Pumping downhill - water hammer

[rgvrider]

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.

 

[Compositepro]

If its a centrifugal pump the flow will not stop rapidly because the pump power is lost. You will also still have flow due to the siphon effect unless you have something to prevent this.

 

[rgvrider]

UPDATE:

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.

 

[rgvrider]

UPDATE 2:

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.

 

[BigInch]

You're not seeing surge pressures because 4 l/s in a 3" line is well within acceptable velocity limits.

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.

 

[ione]

HDPE can mitigate possible surge effects as the low elastic modulus implies a lower wave speed in the system.
Is by chance the line fitted with any valve for air intake to compensate hydraulic grade line drop below a certain threshold?

 

[BigInch]

ione, what are you getting at with this. I don't understand. "Is by chance the line fitted with any valve for air intake to compensate hydraulic grade line drop below a certain threshold?"

 

[dik]

any chance of cavitation?

Dik

 

[ione]

BigInch,
I was just thinking about air intake to compensate for possible line depressurization.

 

[BigInch]

Oh, OK. Its already apparently designed for a vacuum, so I didn't see the need there. Then you'd just be adding air supposedly for eliminating high pressure on vapor column recombination, but that's kind of trading one problem for another. Just didn't see where it would be worth it for such a small system. I would think it would be better to just design for any high pressure as might be required and be done with it.

 

[ione]

I though depressurization could be an issue for a buried HDPE pipe, but I’m probably wrong.

 

[BigInch]

By "depressurization" do you mean vacuum?

 

[ione]

Yeah that's what I meant (lack of language command sometimes produces odd term)

 

[BigInch]

Don't I know it.

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.

 

[unclesyd]

if you are still worried after the response posted above you should checkout this paper from Sioux Chief on water Hammer.

http://www.siouxchief.com/Resource_/ProductMedia/133/Water Hammer FAQ 4-04.pdf

 

[BigInch]

That paper makes some pretty exadurated pressure claims.
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.