False Static Pressure

[sjohns4]

I have a text book pump station and force main - pump station at the low point of the profile, the discharge of the FM is at the high point of the profile to a MH, and there are a few ARV's at the intermediate high points.

We started the station up & did a drawdown, the pumps hit a point on the curve to indicate the pumps are working right, but we were not pumping as much as we should have been by design.

The pressure gauge (with the pumps off) indicated the static pressure was around 20' higher than it should have been. I had the profile surveyed, which matched the County's GIS topo +- a couple feet.

I'm wondering if an air pocket could be somehow causing a false gauge reading? Possibly from a faulty ARV?

It's a submersible station which by design should flow at 1200 GPM (using C=130 which should be conservative for PVC) with one pump through a 16" FM, but we're only getting around 950 GPM.

Any words of wisdom???

Thanks,

Mike

 

[rconner]

Am just curious is the discharge end the high point of the line, and is the discharge end open or somehow valved off/blanked when static pressure reading is taken (I assume near low point)? Also, what is the assumed (for 1200 GPM flow expectation) as well as actual inside diameter of the pvc piping involved?

 

[cvg]

if your static is higher than you expected, then there may be a bust in the calcs somewhere. Also, did you double check that the ARV's are actually working? How did you measure the flow? Have you calibrated your flow meter? What about installing a flume at the discharge manhole to get an accurate flow number.

 

[sjohns4]

The discharge end is at an inside drop to a MH, we checked the only valves, which were at the PS. Data used for the Calcs: ID=15.5 for 16" DR-18 C-905.

We measured the flow by measuring the wet well drop over a given time & calculating the volume.

I'm not sure the ARV's are actually working...that's why I was wondering if an air pocket could cause a false static head reading? If it were more friction loss than expected I'd be pointing the finger real hard at an ARV.

I was just wanting to know if the gauge could lie about the true elevation distance before I send my surveyors back out.

 

[tjkall]

Could there be a stuck check valve? a spring check valve might hold pressure in the line
Tom

 

[cvg]

if you were measuring the wet well drop, did you compensate for inflow into the well?

 

[sjohns4]

Yes,

We measured inflow with the pumps off before and after the test.

Still doesnt explain the screwy gauge.

Thanks,

Mike

 

[rconner]

Has the gauge been checked/calibrated?