Pumping Down Hill 1

[TOHSEngineer]

I'm designing a small pavilion site with a parking lot and bathroom in a remote part of my town. The site is located on top of a hill. At the bottom of the hill there is a stream and on the other side of the stream is the sewer manhole I will be tying into. I was planning on using a grinder pump at the pavilion to pump the sewage downhill, under the stream and back up a few feet to tie into the manhole.

I have designed pump stations before but they were always at a low point and pumped against gravity. What considerations do I need to consider when pumping downhill? The distance from the pavilion to the stream is approx 1300LF with an elevation difference of 80'. Attached is a rough sketch of the project.

Over the past few years I have done limited utility work and am feeling a bit rusty on this one. Thanks in advanvce for any words of wisdom.

 

[TOHSEngineer]

The sketch shows a gravity line for sewer. That was before it was determined impossible to gravity under the stream and tie into the manhole.

 

[civilman72]

Can you gravity feed to a manhole, just before the stream, then pump under stream and to the existing manhole? Seems like this could decrease the size of your pump and length of force main.

 

[TOHSEngineer]

I could, but an 8" gravity line would cost a lot more than a 2" FM. Plus the gravity line would require expensive manholes. The site is remote and the pipeline will be more like a long service since no one in the future will tie into it.

 

[BigInch]

Unless your flowrate is high you may be able to gravity flow downhill rather than "pump downhill". Just pump from your collection basin to the high point, then let it slide down the pipe on its own.

Nice picture, but no elevations and no profile drawing, so its hard to say what's possible or not from here. Is an inverted siphon impossible? Why?

http://virtualpipeline.spaces.live.com

 

[Ron]

Even with a grinder pump, I wouldn't use a 2" line...too much potential to clog.

Pump will be small considering you have essentially no suction head and a negative gravity head. Essentially your only head loss will be pipe and fittings.

 

[msquared48]

Any chance of using a siphon here under the stream. Once charged, if there is a head and the tailwater is always under water, it will flow.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigInch]

I was wondering if the solids would settle in the siphon invert and clog. You'd have to keep a velocity going there to keep it clean, is the only thing to worry about that solution. That might have been why it was deemed impossible.

Ron. Good point. 2" is pretty damned small for anything with solids in it.

http://virtualpipeline.spaces.live.com

 

[bimr]

Make sure you use an adquate pipe velocity.

There have been numerous posters on this forum inquiring about problems with force main. If the velocity is inadequate to push the air downstream, you will have trapped air in the force main. The trapped air will limit the capacity.

You need a velocity at least 4 ft/sec to force air down the force main. Since you are pumping downhill, the air will be attempting to flow backward.

Here are a couple of examples:
thread161-237329: sewer forced main problems

http://www.eng-tips.com/viewthread.cfm?qid=270920

 

[beej67]

By the time you've gone through the hassle of installing a pump and upgrading from a 2" line to something more reasonable (4"?) you might discover that a gravity line of slightly larger size (6"?) is cheaper anyway.

I'd go gravity to avoid the future maintenance issues, personally.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[BigInch]

... and the power cost. With the power required for the pump, payback for the 6"+ line is only a matter of how long it will take. My bet is that it won't be long.

http://virtualpipeline.spaces.live.com

 

[bimr]

I am going to disagree with the gravity recommendation.

It seems that you will have to pump it under the stream whether you have a gravity system or not. So the choice would be between putting a lift station at the bottom of the hill or the top. It would be easier to service the pump if it was installed at the building.

This looks like a good application for one of the low pressure grinder pumps systems. These systems use a 2" discharge force main.

 

[TOHSEngineer]

I don't think the syphon would work since there won't be a constant flow.

I'm thinking the gravity to a mini pumpstation would be best. Do any of you have any suggestions for a mini-pumpstation?

 

[TOHSEngineer]

Bimr, do you have any links for a system like that? Haven't heard of a low pressure grinder pump.

 

[bimr]

Dewberry presented a paper on a similar application:

http://www.dewberry.com/uploadedFil...Affluent Existing Neighborhood_permission.pdf

http://www.eone.com/sewer_systems/extreme/products/grinderpumps/index.htm

 

[beej67]

It seems that you will have to pump it under the stream whether you have a gravity system or not.

Depends on the invert of the receiving junction box, and whether an aerial stream crossing is allowed.

Again, there's no topo on the attached file, and there's no way to tell if a gravity sewer will work without a profile of the proposed alignment. He might be stuck with a force main no matter what.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[bimr]

"Quote: Depends on the invert of the receiving junction box, and whether an aerial stream crossing is allowed."

Do you really think that someone would construct an aerial sewer crossing for a 2" force main considering that the crossing will be located adjacent to a park?

 

[beej67]

Do you really think that someone would construct an aerial sewer crossing for a 2" force main considering that the crossing will be located adjacent to a park?

Of course I don't. If you're going to pump the whole way anyway, then there's no point in doing an aerial crossing. Go ahead and bury the thing.

In fact, if *I* were going with a "2 inch force main," I certainly wouldn't jack and bore under the road to catch that manhole, I'd turn under the road through the culvert and out the other side, bolting it to the underside of the culvert. Then run along the top face of the culvert and follow the northern right of way the rest of the stretch. Easier maintenance, much cheaper to construct, etc.

But that presumes I was going to go with a "2 inch force main" at all. My recommendation was and is to figure out what the grades are like, hoping that a (larger than 2 inch) gravity system is an option. Then make a more informed decision at the pricing phase, by pricing both options out and comparing life cycle costs for your client. You wouldn't need detailed topo, just maybe 3 spots and a site visit.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[bimr]

Strapping it to a bridge might work in the south, but probably would not work in the north.

 

[ClarkBart]

Hi:

The situation you describe concerning pumping downhill is a typical situation is variable grade pressure sewers. There are a number of manufacturers who provide prefabricated pump stations that handle this situation. E-one is one of the most widely known.

There has been a lot of discussion concerning the diameter of the force main. The 2" size is more than adequate when using a a small grinder pump. Using a 4" line will require larger pumps to achieve a flow rate with the appropriate scour velocity. This type of pumping arrangement is very inefficient for the size of your facility and will be an energy hog, be expensive to install because of the large pumps and its associated hardware, large wet well requirements, etc.

Before you make up your mind about the pumps, check out the E-one website, Orenco, or, the Small flows clearing house to get further information about variable grade pressure sewers. Your first instincts about grinder pump and small diameter force main seem to be right on target.