DIFFERENTIAL PRESSURE METERS

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ORIFICE PLATE METERS

Orifice plate meters are the most common meter used in industry today. It is estimated that over 50 percent of the devices used for measuring fluids are orifice plate type. The widespread use of orifice plates provides a great deal of background and operational experience in a variety of situations.

1. Operating Principles: Orifice plates can be used to measure flow because of the velocity-pressure relationship that exists in a flowing fluid. When a restriction, such as an orifice plate, is inserted into a stream, the fluid velocity must increase when passing through the restriction. The increase in velocity is accompanied by a proportional drop in pressure on the downstream side of the orifice plate (Figure 5-l). Since the pressure drop across the meter is proportional to the square of the flow rate, it is possible to calculate the flow rate by measuring the differential pressure (alp) before and after the orifice. Instruments of this type are known as inferential meters as they do not physically measure the flow, but rather “infer’s it from the known relationship between pressure and velocity.

METER DESIGNS: There are different orifice plate designs such as square-edged, one-quarter circle, and conical. For the majority of flow measurements involving gases, air, steam, and water, the square-edged orifice plate is used. Other configurations are primarily designed to address particular situations such as high viscosity, erosive fluids, and fluids containing suspended material.

FIGURE 5-1. Typical Orifice Plate Conditions

1. Square-Edged Orifice: The orifice is sized to meet one specific anticipated flow rate. The upstream face of the orifice is flat with a square edge where the orifice meets the plate surface (Figure 5-2). If the side with a beveled or recessed edge is facing upstream, erroneous data will result.

1.1 Recommended Applications: Square-edged orifice metering is applicable on gas, liquid, and steam flow systems when pipe sizes are greater than 2 inches in diameter.

SPECIAL ORIFICES: These orifices are designed for special flow situations. One-quarter circle and conical entrance devices address low-flow and high-viscosity situations. Their use is limited. In an effort to prevent a buildup of debris on the upstream side of an orifice plate, eccentric orifice plates are used where moisture-laden gases are flowing and segmental orifice plates are used where a liquid containing a large percentage of gas is flowing (Figure 5-2).

LIMITATIONS: Orifice meters cause some permanent pressure loss due to friction. Pressure loss, increased friction, and increased pumping costs may make orifice metering undesirable. The range of this metering system is limited from 3:1 to 4:1. Turndown ratio can be increased by using two or more dp transmitters of different rangeabilities with an obvious increase in cost. They may be mounted in series and connected to a decision processor that will select the appropriate transmitter dependent upon the differential pressure (alp) . Since transmitters are expensive, the use of multiple transmitters is a tradeoff between precision requirements and cost effectiveness. Other limitations are as follows:

• Temperature range is to l,OOO°F.
• Pressure limit is 6,000 psig.

INSTALLATION: The location of the orifice plate in the system is important. Whenever possible, it is preferable to locate the primary element in a horizontal line. For accurate flow measurement, the fluid must enter the primary element with a fully developed velocity profile, free from swirls or vortices. In addition, fluid must exit the bevelled side of the orifice. Such a condition is best achieved by the use of adequate lengths of straight pipe, both preceding and following the primary element. The minimum recommended lengths of piping are shown in Figure 5-3. The diagram in Figure 5-3 that corresponds closest to the actual piping arrangement for the meter location should be used to determine the required lengths of straight pipe on the inlet and outlet. These lengths are those necessary to limit errors due to piping configurations to less than ±%0.5 percent. If these minimum distances are not observed, or if the orifice plate is installed with the bevel on the inlet side, flow equations and resultant flow calculations may produce inaccurate data.

1. Meter Installation: Common methods of installing orifice plate meters are described in the following paragraphs.

FIGURE 5-2. Orifice Plates

FIGURE 5-3. Recommended Minimum Pipe Lengths Before and After Differential Pressure Meters (From ASME Fluid Meters; used with permission) (Page 1 of 4)

Figure 5-3. Recommended Minimum Pipe Lengths Before and after Differential Pressure Meters (From ASME Fluid Meters; used with permission) (Page 2 of 4)

Figure 5-3. Recommended Minimum Pipe Lengths Before and after Differential Pressure Meters (From ASME Fluid Meters; used with permission) (Page 3 of 4)

Figure 5-3. Recommended Minimum Pipe Lengths Before and after Differential Pressure Meters (From ASME Fluid Meters; used with permission) (Page 4 of 4)

FIGURE 5-4. Orifice Meter With Flange Taps

1.1 Orifice Flanges: Special orifice flanges are the most commonly recommended method for meter installation. The pressure taps are drilled into the flanges themselves, which are welded onto the pipe. The orifice is inserted and secured between the two flanges (Figure 5-4).

1.2 Carrier Rings: Carrier rings are the second q ost common method of orifice plate installation. Pressure taps, typically corner taps, are drilled into the rings and the orifice plate is inserted between the rings. The rings and orifice are then inserted between existing pipe flanges (Figure 5-5).

1.3 Existing Flanges and Special Taps: The orifice plate can be inserted between existing pipe flanges and pressure taps drilled into the pipe. This method was widely used in the past, but has since been replaced with the more standardized orifice flanges.

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