water hammer in sewer force main 2

[Chris73]

I am dealing with a recurring problem on a pressurized pvc sewer pipe, which breaks due to water hammering on a regular base. The high pressures/subpressures are caused by immediate pump starts/stops between pump and a 90' bend. Anybody experienced in how to solve such problem enduring and reasonable? Thanks!

 

[RWF7437]

Browse the threads in this and other forums and you'll find much useful information on water hammer. The solution to your particular problem may require a control valve on the discharge side of the pump so that the pump shuts down slowly. This is a common problem and suggests that the oiginal design did not, although it should have, include such valving.

Search on "water hammer".

 

[Nosey]

As RWF7437 has suggested some form of control valve in the line may resolve the issue. The pump may have a minimum flow requirement below which it cannot operate, so consideration should be given to this before installing a valve in the discharge of the pump. It depends upon the type and size of pump.

Changing the rundown time of the pump or closure rate of the downstream valve will improve the situation. A minimum flow recycle loop around the pump (with control valve) will allow the gradual ramp up or down of the pump without having to change the pump motor. However this will involve some additional control logic.

Also consider the installation of a surge vessel or pulsation damper i.e. a vessel pressurised with nitrogen (or similar inert gas) and a diaphragm isolating it from the process fluid. This will only really cope with small fluctuations in volume (how big is the pipe and how dramatic are the pressure fluctuations?).

I hope that this is of use to you.

 

[Gordonba]

Your water hammer problem can be reduced or eliminated by increasing the ramp up and down of the pump motor on start up and stop. Contact your motor supplier and he will fix you up for little cost

 

[stanier]

uPVC has a low fracture toughness and is not a good material where one expects waterhammer or fatigue from frequent starting.

A VSD can be fitted to the pump to control stopping and starting then you are only concerned with power failure. I fhtis is a regular occurrence you may need to consideration surge chamber. Sharing knowledge is a way to immortality

 

[Chris73]

stanier,
Do you mean VSD (variable speed drive) or VFD (variable frequency drive)? How does a VSD work? Without power?

 

[stanier]

VSD can be hydraulc, mechanical , DC or variable frequency. Howwever you achive speed control is of little consequence. It only protects you whilst you have power. That was my point. Then the criteria you need to concern yourself with in managing the risk is what is the likelihood of power failure? This depends upon the quality of supply. Will the pipe break with a single excursion? Or is it the result of fatigue? If the latter risk management exercise will throw up the likelihood of failure. If the former then yiou need a surge chamber, kinetic air valve or other. Whatever you do someone should undertake a formal waterhammer analysis or you just dont know how big the problem is.

Check out

http://www.ventomat.com

for the most modern technology in air valves. Sharing knowledge is a way to immortality

 

[CB2]

"VSD" is generic, "VFD" is specific to variable frequency drive "AC-DC-variable AC" for AC induction motors.

"Soft hydraulic start/stop" options:

Replace the pipe with stronger material.
Install motorized valve bypass system.*
Install a surge tank.
Use a "soft starter" aka, "solid state starter" to ramp start/stop. You are out of luck on power fail**.
Use a VFD to ramp the motor speed up /down (does not start a motor). You are out of luck on power fail**.

* A hydraulic actuator system with enengy storage system to handle a power failure

** Engineer a fly wheel to store energy in the event of a power failure.

The bottom line is your budget.

 

[chip1998]

What is your system static head, total head,pipe class pipe dia and flow: without these you get a lot of general solutions some which do not apply to your situation

 

[BRIS]

PVC pipe is not an ideal material for pumping mains. With PVC pipe you need to de-rate the pressure rating for fatigue and temperature (if the temperature is above 20 C). Waterhammer pressure plus working pressure must be within the de-rated pressure rating for the pipe.

You need a waterhammer analysis to ensure that the working pressure plus surge pressure is within the de-rated pressure rating. Soft start and stop may solve your problem.

Brian

 

[pcbaulch]

I am working on a similar problem, so maybe you can benefit from leveraging my solution. I have a naphtha system that unloads from ships, but towards the end of off-loading, flow is intermittent. The ship's pumps lose flow, and then subsequently regain flow, which causes hammering of my client's pumps. I am designing a holding tank between the ship and main pumps, which will allow for about 30-60 minutes of continuous flow time when flow is lost, thereby, protecting my client's valuable pumps.