Orifice Plate Flowmeters |
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The orifice plate is one in a group known as head loss devices or differential pressure flowmeters. In simple terms the pipeline fluid is passed through a restriction, and the pressure differential is measured across that restriction. Based on the work of Daniel Bernoulli in 1738, the relationship between the velocity of fluid passing through the orifice is proportional to the square root of the pressure loss across it. Other flowmeters in the differential pressure group include venturis and nozzles. With an orifice plate flowmeter, the restriction is in the form of a plate which has a hole concentric with the pipeline. This is referred to as the primary element. To measure the differential pressure when the fluid is flowing, connections are made from the upstream and downstream pressure tappings, to a secondary device known as a DP (Differential Pressure) cell. From the DP cell, the information may be fed to a simple flow indicator, or to a flow computer along with temperature and/or pressure data, which enables the system to compensate for changes in fluid density. In horizontal lines carrying vapors, water (or condensate) can build up against the upstream face of the orifice. To prevent this, a drain hole may be drilled in the plate at the bottom of the pipe. Clearly, the effect of this must be taken into account when the orifice plate dimensions are determined. Correct sizing and installation of orifice plates is absolutely essential, and is well documented in the International Standard ISO 5167.
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