Large Forcemain/Siphon Design Scenario

[davemtu]

A community I am working with, who treats their own sewage, has a very large, undersized, sewage interceptor running through it serving a greater metropolitan area. The community has a ton of extra capacity at their WWTP and there is opportunity to divert 14 cfs of normal flow, up to 47 cfs of wet weather flow, from the interceptor to the communities WWTP. This will alleviate the under sized interceptor, and provide some much needed extra income to the community to treat the sewage and get the plant running at closer to capacity.

The pipe invert at the potential tie-in at the interceptor is actually 23 feet higher than the tie-in at the WWTP, but due to a hill in between, strictly gravity sewer is not feasible.

The initial design has a diversion chamber at the interceptor (invert 896') with gravity flow to a pump station about 100 feet away, then 1100 feet of forcemain to the top of the hill,(elevation 950') then 2 miles of 36" gravity sewer to the WWTP(invert 873')

This design is sufficient, however my colleague believes that we can utilize a siphon to reduce pumping energy costs. Essentially a forcemain the entire length. We have done some basic calculations for velocity and flow, but before we go any further, is there any flaws to this alternative approach? I can provide support info if needed.

 

[dicksewerrat]

Did you look at HDD through the hill? Then it could be gravity all the way. Lower operating costs.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

The HDD through the hill may be attractive. You should make the slope greater than the minimum since it is difficult to maintain grade accuracy when directionally drilling.

It is a probably a mistake to utilize a siphon because of the range of operational flows that you will see. If you were pumping a constant flow, maybe it would sense to consider energy savings.

There are operational considerations for force mains. When your pumps start, you need 3.5 ft/sec velocity to resuspend solids. You can then operate at a continuous 2 ft/sec. The maximum flow is typically around 5 ft/sec. The need to operate within this flow range of 2 - 5 ft/sec adds some complexity to how one operates the force main.

You also have the dry weather/wet weather scenarios to consider. The force main would have to be sized for the wet weather scenario and then you would be operating at low velocities in dry weather. It is difficult to maintain the minimum velocities when you are operating during dry weather. There are also considerations when operations first occur because of low demand.

If you search this forum, there have been many posters that have presented force main operational problems with air entrainment, draining of the force main as it traverses downhill, etc.

 

[davemtu]

Thank you for your quick responses.

HDD through the hill for a full gravity option has been considered, however the size of the pipe, 36", and the subsurface conditions make this not feasible. It would be roughly 60 feet deep. Others have arrived on the conclusion of a forcemain up to the top of the hill, then dumping into a 36" gravity sewer for 2 miles. This project is estimated at 30 million, with energy costs for pumping at roughly $250,000 per year. They will be pumping up 60 feet of elevation head We are thinking that we can cut the energy costs by more than half with the siphon option, if it will work the way we are thinking.

The interceptor we plan on tying into has a very large flow, and we would be diverting 14 cfs (6300 gpm) at all times, this would be constant, and during wet weather events we could potentially divert up to 47 cfs. We are thinking about utilizing two 24-in force mains, one would be always full, siphoning the 14 cfs, and the other would kick on when the flow was higher. By raising the hydraulic gradient during wet weather with the second set of pumps only 20 feet, instead of the 60 feet for partial gravity option, we calculated achieving 40 cfs of flow. So the minimum flow for the primary FM will always be achieved.

With this option, we would be open cutting the force main and grade fluctuations will be at a minimum.

One thing is we have calculated velocities up to 9.5 ft/s of flow in the 24" force main during high flows, what are the problems associated with the high velocities? Is this when air gets into the pipe?

 

[bimr]

Don't understand how you arrived at the $250,000 pumping cost. Most of the pumping cost will be due to the 14 cfs since as you say, it will operate 24/7.

Pumping 14 cfs
8760 hrs per year
60 ft head
$0.07 per kw hr
50% pump efficiency
85% motor efficiency

results in a pumping cost of approximately $102,000 per year.

The economical velocities for pumping cross country are in the 3-5 ft/sec range. At 9 ft/sec, you will have approximately 3 times the head loss for pumping than you will have at 5 ft/sec.

At 9 ft/sec, the friction headloss calculates to 70-80 ft of headloss in the 24-Inch force main over the 2 miles and doesn't seem to make sense if you are attempting to save energy.

The problems with air is related to air binding, two phase flow, and system startup. At system startup, you have to purge the air in order to obtain full flow through the pipeline. Every time you run the storm water pumps, it will be a system startup. Pumping downhill will also require you to pump at a velocity of at least 4 ft/sec in order to push air bubbles downhill.

At the discharge end, you need some type of pressure sustaining valve to keep the force main from draining.

Consider the scenarios should it not "work the way we are thinking".

Have you considered just accepting the 14 cfs and not the storm water?

 

[davemtu]

I would like to point out that this is not a combined system, it is strictly sanitary, and the wet weather flow is a result of I&I. The interceptor collects from multiple communities over a very large area.

As far as only accepting 14 cfs, and none of the wet weather flow, that agreement is out of my reach, and there are other factors in play ultimately coming from the agreements between communities and the people in charge. This is a result of many years of leg work to get to this point of accepting flow from this interceptor.

Can I ask how you calculated the $102,000 in pumping costs? The $250,000 came from another source, and I just assumed it was correct. We have not fully delved into the energy costs/savings yet.

 

[bimr]

You can use one of the online calculators:

http://www.engineeringtoolbox.com/water-pumping-costs-d_1527.html