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Fighting Valves

Fighting Valves

Fighting Valves

(OP)
We have the ML1400, 1762-IF4 and 1762-OF4 modules. The system consists of one pump supplying 2 water systems.

Each system has a flowmeter and a modulating valve. The flow is to be split 50/50 thru each system. There are 3 different flow rates needed.

The PID's are set up to take half the desired flow and adjust the modulating valve to achieve this.

One issue is they are butterfly valves and at the desired flows being 10-20 % open, rather than the desired mid range at desired flow. I'm not sure of the valve resolution at the low end of the valve. (I'm not a valve guy)I haven't been to the site yet, the customer reports the PID's the unit will "hunt" with a large range. Never stabilizing near the set point.


How do we make these guys quit fighting? Will a modulating valve be accurate at 10% open?

Thanks in advance!  

RE: Fighting Valves

I have no idea what the ML1400, 1762-IF4 and 1762-OF4 modules are.  Describe these if important.

Where is the flow measured and how are you controlling the flow?

With two systems you could have one flow meter on each of two lines after a split from the pump.  Instead of two flow controllers you could have one flow controller and the other controlling a ratio against the other flow - or several other combinations of measurement and flow control.

RE: Fighting Valves

One problem is that they are butterfly valves.  Their control range is not linear, most of the modulation happens in the nearly closed position and for 35% to 100% open, flow range is 75% to 100%.   I only use butterfly valves for control when the flow rates are not critical. Smaller valves might work.  Globe valves or special design control valves are needed for precision flow and to eliminate fighting.  Perhaps you can induce some slop in the allowable range so the controllers find a close enough position and stay.  

It doesn't matter how open the valve is, the sensors only measure flow and send a signal to open if flow is low, and close if flow is high.  Often times undersized valves make the system stable.  

RE: Fighting Valves

Do as Mr. Seagull suggests.  Deactivate one flow controller.  Use only one flow controller and dial in the setting for its pipeline and let the other pipeline take the balance of flow.  No hunting possible.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Fighting Valves

I have seen butterfly valves work well at 10% open, but it is not ideal.  The problem may well be something other than the valves.  Ideally, when any one valve changes position it should change the flow in its pipeline only.  If you have undersized piping in the common section, or a pump with a steep curve, as one valve changes the flow in its leg it will have an impact on the flow in the other legs.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

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