The lift station consists of a 8' x 40' Wet well containing two Fairbanks Morse 4" 5435 MV w/ 12.6 Impellers discharging through two 6" American Flow Control 50-SC Check Valves to a 10" C-900 Force main.There are 5 separate ARV's located along the 5,600 ft force main at various locations. The over all elevation change from the pump station discharge to the discharge into the gravity system is around 113 feet.   


  Good Morning thinkers :) I wanted to give you some of the technical blah blah before I got started with the question.  I am currently working on a project that has experienced 4 separate force main failures. The failures being broken 10" pipe. The fractures have been along the pipe and never at the bell. (That's right not the bell) The splits are always located on the bottom of the pipe. The pipe that was removed was sent to private labs to test and the pipe is fine. The second thing that was done was the 3 ARV's located closest to the pump where replaced do to excessive debris. (Municipality maintained) (sheesh).

A monitor was placed at one of the ARV manholes and we sat on it for one month. The measurements never got over 85 PSI. The employer is wanting to have (GA) soft close check valves installed in place of the AFC 50-SC's and two additional inline check valves (Also GA. The pumps will be pulled and new 13.74 Impellers will be fitted. I personally feel like the Inline check valves are over kill because I feel the issue was located in the poor maintenance of the ARV's. How ever. My question is do you think that the addition of the inline check valves coupled with the head pressure will pose a problem for the pumps even with the New Impeller size.

You didn't mention the working pressure allowed on the pipe.  Long splitting would tend to point to radial pressure, rather than longitudinal stress being the cause, which I would presume would manifest as joint separation.

If that's true, then one might also suspect high transient pressure spikes too fast for an unspecialized pressure monitor to catch.  Well, that's my best guess and I'll stick to it until proven otherwise.  I would point to a cause by air in the line, esp. given you've had
ARV problems, the air compressing and driving water columns rapidly when demand is quickly increased downstream, perhaps enough to cause high velocity head pressures when the rapidly moving water colums eventually come to a stop again.

Can you check to see if the breaks were in conjunction with a pump start or stop, or a major valve opening or closing quickly, each of which would also indicate transient pressure causes, but not from air entrainment, just fast velocity changes in the piping.  

I would tend to think that inline pressure-balancing check valves would be more effective on pump starts and valve closings rather than compressed air driven velocity surges, which in that case, you should think about doing more maintaining on those ARVs.  

Regarding "Isn't starting and stopping against a slowly closing/opening shutoff valve much more reliable and cost efficient than taking a 5% power cut over all operating ranges caused by use of a VFD?"

If the piping is not designed for the anticipated operating pressures, he may be forced to use a VFD in order to control the pressure spikes

A force main is normally pumping sewage not clean water. It will be difficult to find a slowly closing/opening shutoff valve that is suitable for such sewage service. Such a vlave is normally used on clean water service without problems.

I would expect that this installation also has a standby power system.

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