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Differential Pressure Flowmeters (Part 3 of 4)

By David W. Spitzer

E-Zine May 2007

Click here to review Part 1
Click here to review Part 2

The construction of differential pressure flow transmitters is such that its wetted parts can be made from materials that can withstand corrosion. In typical installations, impulse tubes are installed such that no flow occurs at the transmitter, so abrasion and wear are usually not important concerns. However, abrasion and wear can affect the performance of a differential pressure primary flow element by affecting its geometry. Differential pressure flow transmitters can measure the flow of many corrosive liquids, gases, and vapors. Differential pressure primary flow elements with appropriate geometries and materials of construction can withstand abrasive fluids.

Differential pressure flow transmitters can be constructed of materials that do not contaminate the fluid. However, they are not generally applied to sanitary service because of limitations on the ability to clean them.

Most differential pressure primary flow elements have straight run requirements, so they are usually not applied where limited straight run is available. In addition, differential pressure primary flow element technology has Reynolds number constraints, so it may find limited application in low flow applications, and where the liquid exhibits high or varying viscosity.

Differential pressure flowmeters measure velocity head, from which the fluid velocity is inferred, after which the volumetric flow rate is inferred. The differential pressure produced is a function of the square of the velocity, so this technology exhibits a relatively small flow turndown as compared with other flowmeter technologies. However, within Reynolds number constraints, the range of accurate flow measurement is relatively easy to change after installation.

Differential pressure flowmeter measurements are inherently affected by fluid density. Density changes in liquid applications are usually small because liquids of its non-compressible nature and because (in many applications) process temperature has a relatively small affect on density. In gas and vapor applications, both temperature and pressure can affect density and significantly degrade the quality of the flow measurement. Notwithstanding the above, note that changes in fluid composition can affect the density of the fluid.

Flow computers can be used to compensate for density (and other operating parameters) in applications where degradation of the flow measurement produces unacceptable flow measurement performance. A flow computer can be implemented as a separate hardware device that calculates the compensated flow measurement from field devices, such as differential pressure, pressure, and/or temperature instruments. These calculations can also be performed in the process control system. In addition to measuring flow, temperature and pressure, some multivariable differential pressure flow transmitters can perform these calculations internally.

Multivariable flowmeters, such as multivariable differential pressure flow transmitters and other multivariable flowmeter technologies are outside the scope of this report because even though there is some overlap with the information contained in this report, additional parameters are used to evaluate the relative performance of the different technologies.

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Excerpted from The Consumer Guide to Differential Pressure Flow Transmitters

ISSN 1538-5280

Spitzer and Boyes, LLC
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