Various Ways to Determine Pump Flow in the Field

Written by Dr. Lev Nelik, P.E., APICS

Page 2 of 2

Power (Amps) Method

The power curve indicates approximately 3.2-hp at the rated point. Power meters (kW-meter) are rarely available, with amps and volts being more commonly displayed at the control panel. Power can be calculated from these readings, although some assumptions of the power factor and motor efficiency would be required:

BHP = (I x V x 1.73 x EFF_motor x PF) / 1000

In our example, a 5-hp 460-V motor is used and we actually read 450-V and 3.9-amps. A typical assumption of the product (EFF_motor x PF) is 0.85, although a somewhat better value can be obtained if one is willing to spend some more time on research work.

Thus, in our example:

BHP = (3.9 x 450 x 1.73 x 0.85) / 1000 = 2.6 hp

This is slightly less than the expected 3.2-hp, meaning a straight horizontal line at 2.6-hp intersects the power curve at flow approximately 50-gpm, depending how accurately you eyeball the curve.

Obviously, too many assumptions and approximations in reading curves bring bad news. However, the good news is that based on two methods, we can state that the flow appears to be somewhere between 50-gpm and 60-gpm. For many troubleshooting purposes, this answer is sufficient.

That's not all. If we also add to this information the approximately 55-gpm data that was registered from the field flowmeter using the technique we discussed (in August) for the less than optimum pipe location, our confidence of the flow actually being somewhere between 50-gpm to 60-gpm will increase even further - a very good thing.

As a note on the power method, some people feel more comfortable simply taking the ratio of actual amps to the motor nameplate (if one is still attached!) amps rating, then multiplying the result on motor rated power. In our example, if the motor rated amps were, say, 8.5-amps, and rated motor power 5-hp, we could then assume the actual power is 3.9 / 8.5 x 5 = 2.3-hp. This is close to the 2.6-hp value we derived earlier by using a power factor and motor efficiency assumption.

The power method can be applied very successfully for field troubleshooting of many pump types, but it has significant drawbacks and cannot be applied for high specific speed (N_s) pumps, such as mixed flow and vertical turbine pumps.

As HI illustrates nearby when comparing impeller profiles for various specific speed designs, pump power is not a nice continuously rising curve, as is the case for most end suction and split case pumps. Instead, the shape of the power curve can be entirely different. It can rise, drop, or stay constant with flow, even making its shape so flat that it becomes difficult to distinguish the difference for a rather wide variation of flows.

The bottom line is that each method has its own place, strength and limitations:

The pressure (head) method is the simplest and quickest, but requires one to have a pump curve and gauges that are not broken or out of calibration. In the realities of the field, these curves are unfortunately long lost or misplaced for the old pumps, and even if they do exist, it is often impossible to know the most recent impeller diameter inside the pump after numerous prior pump repairs and modifications.
The power (amps) method does not require one to "get dirty" around the pump replacing broken gauges, but inaccuracy of the power factor and motor efficiency is a drawback. (Reference power factor fundamentals presented by Joe Evans in "Power Factor: Electricity Behaving Badly (Part One)" (Pump Ed 101, Pumps & Systems June 2007)
Direct flow reading is the most sure way, but most pumps do not have in-line flowmeters installed. Cutting into lines to install them is impractical and expensive. External (ultrasonic) meters are simple, but accuracy is limited due to difficulties in locating a good (HI approved) spot along the pipe of the real field installation.

Often, applying all three methods reduces the error by allowing the user to learn to intelligently interpret the reasons for the differences, be able to explain the peculiarities and inconsistencies of each method, and correct such inconsistencies by solid reason, some understanding of flow mechanics, and reasons for deviations of practice from the theory.

As always, our habit for leaving a parting thought with you: What simplifying assumptions were made in describing the pressure (head) method? What additional errors can these assumptions introduce, and to what magnitude? The first three correct answers get you a winning ticket to our next Pump School session(s). Keep on pumping!

Dr. Nelik (aka "Dr. Pump") is president of Pumping Machinery, LLC, an Atlanta-based firm specializing in pump consulting, training, equipment troubleshooting, and pump repairs. Dr. Nelik has 30 years experience in pumps and pumping equipment. He has published over 50 documents on pump operations, the engineering aspects of centrifugal and positive displacement pumps, and maintenance methods to improve reliability, increase energy savings, and optimize pump-to-system operations. With questions, comments, or to attend his Pump School, he can be contacted at www.PumpingMachinery.com.

Comments (0) Add Comment