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Pumps & Systems, October 2007

In my August column ("Tricks to Taking Flow Measurements in the Field"), I compared the pros and cons of one of several techniques that can be used to estimate pump flow when troubleshooting pump operations under less-than-desirable conditions: directly measuring it with a flowmeter.

One of our readers, Don Casada of Diagnostic Solutions, LLC (Knoxville, TN), responded to that discussion with criticism of the direct measurement method from several perspectives, including the correct positioning of the external flowmeter on a pipe with appropriate requirements for a certain length of straight pipe; the absence of obstructions and bends; and similar HI guidelines.

This month, we continue our discussion by examining two other 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.

Figure 1. Pump performance curve in a combined format. Source: 2004 Goulds Pumps Manual, ITT Industries

Figure 2. Pump performance curve in a single-line format. Source: 2004 Goulds Pumps Manual, ITT Industries

A combined format curve is typically available from a pump OEM generic catalog, while a single-line curve is usually supplied with specific pump quotation, or better yet, a factory-tested solution. In this example, a red triangle denotes the pump-rated point (70-gpm at 100-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-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, SG = 1.0). A horizontal 104-ft head line intersects the H-Q curve (at the proper impeller diameter, which is 5.12-in here in this case) at a little less than rated flow, approximately at 60-gpm.