Determine Pump Flow

Mon, March 03, 2016

How to Determine Pump Flow

All Bell & Gossett Pumps have tappings in the suction and discharge of the pump volutes. These tappings are used for a number of purposes, not the least of which is to help determine how much water is flowing in the system. The best way to determine flow rate in a centrifugal pump is to use a single pressure gauge connected to both the suction and discharge tappings on the volute. A needle valve or gauge cock must be installed between the gauge and each of the tappings. This will allow the operator to close the discharge needle valve and read the suction pressure of the pump, then close the suction side needle valve and open the discharge side valve to determine the discharge pressure. Why one gauge? If two pressure gauges are used, one gauge may tend to read pressures on the low side of its tolerances. Another gauge may tend to read on the high side of its tolerances. The combined error between these two gauges may be sufficient to provide a completely erroneous reading of pressure differential. If we use only one gauge however, the error would be consistent in both the suction and discharge reading. Since we are interested in differential, one gauge is better than two. So how do you determine flow rate from all of the above? With the pump running, open the suction gauge cock and take the pressure reading. Close the gauge cock and open the discharge side gauge cock.. Take the reading from the discharge side. Subtract the suction reading from the discharge reading to find the pressure differential in PSI. Multiply this number by 2.31 and you now know the pressure differential for that pump in feet, TDH, which is what the pump manufacturer uses to publish pump curves. Take a look at the pump curve for this pump (call Bornquist if you don't have one). From the nameplate, determine the impeller diameter. Follow the impeller curve until it reaches the pressure differential (in feet) that you have measured. Draw a vertical line straight down to read the GPM. Naturally, if you have any questions or concerns, give us a call. We'll be happy to help.