API RP 551(R1993) pdf download.Process Measurement Instrumentation
2.2 General
2.2.1 CA1 TEGORIES Certain basic procedures, practices, and precautions apply to the flow 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 flow measurement fall into the following categories: a. Differential-head meters measure flow inferentially from the differential pressure caused by flow 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 float within a vertical tapered tube will assume a posi- tion that is a function of the flow rate through the tube from the bottom. The float must have a density greater than that of the measured fluid. The annular area through which the flow must pass is the difference between the intemal area of the taper tube at the point of balance and the area of the float head. Since the internal area of the tube increases constantly and is continuously variable from bottom to top, whereas the float head area remains constant, the term variable area is used to describe this type of meter. At a constant differential pressure, flow is directly proportional to area. c. Magnetic meters are obstructionless meters that measure the volumetric rate of flow of any liquid that has the required electrical conductivity. Rate is determined using Faraday’s law of electromagnetic induction. d. Turbine meters measure volumetric fuid flow with a pulse train output, the frequency of which is picked up mag- netically from a rotor located in the flow stream and is lin- early related to flow rate.
2.3.1.1.2 Orifice Plate
The sharp- (square-) edged concentric orifice plate is the most frequently used element because of its low cost and adaptability and the availability of established coefficients. For most services, orifice plates are made of corrosion-resis- tant materials, usually Type 304 or 316 stainless steel. Other materials are used for special services. Eccentric orifices or segmental plates should be used for very dirty fiuids or slur- ries or wet gases; quadrant orifices should be used for vis- cous liquids. Advantages of orifice plates include good repeatability, ease of installation, use of one transmitter re- gardless of pipe size, low cost, the wide variety of types and materials available, and the relative ease with which they can be changed. Limitations of orifice plates include their sus- ceptibility to damage by foreign material entrained in the fluid and to erosion. A straight run of upstream and down- stream piping is required for an orifce plate. For details on orifice plates, refer to Chapter 14, Section 3, Part 1, of the API Manual of Petroleum Measurement Standards.
API RP 551(R1993) pdf download
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