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System curves allow correct selection of pumps and are invaluable in troubleshooting of pump problems.
To draw a system curve, follow these steps & refer to fig1:
- 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.
- 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)
- Mark static head. ie: 17m at zero flow.
- 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)
- Join these points with a line. You have completed the System Curve. (Curve may have to be extended to suit higher flow pumps.)
- 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.
- 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.
- Note: 'demand' pressure, ie: sprinklers etc, should be added at each flow calculated to make the system curve. If you can't get the data for the sprinkler / nozzles at various flows, but you know that ie: 10 sprinklers will pass 0.2 L/s each at approx 30psi, then your required flowrate is 2 L/s and add 21m (approx 30 psi) to the static head when you start drawing the system curve (as an approximation).
The 'demand' pressure or "head loss through sprinkler / nozzle at a particular flowrate" is not added for each sprinkler. Only the flowrate of each one is added together. If the pressure is available for the most 'disadvantaged' sprinkler, then it will be available for all sprinklers in that system. Note: this means that a higher pressure will be available to less 'disadvantaged' sprinklers allowing a higher flowrate through those sprinklers. There may be no need to calculate each sprinkler / nozzle, but if there are significant differences in static head / long lengths of pipe / reduced pipe diameters, then the system may require more investigation to allow correct pump selection.
- Note: It is tempting to add extra margins to these calculations, but in most circumstances that can contribute to the wrong pump selection and a big repair bill for a damaged oversized pump operating outside it's operating range. Extra Note: Some applications require a best guess (hopefully) oversize pump with budgeting for pressure gauges, valves, and / or orifice plates, to allow adjustment during commissioning to ensure the duty is achieved with maximum pump life.
fig1: drawing a system curve.
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System Curves for multiple branches
The above method shows the basics for drawing a system curve for one pipe connected to the discharge of the pump. What if there is more than one pipe?.
If 2 or more pipes are connected to the discharge of a pump, the flow through each pipe can be added together at a common head. The combined flowrate can then be plotted as a system curve. Use the highest static head at zero flow on curve.
If there is a common section of pipe before the multiple branch lines, you must first do the above step then calculate friction through the common section (at the combined flow of the multiple branches) then add that to the "common head" used above. Use the common section static head plus the highest static head of the multiple lines as the head at zero flow on the curve.
You will need to use two calculators:
Enter flowrate to calculate headloss
and
Enter headloss to calculate flowrate
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