7 replies to this topic

[Mattgreen286]

Community,

 

I have a question to help me with my understanding of fluids and centrifugal pumping.

 

Let say I have a centrifugal pump, with the dead head pressure of 3 bar(g). I have installed an isolation valve 2 meters downstream of the pump outlet. The piping between the pump and the valve is horizontal. The fluid being pumped is water, at STP.

 

The pump is on 100 % motor speed. I slowly close the isolation valve until fully closed.

 

Since the fluid is incompressible, and there is no head, what is the pressure in the pipe between the pump and the valve?

 

What I can't get my head around, is how can a stagnant, incompressible fluid be at 3 bar(g)?

 

 

[Saml]

It is the same than when you go 30 mts deep in water. You have water at 3 barg.

The only difference is that instead of gravity, the force is caused by centrifugal forces at the impeller.

[Mattgreen286]

Hi Saml,

 

Thank-you for joining my thread.

 

I fully agree that at a depth of 30 mts "pressure" is 3 bar(g), due to the hydrostatic pressure above it.

 

But in my example I have no hydrostatic pressure and, as you rightly say, only have the centrifugal forces at the impeller. So as my valve is closed, and water fills the pipe, how do centrifugal forces generate a stagnation pressure on an incompressible fluid?

 

Surely once the pipe is full with water, the pump can not push more water into the pipe (to increase the pressure).

 

I look forward to your's, and everyone's, input!

[Bobby Strain]

A centrifugal pump won't operate very long with a blocked discharge. Shaft deflection will quickly ruin bearings. That is why pumps are installed with a minimum flow recirculation. And you really have the wrong picture of the discharge pipe. It is usually always full of fluid. And the discharge pressure will be unstable since there is no radial flow in the impeller. Much of the pump head is developed in the discharge diffuser, which requires flow to convert kinetic energy to pressure.

 

Bobby

[Mattgreen286]

 

Hi Bobby, thank-you for joining in!

 

I agree fully that the discharge pipe will always be full of water, my question revolves around stagnant water (rather than flowing water)

 

I want to go deeper into two of your comments which interest me:

 

1) "discharge pressure will be unstable since there is no radial flow in the impeller"

 

Would you be able to expand on the term "unstable"? Are you saying it will fluctuate? If yes, could you explain why it fluctuates and within what kind of range (give my max dp is 3 bar(g))?

 

2) "Much of the pump head is developed in the discharge diffuser, which requires flow to convert kinetic energy to pressure."

 

I agree fully with this point, flow has a dynamic pressure, which has been transferred from the kinetic energy of the impeller. To bring this back to my original question, if there is no flow and therefore no dynamic pressure-is the water at a pressure? And if it is at a pressure, how does this pressure manifest in an incompressible fluid?

[sunny143]

Mattgreen:

 

When you close the outlet valve then the pump is running or stopped already?

[Mattgreen286]

Hi Sunny,

 

In this theoretical scenario, lets say that the pump is running while the valve is closed.

[rdcrags]

The spinning impeller is holding the pressure against the stagnant body of water.  If blocked in too long, the water in the pump will heat up, make bubbles, and damage the internals.