Screw Pump with Recycling Line

[jazimed]

I need your support please.

I'm about to install a screw pump to transfer crude oil through 18 Km of 8" pipeline. As a safeguard, the pump is equipped with an integrated Pressure relief valve plus a high pressure switch which will be installed on the discharge line of the pipe to trip the pump in case of an over-pressure occured. in addition, to control the flow I've foreseen a VFD.


My question is: in case I need to restart the pump with the pipeline already full, the system must provide for the time required to accelerate the liquid from zero velocity to final velocity. There will be 18 km x 8" of crude that needs to reach 4.5 m/s!!! I feel that the pump could not start since it will face a very important resistant torque on its shaft and if so, the pressure will rise quickly so the pressure relief valve will pop up at each start up. At this case, shall I have a recycling line having a control valve linking the discharge to suction?

Thank you

 

[LittleInch]

Normally you would have a separate pressure driven recycle line and not rely on the one inbuilt into the pump.

This is because you can more carefully select how it works, remove it for repair and maintenance and also you can determine where the return flow goes. Most pump systems simply go back to pump suction and hence can heat up very quickly.

whilst you can think of the crude as incompressible, the reality is that it isn't and hence on re-start for an 18km line you could easily pump for 3-4 minutes and only see a relatively slow rise in pressure, which gives you plenty of time to open the downstream valve. Given you have a VFGD as well, I can't see any issues on re-start.

4.5m/sec is a fairly high velocity though so the issue is more one of surge pressures on shutdown. This could very easily pop the pressure relief valves at the pump end.

For both scenarios if you can run a transient hydraulic simulation you can see what is predicted and play with the system to reduce any negative impacts ( valve closure times etc)

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[jazimed]

LittleInch,

Thank you for your reply.

I agree to have a separate recycle line with an external Pressure Safety Valve. at the other hand, I cannot see how the compressibility of the crude oil will allow to pump from 3 to 4 min without noticing a pressure rise!!!

 

[LittleInch]

I didn't say no pressure rise, but it will take a finite time for the pressure to climb to the point where the high pressure trip is activated. It is in part compressibility of the fluid and part expansion of the pipe. Not much per unit volume, but you have a lot of volume.

My point us that your assumption that as spin as you turn on the pump the pressure rise will be up to max in one or two seconds is not correct

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.