suction pressure losses in submersible pump?

[leakyseal]

Dumb q. for the assembled cognoscenti:

I'm working through the hydraulic modelling for my first wet well. Despite a reasonable amount of experience with pumping systems I haven't really worked with sub pumps before. Simple question on its face:

Are there any suction friction losses to calculate with a submersible pump? In a conventional application I would look at, e.g., tank exit loss plus suction piping run loss plus maybe an elbow or two and an isolation valve. In the case of a sub pump the suction is flooded and right there in the fluid, no plumbing in the way. Do I simply assume the suction side friction loss is zero?

thanks
leaky

 

[JRLAKE]

I would determine pump entry losses, but no other friction losses on the suction.

 

[BigInch]

Remember you still must subtract the vapor pressure from the height of liquid over the submersed pump intake too.

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[leakyseal]

JRLAKE, I guess this is my question, is how does one define/calculate the pump entry losses? This is not a tabulated or calculable quantity with which I am familiar.

BigInch, your point is a good one, but I believe I've managed to account for it already. The application is clean(-ish) ambient temperature water so the effects on NPSHa are minimal, and the sump is designed such that the pump suction is flooded well beyond the vendor's recommendations.

 

[JRLAKE]

I always assumed about 1/2 of the velocity head. You could always use a flush pipe entrance. A sharp edged would be, well what do you know, hf = 0.5 v^2/2g.

I guess what I am saying is that it will be pretty negligible compared to your other losses.

Beware of minimum submergence also.

 

[BigInch]

The vendor's recommendations would normally include intake losses in his submerged depth requirements. You would be responsible for adequate additions for VP and viscosities if you were pumping some liquid other than water.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[BigInch]

Valvecrazy suggests this link,

http://www.franklin-electric.com/Manual/AIM_07.htm

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Artisi]

BigInch is on the button, ignore entry losses as they are already accounted for in the vendors performance curve, this curve will be the performance from inlet to the pump discharge, you only need to worry about submergence and the total head from inlet level to discharge level plus friction losses.

 

[JRLAKE]

I never knew about the entry losses already being accounted for in the curve. So I grabbed some Gorman Rupp and Flygt submersible curves to check it out and I don't see anything stating this. Is this true for all manufacturers?

 

[Artisi]

It would make sense for all submersible pump ie Flygt, Grindex, Gorman Rupp etc to have the entry losses included in the performance curve as this is the standard configuration in which they are supplied and usually installed.

Likewise the close coupled submersibles power ratings include both pump and motor inefficiencies - this is the reason some units look to be inefficient in terms of power input to hydraulic output.

 

[JRLAKE]

Thanks for that info. Makes perfect sense to me when I think about it.

 

[EdStainless]

Yes the intake losses are included, you can't test them any other way.
The only time to worry about the submergence is when you are in a tight well, then you may need to be concerned about the flow up past the motor. I am thinking about pumps with very high head as they need all of the NPSH that they can get.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JRLAKE]

NPSHr is directly related to specific suction speed, not head.

The statement about tight wells is inaccurate. I have seen pumps create a vortex in large ponds.

 

[BigInch]

He means high DISCHARGE head.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain