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Beyond Orifices For DP Flowrate Measurements

By Steve Pagano

While the orifice is the most common flowmeter restriction associated with differential pressure (DP) measurements, several others have found a solid place in process applications. Various designs of DP flowmeters provide for an optimum adaptation to the operating conditions and requirements of the user. An important consideration is the pressure drop, which as a rule of thumb should be as small as possible. Flowmeter technologies such as venturi tubes, nozzles, wedges, and flow tubes are characterized by very small pressure drops which leads to reduced energy loss and pumping requirements.

All DP primary elements restrict the flow is some way. According to Bernoulli’s law of conservation of energy, a restriction in a pipe results in an increase in the fluid velocity. The ensuing conversion to kinetic energy reduces the static pressure. This pressure drop is the measured differential pressure, which is proportional to the square root of the flow rate. The differential pressure phenomenon is a universal metering principle for flowmetering. This article reviews the attributes of flowmeter DP elements other than orifices.

Flowmeters based on differential pressure represent a popular choice in the processing industries, constituting a large plurality of the installations. Differential pressure meters are used for liquid, gas, and steam metering in both normal and extreme high temperatures and pressures. The characteristics of DP flow elements have been optimized by extensive research activities over decades and the results are published as standard. Generally they are easy to select for a specific application. They have no moving parts and can be fabricated in a wide selection of materials. Their purchase price is relatively low, even for large pipe sizes. Accuracy is moderate, ranging from 1 to 5 percent, but compensation techniques can improve these values to better than 1 percent.

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