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 parts, any improper functioning of a moving part can be considered a
limitation. Limitations associated with standard operating ranges are as
follows:
Temperature range:
-200C (-328F) to +460C (+860F)
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
5. 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.
6. 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.
7. 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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