Centrifugal Pump and Gear Pump in Series

[JamesDUK]

Hello,
On an offshore oil platform we have an Aero derivative gas turbines with a diesel back-up supply. With the GT vendors scope is a high pressure gear pump which is regulated using a spill back valve. The platform diesel system is circulated by a centrif pump. Manufacturer’s data on the gear pump has a flow rate of 150 l/min or 9m3/hr, however the according to the centrif pump curve we are only supplying 7m3/hr. During turbine start-up we experience pulsing of the gear pump suction pressure between 4 barg and -0.2barg.
Is having these pumps in series a recipe for disaster?
Can we assume the data we're getting from the pump curve is inaccurate with little vibration the gear pump doesn't seem to be cavitating.
Anyone any idea on the start-up problem??


Thanx!

 

[scalleke]

I assume that you are just starting up the gear pump as you need to supply diesel withn the gear pump. Starting up the pump and the spillback valve will open regulated by the pressure in the system?

Would it be possible to start up with the spillback full or partially open?

Then as you need pressure let the spillback valve close so that you build up the pressure that you need. You will see that there is a minimum opening percentage for the spillback valve that will cause you problems in your feed system. Determine that point and fix this minimum opening percentage in your control system.

Best regards.

Scalleke

 

[Techcitizen]

Gear Pp (GP) is a constant flow device whereas Centri Pp (CP) is a variable flow. During start-up, GP is not running and the flow requirement is less, so the CP runs at low flow high discharge pr. The fluctuation in suc pr shows that as soon as you start the GP it tries to draw its required flow, the pr goes down in the suction as the flow delivered by CP is less, as the pr goes down the CP flow increases as per its characteristic curve, cycle repeats and the pr pulsates, eventually stabalising after a while at the flow which is required by GP. It seems that keeping two different type of pps in series is not a good idea.
Having stuck with what you have, solution might be to have a bypass line with a control valve, with quick response characteristic, very near to GP suction which controls the CP discharge pr (i.e. GP suction pr) corresponding to the requirement of GP flow (9 m3/hr) as per CP characteristic curve so that as soon as GP is started the valve will close and provide the necessary flow to the GP. Hope I am making sense ! Not sure whether this solution can work, you have to try and see !
Cheers !!

 

[JamesDUK]

Thanks guys for your answers. This has turned out to be a much more complex problem than I could describe here.
Techcitizen, I neglected to point out that we have a pressure control spill back line already which holds the system pressure and therefore flow, thus this wasn't causing the fluctuations.
We have attributed the fluctuation in the suction pressure to characteristic of the gear pump (bench tests prove this) but still can't pin point it exactly. Anyone with actual operational experience would be fantastic!

 

[enirwin]

JamesDUK,

Since the problem only occurs during startup and goes away (I assume) once the engine gets to operating speed, pulsation from your gear pump may be exciting a resonance in your fuel distribution system. If this is the case, the fluctuation will always occur in the same speed range and the discharge pressure will fluctuate as well. Is this the case? If so, the system resonance would cause your regulator to shuttle back and forth resulting in a fluctuating bypass flow to your pump inlet. Since your inlet is boosted by the centrif pump, the pressure fluctuations are more pronounced than they would be if the gear pump were simply sucking fuel out of a tank.

If pump pulsation is the problem, a pulsation dampener on the pump discharge may help.

 

[Techcitizen]

JamesDUK,
If not already done, would suggest you to ensure that the setting of the pr control in the spill back line (connected to the suction line of the GP) is such that it corresponds with the CP characteristic curve for a flow of 9 m3/hr ! If the pr setting is high, the pusations can still occur specially if the discharge line from CP to suction of GP is long. Also to check is the response time of the control valve, it shall match with the start-up characteristic of the GP.
I know a couple of aeroderivative GTs running with liquid fuel...

 

[JamesDUK]

Techcitizen/Enirwin
Thanks for replies, have been offshore collecting more data and also from another site with a similar arrangement. Data logs show the pressure fluctuations at about 4 Hz, starting when the gear pump starts and continuing until turbine is loaded.
Looks very much like gear pump is inducing these pulses and they fuel valve is struggling to regulate it as you've described. We believe this has caused 2 valve failures at circ $50K each, so problem has become more cost/politics than engineering.

Thanks v much for help.