Check valve u/s Air Release Valve

[SMIAH]

We have an old sewer force main (6 inches CPV). There is an high point/manhole with a check valve placed upstream the air release valve.
It doesn't seem to be working.

I would like to know what is advantage of placing a check valve before the air release valve? Should we replace both or just re-install the air release valve(we'd like a plug valve there to isolate the force main).
There's a head differential of about 9' between the high point and the pump and a length of about 1900 ft.

 

[bimr]

Perhaps the air release valve was leaking and the person was trying to prevent air from being pulled back into the force main.

Air release valves are not a good idea on force mains as they are susceptible to plugging. It does not appear that there is a significant elevation difference in your application to warrant one. You should just install a manual air release valve to let the air out on the initial system startup.

See the attached recommendation from Pumping Station Design.

If you use an air release valve, Vent-o-mat makes a good one.