Pumping downhill - water hammer
Pumping downhill - water hammer
(OP)
Hi,
I have a somewhat interesting problem involving pumping water downhill. I have a system which has a static drop of about 6m from inlet to outlet, with a slight crest rise (2m) in the middle, hence the need to a pump system.
I've modeled a power trip simulation for water hammer. The pipe line is 3", with flow about 4 l/s. With very small inertia, the pump rapidly stops on a power trip. Not surprisingly, this results in close to full vacuum pressure and vapor formation.
My question is: Given I have vapor formation, and resultant rapid volume change on pressure rise and associated pressure spikes, should I be worried if the pipe is designed for vacuum and the modeled pressure spikes are well within pipe design?
Pipe is buried and well anchored.
Cheers.
I have a somewhat interesting problem involving pumping water downhill. I have a system which has a static drop of about 6m from inlet to outlet, with a slight crest rise (2m) in the middle, hence the need to a pump system.
I've modeled a power trip simulation for water hammer. The pipe line is 3", with flow about 4 l/s. With very small inertia, the pump rapidly stops on a power trip. Not surprisingly, this results in close to full vacuum pressure and vapor formation.
My question is: Given I have vapor formation, and resultant rapid volume change on pressure rise and associated pressure spikes, should I be worried if the pipe is designed for vacuum and the modeled pressure spikes are well within pipe design?
Pipe is buried and well anchored.
Cheers.





RE: Pumping downhill - water hammer
RE: Pumping downhill - water hammer
You're right. It seems the elevations have been changed. There is a very small pipe RISE (2m). Not withstanding, should vapor formation be avoided even if resulting surge is within design limits.
Cheers.
RE: Pumping downhill - water hammer
I just remembers something. I'm using HDPE pipe. Does the ductile nature of HDPE mean that it is effectively avoiding surge due to localised elastic expansion?
That would probably explain why I'm not seeing high surge pressures and tending to be worried about their resulting magnitude.
Cheers.
RE: Pumping downhill - water hammer
Pump trip in your pipe segment is going negative, not positive, so it can only go so low before it hits zero, then vapor forms. A positive pressure spike could result, if the vapor pocket collapses rapidly, but being such a small column with probably also little collapse velocity, again no pressure spike.
Siphoning (through the pump) will probably not happen with this small system where the flowing inertia of the downstream column is not great, will not flow far if it was and the suction head does not have enough pressure even with 0 psia in the discharge to keep the pump and motor turning through.
The HDPE will expand/contract more than steel pipe, so pressure spikes will be reduced, but I doubt it is the reason you don't see anything here. Your velocity is very slow and won't make much of a pressure spike even if it came to an instant stop.
RE: Pumping downhill - water hammer
Is by chance the line fitted with any valve for air intake to compensate hydraulic grade line drop below a certain threshold?
RE: Pumping downhill - water hammer
RE: Pumping downhill - water hammer
Dik
RE: Pumping downhill - water hammer
I was just thinking about air intake to compensate for possible line depressurization.
RE: Pumping downhill - water hammer
RE: Pumping downhill - water hammer
RE: Pumping downhill - water hammer
RE: Pumping downhill - water hammer
RE: Pumping downhill - water hammer
Vacuum could be a problem for any pipe, HDPE, buried, underwater, or above, but it seems like this pipe is already designed for a vacuum ... according to the original post.
RE: Pumping downhill - water hammer
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RE: Pumping downhill - water hammer
I doubt you'd ever see anywhere near 400 psi in a line normally running at 50 psig. I'd expect more like 75 psi as a reasonable maximum expectation.