Booster Pump from Ground Storage

[en2e]

I'm working with a small town who is under consent order from the state DEQ for low system pressures. They have a 72' tall ground storage tank, and must keep it at a minimum level of 63' to have even serviceable pressures in the vicinity - when hydrants are opened for fire protection or flushing, water supply to the surrounding services is cut off.

There is plenty of volume in the tank, but the large inactive volume has led to DBP problems (THMs). We're looking at installing a booster pump on the line out of the tank, to increase pressures and allow the tank to drain to about half-full, hopefully killing two birds with one stone. We have an area of influence determined, which gives a GPM, as well as a target HGL in the low-pressure zone, which I figured would be the TDH of the pump (minus the contribution of the static level in the tank, at half full). We're planning on using a multi-stage pump, capable of delivering a consistent head at a broad flow range.

My problem is that when I try to model this configuration in WaterCAD, it doesn't seem to affect the pressures at all. No matter how large I make the pump design point, it won't boost the pressure above about 10 or so PSI. This seems counter-intuitive, which makes me think I'm modeling the system incorrectly. I'm using a steady-state model, with peak hour flows at all nodes, so I'm thinking that the initial level I set in the tank is limiting the pump head. Any ideas?

 

[en2e]

To clarify, I was thinking the pump TDH would equal target HGL, minus ground elevation at the highest point in the low pressure zone, minus static contribution.

 

[BigInch]

Peak flows at all nodes is probably too much for your pump, unless it has the same combined capacity. Total discharge head of the pump will be its suction head (apx. 63ft <the minimum tank level head above your pump centerline> - suction piping loss), plus the Pump's Differential Head at whatever your defined flowrate is for the scenario you are checking.

If you've got minimum pressure now, you'll probably need to install a booster pump with about the same head at a flowrate equal to the sum of all outlet flows.

If you do that and there is still little or no change in the low pressure areas, I would guess you'll need to be increasing those distribution pipe diameters or paralleling those lines with new pipe.

 

[bimr]

The DBP problems are rooted in the geometry of the stand pipe. Unlike elevated tanks, stand pipes are notorious for causing water quality problems.

Changing the pump system will not change the geometry of the tank and will ultimately be a wasted effort.

What you need to do is to look into installing a tank mixing system:

http://www.tideflex.com/tf/index.php?/content/view/171/266/

You have not presented enough information to consider installing pums. Do you supply fire flows? Your peak hourly flow will most likely be in the range of 5-7 times the average daily flow as well.

 

[jonr12]

You should also consider a bypass with a check valve around the booster pump. This way if your pump can’t keep up with fire flows, you still have the head and volume available directly from the standpipe.

 

[en2e]

Thanks for the help everyone. It turned out to be an issue with main diameter and cross-connections. With some line upsizing and valving, pump seems to be working fine. @bimr and @jonr12, thanks for the tips, I'll keep them in mind as we progress.