Pump Recycle Control

[okie]

My question concerns the best configuation to use when controlling a centrifigal pump by way of recycle. Assume that all the economics have been run and it has been decided that recycle control is the option that will be used for pump control i.e. VFD is not an option. In our specific case we are controlling level in a vessel and have a centrifigal pump moving the liquid. The pressure that the pump must overcome is not constant and may range from 10-100 psi. We actually have several teams with very similar situations and as a company we would like for the designs to be as close to one another as practical. As luck would have it, each team has come up with a slightly different control scheme which I list below. My question is "Since this is obviously not the first time that this has been done, is there a generally agreed upon "best way" to configure such a system?" Some of the different proposed configurations are....
1) Control Pump discharge pressure with a PCV on the pump discharge and control level with the recycle valve. The recycle valve to be located upstream of the PCV.
2) Control Pump discharge pressure with the recycle valve and control level with a LCV on the pump discharge. The recycle valve to be located upstream of the LCV. (Basically oposite of option 1)
3) Same as opton 1 but have the recycle (or LCV) located downstream of the PCV.
4) Level control with LCV on pump discharge and minimum bypass flow control with a FCV located upstream of the LCV. Flow meter located upstream of both LCV & FCV.

Clear as mud huh?

Thanks in advance!

 

[tstead]

Why is VFD not an option? You have a tremendous swing of pressures that the pump must operate against - 23 to 231 feet of head - give or take.

That is asking quite a bit from a constant speed centrifugal pump.

So why is a VFD not a feasible option?

Tim

 

[dadfap]

okie,

what type of pump are you using?

I agree with tstead, you are asking a lot of any pump, why is the VSD not an option?

If it is purely the capital cost of the VSD then I would imagine that a lot of this can be offset by the pump efficiency gains.

Where is the recycled fluid going? Ideally it should be plumbed back to the tank.

dadfap

 

[smckennz]

The information you have provided is insufficient to choose the best control method of control.
Can you provide a fuller description of what you are doing, the accuracy you require and the reason for the large pressure variation.
The simplest system for a fixed speed pump would be an LCV with a suitably sized upstream pressure relief valve set at slightly over 100psi for minimum flow protection. This will be the cheapest, but the LCV controllability/rangeability would have to be looked at very carefully with such a wide pressure range.
A fancy solution would be to use a flow control valve and flowmeter downstream of a pressure control valve and pressure transmitter loop.
The possibilities are endless.

You will get some good insight by looking at boiler steam drum level control systems; the 3 element is probably fancier than you need (as they compensate for two phase effects), but the one and two element systems may help you decide.
It really is horses for courses and we dont know what your course is.

Cheers

Steve McKenzie

 

[saxon]

okie, Let's go back to the very beginning and ask the question, "What are you trying to accomplish and why?" Once we know the design issues and parameters we can help you. Until then, you're just chasing your tail.

saxon