API RP 551:1993(R2007) pdf download.Process Measurement Instrumentation
2.2 General
2.2.1 CATEGORIES Certain basic procedures, practices, and precautions apply to the ßow instruments discussed throughout this recom- mended practice. Where applicable, the material covered in this section should be considered a part of the text of the dis- cussions in subsequent sections. Common devices for ßow measurement fall into the following categories: a. Differential-head meters measure ßow inferentially from the differential pressure caused by ßow through a primary el- ement. Flow is proportional to the square root of the differ- ential pressure produced. This differential is sensed by diaphragms, bellows, or manometers. b. Variable-area meters (rotameters) work on the principle that a ßoat within a vertical tapered tube will assume a posi- tion that is a function of the ßow rate through the tube from the bottom. The ßoat must have a density greater than that of the measured ßuid. The annular area through which the ßow must pass is the difference between the internal area of the taper tube at the point of balance and the area of the ßoat head. Since the internal area of the tube increases constantly and is continuously variable from bottom to top, whereas the ßoat head area remains constant, the term variable area is used to describe this type of meter. At a constant differential pressure, ßow is directly proportional to area. c. Magnetic meters are obstructionless meters that measure the volumetric rate of ßow of any liquid that has the required electrical conductivity. Rate is determined using FaradayÕs law of electromagnetic induction. d. Turbine meters measure volumetric ßuid ßow with a pulse train output, the frequency of which is picked up mag- netically from a rotor located in the ßow stream and is lin- early related to ßow rate. e. Positive-displacement meters measure ßow by mechani- cally trapping successive volumetric segments of the liquid passing through the meter body.
2.2.2 TRANSMISSION PRACTICE
Hydrocarbons or other process ßuids should not be piped to any instruments located in a control room. Standard indus- try practice is to convert the ßow measurement to an electri- cal or pneumatic signal and transmit the signal to remote receiving instruments. It is also standard practice to transmit the ßow measurement in local installations where long pip- ing or other methods would otherwise be required. Examples include cases in which solids present in the process ßuid cause plugging or in which differences in elevation could re- sult in head problems. Insulation and heating of long lines to prevent freezing are also minimized or eliminated by the use of transmission systems.
API RP 551:1993(R2007) pdf download
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