Submersible Pump Discharge Velocity 5

[p6g2p6]

Hi, I am looking for some advice on selecting the pipe size / discharge velocity for a sump pump.

Almost every pipe/pump design guideline that I have read suggests that the discharge velocity for a pump should be between 9-12 ft/s. I have desired flow rate of 350 USgpm, therefore using the ASHRAE charts for SCH 80 plastic pipe (file attached), I selected a pipe diameter of 4 inches. This gives a discharge velocity of about 10 ft/s. The problem is that I cannot find any plastic check valves that are rated for velocities in that range. I've reached out to local valve suppliers and they all said that they don't recommend using a check valve with a velocity in that range.

So my question is: Why do all the guidelines recommend discharge velocities in the 9-12ft/s range if it isn't recommended to use check valves in that range?

Should I stick with the my original pipe selection and hope that the check valve doesn't break down? Or, should I listen to the valve suppliers and increase my pipe size to 6 inches?

 

[1503-44]

I believe that would be a normal maximum discharge velocity at maximum rpm speeds. Pumps are often sttached to larger pipes and pipelines, especially when velocities will be in 10 fps or over, to avoid excessive pressure loss in the piping system and the potential effects of water hammer. I'd also tend towards using the lower velocities with plastic pipe.

I note that the doc you attached says, "Therefore at typical velocities of 2-6 fps, the velocity head ...".

It also depends on what type of check valve you might be using.

 

[LittleInch]

I guess you need to look into the actual design of the particular check valves. Most simple check valves end up with a reduction in ID somewhere so the actual velocity in the check valve can be 2-3 times what it is in the pipe.

Some simple flap type checks can start to vibrate or have the disc "bounce" at higher velocities.

10fps is really quite a decent velocity and it won't take many metres of pipe before you have quite a high head requirement.

Also I think the valves look at possible flow reversal speeds and reckon if you doing that velocity on one direction it will lead to potentially high reversal speeds and lead to damage, especially for "plastic" valves.

So you could go to 6" diam just for the check valve section then reduce down again?





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

Check valves cause a lot of turbulence, and they are sensitive to turbulence.
I would at least run a section of 6" for the check, maybe reduce to 4" above that.
I presume that the flow restriction of the 4" is something that you have considered.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[jgailla]

I think you may be confusing discharge velocity for a pump with discharge velocity for the discharge piping.
You should have a reducer after the pump discharge that goes to a 6" or 8" line.
The check valve should be on this discharge piping about 10D past the reducer.
For wastewater pump stations the 10 D doesn't really matter because the reducer is in the pit immediately downstream of the pump and the check valve is farther downstream in a separate valve pit.

 

[bimr]

A typical economical velocity is 3-5 feet/sec. If you use a higher velocity, then your pumping cost (in kw-hours) will be excessive. 9-12ft/s range is probably at the pump exit, not in the discharge piping.

 

[Artisi]

Seems the OP might have vanished, but if still listening some practical information would be more helpful rather than the academic discussion that tell us nothing - - detail like flowrate, head, what's being pumpe, pump detail etc is far more important.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)