Force main Modification

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.

From the originalcheckvalve
.