Suction pipe diameter vs lift height

[eddtan]

Hello everyone! I have a well pump that can generate -0.5barg on its suction side. I am intending to use this pump to suck water out from a tank. The pump is located 3m above the top water level. I want to get some advice on the sizing of the pipe from the tank to the pump.
1) can I just size the pipe based on 1-1.5m/s, assuming NPSHr is met?
2) the pump will be primed, but the vertical section of pipe down to the tank will be empty. Will the water be sucked up into the pipe? would the size of the pipe affect the suction force or lift height? Would a bigger diameter pipe reduce the suction force and hence the lift height?

Appreciate your time reading and comments

 

[LittleInch]

1) for steady state flow, yes

2) difficult to say. A primed pump will pull a certain air volume, but 3m is pushing it unless its a dedicated self priming pump. It might just struggle into life but could easily fail.

Pumps don't like pumping just air....

Far better to have a foot valve and fill the auction line.


Size of pipe would affect air volume dragged through the pump at start up so the smaller the better. But it still might not work. [pre][/pre]


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[eddtan]

Thank you for the super quick response! Noted on the volume on air through pump during start up!

I have to use the same pipe line and pump water back in, so I can't put a foot valve in there.

Only the vertical 3m of pipe will be empty. For the horizontal piping to the pump suction, I intend to install it at a gradient so that water do not flow back to the tank when the pump stops. In this case, do you think I can assume that as long as the horizontal pipe is >3m, water will be sucked into the pump successfully during startup? I am assuming that as 3m of water in the horizontal pipe flows into the pump, 3m of water will travel up the empty pipe as well..

 

[LittleInch]

Not really. The air expansds so if you can increase the length of pipe from the high point to the pump. I would say you want 6m+ of filled pipe to stand a chance. Even then the pump will "cough" and could trip if its big enough.

Or install a NRV on the pump exit and a small vacuum pump to lift the water into the pipe is definitely going to work.

Or buy a self priming pump.

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Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Do a sketch of this arrangement please.

With elevations and lengths.

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Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
A sketch, please!
Pierre

 

[katmar]

The volume of the priming pot is related to the volume of the suction piping. A safe rule of thumb is to have a pot with 3x the volume of the suction pipe, although I have seen recommendations to use as little a 1.5x. There have been many discussions of this problem on Eng-Tips and a search for "sizing priming pot" will find them. See the thread

https://www.eng-tips.com/viewthread.cfm?qid=285861

where the 3x rule was mentioned.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[pierreick]

Hi,
Consider this link, you may want to drop them an email.

https://www.munsch.de/en/products/priming-pot

Pierre

 

[eddtan]

Wow that is very helpful guys!! Now I have a much better idea. I will look at those links.

Thank you!

 

[LittleInch]

Think about it.

To get a lift of 3m you need a diff pressure of 0.3bar, say 0.5 for good measure.

That means the 3m air volume in your pipe needs to be 6m long.

So if have got 6m of liquid filled pipe, or the equivalent volume in a suction pot then you should get liquid to the top of your fill pipe which will then start flowing into your pump.

But the pump won't like it to start with and will "chugg" quite a lot until all the air gets through the pump and could trip if you're not careful or create pressure pulses which might do the same thing.

Vacuum lift is so much better.

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