TABLE OF CONTENTS Differential pressure or head-type flowmeters represent one of the most commonly used flowmeter technologies. Differential pressure is the basis of flow because without it there can be no flow. For example, picture a thin section of fluid in a pipe. If an equal pressure were applied to both sides of the section (P 1 = P 2), the fluid would just sit still. However, if one pressure is greater than the other (P 1 > P 2), flow would take place from the high-pressure region to the low-pressure region. The greater the differential pressure, the higher the rate of flow. Given this information, it is easy to see that flow rates are proportional to differential pressure. Mathematically, flow is expressed as a square root function of the differential pressure, as shown in the following equation:
Some type of primary element is used in the pipeline to restrict flow and create the differential pressure. This pressure is measured by a differential pressure transmitter and a flow rate is indicated.
General installation practices that apply to pressure transmitters should also be applied to differential pressure transmitters measuring flow. In addition, some special considerations are listed below.
A general rule is that there should be four pipe diameters of straight pipe upstream of the orifice plate (or other primary element) and two pipe diameters of straight pipe downstream.
The most popular type...
