on the dial. The data point does not have to be the maximum flow on the dial, as many installations want to have the normal flowrate occur near the mid dial point. Differential pressure gauges with a magnetic movement are very practical for these applications because they can be pushed well beyond the max flow reading without damage (i.e. they can be “pegged” with no damage) as long as the max system pres- sure is not exceeded. The application shown here is intended to periodically prove a fire pump flow capability, in a closed loop, in case the sprinkler system is actuated. The typical magnetic pressure gauge is +/- 2 percent full scale accuracy, but when used as a flow gauge, the square root data is +/- 1 percent full scale, owing to the fact that the square root of 1.02 is approximately 1.01. The range of the pressure gauge is usually 1: 10, so the flow range is about 1: 3, and is sometimes called the turndown ratio.
Flow rate can be measured by
collection or by calculation,
based on known pipe sizes.
Pre-engineered bulges Second, the pressure differential does not have to be caused by a restriction, and in piping systems it actually should be a result of a “bulge” in the system which will not be a parasitic loss in flow. Picture two shows the set-up with the differential gauge cali- brated in flowrate in a non restrictive configuration. The system shown is calibrated for water flow at 4.0 psid cor- responding to 1,000 gpm, and is a permanently installed gauge. But in the case of a large piping system, the gauge may also be portable, connected to tap points when needed. The cost to hire consultants to measure the flowrates in waste- water systems can be quite high. However, if the piping systems have strategically placed pre- engineered bulges, the occasional measurement could be per- formed easily and inexpensively. WT
Joe Gordon is the owner and founder of Differential Pressure Plus Inc.
( www.differentialpressure.com), a manufacturer of differential pressure
gauges and switches. He holds a BSME and MSME from the University
of Connecticut and a MBA from the University of New Haven and has
worked in the fluid process industry for 40 years. Gordon volunteers on
the Branford Water Pollution Control Authority and is the chairman of
the Branford Economic Development Committee.
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