This column will examine two indirect methods of determining pump flow in the field:
- pressure (head) measurement
- power (amps) measurement
In both of these cases, you must obtain the pump performance curve for the respective application, which normally comes in combined or single-line formats, as shown in Figures 1 and 2.
A combined format curve is typically available from a pump original equipment manufacturer (OEM) generic catalog, while a single-line curve is usually supplied with specific pump quote, or better yet, a factory-tested solution. In this example, a red angle denotes the pump-rated point (70 gallons per minute [gpm] at 100 feet [ft] head) where a pump is expected to operate, but the actual flow is found suspect by operators.
Pressure (Head) Method
Let’s assume that the discharge gauge reads 55 pounds per square inch gauge (psig) and the suction gauge reads 10 psig, thus a 45 psi pressure differential exists. This would correspond to 45 x 2.31 = 104 ft head (assuming cold water, specific gravity = 1.0). A horizontal
104 ft head line intersects the H-Q curve (at the proper impeller diameter, which is 5.12 inches in this case) at a little less than rated flow, approximately at 60 gpm.
Power (Amps) Method
The power curve indicates approximately 3.2 horsepower (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 EFFmotor x PF) / 1,000 Equation 1
In our example, a 5 hp 460 volt (V) motor is used and we actually read 450 V and 3.9 amps. A typical assumption of the product (EFFmotor 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) / 1,000 = 2.6 hp Equation 2
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
As a note on the power method, some people feel more comfortable simply taking the ratio of actual amps to the motor nameplate amps rating, then multiplying the result on motor rated power. In our example, if the motor rated amps were 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 (Ns) pumps, such as mixed flow and vertical turbine pumps.
As HI illustrates in Figure 3, 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 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 often long lost or misplaced for the old pumps. Even if they do exist, it can be 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, read it here)
Direct flow reading is the most sure way, but most pumps do not have in-line flow meters 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.
Editor’s Note: This column originally ran in the October 2007 issue of Pumps & Systems.