Plate orifice

Enough space must be available to properly service the flow meter and to install any straight lengths of upstream and downstream pipe recommended by the manufacturer for use with the meter. Close-coupled fittings such as elbows or reducers tend to distort the velocity profile and can cause errors in a manner similar to those introduced by laminar flow. The amount of straight pipe required depends on the flow meter type. For the typical case of an orifice plate, piping requirements are normally Hsted in terms of the P or orifice/pipe bore ratio as shown in Table 1 (1) (see Piping systems). 

Table 1. Required Straight Pipe Lengths for Orifice Plates, Nozzles, and Venturis ...

Fig. 6. Orifice plates (a) concentric, (b) eccentric, (c) segmental, (d) universal, and (e) quadrant-edge.

Fig. 7. Orifice plate pressure tap locations. A, corner taps B, flange taps C, D taps D, 1/2 D taps and E, pena contracta taps. See text.

For very low flow rates the orifice plate is often incorporated into a manifold, an integral part of the differential-pressure transmitter. This provides a convenient compact installation. 

Flow Nozzles. A flow nozzle is a constriction having an eUiptical or nearly eUiptical inlet section that blends into a cylindrical throat section as shown in Figure 8. Nozzle pressure differential is normally measured between taps located 1 pipe diameter upstream and 0.5 pipe diameters downstream of the nozzle inlet face. A nozzle has the approximate discharge coefficient of an equivalent venturi and the pressure drop of an equivalent orifice plate although venturi nozzles, which add a diffuser cone to proprietary nozzle shapes, are available to provide better pressure recovery. 

Measurement of Fluid Flow hy Means of Orifice Plates, Nobles and Venturi Tubes Inserted in Circular Cross Section Conduits Punning Full, ISO 5167-1980(e), International Organization for Standardization, Geneva, Switzerland, 1980. 

Continuous processes for copolymer production were developed initially for the microporous resins. The system generally involves injecting the monomer mix into the aqueous phase through orifice plates. Droplet size is controUed by the diameter of the holes in the plate and the rate at which the monomer is injected into the aqueous phase. The continuous process produces copolymer beads which have greater uniformity in size than those produced in batches. 

Flow. The principal types of flow rate sensors are differential pressure, electromagnetic, vortex, and turbine. Of these, the first is the most popular. Orifice plates and Venturi-type flow tubes are the most popular differential pressure flow rate sensors. In these, the pressure differential measured across the sensor is proportional to the square of the volumetric flow rate. 

The proper installation of both orifice plates and Venturi-type flow tubes requires a length of straight pipe upstream and downstream of the sensor, ie, a meter mn. The pressure taps and connections for the differential pressure transmitter should be located so as to prevent the accumulation of vapor when measuring a Hquid and the accumulation of Hquid when measuring a vapor. For example, for a Hquid flow measurement in a horizontal pipe, the taps are located in the horizontal plane so that the differential pressure transmitter is either close-coupled or connected through downward sloping connections to allow any trapped vapor to escape. For a vapor measurement in a horizontal pipe, the taps should be located on the top of the pipe and have upward sloping connections to allow trapped Hquid to drain. 

Capacity. Pumps deHver a certain capacity, Q, sometimes referred to as flow, which can be measured directly by venturi, orifice plate (11), or magnetic meters (12) (see Flow measurement). The indirect way to determine capacity is often used. Whereas this method is less accurate than applying a flow meter, it often is the only method available in the field. The total head is measured and the capacity found from the pump head—capacity (H— curve (Fig. 2). More recently, sonic flow meters (13) have been used, which can be installed on the piping without the need for pipe disassembly. These meters are simple to use, but require relatively clean single-phase Hquid for reHable measurements. 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

A regulator is a compact device that maintains the process variable at a specific value in spite of disturbances in load flow. It combines the functions of the measurement sensor, controher, and final control element into one self-contained device. Regulators are available to control pressure, differential pressure, temperature, flow, hquid level, and other basic process variables. They are used to control the differential across a filter press, heat exchanger, or orifice plate. Regulators are used for monitoring pressure variables for redundancy, flow check, and liquid surge relief. 

Corner taps. Static holes drilled one in the upstream and one in the downstream flange, with the openings as close as possible to the orifice plate. 

FIG. 10-19 Coefficient of discharge for square-edged circular orifices for Nrs > 30,000 with the upstream tap located between one and two pipe diameters from the orifice plate [Spitzglass, Trans. Am. Soc. Mech. Eng., 44, 919 (1922).]... 

The discharge coefficient of sharp-edged orifices was shown by Benedict, Wyler, and Brandt [J. Eng. Fower, 97, 576-582 (1975)] to increase with edge roundness. Typical as-purchased orifice plates may exhibit deviations on the order of 1 to 2 percent from ASME values of the discharge coefficient. 

Gas-Liquid Mixtures An empirical equation was developed by Murdock [J. Basic Eng., 84, 419 33 (1962)] for the measurement of gas-liquid mixtures using sharp-edged orifice plates with either radius, flange, or pipe taps. 

An equation for use with venturi meters was given by Chisholm [Br Chem. Eng., 12, 454—457 (1967)]. A procedure for determining steam quahty via pressure-drop measurement with upflow through either venturi meters or sharp-edged orifice plates was given By Colhus and Gacesa [J. Basic Eng., 93, 11-21 (1971)]. 

Rietz Extruder This extruder, shown schematically in Fig. 18-49, has orifice plates and baffles along the vessel. The rotor carries nmlti-ple blades with a forward pitch, generating the head for extrusion through the orifice plates as well as battering the material to break up agglomerates between the baffles. Typical applications include wet... 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

Systematic Measurement Error Fourth, measurements are subject to unknown systematic errors. These result from worn instruments (e.g., eroded orifice plates, improper sampling, and other causes). While many of these might be identifiable, others require confidence in all other measurements and, occasionally, the model in order to identify and evaluate. Therefore, many systematic errors go unnoticed. 

Instrumentation Calibration may be required for the instruments installed in the field. This is typically the job of an instrument mechanic. Orifice plates should be inspected for physical condition and suitabihty. Where necessary, they should be replaced. Pressure and flow instruments should be zeroed. A prehminary material balance developed as part of the prehminary test will assist in identifying flow meters that provide erroneous measurements and indicating missing flow-measurement points. 

Orifice plates. Either the concentric orifice, eccentric orifice, or segmented orifice-type. Choice depends on the quality of the fluid handled. Venturi tubes. These consist of a well-rounded convergent section at the entrance, a throat of constant diameter, and a divergent section. Their accuracy is high however, installation, unless planned for in advance, is very difficult in the field. 

ASMEflow nozzle. These nozzles provide for accurate measurements. Their use is limited because they are not easily placed in a process plant however, they are excellent for shop tests. Venturi meters and nozzles can handle about 60% more flow than orifice plates with varied pressure losses. 

Velocity traverse techniques must be used when, due to the configuration in piping, nozzles, or orifice plates, etc., cannot be used. 

Primary sensory device not installed (i.e., orifice plate, temperature bulb). 

Orifice plates (install after checking bore and location)... 

Orifice Mixing Column An orifice mixing column consists of a series of orifice plates contained in a pipe. The pipe normally is fabricated of two vertical legs connected by a return bend at the bottom with the orifice plates installed between flanges in the vertical legs. Typical use is for cocurrent contacting in caustic and water washing operations. 

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