As long as you keep providing feedback (positive or negative) about the responses to your questions, the answers will keep coming!

Sincerely,
markvguy

 

I understand that the problem occured while starting the unit. The problem frm the servo can be ruled out. Then it could be frm the flow divider. I feel that during start up the flow divider actually rotated resulting in the PUs sending speed signal >3%. This is bçause of the drag phenomena mentioned by markvguy. So during start up the fuel accumulated in the lines before the flodi would get pushed thru it, resulting in the PUs detecting rotation. I would suggest u to check for any fuel passing in ur liquid fuel stop v/v and also 20FL solenoid. At higher shaft speeds - if the unit is on gas fuel - the residual magnetism would not have enough power to engage the clutch plates and thus the fuel p/p will stop rotating.

During next start up isolate as many v/vs as possible in ur liquid fuel system and try. Pl revert with ur feedback.

GT Specialist.