Low Pressure Sewer Question

[EngWade]

This has probably been stated before, but for the life of me I cannot find any answers online anywhere. But could someone give me an idea what the operating pressures are for a low pressure sewer? Or maybe there is no "rule of thumb" with these? I'm being challenged to design a force main to connect into a municipal LPS which I cannot get any information on other than it's size (6") and that it is a LPS. Is there a typical operating pressure for an LPS? Say, 20psi? 30psi?

Any input is greatly appreciated.

Thanks.

 

[bimr]

It will depend somewhat on which pump manufacturer is used.

In operation, the E-One system grinder pump station will handle sewage and many items that should not, but often do, appear in domestic wastewater. For example, plastic, wood, rubber and light metal objects can be routinely handled without jamming the grinder or clogging the pump or piping system. The grinder pump will discharge this slurry at a maximum rate of 15 gpm or 11 gpm at a pressure of 40 psig. Transporting sewage several thousand feet to a discharge point at a higher elevation is possible as long as the sum of the static and friction losses does not exceed design limits of 185 feet TDH (80 psig).

http://www.eone.com/sewer_systems/extreme/products/performance_curve.htm

http://www.eone.com/sewer_systems/extreme/products/lpsdesign/#advantages

 

[EngWade]

Thanks for the reply Bimr.

So you are saying then, at least according to E-One, a pressure sewer can be considered a LPS up to 80psig? That doesn't seem like a low pressure to me at all, considering water mains are typically held to a maximum pressure around 80-100psig.

I think my design is adequate, as I've calculated being able to effectively pump up to 60psi, and the topography that the municipal force main follows essentially goes down hill. But it's nerve racking not being able to know what the pressures I'm tieing into are.

 

[bimr]

Generally, the LPS system will operate at 40 psi.

The system supplier designs the system with an estimate of the total number of pumps that will be discharging into the system at any one time. To eliminate the possiblity of a pump operating at a slow rate at such high flow times, the pumps are semi-positive displacement and are capable of pumping at the higher discharge pressure.

http://lacanadaflintridge.com/city/public_works/sewer_docs/LPS Technology 101.pdf

 

[davefitz]

Its been a while since I've reviewed the E-one system handboook, but the individual system pump performance curve needs to match the municnipal system max system pressure , to be obtained from the municipality- and it likely was designed using the E-one handbook. Other issues may be:
-pump dead head pressure may not exceed design pressure of the LPS piping at 80 F
-individual site to have an outlet check valve
-individual site pump inlet to have a screen to avoid ingesting plastic toiletries
-municipal downhill flowing piping to have gas voiding vents spaced at ( 1500 ft) , not required on uphill legs.