Adding a screen to a pipe and calculating NPSH

[Earmouse]

Hey all. We're looking to add a screen in one of our pipes and we have to figure out whether the change in pressure caused by adding the screen will starve the pump. I believe this is an NPSH problem, and I am using the equation...

NPSH = 1/g * [(Pa - Pv)/density - hf] - Z

... where g is gravity, Pa is pressure at the surface of our pond, Pv is the vapor pressure, hf is friction in fittings, and Z is the height of the pump above our pond.

The trouble I'm having is figuring out how adding the screen will change the variables in this equation.

Any and all help is greatly appreciated!

 

[Compositepro]

The screen will eventually starve the pump. It is only a question of when. That depends on screen area and how much debris it is catching and the type of debris. One leaf can plug a lot of area.

 

[katmar]

The pressure drop through the screen is part of hf

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[ione]

A screen represent a so called “minor loss” or “local loss” (same as losses due to fittings and to distinguish them from those “major losses” caused by friction). As already suggested by katmar it has to be considered amongst the term hf in your equation.

In the link below you’ll find a free calculator which could help you to calculate what you’re looking for.

http://www.pressure-drop.mobi/

 

[Earmouse]

Thanks guys!

 

[rmw]

I think I would reword ione's statement to "A clean screen is a minor loss...". To katmar's point, once clogged with leaves, pipe scale, mechanics rags, pieces of wood, lunch buckets, hard hats, welding gloves, etc. (I could go on and on) it could easily become a 'major' loss and cause significant damage to a pump especially if the pump was borderline with respect to NPSHa already.

rmw

PS: The thought occurred to me to start a thread to get the members to list what they have seen caught by strainers and/or screens. I didn't use my whole list above.

 

[rmw]

Oh, and ione, that is a good site. One disappointment however in the case of a strainer or screen was that it didn't give the option to select a screen size. I have to assume it defaults to some particular value but it is not stated what it is. Still, a good site.

 

[ione]

rmw,

I agree that minor loss is an unfortunate use of words to describe something which could become not trivial at all in computing head loss. Anyway that's the way it is: I've not coined the term myself

 

[11echo]

Are you talking a flat screen or a "witch hat" cone strainer?

 

[Longinthetooth]

Permanent screens in pump suctions are generaly a bad idea for all the reasons stated above. The only place I have seen them is in the exit of cooling tower basins feeding the CW recirculation pumps. In this case they are massive and easily pulled for regular cleaning. Startup screens are frequently installed in pump suction lines to protect aginst gloves, welding slag etc, but they should be removed ASAP no matter what operations want.