New Pump Along Existing Force Main 1

[ChrisTN]

I have a project where the most feasible access to sanitary sewer is to install a pump station to serve the new building and tie into an existing 2" force main that serves the adjacent business. The existing main is 1000' and the new connection will be about 650' from the discharge point (350' from the existing pump). Very low flows and very flat topography along the force main to the discharge point. I have obtained the model number and pump curve for the existing pump and have also picked a preliminary pump design for the new building.

Where I'm getting lost is how to properly calculate the worst case scenario of both pumps operating simultaneously. How do I calculate a TDH at each pump under those conditions? Do I simply add the to operating flows for both pumps and use that flow to determine the friction loss, then check to see if that point is still on the curve for each pump? I'm rather green when it comes to pump station design , so any nudge in the right direction would be greatly appreciated. Thanks.

 

[bimr]

Yes, that is the correct method if you are saying that there is one pump in each of the lift stations.

It is important to note your comment regarding "most feasible access". The option that you described sounds like a bad one.

Your option relies on the ability of the check valves to hold the back pressure when the opposite pump is working. In safety reviews of process systems, it is generally assumed that check valves will fail at some point, so it good practice not to rely on check valves. You should be prepared for the consequences should the check valves fail.

 

[redbridge]

It is very common to connect new pumps on existing force mains. To calculate the flows of both pump stations running at the same time is more complicated than adding the operating flows. The operating flows will be different because they are pumping against more head than they would be when pumping alone. The easiest way would be to model this with some type of software. If you give me the three points off each curve, lengths & diameter of the force mains, where the new force main connects to existing force main, and elevations at each point I will explain in detail.

 

[ChrisTN]

The data is as follows:
Existing force main= 1000' SDR-21, 2" dia.
Elevation of entire force main= 1239' (flat run)
Elevation of existing pump= 1233'
Existing pump curve:
Q= 0 gpm, H= 64'
Q= 16 gpm, H= 55'
Q= 32 gpm, H= 45'
Q= 48 gpm, H= 15'

Proposed pump will connect along existing force main at 635' from the discharge (365' downstream from the existing pump). Proposed pump will connect to existing main via additional 170' of new piping, 1.5" or 2" SDR-21.
Elevation of proposed pump= 1234'
Elevation at tie-in point= 1239'
Proposed pump curve:
Q= 0 gpm, H= 62.5'
Q= 15 gpm, H= 57'
Q= 30 gpm, H= 44.5'
Q= 52.5 gpm, H= 3'

I appreciate your help tremendously. Just out of curiosity, what software package do you use? Can this be modeled with EPANET? Thanks.

 

[Yellowtetra]

ChrisTN,

This can be modeled with EPAnet or any kind of pressure hydraulic modeling software like SewerCad, WaterCad, KYpipe, etc.

Dan Barr, PE
Burgess and Niple

 

[howertoneng]

The easiest way is to use SewerCad. We use this on multi-pump, multi-forcemain analysis.

It allows you to plug in the pump curves, geometry, etc.

1. do an existing system analsis. Determine what the pump run times, operating points are.

2. do a proposed system analysis, your new pump selection will need to operate at multiple points on the curve. check with just the one pump station on, then with both running. Check to make sure the existing pump station is not affected. Make sure it will still operate effectively on its own curve. ensure that you are creating a situation where the existing pump station cannot run, or overflows, beacause the new pump station is "overpowering" it.