Orifice Flow

6 Key excerpts on "Orifice Flow"

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  eBook - PDF

  Solved Practical Problems in Fluid Mechanics

  • Carl J. Schaschke(Author)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  The pipeline has an inside diameter of 50 mm, and the orifice plate has a diameter of 35 mm. FIGURE 2.3 Location of a Venturi to a Bend 31 Flow Measurement The pressure drop across the orifice is measured using a U-tube manometer containing mercury. If the difference in levels of the legs of the manometer is 100 mm, determine the mass flow rate of the water. Solution Orifice meters are a form of constriction across the flow of a fluid in a pipe. The placing of such constrictions in pipelines is a common way to measure the rate of flow of process fluids. The measurement of flow is by way of measuring a difference in pressure caused by the fluid increasing its veloc-ity through the constriction and is therefore not a direct measure of flow. The pressure difference can be measured using pressure gauges or U-tube manometers from which the rate of flow can be conferred. Orifice meters are low-cost meters noted for their simplicity with no mov-ing parts and their accuracy. They are straightforward to install and main-tain and are capable of covering a wide range of flow rates (see Figure 2.5). They are, however, an intrusive form of measurement and a flow constriction results in large permanent energy losses. Depending on the application, the orifice itself can become eroded, for example, by sand or corrosive fluids or may be obstructed by solids such as wax or hydrate in the case of hydrocar-bon flow applications. Using a fixed aperture for the measuring of differential pressure, the rate of flow can be calculated. The converse is the variable area flowmeter or rota-meter that consists of a tapered tube within which a float is suspended by the upward flow of fluids. There is a fixed pressure drop across the float and vari-able area around the float in the tapered tube, which varies with elevation. The rate of flow through an orifice in a horizontal pipe is determined by applying the Bernoulli equation at an upstream position and at the orifice h d FIGURE 2.4 Venturi Lift

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  eBook - PDF

  Flow Measurement Handbook

  Industrial Designs, Operating Principles, Performance, and Applications

  • Roger C. Baker(Author)
  • 2016(Publication Date)
  • Cambridge University Press

    (Publisher)

  Downstream of this point, diffusion takes place with considerable total pressure loss. The data on which the orifice predictions are based may be presented in three ways (cf. Miller 1996): 1. The most accurate method is to use a discharge coefficient-Reynolds number curve for the required geometry, which includes all dimensional effects and other influences. 2. To reduce the number of curves, a datum curve is used in conjunction with cor- rection factor curves. This was essentially the procedure adopted for the British Standard 1042: Part 1:1964. 5.1 Introduction 117 3. For convenience with the advent of modern flow computers, the data are reduced to a best-fit equation. This is essentially the procedure with the most recent versions of the international standard, and its form will be dealt with in the following pages. The most common orifice plate is a metal disc spanning the pipe with a precisely machined hole in the centre of the plate; it is usually mounted between flanges on the abutting pipes, with pressure tappings fitted in precisely defined positions and to precise finishes. The differential pressure is measured by manometer, Bourdon tube or a pressure transducer, and the flow is deduced from the equations and probably computed using a flow computer. The importance of the orifice is its simplicity and predictability, but to achieve high accuracy it is essential that the detailed design of the meter is the same as that from which the original data were obtained, and that the flow profile entering the meter is also the same. To ensure that the details are correct, the national and inter- national standards lay down the precise requirements for constructing, installing and operating the orifice meter. It must be stressed that departing from the standard requirements removes the predictability and prevents the standard from being used to obtain the flow prediction.

