The two main aspects to selecting a pump are the required flow rate and the required head. As an indication of what a particular pump is capable of the manufacturer will often quote a maximum head figure and a maximum flow figure however it is important to note that you will not get the maximum flow and the maximum head at the same time.
Required HeadThe required head for your water feature is defined as the vertical distance from the surface of the body of water in it to the point at which water is to be released. The head you require can therefore easily be determined by simply measuring this distance. Consider the example of a feature consisting of a pond with a tube carrying water up to a spout above the pond from which the water is to fall back to the pond. The required head in this case would be the vertical distance from the surface of the water in your pond to the end of the spout from which the water is to be released.
In addition to this it should be noted that the tube being used will add resistance to the system and should therefore be kept as short as possible. The amount of resistance generated by the tube depends on its diameter. However as an approximation every 1m of tube used will require an additional 0.1m of head.
So for example if the pond used in our example has a spout 0.5m above the water surface and 1m of tube is used to lift the water to the spout then the required head will be 0.6m.
Required FlowThis is the rate at which you require water to flow through your feature and is generally measured in litres per hour. It is somewhat more difficult to estimate than the required head. You can try to estimate it by using a garden hose to get water to flow through your feature or by just looking at the flow from the end of a garden hose and adjusting the tap until you get the flow you think you require. You can then measure the time it takes to fill a 1-litre bottle, for example, and use this to calculate the flow in litres per hour. The flow in litres per hour is then given by:
Volume of bottle in litres * 3600 / time taken to fill in seconds
If you are using the pump to circulate water through a garden pond then in order to maintain a healthy pond, experts recommend circulating the water at least once every two hours.
Flow Verses HeadConsider attaching a length of loose tube to a running pump. If the open end of the tube were held at the same level as the surface of the water in to which the pump is submerged then the flow obtained would be the maximum flow of the pump. A pump's maximum flow is therefore only obtained at zero head.
A pump's maximum head indicates how high the pump can push water. If the open end of the tube is now lifted up, the flow from the tube will begin to reduce. Eventually a height will be reached at which the flow stops. This is defined as the maximum head of the pump. A pump's maximum head is therefore only obtained with zero flow.
In practice your pump will generally be used somewhere between the two, i.e. at less than maximum head and at less than its maximum flow. It is possible to determine what flow you will get at a particular head using the pump characteristic graph. This is a graph of head against flow for a particular pump and you can use it to look up your required head and find the flow that the pump will give you.
Some pumps have a flow adjustment control knob. This may be used to turn the flow down if the flow is too great in your system. Pumps with a flow adjuster therefore have characteristics that have two curves. One curve showing the flow for a particular head when the flow adjuster is turned to maximum and the other showing it for the condition when the adjuster is turned to minimum. So for a particular head you can find the maximum possible flow and minimum possible flow for that particular pump at that head (height). It should then be possible to adjust the flow anywhere between the two using the flow adjustment knob.
Pumps that do not have a flow adjuster have one curve on their characteristic, that being the maximum flow for the pump. Adjustment for these pumps can be achieved if necessary by using a flow control valve at the outlet or in the outlet pipework.
ExampleIf your required head is 0.5m then range of flow available from a Sacem Winds 400 would be determined by drawing a horizontal line across the head axis of the pump characteristic at 0.5m.
Vertical lines are then drawn down from the points of intersection with the two curves. The flow from the pump with the flow adjuster set to minimum and maximum can then be read from the flow axis. In this case the Sacem Winds 400 flow adjuster will give a range of adjustment from 130 to 225 litres per hour at a head of 0.5m.
While the above curve is for a specific model of pump its shape is typical for all the pumps we have available.
Other criteria to consider include power supply, outlet diameter and mounting type:
Power SupplyMost pumps are mains, 240Vac operated. This means they can operate from a domestic power supply without the need for a transformer. Outdoor mains powered pumps have 10m of power flex. Solar powered pumps have the advantage of not requiring a long cable but generally only work in bright sunshine; they have limited power and therefore generally only produce a small flow of water at a small head.
Power RatingThis is generally measured in Watts (W). The power requirement for the pump will give an approximate indication of the capability of the pump. It is used by many when selecting a replacement for an existing pump that has failed. A pump with a similar power rating will generally have a similar capability but this is not as accurate a measure as the head and flow figures.
MountingMost pond pumps are submersible and when installed lie submerged in water. Water is drawn in through a grill in the pump casing and exits through an outlet port, into which a pipe or fountain head can be fitted. There are however also surface mount pumps available. These have an inlet port and an outlet port. This allows a pipe to be connected to the inlet as well as the outlet. The pump can therefore stand in a dry environment and does not have to be submerged.
Outlet SizeThis is the diameter of the pump’s outlet. Pumps are usually supplied with at least a small length of rigid pipe (hose-tail) at the outlet on to which a flexible hose can be attached. Some larger pumps are supplied with several sizes of hose-tail and these push into the body of the pump allowing different diameters of flexible hose to be attached.