Pump down hill 1

[NAH229]

thread161-24447
Hi,

I have designed a pump station pumping down hill similar to the above thread but it's smaller. What I found is there are two major system curves for this kind of PS:

1. Initial system curve using static head to highest intermediate point - positive static head.

2. Operating system curve: static head is head between wetwell reservoire elevation and discharge elevation - negative (assume it's steady state hydraulic condition).

It would be risky to select pump duty point using negative static head because the selected pump may fail to prime or pump head may exceed shutoff head, best selection is static head condition #1.

If you try to use WaterCAD or EPANET to solve this problem, the software will select condition #2 as default which is wrong in some cases. This problem should be calculated by traditional way: paper, pencil and calculator.

There two main issues pumping down hill:

1. Siphon after pump off. This can be controled by a VAC set at highest point should be near pump station. I would not worry about positive surge pressure but vacuum pressure that could collapse the forcemain downstream.

2. During pump running cycle: Hydraulic condition may not steady state because of collum separation toward downstream discharge. negative pressure near downstream discharge increases flow velocity downstream - corrosive conditon.

I think for a large flow rate, downstream piping should be reinforced to work with both corrosive and vacuum condition.

This kind of PS very common in mountainous areas where it's impossible or risky to build gravity sewer system. I'd appreciate any new experienced feeds for this matter, I'd like to learn more about it.

Thanks
NAH

PS. I appology to reopen this thread. Honestly, I'm still crazy with this problem - not much studies about this matter.

 

[stanier]

Such systems have been designed as a pumped system to a high point manhole and then a gravity main from there. Alternatively a pressure sustaining valve could be used at the end of the pipeline to keep the HGL above elevation.

Such a valve could be a Red pinch valve or Dezurik eccentric plug valve both of which have been used to throttle sewage.

The danger in allowing air into such systems is that when they egress through air release valves they carry odours. Also such air pockets can hydraulically bind such systems.

Sewage rising mains may not run constantly as the diurnal peaks cause pumps to turn off and on. When the pump starts it may be pumping into a partially empty line and hence the friction head is reduced. The pump can run down its curve and trip on overload. The pump may not run long enough to refill the line before the sump low level is reached. Such a system is very difficult to control.

If you use a PSV at the end of the line it is easy to control the system; model it in Epanet and avoid the risks.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[davemtu]

Hi, I am currently working on something very similar to the thread you referenced, and the parameters are actually eerily similar to that thread. We have a community that treats its own sewage, and their WWTP is only at about 1/3 of capacity, there is potential to divert flow from a nearby county sewer interceptor to alleviate the over-capacity interceptor and add to the under capacity WWTP. We are looking at 9-25 MG

The invert at the interceptor is actually 23 feet higher than the proposed exit at the WWTP, but there is a 50 foot hill in between and poor subsurface conditions, ruling out gravity the whole way. Initial design is a 1100 foot force main to the top of the hill, then 2 miles of gravity to the WWTP. My colleague is thinking that we can do force main the whole way, turning it essentially into a siphon to conserve energy. We are thinking about using two 24-inch force mains, one would be constant flow/siphon of 14 cfs, the other would kick on during wet weather flow. Are there any flaws to this thinking, what are some of the major considerations?

 

[bimr]

Excellent points present by Stanier.

See the articles about air in force mains.