Centrifugal pumps are rated by flow output at various resistance levels. Charts provided by the manufacturers, called "pump curves," show the flow rate along the horizontal axis and the resistance level along the vertical axis. The resistance level is in head, sometimes labeled "TDH", "head in feet" or "feet of head." Properly sizing a centrifugal pump requires finding the head for all the components and elevation changes in the system. The total head of the system is compared to the pump curve to determine if the pump can supply the required flow.
Things You'll Need
- System plan
Determine the total head in the system by adding up the resistance of each component, including the pipe and fittings. Components such as filters and separators will have published head loss charts. Compare the system plan's designed flow rate to the chart to find the head loss. For pipe and fittings, refer to "friction loss charts" for the specific type and size of pipe. If resistance values are stated in pounds per square inch (psi), multiply the value times 2.31 to convert to head.
If the system discharges at a point higher than the centrifugal pump, add the elevation of the discharge point to the calculation. One foot of elevation change is equal to one foot of head.
Do not add additional elevation for the pipe extending above the pump and then descending to the discharge point. The "siphon effect" of the water descending compensates for the elevation change. Note this total elevation change separately.
Compare the total head of the system to the pump curve chart. Read across the horizontal axis and find the system plan's flow requirement. Read up on the vertical axis to find the head of the system. Pump curves at or above the point where these values intersect provide enough flow for the system.
Verify the centrifugal pump can overcome the total elevation change not included in the head calculation. Looking at the chart, add this extra elevation to the vertical axis and read across to where the selected pump curve intersects this value. Match that point along the bottom axis of the chart to find the initial flow rate of the pump. The initial flow rate must be sufficient for any check valves or pressure sensors in the system.
Calculate the head of an existing system using vacuum and pressure gauges if determining the components is not possible, such as with buried pipe and fittings. Install a vacuum gauge near the suction port of the pump and a pressure gauge near the discharge port. Note the values with the pump operating. The vacuum gauge will display vacuum in inches of mercury, or Hg. Multiply this value times 1.133 to convert to feet of head. The pressure gauge will display pressure in psi. Multiply this value times 2.31 to convert to feet of head. Add the two values together for the total head in the system.
Tips & Warnings
- If the resistance of a component is given in a range of values, use the middle of the range.
- Filter manufacturers might recommend an additional resistance value for dirty filters. Add this value to the total elevation change calculation.
- Photo Credit Flowing water pump for hot water. Isolated on white. image by diter from Fotolia.com waterfall image by Lisa Batty from Fotolia.com pressure guage image by Jeff Davies from Fotolia.com
How to Calculate Pump Pressure
There are a number of contexts where the calculation of pump pressure can be key to minimizing expenditure and maximizing safety. Unfortunately,...
How to Size a Submersible Well Pump
Submersible well pumps are seated beneath the water level in the well. These pumps are considered to last for a long time...