Multiple Sanitary Force Main Connections 3

[GoldDredger]

Has anyone heard of connecting a new sanitary force main into an existing sanitary force main?

I've got a residential development that will need to use a force main and lift station to pump some distance to a treatment plant or gravity main.

There is a closer connection to an existing force main, which also connects to said treatment plant.

Connecting to the existing force main would be a shorter distance, but I can think of a number of problems in making such a connection.

Some of them being backflow prevent needed on both lines at connection (could a check valve work on fluid with solids?). Or if one pressure was higher than the other (both systems on simultaneously) one charging the other, or preventing flow into one line (backups, blown pumps, blown lines). Or the combined outflow line being undersized for the new connections.

Any opinions?

 

[bimr]

This option would have to be evaluated on a case by case basis since most pumping systems have unique parameters. You can not make any generalized comparisons for all pumping and force mains.

My opinion is that it would have to be a significant advantage to consider combining the force mains. It will probably be more practical to combine smaller force mains than larger systems.

 

[jgailla]

Connecting a new force main into an existing force main is done frequently in my area (North Florida). You should have a check valve at the lift station itself, which takes care of the backflow issue. I have not seen a check valve put farther downstream. The maximum distance I can remember going for this scenario is about a mile. This has typically been a low rate connection (500 to 1000 gpm) into a large rate manifold (2500 to 5000 gpm).
The issue comes if the operator of the system at the connection point does not have his system modeled. The operator should know the manifold pressure that your pump will have to push against to get your sewage into the main line. There are a few proprietary modeling systems that utilities normally use (I can't think of the names right now). WaterCAD can also be used to model the forced main system if you know the parameters. If you have a system operator that does not know his system, you may have to model it yourself. You would need the pump information of all the lift stations and the sewage generation characteristics of the area served.
The connection is made using a tapping sleeve and valve, just like with a potable water line.
Your concerns are valid concerning undersizing or overpressuring of the existing force main if the operator has not modeled his system.

 

[francesca]

Yes, it can be done. You need to analyze the system/trunk forcemain in various scenarios, including all pumps pumping (this will involve iteration) and pumps individually pumping. You need to make sure that none of the combinations falls off the pump curves for each pump, and that each lift station still operates acceptably, i.e. with the pumps not cycling excessively as that will wear the motors out.

If the system requires upgrading one of the other lift station pumps, it's probably cheaper for the municipality to upgrade the size of the trunk main (in the long run) unless that pump set has reached the end of its useful life. Either way, the municipality may require the developer to make the improvements and provide compensation in some for. In the area I used to do this, the municipality would give the developer vouchers to offset the cost of connecting the houses once they were constructed. There was even some black market trading in these vouchers, where the developers who only sold lots would pay us with vouchers and we'd sell them on to home builders at less than face value.

 

[GoldDredger]

Thank you all for the responses. They were very helpfull.