Pump has a suction lift of more than 10.3 m

  • #1
suryanarayan
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As I was going through a course on Hydraulics, there was a brief explanation about how the atmospheric pressure can only support a water column of 10.3 m.A few more points that are prerequisite to the discussion are as follows

1)A fluid cannot be pulled;it has to be pushed.
2)A centrifugal pump's impeller ,on rotation,throws fluid away from the eye(due to the centrifugal force) and creates a partial vacuum at the eye.

So ,consider the following situation given in the figure
Untitled.jpg

The experiment is conducted in normal atmospheric conditions
1)We fill the suction side and the discharge with water completely.This is possible because the check valve prevents the fluid from escaping.Now the impeller is completely immersed in water.
2)The pump is started runs at a very high rpm.

What is going to happen in the situation?

My thoughts are as follows:
The impeller throws the fluid to the discharge ,partial vacuum is created at eye,the check valve opens and since the weight of the water column is higher than what the atmospheric pressure can hold,the water drains to the tank.This creates a flow discontinuity in the suction side.

Will that be the case or will something else happen?
 

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Answers and Replies

  • #2
BvU
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Hi,
the check valve opens and since the weight of the water column is higher than what the atmospheric pressure can hold,the water drains to the tank
that valve opens only if the pressure outside is higher than inside. That never happens: the pump can't go further than 'zero pressure' at the inlet of the pump.
 
  • #3
russ_watters
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What is going to happen in the situation?

My thoughts are as follows:
The impeller throws the fluid to the discharge ,partial vacuum is created at eye,the check valve opens and since the weight of the water column is higher than what the atmospheric pressure can hold,the water drains to the tank.This creates a flow discontinuity in the suction side.

Will that be the case or will something else happen?
The purpose of the check valve is to prevent flow down the pipe and your answer as the water flows down the pipe?

What I see happening here is essentially nothing: when you turn on the pump, the water expands a little until the pump starts cavitating.
 
  • #4
Rive
Science Advisor
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...since the weight of the water column is higher than what the atmospheric pressure can hold...
Since this the foot valve will remain closed. In this setup if the suction leg is full then there is no way that valve will ever open.
 
  • #5
anorlunda
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1)A fluid cannot be pulled;it has to be pushed.

I think you misunderstand. It can be pulled a little, but if water is pulled more than about 10m it begins to boil and turn into a gas. That causes what @russ_watters mentioned, as "pump cavitation" But you can use a pump to suck water up 1m, no problem.
 
  • #6
russ_watters
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I think you misunderstand. It can be pulled a little, but if water is pulled more than about 10m it begins to boil and turn into a gas. That causes what @russ_watters mentioned, as "pump cavitation" But you can use a pump to suck water up 1m, no problem.
Quick clarification; from a practical standpoint we often consider the suction pressure to be negative, but for this problem it probably helps to use absolute pressure, which can only be positive (save for the tiny effect of intermollecular forces). Either is fine as long as the OP understands that "pull" is more a colloquialism than a physical reality.
 
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  • #7
Averagesupernova
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If the centrifugal pump here is designed for water only then it is unlikely the level of water in the suction pipe will rise even a meter. Try getting a centrifugal pump to prime in this manner even with only a meter of lift. It is all but impossible. Once the pump is primed things are different and it will work.
 

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