System curves allow correct selection of pumps and pipes, and are invaluable in troubleshooting pump and system problems. To draw a system curve, follow these steps & refer to fig.4:

1. Find details of duty. ie, in this example: Water, 2m suction lift, 15m static discharge (17m total static head), 360 metres of 150mm schedule 40 steel pipe.
2. Draw a chart with flow on bottom scale and head on left scale. (estimate scale required based on size of existing pump, or guess maximum flow expected - example shows max flow as 100 L/S and max head as75m - sometimes you just have to guess to get started)
3. Mark static head. ie: 17m at zero flow.
4. Mark 2 or 3 other points.ie: at 20L/S friction loss is 0.73 m / 100m of pipe, therefore 0.73 x 3.6 + 17 = 19.6 metres. Put mark at junction of 20 L/S and 19.6 m. Repeat for other points. (remember to add static head each time)
5. Join these points with a line. You have completed the System Curve. (Curve may have to be extended to suit higher flow pumps.)
6. The pump operating point is where a pump curve crosses the system curve. Draw as many pump curves over the system curve as you like, to see where different pumps will operate, or draw system curve over pump curve.
7. If pump curve does not cross system curve, the pump is not suitable. If the pump curve crosses the system curve twice, then the pump will be unstable and is not suitable.

The use of two or more pumps to increase flowrate is called Parallel pumping. The use of two or more pumps to increase head is called Series pumping. Operation of pumps under these circumstances may appear simple, but there are more complex issues to consider, ie:

In series applications: consider the pressure rating of pump, shaft seal, pipework and fittings. Placement is critical to ensure both pumps are operating within their recommended range and will have a constant supply of water.

Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the head each pump produces at the same flow. More curves can be added in the same way Fig.5.

In parallel applications: confirm suitability of pumps by drawing a system curve (often 2 pumps will only deliver slightly more than one pump due to excessive friction loss. Also you can confirm that pump operation will be within its recommended range.). Non return valves are required especially if one pump operates alone at times. Dissimilar pumps or pumps placed at different heights requires special investigation.

Once the curve for two pumps has been drawn, add the system curve, the point where the system curve crosses the curve for two pumps , indicates the total flow from two pumps. Draw a horizontal line from this point back to the head axis. Where this horizontal line crosses the curves for a single pump indicates the amount of flow contributed by that pump to the total flow.

Fig.7 shows a system curve crossing a pump curve twice. This is an example of unstable operation. Note that if the first pump is operating at point 'C' when the second pump is started, the second pump will operate at shut head, delivering no flow as it will never be able to open the non return valve (required to prevent one pump discharging through the other when only one pump is operating). If this was to occur, the pump could eventually explode!

In some cases it may be possible to change the order of starting the pumps, and the curves can be drawn to check this operation, however if there is any indication of unstable operation or possibility of one pump being 'over powered' by another, the system may need to be changed or different pumps will have to be used.

A: NPSH (r) EXCEEDS NPSH (a)

Due to low pressure the water vapourises (boils) and higher pressure implodes into the vapour bubbles as they pass through the pump causing reduced performance and potentially major damage. For more see NPSH - Nett Positive Suction Head

B: Suction or discharge recirculation

The pump is designed for a certain flow range, if there is not enough or too much flow going through the pump, the resulting turbulence and vortexes can reduce performance and damage the pump. For more see Centrifugal pump Operating Range

The damage to a pump from cavitation can be severe. It may shorten seal and bearing life and damage volute, backcover, and even pipework beyond repair. The implosions due to cavitation can sound like gravel passing through the pump. Pumps are not able to pass vapour or air same as an air compressor is not able to pass water.

NPSH - Nett Positive Suction Head

NPSH is a dirty word? There is enough fear of it to suggest it is. But why? Because some people will not accept that pumps don't suck!
If you accept that a pump creates a partial vacuum and atmospheric pressure forces water into the suction of the pump, then you will find NPSH a simple concept.

Where:
'p'= atmospheric pressure,
's'= static suction (If liquid is below pump, it is shown as a negative value)
'v'= liquid vapour pressure
'f'= friction loss

NPSH(a) must exceed NPSH(r) to allow pump operation without cavitation. (It is advisable to allow approximately 1 metre difference for most installations)

NPSH(r) is the Nett Positive Suction Head Required by the pump, which is read from the pump performance curve. (Think of NPSH(r) as friction loss caused by the entry to the pump suction.)

If the speed or impeller diameter of a pump change, we can calculate the resulting performance change (at the same efficiency) using:

Affinity laws:
1. The flow changes in proportion to speed ie: double the speed = double the flow
2. The pressure changes by the square of the difference ie: double the speed = multiply the pressure by 4
3. The power changes by the cube of the difference ie: double the speed = multiply the power by 8