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Control Valve Automation

11/04/2009 10:52 AM

Ive been having problems automating a control valve used on a line thats pumped using a Gould's Aquavar CPC. The CPC changes the pump frequency when a pressure drop is sensed, and that changes to flow rate. The control valves react to the flow rate and change positions,causing both to oscillate endlessly.

Is there a way to control the frequency or lock the CPC once a minimum value is reached? Or should I figure out how to increase the time basis for the control valves to work on? Im relatively certain that if I increase the time it takes for the control valve to reach full scale it will reduce the oscillation,but It would still oscillate due to the CPC trying to maintain constant pressure.

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#1

Re: Control valve automation

11/04/2009 11:01 AM

This is a controller tuning problem.

http://en.wikipedia.org/wiki/PID_controller

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#2

Re: Control Valve Automation

11/04/2009 2:16 PM

mechwarrior-

You have two control systems fighting against each other. The CPC drives the pump speed, therefore the gpm and system pressure drop. The control valves react to the pressure drop and modulate accordingly to control for setpoint and their actuation changes the system pressure drop, etc.(assuming 2-way valves). As one system attempts to control to its setpoint, it affects the second system which, again, affects the other.

Solution:

The CPC should ramp the pump speed to control only the system pressure with a differential pressure transmitter at least 2/3 of the total pipe distance from the pump. (I prefer the sensors to be at the last valve.)

You should have one valve at the end of the loop to bypass a minimum of 20% of the total flow of water from the supply side to the return side. The valve should be actuated by a "high limit" when the pump is at its minimum frequency (or on bypass) and still cannot reduce the pressure to setpoint. This valve is 2-position actuated by the high limit.

The control valves should only control from a temperature sensor with proportional control, only. Leave integral and derivative at "0".

Send more info concerning the system for a detailed solution.

1. Number of valves, Cv of each valve, designed system pressure drop.

2. Pump specifications, GPM, etc.

3. Pipe size of main system.

4. Original design of the piping system & local valves.

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#3

Re: Control Valve Automation

11/04/2009 3:22 PM

First, determine which of the 2 (pressure or flow), is the more important variable to control.

Second, tune that loop to be at least 3 times faster than the less critical loop. Or the less critical loop 3 time slower.

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#4

Re: Control Valve Automation

11/05/2009 12:43 AM

If you can find a way to display the P&ID (Piping and Instrumentation Drawing), together with the desired control settings and flow rates, this will help responders to understand your total system. Both of the preceding posts are GAs, each covering different aspects of your situation.

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#5

Re: Control Valve Automation

11/05/2009 11:58 AM

Your using the CPC to control the pump, what are you using to control the valve? And what type of process is this for?

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