Pumps are used to increase the pressure of water in a pipeline
to overcome static and dynamic lift requirements. Multiple
pumps are grouped at pump stations with a common suction
manifold and often a common header manifold, although splitting
the output of a pump station is sometimes used where different
discharge head requirements are present for separated hydraulic
parts of the distribution network. Large water pumps tend to be centrifugal, with either horizontal or vertical shafts to the motive drive and split cases to make maintenance of the large impellors easier.
Pumps at a single pump station can be equally sized, mixed in size in terms of either head or flow characteristics, and have a wide variety of motive sources. Typically, electric motors are used to drive pumps but systems with gas engines, diesel engines, variable speed electric drives, and even hydro-power are commonly found.
Pump stations have many possible duties, including:
Raw Water Pumping
Pumps deliver raw water from a variety of sources, such as rivers and lakes. They may supply a treatment plant directly, or transfer water into embankment storage ponds to allow turbidity to settle out. These tend to be low head, high volume pumps, lifting water only a short distance to the plant. Aquifer pumps (wells) are another common form of raw water pump, but they can have very significant static head requirements, sometimes hundreds of meters. They require multistage pump units on a common shaft to achieve the discharge head required.
Pumps are used in treatment plants where gravity alone is not sufficient to provide the head or flow to get water through the entire treatment process.
Clear Water Tank or High Lift Pumps
These are the first of the treated water pumps and are used where the treatment plant is below the hydraulic grade line of the distribution network. They provide the initial head in the distribution network. These tend to be very large centrifugal pumps with high efficiency electric motors. These pumps are often the largest single energy users in an entire source-to-tap water network.
Rather than lifting all water from a treatment plant to a pressure high enough to serve the highest customer, it is preferable to split a distribution network into pressure zones and only raise the actual water required by the customers in the highest pressure zone. Booster pumps draw off the pipe network or from intermediary storage to a pressure suitable for the last customer.
Splitting a distribution system into pressure zones and only pumping what is needed for the higher zones reduces the amount of pumps and pumping required, while also reducing energy consumption. Variable speed-driven pumps allow a greater range of flexibility in both discharge flow and head than a pump station or fixed speed pumps and can often replace combinations of fixed speed pumps at a single pump station.
Selecting pumps to operate efficiently and effectively requires considerable technical skill and forecasting, hydraulic modelling and planning. Treated water pumps can operate for many decades and must cope with changing distribution network demand and discharge pressures during that time. Selecting a pump to operate at the Best Efficiency Point (BEP) works best if the pump station supplies a top-filled tank directly with no intermediary demand. In all other cases, trade-offs must be made between varying head and flow (i.e. the system curve of the network into which the pump is delivering).