Section 4. Variable Annular Orifice Meters

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VARIABLE ANNULAR ORIFICE METERS
Section 4.
1.  INTRODUCTION.  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
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range.  The-flow rate measurements may be read and recorded locally or
remotely.
1.1 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.
2.  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.
2.1 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.
2.2 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.
3.  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.
4. 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
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