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McCrometer Newsletter April 2002

Permanent Pressure Loss or Head loss with Flow Meters

Head loss is an important factor to consider when designing a flow measurement system. Its effects can be very serious, in the worst case, causing fluid to simply stop flowing. Here we look at how the choice of flowmeter affects head loss.

Head loss can be regarded as the gradual drop in fluid pressure due to friction effects from a pipe wall when fluid flows through a closed pipe. Any deviation from a perfectly smooth, straight pipe will increase the head loss, also known as the permanent pressure loss. For example, bends, T-pieces, headers, valves, very rough pipes or steps between pipe-sections all contribute to head loss. And of course, flowmeters that have parts in the flowing medium also have an effect.

Here's an example of the costly significance of head loss. When Canadian engineers were planning to increase the quantity of gas delivered to the US, they found that if they continued using the existing Orifice meters, the additional head loss would force them to buy an expensive compressor. Instead, they decided to replace the Orifice meters with Ultrasonic Gas Meters, which use a non-intrusive measuring method with a head loss equivalent to a straight length of pipe. Buying the Ultrasonic meters rather than a new compressor turned out to be the most cost effective solution.

System designers should consider not only the intrusiveness of the flowmeter in the flow stream, but also the additional length of pipe required to give an acceptable reading. For example, V-Cone meters and Venturi meters have similar permanent pressure drops through the device. However, the V-cone requires simply 0 to 3D length of pipe upstream, whereas the Venturi may require from 4D to 19D upstream. The V-cone is very effective in minimizing pressure loss due to the design of the cone and the minimal pipework required when it is installed.

To find out more about McCrometer's line of flowmeters and the effect of head loss on flow measurement design, contact McCrometer at 951-652-6811 or visit our web-site at <http://www.mccrometer.com>.

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Copyright 2002 McCrometer