(1) Design and construction. There are typically three basic subassemblies in a positive displacement
meter.
(a) External housing. This is the pressure vessel with inlet and outlet connections. Meters may
be of single or double case construction. The single case acts as a pressure vessel and as the outer wall of the
measuring element. Small meters (less than 6 inches) are normally single case. Double case construction is used
on meters over 6 inches. The advantage of double case construction is the piping stress or pressure is not
transmitted to the measuring element.
(b) Internal measuring element. This measures the volumetric flow by continuously separating a
flow stream into discrete volumetric segments and counting them. It is also used to drive the counter.
(c) Counter drive train (counter or totalizer). The gear ratio of the drive train is designed to
convert the fixed volume per revolution to the number of gallons or barrels and is transmitted to the counter. The
calibrator (adjustor) is used to adjust the counter for slippage through the meter. It may be used if the meter is
outside the authorized tolerance.
(2) Operation and maintenance.
(a) Because of the design of the positive displacement meter, it must be protected from dirt and
trash. Any solid material passing through the meter will cause it to malfunction.
(b) The meter will also register any air passing through the housing. The clearance between the
housing and the vane is usually .004 inches to .005 inches.
(c) Because of their simplicity, positive displacement meters are the most commonly used in
petroleum operations and are used as master meters in the place of provers.
b. Turbine meters. Turbine meters are precise measuring devices, even though they are classified as
inferred rate meters. They go from simple measuring devices to very complex instruments, and are much smaller
than positive displacement meters. There are three basic subassemblies in a conventional turbine meter.
(1) Design and Construction.
(a) Meter housing. The mete housing assembly is constructed of a flanged pipe spool and houses
the internal parts.
(b) Internal parts. The heart of the internal parts subassembly is the rotor blade suspended in the
flowing stream on the platform bearing and rotor shaft.
(c) Detector Subassembly. In the more complicated meters, you have the viscosity compensator
and the magnetic reed switch. In this kind of meter, the flow is corrected to 60ƒF.
(2) Operation and maintenance.
QM5200
17-2
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