2 Pumps as Primary Control Valve as trim

[rmarquis1983]

Does anyone have any experience in this?

I have a lead lag pump setup for water that is maintained by pressure using VFDS, the idea is to have the control valve act as a trim and letting the pump VFDS set as the primary loop.

 

[TheWaterboy]

I am not following, why not let the VFD do their thing and maintain pressure?

 

[rmarquis1983]

I agree 1000% and thats how we always do it. However per spec this is how they want it to operate.

The system will have two pumps that are 100% redundant and can be operated separately or in parallel. The pumps will be equipped with variable frequency drives and control valves that will modulate to maintain the desired pressure. The pressure controller will modulate the VFD as the primary means of control and the control valve as the secondary means of control to maintain minimum VFD current consumption while satisfying the flow rate setpoint. The pressure controller will have a low-speed limit on the VFD to prevent operation at inadequate pressure. (Low-speed limit on pumps TBD at startup). Alarming for both pumps, VFDS, and pressures will be provided. The feedwater control will allow the operator to modify the required pressure setpoint, switching of controller modes (auto and manual), and direct speed control of the pumps in manual mode.

 

[rupej]

You're not going to have good results if the valve and the pumps have their own PIDs operating on the same PV.

You may be better off with a split range control, or if flow is the concern, letting the valve's PID use flow as the PV while the pumps use pressure.

 

[Peter Nachtwey]

The valve will also waste energy. Keep the valve but keep it open and control with the pumps if you can. This should work unless the pressure much change quickly.


Why doesn't anybody consider using an accumulator on the output of the pump. This is like putting a capacitor across a noisy circuit. It filters noise or in this case pressure spikes. If an accumulator is used then the valve should be replaced by a dump valve that is energized open to permit flow but will divert flow to a tank or drain when power is lost . This will let the accumulator discharge safely.

 

[g.mccormick]

Do you have a P&ID of the circuit or atleast a sketch? Where is the valve in relation to the pumps and loads? It sorta sounds like the valve may be a bypass valve dumpung to return/drain.

 

[Saffa]

I have once seen a legitimate need for a valve in a scenario where the discharge head on a large irrigation pump was too low when operating a particular field. A control valve was needed to keep the pump from operating too far right on the curve.

But, this doesn't sound like that kind of thing unless there are some weird hydraulics.

Agree with Peter that an accumulator is generally a good idea if this is a closed system with variable demand. Helps prevent hunting around the SP if there are short, small draw offs like small solenoid valves opening for short periods