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Variable annular orifice meters are permanently installed in piping to measure mass or volume flow of gases, liquids, or vapors. Variable annular orifice meters do not have a fixed orifice area as do other differential pressure meters, i.e., orifice plate, pitot tube, and venturi. Variable orifice meters produce results on a linear scale, with large rangeability, reliable repeatability, and greater operating temperature range. The-flow rate measurements may be read and recorded locally or remotely.
1. Meter Designs: Changing the area of the orifice with relation to flow rate is accomplished in variable annular orifice meters by two main designs. One design uses a movable plug, the other uses a movable orifice.
OPERATING PRINCIPLES: Introduction of an orifice into a flow line produces a differential pressure that is measurable and relative to the flow rate. If the area of the orifice is fixed, the differential pressure produced is dependent upon inlet swirl and velocity profile and is proportional to the square of the velocity of the fluid. This results in a nonlinear scale and small rangeability. However, if the orifice is annular and its area increases with the flow rate, the differential pressure is directly proportional to the flow rate. Variable orifice meters do change the area of the orifice in relation to flow rate by axial movement of a plug or bellows.
1. Movable Plug: The movable plug design has a fixed orifice with a contoured plug mounted on a movable shaft (Figure 5-17)0 AS the flow rate increases, the plug moves axially in the direction of flow resulting in a larger orifice area. This produces a pressure drop that is linear and directly proportional to the flow rate.
2. Movable Orifice: The movable orifice design has a fixed contoured plug and provides for the movement of the orifice that is attached to a bellows (Figure 5-18). As the flow rate increases, the bellows compresses causing an increase in the annulus. This enlargement of the annulus results in a pressure drop that produces a differential pressure change directly proportional to the flow rate.
APPLICATIONS: Variable annular orifice meters are versatile in their application. Specific operating characteristics and capabilities are dependent upon the manufacturer. Generally, they can be used with most liquids such as water, petroleum distillates, chemical compounds, and crude oil. Use with gases is virtually unlimited. Metering of vapors includes steam, ammonia, and Freon.
LIMITATIONS: The major limitation of the variable annular orifice meter is that it requires system shutdown for installation since it is located axially in the pipe. Because variable annular orifice meters have moving parts, any improper functioning of a moving part can be considered a limitation. Limitations associated with standard operating ranges are as follows:
  • Temperature range: -200°C (-328°F) to +460°C (+860°F)
  • Pressure range: 0 to 2,500 psi
  • Maximum viscosity: 100 cps
  • Turndown ratio: 100:1
  • Pipe size: 1/2- to 16-inch
  • Straight pipe requirements: 6D upstream and 3D downstream
Movable Plug
FIGURE 5-17. Movable Plug
Movable Orifice
FIGURE 5-18. Movable Orifice
INSTALLATION: Installation of a variable annular orifice meter is similar to repairing a section of pipe or installing a T-section. The line must first be shut down. A section of the pipe is removed and mounting flanges that accept the flanges of the respective meter are put in place. The meter is then attached according to manufacturer’s recommendations. A distinct advantage of the variable orifice meter is that usually only six diameters (6D) of straight pipe are required upstream of the meter installation and three diameters (3D) downstream. Sensor mountings can be for local or remote readout. Attachments for remote reading may be for pneumatic or electronic transmission.
MAINTENANCE: Maintenance on the most common variable annular orifice meters is negligible, but the dp cell , sensors, data transmission lines, and processor require regular monitoring and maintenance. Maintenance schedules should be developed according to manufacturer’s recommendations. The only components that may require attention are the contoured plug and the movable bellows. Since inspection requires system shutdown, it is warranted only if an unreasonable or unexplainable change occurs during regular monitoring of meter data. Monitoring of data is a viable indicator for needed inspection since the repeatability of these meters is usually 0.25 percent of full scale.
ACCURACY AND RELIABILITY: The accuracy of variable annular orifice meters is ±%1.0 percent of full scale if calibrated to specified tolerances. No specific quantitative data is available on the reliability of variable annular orifice meters, but the mean-time-between-failures is usually a matter of years. Repeatability over the entire range contributes significantly to the large turndown ratio.
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