Orifice plate meter

The orifice plate meter factor should be adjusted for actual operating parameters. For liquid streams, the flow meters should be adjusted for °API gravity, temperature, and viscosity. For gas streams, the flow rate should be adjusted for the operating temperature, pressure, and molecular weight. 

The Continuity and Bernoulli Equations may be used to derive equations relating flow rate to measured pressure difference for the Venturi meter (and the orifice plate meter discussed below). 

Orifice Plate Meter. Figure 9 illustrates an orifice plate meter. Here fluid is suddenly accelerated as it passes through an orifice in a plate, which is normally held between two flanges. Pressure is measured upstream (point 1) and downstream (point 2) of the orifice. There are a number of standard positions for the pressure tapings, D 79/2 ,... 

Figure 9 Orifice plate meter showing various tapping locations and pressure profile...

Orifice plate meter. The orifice plate is located between the flanged connection behind the pressure transmitter. The flanges are designed with pressure taps that connect to the pressure transmitter, which monitors the pressure drop across the orifice. The calibrated data acquisition systems continually record the flow. 

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). 

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. 

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)]. 

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. 

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. 

Flow Meter Orifice Plate - A flow meter orifice plate is permissible in normal process flow pressure reheving path, provided that it can pass the required emergency flow without exceeding pressure limits of the upstream equipment. However, it is not acceptable in PR valve inlets and flare headers. 

The orifice plate is simple to manufacture and has a relatively low cost. It does, however, create a quite large permanent pressure loss when installed in the ductwork. The venturi is smoothly shaped with a low permanent pressure loss but requires more space and is more expensive. The nozzle is a compromise betw een the orifice and the venturi. All three devices are standardized flow meters with very detailed descriptions of their geometry, material, manufacturing, installation, and use. -... 

Pressure-reducing stations, orifice plates, pitot tubes, and steam meters may all suffer from excessive mechanical wear and may therefore malfunction. 

Figure 6.19. Pressure distribution using orifice plate, venturi meter, and Dali tube. Pressure falls by 10% from upper pressure tapping to throat in each case...

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 total friction loss in an orifice meter, after all pressure recovery has occurred, can be expressed in terms of a loss coefficient, ATr, as follows. With reference to Fig. 10-12, the total friction loss is P — P3. By taking the system to be the fluid in the region from a point just upstream of the orifice plate (Pj) to a downstream position where the stream has filled the pipe (P3), the momentum balance becomes... 

A 2 in. sch 40 pipe carries a 35° API distillate at 50°F (SG = 0.85). The flow rate is measured by an orifice meter which has a diameter of 1.5 in. The pressure drop across the orifice plate is measured by a water manometer connected to flange taps. 

The coefficient of discharge Cd for a particular orifice meter is a function of the location of the pressure taps, the ratio of the diameter of the orifice to the inside diameter of the pipe dJdY, the Reynolds number in the pipeline Re, and the thickness of the orifice plate. 

A series of tap connections in an annular pressure ring gives a mean value for the pressure at point 1 in the approach section and also at point 2 in the throat. Although Venturi meters are relatively expensive and tend to be bulky, they can meter up to 60 per cent more flow than orifice plates for the same inside pipe diameter and differential pressure [Foust et al. (1964)]. The coefficient of discharge Cd for a Venturi meter is in the region of 0.98. Venturies are more suitable than orifice plates for metering liquids containing solids. 

GCa. Turbine meters are used for oil, ga and water measurement. While this is common practice for oil and water measurement, orifice plates have historically been used for gas measurement. By using turbine meters, much greater turndown ratios (approximately 30tl), are possible than with orifice plates, avoiding the need for an operator to change plates to accommodate different gas rates. 

The lesson is that near the top end of its range, the indicated flow is likely to be accurate, even if the meter is not well zeroed, or the measured AP is not too accurate. On the other hand, flowmeters using orifice plates cannot be very accurate at the low end of their range, regardless of how carefully we have zeroed them. Digitally displayed flows also follow this rule. 

Differential pressure transmitters (or DP cells) are widely used in conjunction with any sensor that produces a measurement in the form of a pressure differential (e.g. orifice plate, venturi meter, flow nozzle, etc.). This pressure differential is converted by the DP cell into a signal suitable for transmission to a local controller and/or to the control room. DP cells are often required to sense small differences between large pressures and to interface with difficult process fluids. Devices are available that provide pneumatic, electrical or mechanical outputs. 

The equipment system scheme is essentially the same as that shown in Fig. 6.14 but with two differences (1) The orifice plates are used for metering airflow rates and (2) Since the equipment is much larger than that used in the model investigation and therefore the feeding rate is much larger, the screw feeder for wet material feeding in the quasi industrial test is not as complex as that shown in Fig. 6.14 in structure, but is a common one. 

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