Pump Stations Modelling Scenarios

[westend392]

Hi everyone,

I just started modelling transmission system for wastewater collection system thus rather green at it. When I run my hydraulic model for the transmission systems I usually run it under 2 scenarios:
1. With all of the pump stations in the system running.
2. With individual pump station in the system running by itself.
I'd then select the pump for each station than can serve those two extreme conditions.

My question is whether I'm being way too conservative in just using those two scenarios for my pump selection? I think I can reduce my power requirement for the pumps that I have selected by running the model under scenarios where, for example, I have 85% and 25% of the pumps operating instead of all or just one.

 

[francesca]

It really depends on your design condition: are you designing for normal peak flow or wet weather peak flow?

You need to design for what the local authority will let you get away with (usually a single pump at all stations running simultaneously), but you also need to know where the bypasses will occur in large storms when infiltration is high and most stations will have multiple pumps running.

The basin-wide sewer model I built (SewerCAD -- miserable experience) we designed for a system that would have no bypass in the 2-year flood. We used real flow data taken from three months of monitoring with an actual 2yr storm (intensity - duration). We allowed multiple pumps to turn on at the stations for the 2yr storm, but for regular flow conditions, we needed just a single pump to turn on, with the back-up pump allowed to come on for limited periods during the peak. It was a 24-hr simulation.

 

[stanier]

Perhaps you should Epanet and run the model to represent a number of days operation. The size of your sumps can also be checked out.Epanet is a free network analysis software.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[westend392]

Thanks everyone,

Fransesca,
Our collection system is actually going to be a brand new wastewater gravity collection system. So I would think I&I will be minimum for the first few years of operation. We design the entire system for a normal peak flow with a preak factor of 3.5. Since this system is going to be new, do I need to model it to include major I&I occurrence? The system will be located in coastal area.

 

[francesca]

In the (new) subdivisions I designed, we did not design either the gravity or the lift stations for I&I, so you should be fine. However, I wouldn't push the limits either, as some I&I is unavoidable.

 

[stanier]

If the system is designed and constructed using thermoplastic materials such as PE or ABS with swelded joints I&I will alwys be zero. You may need bolted manhole covers to stop water ingress there. the other nebenfit is that for the early years of low flow the hydrogen sulphide build up wont corrode the plastics like it attacks cement linings.

You may need to consider odour control at the treatment plant forth early years as your detention times will be quite long.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[westend392]

One thing that I have considered is to control the detention time in the lift station via the control panel to minimize odor problem. I am concern about low flow. Some of our lift stations' peak flows are even less than the minimum flow to avoid settling velocity. There are two solutions that have been suggested to me:
1. Lower the elevation of the liquid level device for pump-activation condition.
2. Program the Control Panel so the lift station will pump the wastewater to the minimum water level at least once every maximum detention period (for ex., 30 minutes).

 

[francesca]

What about future growth?

Even with pumping to the minimum water level every 30 minutes, you'll have about a foot of storage in the wet well. If you have low flow, you can still have an odour problem.

 

[westend392]

Our peak flow has already incorporated some future growth allowance. This is why I'm even more concern with low flow. Any suggestion for the type of odour control to use? Most of our lift stations are small (w/ 3-5 HP grinder pumps) except for a few that I would consider medium ( w/ 10-20 HP non-clog pumps).

 

[stanier]

In respect of odour the golden rule is "dilution is the solution to pollution". Thats is if the odours can be vented through a vent stack and taken away by the wind then that is the cheapest. Environmental impacts of a stack have to be considered. This is common practice in municipal pump stations.

Odour control at treatment plants is by chemical injection into the main, odour scrubbing with chemicals or odour beds.

You can also inject chemicals at a pump station if it is a residential area and stacks are an eyesore.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au