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7 comments

Is a Shut Discharge Valve Counterproductive?

Posted December 21, 2011 8:38 AM

Flow Control columnist Larry Backus writes that he would never recommend starting a centrifugal pump with a shut discharge valve. Do you agree, or have you ever found it productive to start a pump against a completely shut discharge valve?

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#1

Re: Is a Shut Discharge Valve Counterproductive?

12/21/2011 11:26 PM

Not completely shut, but cracked open to bleed the bubbles without suddenly dumping the prime.

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Guru
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#2

Re: Is a Shut Discharge Valve Counterproductive?

12/21/2011 11:33 PM

Yes I agree, but with a caution not to start with wide open condition, not even half open. A little open is the best before start and then to open further as per requirement after starting.

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#4
In reply to #2

Re: Is a Shut Discharge Valve Counterproductive?

12/22/2011 8:11 AM

You can't do it in an automated system where pump is started by a signal from a pressure switch or a float(or level) switch.

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#7
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Re: Is a Shut Discharge Valve Counterproductive?

12/24/2011 7:38 AM

A good doubt. I would have added a little clarity on my comment. The condition I recommonded was for an empty discharge line before starting the pump. A little open condition of discharge valve is to creat a back pressure which is already available in case of an automated system where pump is started by a signal from a pressure switch or a float(or level) switch. Hence, on such conditions there is no harm keeping discharge valve fully open.

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#3

Re: Is a Shut Discharge Valve Counterproductive?

12/22/2011 4:41 AM

A few years ago, I was designing a control system for a toluene manufacturing plant. The process starts with the chemicals flowing in a loop until stable conditions are established, then the toluene is diverted from the loop to an output storage tank. The Chemical Engineer asked me to program the PLC to close the loop valve and then open the outlet valve. I refused because if the outlet valve failed to open when requested, the pump would have been operating against a shut discharge. He argued that a centrifugal pump would not generate significant pressure and would do no harm. I argued that thrashing toluene about in the body of a 22kW pump would raise the temperature of the toluene to above it's flashpoint in less than 2 minutes. Not just counter productive consequences, literally explosive consequences. We settled on verifying that the outlet valve was open before closing the loop valve.

Moral of the story: Each application has to be assessed independently. There is no 'one answer fits all' solution for operating pumps.

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#5
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Re: Is a Shut Discharge Valve Counterproductive?

12/22/2011 10:02 AM

Moral of the story: Each application has to be assessed independently. There is no 'one answer fits all' solution for operating pumps.

GA. I couldn't agree more. Something I've learned is that statements that use "never" or "always" in them are rarely true. The pump guy's advice is good as a thumb rule. I've seen many systems designed such that the pumps are supposed to be started against a shut discharge (with the understanding the pump is not dead headed) similar to what you describe.

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#6

Re: Is a Shut Discharge Valve Counterproductive?

12/22/2011 3:16 PM

In some situations it is better to start with the pump in cavitation so the load is less on both electrical and process involved.

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