Interceptor Forcemain Design Criteria 1

[bccwwes]

I am looking for a good reference on the design of an interceptor forcemain.

I am currently working on the design of a brand new 6 mgd WWTF in the coastal plain (flat). My initial service area is 6 miles from the treatment and disposal site. The forcemain between this service area and the site will ultimately become a interceptor FM with smaller forcemains interconnecting along the way.

I am looking for a reference relating appropriate standards for the interconnection of these smaller FMs, since they will not maintain minimum velocity in the interecptor.

 

[Maury]

Are you sure your terminology is correct? I've never heard of a network of force mains. How will you design pumps for all of the possible head conditions that are going to occur?

 

[bccwwes]

Interceptor Forcemain is the best terminology I can think of. In doing Google search I see that Hampton Roads Sanitation District (Norfolk Va area) actually has Interceptor Engineers for various service areas (I am considering contacting them directly).

I believe I would be designing based on all pump stations operating, and that the main pump station would be providing scour velocity, but smaller interconnecting pump stations would not create enough velocity in the inteceptor FM. I would likely be overdesigning the main PS so that future interconnections would not limit the flow unreasonably.

I know we have similar situations in our local jurisdiction, in fact I designed an interconnecting PS awhile back. They just told me what the FM pressure was at my connection point and I handled it as a static pressure.

I am hoping that there are some guidelines as to how much interconnection is appropriate as compared to the primary flow, etc.

Thanks.

 

[sjohns4]

I've designed several pump stations along interceptor force mains.

The area I'm working does not require adequate scour velocity be maintained during all conditions, but it must occur - maybe one station won't provide scour velocity, but two will operating together. The feeder lines from each pump station can operate the oppisite: when both stations or on each station will not provide adequate scour in it's dedicated pipe, but when they are operating independantly they will.

I've also seen parallel forcemains installed. Start the station on the small pipe, when additional flows come along swith to the larger pipe, and when ultimate build out flows reach the station you operate both force mains. Some times pump changes are required between each operational phase so you might want to think about sizing certain components such as electrical service entrance for ultimate anticipated conditions.

There has been a lot of discussion about parallel force mains with local authorities, the pro is you can provide valving in case of a brake often normal daily flows can be routed through a short section of a single pipe, however, it's consideribly more expensive to install two pipes than to pig the slow moving pipe from time to time until you get enough flow it will clean itself.

The conservative method is to assume all pumps on, but careful b/c when all pumps are not on the stations may run to the far right of the curve, so I always specify non-overloading pumps for manifold systems.

Mike