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Entrapped air bubbled causing high head?

Entrapped air bubbled causing high head?

Entrapped air bubbled causing high head?

(OP)
I have a sump pump discharging at a head 50% higher than the system curve calculated (147' instead of 95').  The sump pump is self priming and tees into a line charged at ~30 psig.  There is a 16' elevation change from the pump discharge into the tee.  There are no dips in the piping, it is all 90s and straight pipe.  However, I was thinking that it could be possible to run the pump dry when priming and introduce an air bubble that would be trapped in the line, causing high dP and low flow.  There is a check valve immediately downstream of the pump discharge.  Has anyone experienced something like this?  Is this correct thinking?

-Mike

RE: Entrapped air bubbled causing high head?

Let me make sure that I'm understanding your piping.  It looks like you expect the pump discharge to be 95 ft which includes the pressure required to get into the 30 psig (68 ft) header.  So your expected pump system load is 27 ft which is made up of 16 ft for the elevation change and 11 ft for flow resistance.  Now you are seeing 77 ft of head which is still 16 ft of elevation change, but now resistance to flow is 61 ft of head or 450% increase in resistance.

I'd look either at scale in the pipe (reducing the pipe ID and increasing velocity and friction) or a problem with an outlet valve or an increase in the header pressure.  

Is there anywhere to put a gauge immediately downstream of the check valve (sometimes there is a plug in the valve body that can be used) to see if maybe the discharge check is jammed partially shut when it should be fully open.  If that isn't possible, I'd look at either running a camera or dropping out a section of pipe to inspect it if this is a big enough problem to shut down for.

David

RE: Entrapped air bubbled causing high head?

I tend to agree with zdas04.  I don't see any way that an air bubble in the discharge piping could cause this sort of resistance.  In addition to the excellent suggestions above, you should check for a dropped gate on a gate valve.   If you have multiple taps for high points, low points, etc., a full pressure survey of the downstream system may reveal the location of the added pressure drop.   

Johnny Pellin

RE: Entrapped air bubbled causing high head?

Whether you can have a vapor lock in the system will depend on the profile of the entire discharge piping.  The first 16 ft of elevation change wouldn't be enough to cause a persistant vapor lock, but if you have a number of uphill sections farther downstream with vapor space in each uphill segment, heads of each add and a persistant vapor lock could become a problem.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Entrapped air bubbled causing high head?

(OP)
Thanks for the replies.  I think it has to do with a failed valve, but before I had the line shut down I wanted to rule out the possibility for anything else.

I'll follow up on the post once the work is complete.

-Mike

RE: Entrapped air bubbled causing high head?

entrapped air is a very common problem in waterlines and sewer force mains.  The air is not just from priming of the pump. All water contains dissolved oxygen and most also contains entrained air bubbles. Changes in pressure of the water may cause the air to come out of solution. The small bubbles coalesce and then can be trapped at high points in the line if the flow velocity is not high enough to move it through or if there are no valves to release the air. Biginch touched on this. This effectively reduces the cross sectional area of the pipe and can cause large increases in the pressure. This has been discussed at length in the context of sewer force mains in the wastewater formum.

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