How Does Opening a Valve Affect the Force Due to Fluid Pressure?

[KingBongo]

I have been thinking about this a lot. It is about conservation of energy. A simplified analysis using Bernoulli's Principle for one-dimensional incompressible liquid fluids should be good enough to gain some understanding (at least for me). Let us also neglect gravitational effects.

Assume you have a pressurized pipe filled with liquid plugged by a spring loaded valve at one end. What is the (fluid) force on the valve?

I understand that when the valve is CLOSED the force would simply be F = P*A, where P is the STATIC pressure and A is the Area on the valve. But what if you OPEN the valve a little so that liquid begins to flow? Is the force on the valve determined by the STATIC pressure only, or is it determined by the TOTAL pressure, i.e. including the kinetic part of the fluid motion?

What is the Force? Please help.

 

[genghiskron]

Make sure you state exactly what you know from the problem. A problem like this can be solved many ways if you know most of the information, but a homework problem usually gives you the bare minimum and can only be solved one way.
If fluid is flowing through a control volume, a valve for example, the net resultant force on the valve is equal to the difference in momentum rates (mass flow rate times velocity) between the inflow and the outflow.

 

[KingBongo]

genghiskron:
Thank you! Well, I am actually not a student. I am a Doctor in control engineering. But this flowing liquidsthing is killing me. No wonder, since I only took a basic course like forever ago, :) Is it thermodynamics we are talking about here?

Are there any good books I could read about this? A little bit more advanced would be just fine.

Further questions:
I suspect the "momentum rates approach" can only be used if the valve is open, because else there would be no flow at all. Is it so?