I am working with a local council in an area of New Zealand that is very flat, so everything needs to pumped for reticulated water to houses.
In this system, water is transferred (using pumps) from a bore and stored in a main reservoir (3,500cu m)where it is treated. From the main reservoir, the water is pumped to a small town reservoir which is located in a tower with approx 38m head. The town reservoir holds 385cu m. We are only talking about a small town in rural NZ with a population less than 2,300 people.
The pump station is due for an upgrade and one option on the table is to pump directly into the reticulation loop and remove the storage reservoirs. Effectively the pumps would provide all the pressure (approx. 420kPa) for the town.
My questions are these:
- Will energy use decrease/increase with a direct feed into the reticulation loop and why?
- One disadvantage of a direct feed relates to the lack of capacity if there was a power outage, earthquake, disaster etc. What are the advantages?
- Any other consideration greatly appreciated.
I work for a water district that has about 100 pump stations, and uses both approaches (1) pumping to elevated storage, and (2) constant pumping in a closed loop system. While in theory both ways of pumping do the same job (same quantity of water delivered at the same pressure), in practice, the pumping to a tank is much more efficient.
With a tank, you can operate a pump in its sweet spot, and as demands vary throughout the season, you adapt by changing how many hours per day the pump operates. Pumping to a tank typically has an efficiency of 75% or greater. Plus, since the fire storage is in the tank, we only need a 2 pump station to have redundancy.
With constant pumping, you need to operate a pump 24/7, regardless of the demand (the 2,300 household demands are too large for an effective hydropneumatic system). So you’ll need to specify pumps that meet the full range of demands. For our system, the ratio of summertime peak demand to wintertime minimum demand is about 20:1. Add in fire flow, and the pumping demand ratio gets to 200:1 (or larger). So how many pumps are you planning to use? We use 4 at most - including two redundant fire pumps and a backup genset. With that range of flows, and that few pumps, there’s no way to keep a pump station operating efficiently (VFD or not). Our typical constant pumping zone efficiencies are less than 20% on an annual average.
There are a few advantages to constant pumping. Lower capital costs. Storing water can give rise to quality concerns - disinfectant residual and disinfection byproduct problems are smaller without storage tanks. Sometimes, there just isn’t a good place to put a tank.
Without the tank, your new pump station just got more expensive due to fire flow, and your operations just got more expensive due to poor efficiency. I’ll leave it to you to do the economic comparison between tank maintenance and pump station costs. But unless you have serious water quality concern with your tank, I bet you’re better off keeping it.
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