Flow measurement
Strengths
Limitations
Large installed base
Subject to wear
Comparatively inexpensive
Medium accuracy
Wide range of flow rates
Complex mass calculations
Simple, straightforward
Possible regular maintenance/
inspection
Extensive pipe diameters
Poor turn-down
Table 2: Strengths and limitations of differential pressure flowmeters.
Positive displacement flowmeters
PD flowmeters measure volumes of fluid by repeatedly filling and
discharging compartments of known volume, with gas from the
flowstream. There are various types of PD meter, using vanes, gears,
pistons, paddles or diaphragms to separate the fluid. They provide
high accuracy, but cannot handle dirty fluids and incorporate moving
parts that are subject to wear.
Strengths
Limitations
High accuracy
Clean gases only
Unaffected by flow profile
Subject to wear
High rangeability
Pressure drop issues
Higher maintenance costs
Low flow rates
Table 3: Strengths and limitations of positive displacement flowmeters.
Thermal flowmeters
Thermal flowmeters measure the mass flow of gases, employing
a combination of heated elements and temperature sensors, with
thermo-dynamic principles used to derive actual mass flow. They
do not require correction for changes in gas temperature, pressure
or density and are extremely accurate, especially when measuring
low and very low flow rates, and are no longer regarded as high
cost. Thermal meters are often limited to clean gas; however, with
the ‘non-bypass’ principle, there are no narrow pipelines to clog with
particulate, and they, therefore, can handle most gases.
Strengths
Limitations
Measure mass flow
Clean gases (bypass types only)
Highly accurate and repeatable High flow rates (above 11 000m
3
/hr)
No moving parts
Minimal maintenance
Low to very low flow rates
Table 4: Strengths and limitations of thermal flowmeters.
Turbine flowmeters
Fluid passing through a turbine flowmeter spins an axial rotor, the
rotation speed of which indicates flow velocity. They have a wide flow
range and offer a reasonable level of accuracy at an affordable price,
although they are restricted in use to clean, non-corrosive fluids and
measure only volumetric flow. Similar comments apply to paddle
wheel and pin wheel flowmeters, which translate the mechanical
action of paddles/ wheels into volumetric flow.
Strengths
Limitations
Familiar technology
Clean gases only
Medium purchase price
Need consistent velocity
Reasonable accuracy
Require straight pipe runs
Easy to install and maintain
Bearings subject to wear
Wide-flow rangeability
Need enough flow to spin
Table 5: Strengths and weaknesses of turbine flowmeters.
Ultrasonic flowmeters
Ultrasonic flowmeters use sound waves to determine the velocity of a
gas, employing either transit-time sound travel or Doppler frequency
shift techniques. Volumetric flow is determined by multiplying velocity
with the pipe area. Available in inline and clamp-on formats, ultrasonic
meters are non-obstructive and very accurate, although thismay require
multipathmeters, where flowrate is determinedby averaging the values.
Strengths
Limitations
Non-invasive (clamp-on)
Higher set-up costs
No moving parts
Pipe-wall interference (clamp-on)
Detect zero flow
Clean gases only (transit)
High life expectancy
Need particles (Doppler)
Wide range of larger pipe sizes
Known gas profiles only
Table 6: Strengths and limitations of ultrasonic flowmeters.
Variable area flowmeters
Also known as rotameters, VA flowmeters typically comprise a ta-
pered glass or plastic tube and an internal metering float, with the
volumetric flow rate proportional to the displacement of the float.
Among the oldest flow technologies, it is inexpensive and easy to
install, although historically had to be fitted vertically, and is sensi-
tive to changes in temperature, pressure and density. Interestingly,
a digital alternative has been recently introduced, the ‘mass-view’,
which offers greatly improved accuracy, electronic output signals
and no fragile glass components in the flow path, eliminating some
of the limitations of VA flowmeters.
Strengths
Limitations
Ease of installation
No data output (glass/ plastic
types)
Large installed base
Glass breakage issues
Low maintenance
Sensitive to condition changes
Simple technology
Moderate accuracy
Float sticking problems
Table 7: Strengths and limitations of variable area flowmeters.
Electricity+Control
December ‘13
16
