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FM design question

FM design question

(OP)
A client has asked us to look at modifying an existing submersible pump station to have the FM come out of the top of the wetwell (above ground piping with check, plug, and ARV installed) and then dive back below grade. The existing setup has the 4" FM going through a valve vault. The 4" FM ties into another 6" FM and the elevation stays pretty constant (flat) until it dumps into an existing MH.

Bringing the 4" FM above ground puts the high point of the FM (+23') at the PS. The discharge at the MH is (+14'). A couple of questions come to mind:

  1. Should we be concerned that the ARV is at the high point right at the PS?
  2. Is siphoning a concern? Should the ARV be a combination valve?
  3. Other concerns?
Thanks in advance for any input/suggestions!

RE: FM design question

One concern would be the potential for freezing, which you have not mentioned.

1.Should we be concerned that the ARV is at the high point right at the PS?

The ARV is probably not needed in the first place, You can bleed the air out during the initial startup. After startup, you should ensure that you adequate velocity in the FM to push the air downstream. ARV are a maintenance problem and should be avoided on force mains.

2.Is siphoning a concern? Should the ARV be a combination valve?

How is possible to siphon when you are pumping into a forcemain?

A combination ARV will put air into the force main and cause problems.

Another concern should be the possiblity that the 6-Inch forcemain may force the wastewater backward into your lift station. A check valve is not a reliable stop. Consider the use of an automatic valve that will close when the pump staton pump(s) deenergize.

RE: FM design question

(OP)
Thanks for the reply bimr!

There is really no potential for freezing where we are. The above ground setup copies other municipalities in the area, but I am also questioning the wisdom of the ARV.

My concern was that a siphon could occur when the pumps shutoff.

The discharge of the 6" forcemain is into a receiving manhole. We'll have check valves at the PS and also at the interconnection of the FMs.


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