Combining forced sewer pipes 3

[mirza057]

Guys,

Is it ok to combine the forced sewage pipes from 2 sump pits before they get into the inspection chamber(manhole), i think they must be taken separately till the inspection chamber.

 

[PEDARRIN2]

Do I all the time with discharge from elevator sumps.

If the sumps are located far apart, it might be advisable to keep them separate to minimize the piping.

 

[mirza057]

Peddarin,

The first sentence in your reply is unclear, can you please rephrase it?

 

[PEDARRIN2]

It should read, "Do it all the time with discharge from elevator sumps."

These are commonly located in the same proximity so combining them makes sense.

 

[racookpe1978]

But if the two pipes are both pressurized, what prevents backflow from pipe A into Pipe B when the outlet to the sump clogs or get filled with storm water? Seems like the idea works only if gravity is draining both A & B into the sump at an adequate rate all the time.

 

[bimr]

It is possible to combine forcemains. The combined forcemain must be sized for the combined flow.

If you have solids, there may be some concern for plugging when operating at low velocities in the combined forcemain.

 

[mirza057]

Thank you guys

Bimr, kindly can you let me know what do you mean by plugging? Is it blocking?

 

[bimr]

If there are heavy solids, you need a velocity of approximately 3 ft/sec to keep the solids in suspension. If the velocity is reduced the solids will build up and plug the forcemain.

If you size the combined forcemain for 2 pumps, you will see a reduced velocity if only one pump is operating.

The design is more simple if there is only one pump per forcemain.

 

[PEDARRIN2]

Typically, water in an elevator will not have solids so there should not be much risk of clogging.

Backflow is prevented by a check valve installed downstream of each pump, typically in the horizontal portion of the piping in the elevator shaft.

This is likely a specialized situation where the pumps are close in location. If they are not, separate pipes would likely be a better design.

Typically, sewage pumps, if receiving black water, would be designed that the pumps can pass 2" solids and possibly have impellers which will shred/grind/etc. solids to less than 2".

I agree with bimr to size the piping for one pump to keep the velocity up and push the waste downstream, especially if the pumps are not going to operate often. Once solids settle out and dry out, it is very difficult to dislodge them.