Reactive Force for a Piping Discharge
Reactive Force for a Piping Discharge
(OP)
Friends,
What is the formula for calculating the reactive force from a liquid discharge? I am expecting a function of density, flow, velocity squared, etc and am asking for some quick help.
This is the story: I come back from holiday to find a piping wreck at one of our cooling towers. The story I get is that a pump was started using a "minimum flow line" back to the tower. This is a 12" line without any restriction running from a pump discharge (rated at 3050 cubic meters/minute) and about 4 or 5 meters up the side of the tower. It has two horizontal bends at the top that seem to match a reactive twisting action of the piping wreckage. The story that I got from the RCA team was that some sort of a hammer occured, but I think the answer is much simpler. The team did not consider a reactive force acting on a long moment arm. I made an inquiry to the manager as to whether the piping anchor at the top of the line might have failed, and was surprised to be told that the piping was not anchored. There was supposed to be a U-bolt anchor, but apparently it was never installed. This makes me even more suspicious of a reactive force causing the piping wreck. Anyway, this is the story, so any help with a formula or comment is appreciated. If I calculate the force, I can get a mechanical engineer to comment on whether my theory is plausible.
best wishes always,
sshep
What is the formula for calculating the reactive force from a liquid discharge? I am expecting a function of density, flow, velocity squared, etc and am asking for some quick help.
This is the story: I come back from holiday to find a piping wreck at one of our cooling towers. The story I get is that a pump was started using a "minimum flow line" back to the tower. This is a 12" line without any restriction running from a pump discharge (rated at 3050 cubic meters/minute) and about 4 or 5 meters up the side of the tower. It has two horizontal bends at the top that seem to match a reactive twisting action of the piping wreckage. The story that I got from the RCA team was that some sort of a hammer occured, but I think the answer is much simpler. The team did not consider a reactive force acting on a long moment arm. I made an inquiry to the manager as to whether the piping anchor at the top of the line might have failed, and was surprised to be told that the piping was not anchored. There was supposed to be a U-bolt anchor, but apparently it was never installed. This makes me even more suspicious of a reactive force causing the piping wreck. Anyway, this is the story, so any help with a formula or comment is appreciated. If I calculate the force, I can get a mechanical engineer to comment on whether my theory is plausible.
best wishes always,
sshep





RE: Reactive Force for a Piping Discharge
F=force m=mass a=acceleration d=derivative(change in)
ma= momentum
At elbows force is caused by acceleration.
At nozzles (jets) force is due to the change in momentum.
RE: Reactive Force for a Piping Discharge
Per API520 relief pipe reaction force calculation:
F=W^2/(12.96*A)*(x/rhog+(1-x)/rhol)+A/1000*(Pe-Pa)
where
W=flow, kg/hr
A= pipe exit area, mm^2
x= vapor mass frac
rhog, rhol= density gas and liquid, kg/m^3
Pe= pressure at exit, kPaa
Pa= ambient pressure, kPaa
So it would be a simple plug and chug, except for the exit pressure (Pe). How is this calculated?
You would think I should know that sort of detail after 25years, except that in most problems I am solving for flow using an exit pressure from the pipe equal to atmospheric pressure- i.e. Pe=Pa and the only residual energy is kinetic. The only cases where it has ever come up for me is when the flow is at a sonic limit (choked flow), and even then I am usually left wondering what exit pressure is reasonable. Is that what the (Pe-Pa) term means, and if so how do I calculate Pe? Any help is appreciated.
best wishes,
sshep
RE: Reactive Force for a Piping Discharge
Good luck,
Latexman
RE: Reactive Force for a Piping Discharge
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Reactive Force for a Piping Discharge
Katmar, I am ending my day and don't normally work in metric units. How do you convert from velocity to kPa- i.e. from velocity head to pressure?
best wishes,
Sean
RE: Reactive Force for a Piping Discharge
with v in m/s and g = 9.81 m/s^2. Then convert from metres of H20 to kPa. You can use this valid free converter
http://www.katmarsoftware.com/uconeer.htm
RE: Reactive Force for a Piping Discharge
RE: Reactive Force for a Piping Discharge
Take a look at your operations manual