force sewer line breaks recurrently 5

[civiliankdg]

The system is simple,one centrifugal pump station(2400gpm thru 4800gpm) and 2km foce main line(165psi rating pvc) which climbs 65m elevation and discharges at the top. The pump turns on/off more than 100 times a day.
And after 7 month operation, the force sewer line starts breaking recurrently in a certain area(within 150m), ring type crack at hub joint or longitudinal pipe spilt.
I suspected the surge pressure, but the actual pressure reading in the leaking area is that the static pressure during pump off is 92psi and the pressure fluctuation at pump stop is 40psi thru 135 psi due to the surge which is less than the pipe rating. There is a surge relief valve in pump room set-up at 128psi and it works at every pump stop.
What is the cause of these pipe failures? and what is the solution?

 

[civilperson]

The vertical change in grade should cause a 92 psi increase in relative pressure. Thus the static reading. I am guessing a fatigue failure of the PVC pipe due to cyclic loads induced by the pump.
A cheap fix to try is concrete or flowable fill embedment to restrain the expansion of the PVC. Backfill quality directly influences the strength of the pipe system. Next fix to try is stronger pipe, steel or Ductile iron.

 

[rconner]

More than a quarter century ago I think some (pre)disposition towards problems in pumped sewage systems was discussed in a paper "PVC Pipe Performance" in the (I think maybe first) International Conference on Underground Plastic Pipe sponsored by ASCE in New Orleans, LA in 1981. In laboratory examinations at the time of pipe samples from 332 pvc pipe failures in the UK, the authors reported the three highest categories were "laying and jointing faults", "manufacturing faults", and "operational factors". Of the "operational factors", the most prevalent sub-cause was "surge fatigue". A quote from that paper, "The most important operational factor found was fatigue due to surge pressure fluctuation. This accounted for 5.7% of all failures but was found to be particularly associated with pumped sewage systems and accounted for a much higher percentage of failures in this type of application."
While I have much more recently seen a sort of claim or inference that US-made pvc pipe is somehow better than that made abroad, some at least similar-sounding issues were apparently discussed in a subsequent report from Sandia National Labs for the United States Department of Energy in 1984, entitled “PVC (Polyvinylchloride) Pipe Reliability and Failure Modes”. I think if you do a search of these forum archives with the “Advanced Search” feature you will also find that pvc pipe is still being sold for these conditions, and many others have unfortunately been learning these same lessons in the many years since. I am additionally very skeptical for many reasons that a particular “fix” proposed, i.e. to make this pipe work by somehow digging up this entire plastic pipeline and encasing it with stiffer flowable fill, will work!

 

[redbridge]

Are there check valves at the pump station? Do the pumps have vfds? What size is the force main? Sounds like water hammer.

 

[SteveWag]

I agree with Lburg, the kind of failures you describe are do to pressure exceeding the burst capacity of the pipe. Most large clear water pump facilites have cleverly designed systems to limit this kind of thing, while most sewage facilities don't. Most common cause of hammer when stopping is slow closing check valves. If you have external arm swing checks, put a strong spring on the arm, tending to force the valve shut. Anything to speed the closing should help.

If you have ball checks, good luck.

VFDs are nice, but power failures do happen. The piping system must be designed to stand alone with out relying on power to work.

Steve Wagner

 

[civiliankdg]

Let me supplement to explain the situation.
There is a check valve at pump station.
The pump doesn't have VFDs. Just on & off.
And the pipe is 16" DR25 PVC pipe.

 

[bimr]

You probably have the wrong type of check valve or an improperly adjusted check valve.

The check valve slams when the check valve is caught in a partially open position and a flow reversal occurs. It is essential that the check valve closes before a reversal occurs (such as during the pump shut down).

You need to install a check valve with a spring or lever and weight that will assist the check valve in closing quickly.

 

[CivEnvEng98]

I know of a few utilities that have abandoned weighted check valves for all non-plant pumping stations and went with the spring exclusively.

I'm surprised the Utility builds stations of this size without a VFD or RVS.

Please let us know what you ended up doing and how it worked out.