Flow Measurement and Control

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Flow Measurement and Control
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Orifice Meter
The orifice meter consists of an accurately
machined and drilled plate concentrically mounted
between two flanges. The position of the
pressure taps is somewhat arbitrary.
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Orifice Meter
The orifice meter has several practical
advantages when compared to venturi meters.

• Lower cost
• Smaller physical size
• Flexibility to change throat to pipe diameter
  ratio to measure a larger range of flow rates

• Disadvantage
• Large power consumption in the form of
  irrecoverable pressure loss

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Orifice Meter
The development of the orifice meter equation is
similar to that of the venturi meter and gives

• where
• ratio of orifice diameter to pipe diameter
  0.5 usually
• S0 cross sectional area of orifice
• V bulk velocity through the orifice
• C0 orifice coefficient 0.61 for Re gt 30,000

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There is a large pressure drop much of which is
not recoverable. This can be a severe limitation
when considering use of an orifice meter.
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Pressure Loss inOrifice Meters
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ASME Design Standards

• Fluid Meters Their Theory and Applications, 6th
  ed., American Society of Mechanical Engineers,
  New York, 1971 pp. 58-65.

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Rotameters
Rotameters fall into the category of flow
measurement devices called variable area meters.
These devices have nearly constant pressure and
depend on changing cross sectional area to
indicate flow rate. Rotameters are extremely
simple, robust devices that can measure flow
rates of both liquids and gasses.
Fluid flows up through the tapered tube and
suspends a float in the column of fluid. The
position of the float indicates the flow rate on
a marked scale.
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Rotameters
Three types of forces must be accounted for when
analyzing rotameter performance
Buoyancy

• Flow
• Gravity
• Buoyancy

Gravity
For our analysis neglect drag effect
Flow
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Rotameter
Mass Balance Assume Gradual Taper
Flow Between Float and Tube
S3 is annular flow area at plane 3
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Rotameter

• Momentum Balance
• Note
• p3 p2
• Must account for force due to float

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Rotameter
Mechanical Energy Balance
0
(Base velocity head on smallest flow area)
Assume
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Rotameter
Combining Momentum and Mechanical Energy Balance
After Some Manipulation
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Rotameter
Assuming Sf S a discharge coefficient can be
defined
CR must be determined experimentally. As Q
increases the float rides higher, the assumption
that Sf S is poorer, and the previous
expression is more nearly correct.
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Other Flow Meters
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Turbine Meter
Measure by determining RPM of turbine (3) via
sensor (6). Turbine meters are accurate but
fragile.
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Coriolis Meters
When fluid is passed through a U-bend, it imposes
a force on the tube wall perpendicular to the
flow direction (Coriolis force). The deformation
of the U-tube is proportional to the flow rate.
Coriolis meters are expensive but highly accurate.
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Pneumatic Control Valves
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Orifice Meter Example

• A 2 in. Schedule 40 pipe carries 35º API
  distillate at 50 F (SG0.85). The flow rate is
  measured by an orifice meter which has a diameter
  of 1.5 in. The pressure drop across the orifice
  plate is measured by a water manometer connected
  to the flange taps. If the manometer reading is
  20 in. of H2O, what is the flow rate of the oil
  in GPM ?

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Now what ???