pump trip surge loads

[laura80]

Hello

I am concerned about the following issue.

I easily undertand when a valve closes the loads that this produce in pipe. The first loads begins just against the valve that closes and it is propagated upstream the valve, with the wave of pressure.

But I do not undertand when a pump trips, the pressure falls near the pump (in the discharge), and a negative pressure wave is propagated downstream the pump. But, what about the forces? What are the direction and the aplication point for these loads?

Is the same compression wave that is produced when the valve closes? I don't think so.

Do you now any book or paper where the pump trip with the aplication of the loads is well explained? I always find the explanation of water hammer with a closure of the valve, but never explain the effects on the pipe when a pump trip, or downstream the valve that closes.

If I want to perform a static analysis of the pipe, and input these loads, and need to Kown in which direction and point of aplication input the loads.

Thank you in advanced

 

[BigInch]

You know what a wave looks like, right?
Take a "sine wave".
What you see in front of a valve is the pressure building, just as a sine wave builds y = sin( [θ] ) starting from [θ]=0
What you see downstream of the pump is the trough of a similar wave. y = sin( [θ] ), but starting from [θ]= [π]
If the discharge pressure when the pump trips is less than 1/2 the amplitude of the wave, the pressure will fall to 0, or rather the fluid's vapor pressure, as a minimum. Just like the suction effect you might have noticed as you're standing on a platform at a train station as the end of the express train wizzes by.
If you wait long enough, the wave created by the fluid coming to rest against the downstream valve will be reflected back to the pump and you will see the pressure build again as that wave arrives at the pump.


What would you be doing, if you knew that you could not fail?

 

[bimr]

The starting and stopping of a pump is more or less the same scenario as a valve closure. The difference being the time of closure or shutoff.

In general, any disturbance in the water generated during a change in mean flow conditions will initiate a sequence of transient pressures ~waves! in the water distribution system. Disturbances will normally originate from changes or actions that affect hydraulic devices or boundary conditions. Typical events that require transient considerations include:
1. pump startup or shutdown;
2. valve opening or closing ~variation in cross-sectional flow
area!;
3. changes in boundary pressures ~e.g., losing overhead storage
tank, adjustments in the water level at reservoirs, pressure
changes in tanks, etc.!;
4. rapid changes in demand conditions ~e.g., hydrant flushing!;
5. changes in transmission conditions ~e.g., main break or line
freezing!;
6. pipe filling or draining—air release from pipes; and
7. check valve or regulator valve action.

Rough calculations can be made either using the Joukowsky equation or more accurate ones using the method of characteristics.

http://help.intellisitesuite.com/Hydrocarbon/papers/6020.pdf

The wikipedia article has references for you:

http://en.wikipedia.org/wiki/Water_hammer

Waterhammer Analysis by Don Wood

http://www.kypipe.com/new_stuff/SimonFreese_Journal_Article.pdf

 

[stanier]

AFT Impulse allows you to link the transient effects of pump trip or valve closure to a Caesar II stress input file. Using CII you can analyse the dynamic loads on the system.

http://www.aft.com

allows you to download a demo copy. They also have a technical paper on the topic.

Pump trip can be synonamous with "Thomas the Tank" engine pushing troublesome trucks up an incline . When Thomas stalls from lack of steam or wheeel slip the trucks keep going for while then run back down the hill and crash into Thomas!

References are Fluid Transients in Pipeline Systems ARD Thorley
Pressure Transients in Water Engineering Ellis


"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[stanier]

http://www.aft.com/training/white-papers

is the paper on evaluating loads.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[BigInch]

fluid Pressure Forces exerted on the pump during a trip, neglecting pipe pressure expansion force, is the change in pressure times cross-sectional area of flow.

-1000 psig * 56.25 inch^2 = -56250 lbs

What would you be doing, if you knew that you could not fail?