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  Mechanical Measurements

  Jones' Instrument Technology

  • B E Noltingk(Author)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  This is termed a concentric orifice plate, see Figure 1.5(a). The plate would normally be clamped between adjacent flange fittings in a pipeline, a vent hole and drain hole being provided to prevent solids building up and gas pockets developing in the system, see Figure 1.5(b). The differential pressure is measured by suitably located pressure tappings on the pipeline on either side of the orifice plate. These may be located in various positions depending on the application (e.g. corner, D and D/2, or flange tappings), and reference should be made to BS 1042 Part 1 1964 for correct application. Flow rate is determined from equation (1.24). This type of orifice plate is inadequate to cope with difficult conditions experienced in metering dirty or viscous fluids and gives a poor disposal rate of condensate in flowing steam and vapours. Modified designs are utilized to overcome these problems in the form of segmental or eccentric orifice plates as shown in Figure 1.5(a). The segmental orifice provides a method for measuring the flow of liquids with solids in suspension. It takes the form of a plate which covers the upper cross-section of the pipe leaving the lower portion open for the passage of solids to prevent their build-up. The eccentric orifice is used on installations where condensed liquids are present in gas-flow measurement or where undissolved gases are present in the measurement of liquid flow. It is also useful where pipeline drainage is required. To sum up the orifice plate: Advantages 1 Inherently simple in operation 2 No moving parts 3 Long-term reliability 4 Inexpensive Disadvantages 1 Square root relationship 2 Poor turn-down ratio 3 Critical installation requirements 4 High irrecoverable pressure loss 1.3.1.2 Venturi tube The basic construction of the classical venturi tube is shown in Figure 1.6. It comprises a cylindrical inlet section followed by a convergent entrance into a cylindrical throat and a divergent outlet section.

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  eBook - ePub

  Process Engineering and Plant Design

  The Complete Industrial Picture

  • Siddhartha Mukherjee(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  11 Instrumentation and Controls

  DOI: 10.1201/9780429284656-11

  11.1 Introduction

  The term “instrumentation” in a process plant refers to a set of devices each of which measures, indicates and records parameters such as temperature, pressure, flow and level. Analyzers also fall in this category which measure the concentrations of one or more constituents of a process stream in gaseous or liquid phase.

  11.2 Flow Measurement

  11.2.1 Differential Pressure Type – Orifice Flowmeters

  In an Orifice Flowmeter, the pressure drop across an orifice installed in the flow path is used to estimate the flow (Figure 11.1 ). P1 is the pressure upstream of the orifice, P2 is the pressure at the vena contracta where the velocity is the maximum, and consequently the pressure P2 is the minimum.

  (11.1)

  P 1

  + ½ ρ

  v 1 2

  =

  P 2

  + ½ ρ

  v 2 2

  Assuming that the velocity profiles upstream and downstream of the orifice are uniform, the equation of continuity can be expressed as

  (11.2)

  Q L

  =

  v 1

  A 1

  +

  v 2

  A 2

  Combining Equations (11.1) and (11.2), and assuming A2 < A1 we arrive at the following equation:

  (11.3)

  Q L

  =

  A 2

  2

  P 1

  −

  P 2

  / ρ

  0.5

  1 −

  A 2

  A 1

  2

  0.5

  Equation (11.3), however, gives only a theoretical value of the flow. The area at the vena contracta A2 is not known since this is not easy to measure. Therefore, the area A2 is more conveniently taken as the area of the orifice itself. Furthermore, while P2 is the pressure at the vena contracta, the measured value of P2 may not exactly reflect the actual pressure at this point. In addition, losses may not be negligible as we have assumed.

  Figure 11.1 Orifice meter.

  To compensate for this error, Equation (11.3) is modified by adding a discharge coefficient, C

  d

  . Table 11.1 provides values of C

  d

  at different Reynolds numbers [1 ].

  (11.4)

