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

[bcarwell]

I have a new cheapo non-self-priming water pump. Instructions say to install a "foot valve" on the input side, and that this will prevent loss of prime.

What exactly IS a "foot valve" ? And why would it prevent loss of prime ?

Bob

 

[beenthere]

As I understand it is like a one-way flapper valve that holds a column of water in the pipe. It as the lower end so holds that column of water above it.

http://www.inspectapedia.com/water/Foot_Valves.htm

 

[Chilly807]

The foot valve goes at the "foot" of the piping in your well/cistern/lake, etc. It acts as a one-way valve to prevent water from draining out of the pipe when the pump isn't running.

Sean

 

[rjmack]

check valve.

 

[Mace Canute]

If you Google "foot valve" you will find many entries that describe what it is and how it works with several pictures and illustrations.

 

[Qapla]

The foot valve goes at the "foot" of the piping in your well/cistern/lake, etc. It acts as a one-way valve to prevent water from draining out of the pipe when the pump isn't running.

This is correct. Keep in mind that a "check valve" is a "one way valve" that keeps the water from returning to the source. The main difference in the "check" and the "foot" valve is the location. A "foot valve" is technically a "check valve" that is located at the bottom of the well drop pipe - thus the name "foot".

A check valve can be installed near the pump since it is mostly a safety device. A "foot" valve is installed on the "bottom" of the drop pipe.

Since a check valve will also stop the water from going back down the well, you might wonder why you need the foot valve.

The main reason for the foot valve being at the bottom of the well is to maintain a drop pipe that is filled with water. The weight of the water in the pipe will help keep the valve closed when the pump is off and keeps the pipe full. This way, when the pump turns on, the water in the pipe helps keep the pump from losing it's prime while starting a flow of water.

If you only have a check valve at the top, it is true that the water in the storage tank won't drain back down the well, but gravity will leach the water from the drop pipe. When the pump turns on, there may not be enough water in the pipe to maintain the prime while the pump pulls a fresh flow. Even if it does prime, you will most likely send a pipe full of air into your storage tank. A few starts like that and you will replace the water in the tank with only air.

If you have no check or foot valve, there will be no water available for the pump to prime itself.

 

[bcarwell]

Ahhhh. Thanks to all, now I understand. Since I am running my pump from a (house) pressurized input source (e.g. a garden spigot located right next to the pump) I assume that I can dispense with the "foot" valve once I've primed the pump.

Thanks again for all your help !

Bob

 

[Egon]

With a pressurized suction line no foot valve/check valve should be required.:thumbsup:

 

[Argosy]

With a pressurized suction line no foot valve/check valve should be required.:thumbsup:

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?

 

[J_J]

The weight of the water helps to keep the check or foot valve closed. The stand pipe is not pressurized by the pump. You need a check valve to keep water in the tank and the suction side of the pump. There is pressure on one side of the check, and a vacuum on the other side. If you have both valves, the pressure on the foot valve is only the weight of the water in the stand pipe. The pressure on the upper check valve is the tank pressure.

I think What Egon is saying is, that if you only have a foot valve, then the pressure from the tank is on the top of the foot valve, and therefore, the suction side has tank pressure. You have to have a check of some kind, unless it is an artesian well.

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

 

[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.