Manifold force main modeling 2

[joyo2]

I have about 11 pump stations using the mainfold force main. I was asked to do an hydraulic analysis to evaluate the two at end of the force main (farthest) by watercad.
How should I do? I mean there are 9 other pump stations, which one(s) should I put on, which one should I put off?
Is there any rule to set up the stutus of the pumps (on/off) to evaluate the other pump (the one I want to valute)?
Thanks,

 

[francesca]

WaterCAD will let you attach all pumps to the manifold and then turn them on and off individually/in groups/whatever permutation you can come up with.

 

[Lan123]

This is very good question. I did also faced a similar situation sometime back. Howver mine was much more simpler than yours. It had only four LS. I assumed that all four LS did discharge simultaneously. If you assume this you will end up with a very conservative answer. But PE who is going and sign and seal, the client, and the State EPA have will have to agree on approach.

 

[francesca]

If you only model all four discharging at once then you miss the case of only one pump discharging and spinning off its curve because the head isn't high enough.

 

[joyo2]

Should the system be designed at the worst scenario, such as all the pumps on? although this is not the real case.
It is difficult to decide which pump(s) should be on or off. In my idea, I checked the pump run time, if the run time is very short, like 2~6hr/day, I would set it off. Is this approach reasonable?
Thank you for your response.

 

[francesca]

All pumps will run during a 10-yr storm event. And I do mean ALL of them, not one at each lift station. You have to design for ALL conditions; all pumps, no other pumps, etc. Any case that CAN occur is a real case, just not the most common case. You also need to test all the existing pumps on the manifold to make sure that adding a pump doesn't push the head too high for it to operate effectively.

 

[sjohns4]

I've heard this debated quite a bit over the years. Chances are, with 11 pump stations in some cases you will have a pump station or two running but not discharging. This does happen in practice, those stations will empty when the others are off, hopefully. That's all dependant on the time the ohters are off and the pump capacity vs. actual inflow. I've seen, in practice that happen without spills. I dont know that I'd be comfrotable designing a system like that however? When designing a new station on a manifolded system I'll check the operating conditions with all other pumps on, and all other pumps off, make sure my station will pump without overloading, and be assured it will run somewhere between those points most of the time.

Always check your results! I have got screwy results on occasion when puting multiple pumps in the same manifold from water CAD. Just make sure the pumps will operate at the point Water CAD says they are going to and the friction losses in each section of pipe match the flows that are listed.

Mike