  Q L

  =

  C d

  A 2

  2

  P 1

  −

  P 2

  / ρ

  0.5

  1 −

  A 2

  A 1

  2

  0.5

  Table 11.1 Discharge Coefficients – Orifice Meters

  Pipe Diameter, in	β Ratio D 2 /D 1	Reynolds Number

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  Flow Measurement

  By Square-Edged Orifice Plate Using Corner Tappings

  • W. J. Clark(Author)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  FART I-BASIS OF MEASUREMENT This page intentionally left blank INTRODUCTION Of all the methods available for the measurement of steady rates of flow of single-phase fluids, those based on determination of the pressure-drop developed across a constriction in a pipe through which the fluid is flowing full-bore are the most widespread. Constrictions may be of various types, viz. venturi tubes, nozzles, orifice plates, etc. Of these the orifice plate with a circular concentric square-edged orifice, using corner tappings connected by pressure pipes to the pressure-difference meter, is in considerable use in Britain and parti-cularly on the Continent where many thousands of installations are in continuous operation. The reasons for its choice can readily be seen after consideration of Fig. 1 which illustrates a number of typical constriction devices. Considering these in turn: A- ORIFICE PLATE B - NOZZLE C - VENTURI TUBE Fig. 1 — Examples of Primary Throttling Devices in General Use in Industry The venturi tube (Fig. 1C) is designed for good pressure recovery. It is expensive to manu-facture however and its very considerable length (up to 30 ft in a 3 ft diameter pipe) often introduces difficulties in installation. Its high cost is also a drawback especially if it becomes necessary to change the flow range of an installation to such an extent that a new tube having a different throat diameter is needed. It serves its most useful function in large pipes where power losses due to metering pressure-drop represent an appreciable operating cost or where satis-factory plant running necessitates a minimum overall pressure loss. Its use should also be con-sidered where the approximate measurement of the flow of dirty fluids is required as it does not obstruct the passage of suspended matter. The nozzle (Fig. IB) is compact but its specially designed inlet profile makes it much more expensive to manufacture than an orifice plate.

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  eBook - ePub

  Chemical Engineering Fluid Mechanics

  • Ron Darby, Raj P. Chhabra(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  In addition to the bulk flow meters, highly sophisticated techniques like laser Doppler anemometer (LDA), magnetic resonance imaging (MRI), and particle image velocimetry (PIV) are also available which allow the measurement of point velocities as well as the fluctuating components in complex flow geometries. As of now, owing to their high costs and limited range of applicability in terms of fluid medium, etc., their use is limited to research only. Detailed descriptions together with their advantages and disadvantages are available in the literature (Adrian and Westerweel, 2010; Zhang, 2010; Smits and Lim, 2012).

  SUMMARY The significant points that should be retained from this chapter include the following: •  The basic principles governing the operation of flow meters such as the pitot tube and obstruction meters such as the orifice, venturi, nozzle, etc. •  Application of the orifice meter for incompressible and compressible flows •  Determination of the unknown pressure drop, unknown flow rate, or unknown diameter for the orifice meter or other obstruction meter •  Awareness of the large variety of other non-invasive flow meters such as vortex shedding, magnetic, Coriolis, ultrasonic, etc. meters and where to find information on them PROBLEMS

  FLOW MEASUREMENT

  1.  An orifice meter with a hole of 1 in. diameter is inserted into a 1½ in. sch 40 line carrying SAE 10 lube oil at 70°F (SG = 0.93). A manometer using water as the manometer fluid is used to measure the orifice pressure drop reads 8 in. What is the flow rate of the oil in gallons per minute (gpm)?

  2.  An orifice with a 3 in. diameter hole is mounted in a 4 in. diameter pipeline carrying water. A manometer containing an immiscible fluid with a specific gravity (SG) of 1.2 connected across the orifice reads 0.25 in. What is the flow rate in the pipe in gpm?

  3.  An orifice with a 1 in. diameter hole is installed in a 2 in. sch 40 pipeline carrying SAE 10 lube oil at 100°F. The pipe section where the orifice is installed is vertical, with the flow being upward. Pipe taps that are 10.5 pipe diameters apart are used, which are connected to a manometer containing mercury to measure the pressure drop. If the manometer reading is 3 in., what is the flow rate of the oil in gpm?

  4.  The flow rate in a 1.5 in. ID line can vary from 100 to 1000 bbl/day, and you must install an orifice meter to measure it. If you use a differential pressure (DP) cell with a range of 10 in. H2 O to measure the pressure drop across the orifice, what size orifice should you use? After this orifice is installed, you find that the DP cell reads 0.5 in. H2

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