Orifice metering

Section 10 of this Handbook describes the use of orifice meters for flow measurement. In addition, orifices are commonly found within pipelines as flow-restric ting devices, in perforated pipe distributing and return manifolds, and in perforated plates. Incompressible flow through an orifice in a pipehne as shown in Fig. 6-18, is commonly described by the following equation for flow rate Q in terms of pressure drop across the orifice Ap, the orifice area A, the pipe cross-sectional area A, and the density p. 

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. 

Orifice Meter The most widely used flowmeter involves placing a fixed-area flow restriction (an orifice) in the pipe carrying the fliiid. This flow restriction causes a pressure drop that can be related to flow rate. The sharp-edge orifice is popular because of its simplicity, low cost, and the large amount of research data on its behavior. For the orifice meter, the flow rate for a liquid is given by... 

Orifice Meters A square-edged or sliarp-edged orifice, as shown in Fig. 10-18, is a clean-cut square-edged hole with straight walls perpendicular to the flat upstream face of a thin plate placed crosswise of the channel. The stream issuing from such an orifice... 

Rate of discbarge through an orifice meter is given by Eq. (10-8) for either hquids or gases. For the case of subsonic flow of a gas (/ < / < 1.0), the expansion factor Y for orifices is approximated by... 

General Principles The underlying principle of an ideal area meter is the same as that of a head meter of the orifice type (see subsection Orifice Meters ). The stream to be measured is throttled by a constriction, but instead of observing the variation with flow of the differential head ac-ross an orifice of fixed size, the constriction of an area meter is so arranged that its size is varied to accommodate the flow while the differential head is held constant. 

Sharp-edged weirs have edges like those of square or sharp-edged orifices (see subsection Orifice Meters ). Notched weirs are or(iuarily sharp-edged. Weirs not in the sharp-edged class are, for the most part, those described as broad-crested weirs. 

AGA. Manual of Petroleum Measurement Standards. Chapter 14—Natural Gas Fluids Measurement, Section 3 Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids. AGA Report No. 3. (ANSI/API 2530, 1985 and CPA 8185, 1985). American Gas Association, Cleveland, Ohio. 

In the sampling train itself, the gas flow must be measured to determine the sample volume. Parhculates and gases are measured as micrograms per cubic meter. In either case, determination of the fraction requires that the gas volume be measured for the term in the denominator. Some sample trains contain built-in flow-indicahng devices such as orifice meters, roto-meters, or gas meters. These devices require calibration to assure that they read accurately at the time of the test and under test conditions. 

Dali Flow Tube - The advantage is this type of flowmeter is that it has a permanent head loss of only 5 % of the measured pressure differential. This is the lowest pressure drop of all orifice meter designs. Flow ratios as high as 1 10 (e.g., 1.0 to 10 kg/s) can be measured within + 2% of actual flow. Dali flow mbes are available in different materials and diameters up to 1500 mm. 

Figure 13-6B. Effect of pulsation filter on gas compression pulsations at orifice meter from 200-550 psig. (Used by permission von Nimitz, W. W., and O. Flanigan, Oil and Gas Journal, p. 68, Sept. 8, 1980. PennWell Publishing Company. All rights reserved.)...

Design suction and discharge bottles for an ethylene compressor. Suction drum is to be sized for an orifice meter a short distance upstream. Discharge drum is to be sized for prevention of piping vibration. 

Meter for Measuring Air (or Gas) Volume. A standard orifice meter is generally used to measure air (or gas) injection volume rates. 

The orifice meter, in which the fluid is accelerated at a sudden constriction (the orifice) and the pressure developed is then measured. This is a relatively cheap and reliable instrument though the overall pressure drop is high because most of the kinetic energy of the fluid at the orifice is wasted. 

Figure 6.14. (a) Orifice meter (b) Venturi meter (c) Nozzle... 

Reynolds number through orifice Figure 6.16. Coefficient for orifice meter... 

Because of its simplicity the orifice meter is commonly used for process measurements, and this instrument is suitable for providing a signal of the pressure to some comparator as indicated in Figure 6.1. 

This meter may thus be considered as an orifice meter with a variable aperture, and the formulae already derived are therefore applicable with only minor changes. Both in the orifice-type meter and in the rotameter the pressure drop arises from the conversion of pressure energy to kinetic energy and from frictional losses which are accounted for in the coefficient of discharge. The pressure difference over the float —AP. is given by ... 

The flow of water through a 50 mm pipe is measured by means of an orifice meter with a 40 mm aperture. The pressure drop recorded is 150 mm on a mercury-under-water manometer and the coefficient of discharge of the meter is 0.6. What is the Reynolds number in the pipe and what would you expect the pressure drop over a 30 m length of the pipe to be ... 

The flow is also metered using a 15 cm orifice plate across which the pressure differential is 50 mm on a mercury-undcr-waler manometer. What is the coefficient of discharge of the orifice meter ... 

The flow of liquid in a 25 mm diameter pipe is metered with an orifice meter in which the orifice has a diameter of 19 mm. The aperture becomes partially blocked with dirt from the liquid. What fraction of the area can become blocked before the error in flowrate at a given pressure differential exceeds 15 per cent Assume that the coefficient of discharge of the meter remains constant when calculated on the basis of the actual free area of the orifice. 

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