What is "foot valve",why it prevents loss of prime

[Chilly807]

Maybe I'm missing something. What would maintain the pressure in the line when the pump shut off if it was open on the end? And if the suction line was pressurized wouldn't it be a discharge line?

What's missing is what the OP is trying to do with the pump.. from what I read between the lines, he's either trying to boost the output pressure or installing a pumping station to get the water up to a higher level with enough pressure to still do it's job. High rise buildings are like this, there'll be a pumping "station" every so many floors to ensure the upper floor occupants still get decent water pressure, otherwise all the pressure is lost trying to get water up say 30 floors.

As to boosting the output pressure, if you have two pumps in series, the output pressure capacity of the second pump will be added to the feed pressure from the first. For example, 50 psi going into a pump capable of creating 50 psi as well will give you a possible 100 psi output before any efficiency losses.

One thing you have to be careful with is pipe sizing. For example, your feed piping (can be either pressure or suction feed) needs to be larger diameter than your discharge pipe. The "feed" pump must have higher GPM rating than your "boost" pump, or your pressure feed turns into a suction and you have cavitation troubles since the output rate of the first pump is not infinite.

Keep in mind that even a pump with a "suction" feed actually has a pressure feed of sorts, since the pressure the atmosphere exerts on the water entering the pump is greater than the low pressure the pump is creating ahead of it. The pump only creates flow, pressure is caused by restriction. A vacuum is defined as "lack of pressure".

Sean

 

[mmurphy]

Chilly807,:thumbsup::thumbsup:

 

[Egon]

Chilly:thumbsup::thumbsup:
:thumbsup::thumbsup:

It's all about NPSH.

most of the downhole pumps are a series of individual pumps stacked on top of each other. Mostly referred to as stages.

 

[J_J]

Actually, the question is does he need a check or foot valve if the new pump is using pressurized water from the house supply. The answer is, no check is required.

However, if the house water is low or off, the new pump will suck the hose flat, and may burn up due to lack of water.

 

[shade tree welder]

Most shallow well pumps, can draw from about 25 ft.

Just for a bit of trivial, the maximum pump on water is 32 ft. Trying to
obtain suction of water higher than that you are just creating a
barometer.

Maximum suction head varies for different liquids based on the specific gravity
of the material in question.

 

[mmurphy]

Just for a bit of trivial, the maximum pump on water is 32 ft. Trying to
obtain suction of water higher than that you are just creating a
barometer.

Maximum suction head varies for different liquids based on the specific gravity
of the material in question.

In all actuality, probably closer to 24 foot is best. That does depend on atmospheric pressure, friction loss in pipe, vapor pressure from temperature. Of course you do have the idea how it works on a suction lift condition.

I will add, pumps actually do not "suck" the water, but creates a low pressure zone in the pipe. Suction lift conditions rely on the atmosphere pressure to push water up the tube or pipe that no longer is effected by the atmosphere.

In a suction lift condition, you do need a foot valve at bottom or pump will lose prime.
As Egon mentioned, NPSH (net positive suction head) is a pressure or volume of water above the impellar of the pump that will keep giving it a positive pressure while running.

 

[rjmack]

Just for a bit of trivial, the maximum pump on water is 32 ft.

you mean, the maximum height water can be drawn, with a vacuum, is roughly 32ft @sea level.

15psi/.42psi per vertical ft of column?

 

[MikeD74T]

...I will add, pumps actually do not "suck" the water, but creates a low pressure zone in the pipe... quote]

Isn't 'creating a low pressure zone' the definition of 'sucking'?? MikeD74t

 

[rjmack]

Isn't 'creating a low pressure zone' the definition of 'sucking'??

you can only 'suck' to zero, you can push to infinity... or somewhere in that neighbourhood.

i think it's all about manipulating the balance between the 'pushes'.

i wish i knew more about jet pumps, but i think once it has utilized the correlation between velocity and pressure to enable, by spinning the impeller, atmospheric pressure to fill the inlet it uses fluid shear to provide pressure up to the point where the non-compressibility of a fluid effectively acts as a force to relief back past the impeller.

if somebody can expand/correct it sure would be nice to know the nuts and bolts of this.:thumbsup:

 

[mmurphy]

rjmack, you are getting it.
Impellar size and rpm determines head of how high water is thrown.
6" impellar at 3600 rpm throws a head the same as 12" impellar at 1800 rpm. Now you take the 12" impellar, change to 3600 rpm, your head will double. With out getting calculator out, hp requirement will be about triple or more.

Far as sucking water, I am not the scientist who came up with that, but I understand what they is saying because you reach a point when atmospheric pressure no longer has a affect on pushing water up a column, water starts vaporing in to gas and then you chew impellar up to cavitation.

As pressure builds up you lose ability to keep pushing water in a centrifugal pump. (impellar) High pressure side will started leaking back to low pressure side of pump untill no water will flow.

If you are moving water 100' elevation, and have a 7' suction lift (top of water table to center of eye on impellar) You now have 107'of work to do. Just will point out no friction losses added in this example.(also needs a foot valve here)

Now you have 7' positive suction, or 7' of water in a table above eye of impellar and need to move from pump of 100' elevation, you now have 93' of work to do.