Holding suction after pump shuts down 2

[mrelet]

I have a PacFab Challenger CHII-NI-3FE high head pool pump that has a "impellar screw that prevents reverse spinning of impellar in back-flow applications". I'm running tests that require the drain to be blocked with a pad and the pump to be turned off-- the pad should then be freed. Curious to me is that the pressure in the suction pipe stays negative after the pump is stopped-- sometimes for minutes. I would think the pressures in the pipe and the pool water level pressure would equalize rather quickly.

Can this pump act as a check valve? Is something else going on?

 

[thewellguy]

I'm not clear from your post if the pad is blocking the suction or discharge side of the pump. If you are blocking the discharge side then the pad could act like a check valve and keep pressure on the system. The anti-back spin impeller does not have anything to do with the pump holding prime.

 

[mrelet]

Thank you thewellguy. The pad blocks the drain on the suction side to the pump. Is there any way the impellar can block the suction/discharge sides?

 

[Bjegovic]

centrifugal pump? no.

 

[Artisi]

It sounds to me like you are saying the once the pump suction is blocked and the pump shuts-down, the backflow from the discharge side of the unit is expected to dislodge the obstruction. Is this a correct assumption??

Is there a non-return (check valve) on the pump discharge?


Naresuan University
Phitsanulok
Thailand

 

[mrelet]

Thank you Artisi.
Yes, backflow of sorts. Either a pressure shock reflected back to the drain or final pressure equalization should occur. There is no check valve. There is a globe valve. If it matters, we have the pump discharge volute rotated 90 degrees from straight up to sideways. This holding suction occurs when we have the pump a few feet below the tank water level and with a moderately oversized pump (3 hp to deliver 60 gpm).

 

[KiwiMace]

mrelet wrote: "This holding suction occurs when we have the pump a few feet below the tank water level"

Sorry to ask the obvious - it's not the suction side head holding the pad on? If your pumped system has low lift (say you are pumping a pool filter at an even level), the suction side static head might be preventing backflow.

 

[mrelet]

Thanks CinciMace.

My exact configuration is: 2-foot water level; pump is 3 feet below water level. Discharge is 2-feet above water level and is all pipe with a globe valve; no filter. When the drain is blocked, the pump runs 1/2 sec then turns off. Pressure at the drain drops to (-) 20-25 in Hg after the drain is blocked and, after pump is shut down, then degrades over 1-2 minutes to the tank water head pressure (2-foot). Thus, pressures well below the static head are maintained. The static head could only add 2 in Hg (about 2-foot).

This is a mystery.

 

[Zapster]

This should be an easy problem to identify.

1. Contact the pump manufacturer and ask them
2. After the pump shuts down is the water level in the discharger pipe above the level in the pool? Or does it slowly rise as the pad comes loose? These must be verified.
3. Take the pump apart and look at the construction to understand what is taking place

 

[Artisi]

The difference in head from water level to the highest discharge point is only 2 ft - so you are expecting a pressure of less than 1 psi to dislodge an obstruction that has been forced into the inlet by atmospheric pressure of around 10psi.

Also - it sounds to me like there must be an inbuilt non-return valve somwhere - otherwise the vacuum of 20"-25" would erode immediately the pump shut down.

You should follow Zapster's advice and investigate the pump construction.

Naresuan University
Phitsanulok
Thailand

 

[mrelet]

Thank you Zapster and Artisi, I'll tell you what I find.

 

[checman]

mrelet You may have the siphon effect working against you. If you notice that the pad is released after the discharge line drains it may be the problem.