Forcemain broken problem

[yyan]

An engineer was hired by a developer to design a sanitary pumping station for a new subdivision in 2000 with capacity of 32L/s. The forcemain was designed and installed by developer.
In 2002, this Engineer was hired by our company, later the developer wanted to upgrade the pumping station to 60L/s, so they gave this job to our company and the same engineer did the upgrade, the forcemain was no change.
Recently the forcemain was broken twice. Once is just before New Year, the second time is early March. Now the City Engineering Department thought it is the upgrade caused the forcemain broken. I was signed to review this work. The forcemain is 10" dia. PVC C906 DR26 and 1600m long, design velocity is 4ft/s. Forcemain is continuously up to the top of a hill with two 90 degree and one 135 degree direction change. I did the water hammer calculation and it showed the DR26 pipe is good enough to withstand the surge pressure. But I found the forcemain has no pressure control devices installed along its 1600m running. I asked the engineer who designed the pump and he thought because the forcemain is up all the way, the high point is at the discharge point of the forcemain, so air release is not necessary. I didn’t agree with him. The two broken points are 300m and 350m away the pumping station at the smooth running segment, no grading or direction change.
Hope somebody can give some idea. It will be really appreciated.

 

[bimr]

Are you able to explain the failure mechanism? For example, pipe rupture or joint slipped apart?

 

[Maury]

What do you mean by a "pressure control device". If you're talking about an air release valve, it shouldn't be necessary if there are no intermediate high points. An air release valve won't control pressure.

 

[yyan]

To bimr:
The forcemain broken is pipe rupture. The crack is around 2 metre long.

 

[bimr]

You have not said the elevations that you are pumping to and from.

If indeed, the force main goes up the hill at a constant slope, you probably do not need an air release. If you have a section of force main, that levels off as you go up the hill and then rises in slope, you may need an air release.

If you are pumping up to a significant elevation, you probably need a pressure control device on the pump. The pressure controller will prevent the water from coming back down the force main when the pump stops and causes a slamming of the lift station check valves. If indeed this is what is happening, you would get the highest water hammer pressures close to the lower elevations, near to the lift station.

Note that these are just general comments since you have not provided all of the details that are necessary to analyze this

 

[yyan]

To bimr:
Thanks for your reply.
The pumping station discharge elev. is 267.2m and forcemain discharge to a Manhole at top the hill and invert elevation is 308.9m. The forcemain running has three locations with steep up, around 25% slope in short distance. But the two broke points are over 60 meter away the sharp change points.
The pumping station has installed the air/vacuum valve and check valve.

 

[bimr]

yyan,

This is the type of pump control valve that I was referring to. This type valve will eliminate pressure surges on pump startup and shutdown and eliminate the slamming of check valves.

http://www.gaindustries.com/html/08a_electricchecksewage.htm

 

[dicksewerrat]

Was the outside of the pipe scarred? this may bea case of rough handling of the pipe prior to and during construction. Pressure test the main to about 1.5 times the working pressure and see what happens. PVC pipe should not leak at all.

 

[rconner]

It appears to me there may be some confusion with the sort of combined description "PVC C906" in the initial inquiry (I believe AWWA C906 is a standard for polyethylene plastic pipe, whereas C900 and C909 are AWWA standards for different types of PVC plastic pipe of this size) -- you may wish to clarify.
Randy Conner