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Hello all,

We are in the midst of commissioning several diesel-fueled LM1500's to generate power and cannot accurately monitor net fuel flow.

The supply system has been completely vented and the flow readings are stable and seem accurate based on supply pump performance. The return system flow meter is unstable.

The present fuel supply system configuration is a pressurized main fuel header (750psig) and each branch line equipped with a Endress-Hauser 92F Ultrasonic flow meter. The excess fuel from the Woodward LQ6 main control valve flows in an underground gravity drain to a central holding tank. The return line is equipped with a second 92F. The design intent was to deduct the return flow from the supply flow to calculate the net fuel consumtion.

All personnel I have spoken with agree that it is most likely that the return line is partially filled with air. That would explain our condition. Unfortunately, the return line operates at ambient pressure so we cannot simply vent out the air.

Our first solution is to add a back pressure device to the fuel return line downstream of the flow meter to maintain liquid level and then completely vent the line of air.

Does this sound reasonable? To be honest, this design error is on a primary design feature and prompts me to investigate the system a bit. Is there any fuel system schematics available online that identify typical issues?

Specifically, does anyone know what sort of backpressure can be added to the LQ6 without performance being effected? With 750psig on the supply side, an extra 10 or 15 psig on the outlet seems trivial.

I'd appreciate any input you can give.

Thanks very much!