Centrifugal Pump - Liquid Level Control Valve (Recycle)

[agavi]

Ladies and Gentlemen,

I have a centrifugal pump pumping liquid out of a pressure vessel @ 100 psig into a pipeline @ 900 to 1,350 psig. Currently there is a discharge pressure control valve set @ 1,000 PSIG installed. It is wide open unless the pressure in the pipeline drops below 1,000 psig. The pump has a VFD that slows the pump down if the level in the tank starts dropping. Since the pump can only slow down so much I’m planning to add a recycle valve that operates on liquid level and sends fluid back to the inlet of the pressure vessel. It will start operating if the pump is at its minimum speed and the liquid level in the vessel is still dropping. I’m getting a little bit of push back from some of my colleagues. They are saying this won’t work because it will allow the pump to go off the right side of the curve during a full recycle scenario. They can’t tell me how this will happen, though. In doing a bit of google research, I have found numerous schematics showing different control schemes but I haven’t seen one where the recycle valve operates on level. Will this setup work well? If not, why won't it?

Also, the flow range of the pump is about 40 – 90 gpm and the pressure vessel is 18,000 gallons and horizontal.

Thanks for the help!

 

[LittleInch]

Well it's up to you to set the Cv of the control valve to avid this and you may also need an override to prevent excess flow.

If there is no flow coming into your vessel then at low level your pump is on min speed but in full re-cycle - i.e. no forward flow.

So with 900 psi across the valve, set the max Cv so that max flow through the valve equals max pump speed which still allows 100 psi to be produced. Then as flow into your vessel increases the valve can start to close, pressure increases a little and forward flow re-commences.

full recycle for any length of time will result in increased temperature.

Just work some options wrt what flow can happen at different conditions.

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

 

[mk3223]

IMO, it makes no sense to add the recycle line to maintain the liquid level in the vessel.
Instead, one option is to design the process of the adequate source for maintaining a steady level. The other is to add a level control function to the existing pressure control valve in the Discharge line.

 

[georgeverghese]

Can you clarify :
Currently there is a discharge pressure control valve set @ 1,000 PSIG installed. It is wide open unless the pressure in the pipeline drops below 1,000 psig.

A sketch showing this scheme with the feedback / feed forward mechanism at this PCV will also help.

 

[georgeverghese]

Suspect this pump's VFD speed turndown is set such that it will produce a min dp of 1000-100 = 900 psi at min speed, with the intent of keeping line pressure always above 1000psig. In this case, I can see the rational for a low level recycle valve.

There is no way the pump will operate at the right side of the pump curve at min speed, if the level controller output is split ranged to the 2 devices ( i.e the pump VFD or the LCV). Assuming the LIC is a direct acting controller, and the LCV is FC, then suggest 0-50% LIC output is linearly reversed and acts on the LCV, while the 50-100% range from the LIC acts on the VFD. Obviously you would size the LCV to recycle the flow seen at the current pump min speed lockout setting at the VFD with a dp of approx 850-900psi.

Presume you will have the LCV forced open at DCS on pump startup to allow for a min flow recycle for say 15-20seconds (or until line pressure builds up beyond 1000psig) and the backpressure PCV starts to open and forward flow is established. Similarly, you would force open this LCV whenever there is a pump trip.

The pump min backpressure CV set of 1000psig and the corresponding min speed setting at VFD are probably to maintain some min flow of condensate coolant /lubricant through the primary seals on the pump shaft?

Presume the Q-h curve is not flat at around the min flow point at min speed set.