Sewer Lift Stations and Force Mains 1

[GoldDredger]

I have a proposed 10 acre commercial development that will require a lift station and force main. The nearest town gravity main is about a mile away.

However, near the property is an existing 4" force main to the gravity main, which is fed by a town lift station serving a residential neighborhood.

Rather than construct a separate and parallel force main from the commercial development, I thought maybe we could tie into the existing 4-inch force main.

A few concerns come to mind. First a shared force main could run into problems if both lift stations are operating at the same time. Differing head pressures, capacity, etc. And of course, when one is operating and the other is not (perhaps a check valve needed to prevent sewer going up wrong main)

Has anyone had any experience with this situation, is it even feasible?

 

[jgailla]

It may be feasible. I have experienced this issue in different forms on most of the sewer projects I have done.
You will need the system curve from the existing pump and system to analyze what happens when you add your pump in.
You will need a check valve at your pump station for when their pump is on. If they have no existing check valve, you will need to add one at their system, which could cause trouble.
It is likely that their pump will be unable to discharge at design flow when your pump is hooked up to the system.
You could consider providing them with a new, larger, pump. It depends on expense and your relationship with the existing system owner.
It is possible that the 4" line is undersized for both flows. If this happens, you'll have to lay another line anyway. In this case, a cost sharing contract can be drawn up with the town if future development is expected to put in a larger common line and abandon the existing.
The town engineer should help you determine what is in the best interests of the community and the various stakeholders.

 

[bimr]

It may be feasible as jgailla posted but you have not presented enough information to develop an opinion.

Allowing two separate pump stations to tie into a common sanitary forcemain may have the potential for problems. 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.

Centrifugal pumps are not positive displacement machines. The flow from the centrifugal pumps varies with the back pressure that the centrifugal pump is pumping against. When two different mismatched are pumping against each other, the stronger pump will be the one that passes more flow. How would you propose to maintain flow rates in the force main when you have independent pump stations turning on and off?


You also have the dry weather/wet weather scenarios to consider. There may be an additional issue having to do with peak flows from storm water. 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.

Do you know if the rxisting lift station is operating at maximum capacity? You may be required to prepare a report to evaluate the existing capacity of the forcemain and the number of users that discharge into the lift station collection system.

These are some of the issues that need to be addressed.

 

[BenJohnson]

As bimr says, you need to check out a number of factors. A 4" force main is very common and normally the minimum allowable diameter. So, the other pump station may be using the pipeline perhaps 10% of the time. In that case, you have lots of extra capacity. If the other pump station is pumping 50% of the time, then you will want to examine as-built plans for the other pump station. You must be sure that your pump will not interfere with the other pump. To do that, you must understand exactly how he other pump station works - from pump curves to design flow and operation times, etc.

If you understand how the other pump station works, then I am pretty sure you can design up your system to complement, not conflicts with the other pump. You will undoubtedly need some telemetry between pump stations so the can say, "I'm turning on, I'm turning off".

One word of caution: if things start to get complex because you are need everything to run perfectly for both pump stations to work properly, then put in the parallel forcemain. Don't try to shoehorn in a tight system just to save a little money on a forcemain. That is a recipe for problems in the future.

 

[chicopee]

So, in essence it becomes a question of either concurent or noconcurent loading if you were to use the elements of the existing installation. That's what must be determined. Of course,you can always review the original drawing details of the existing installation to see if the designer made provisions for future expansions.
Am I understanding that this is strictly for sewage and not combined with storm flows?

 

[stanier]

You would also need to consider a waterhammer analysis of the combined operation. A surge at your pump station could impact the other pump station, thrust blocks and pipeline.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

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

 

[jgailla]

It is a bad idea to assume that the lift stations will not run at the same time.

 

[stanier]

Also that they dont both trip att he same time and create a high surge pressure. Any revised system needs to be engineered to take into account all possible operating scenarios.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

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

 

[BenJohnson]

One idea is to put in some simple instrumentation at your pump station that measures the ambient pressure in the force main. If pressure in the force main increases above normal, then the other pump station must be pumping. In this case, your pump station must wait until the pressure drops back to ambient before it can begin pumping. Your wet well would need to be sized to accommodate this temporary delay.

 

[fel3]

Alternatively, instead of pumping in parallel (with all its attendant issues) you could pump in series with a wetwell in between to simplify operation. You would still need to analyze the existing lift station as described above. The existing pumps may still be OK, but it's also possible that upsizing is warranted.
-- If there's enough wetwell capacity in the existing lift station, you could discharge directly into the existing wet well.
-- If there isn't enough wetwell capacity in the existing lift station, you would need to construct a new adjoining wetwell and connect the two with a big pipe.

I've done both: pumping in parallel and pumping in series. The choice depends on the results of your analysess, both hydraulic and cost.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[dicksewerrat]

Ask the owner of the pump station if you can tie in by a gravity line? Offer to rebiuld his wet well to accommodate all the flow.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com