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Positive
displacement (PD) flowmeters are truly a member of the class of traditional
technology flowmeters. Their history
goes back to 1815, when Samuel Clegg invented the first PD gas flowmeter. This was a water sealed rotating drum meter.
Clegg’s son-in-law, John Malam, together with Samuel Crosley, invented
an improved model in 1825. Problems
remained, however, with high cost, freezing, and large size.
Thomas Glover
invented the first “dry” gas diaphragm meter in 1843.
Glover’s meter contained two diaphragms and a sliding valve.
In 1844, the first actual “dry” gas meter was developed by engineers
Croll & Richards. The
diaphragm meters used today are similar to these early meters, although major
improvements have been made in material of construction.
Early meters had diaphragms made of sheepskin with steel metal
enclosures. More recent meters have
synthetic rubber-on-cloth diaphragms and are made of cast aluminum.
Bopp &
Reuther of Germany
holds the earliest patent for the oval gear meter in 1932.This meter was later popularized by Oval Corporation of
Japan
, which introduced the oval gear meter in the early 1950s. Oval gear meters are used for liquid measurement.
They are quite popular for oil applications, especially for downstream oil distribution
involving custody transfer. Here they compete with Coriolis meters, which are gaining market
share in downstream oil measurement.
Click to learn
more about
How they
work
PD flowmeters operate by repeatedly filling and emptying
compartments of known volume with the liquid or gas from the flowstream.
Flowrate is calculated based on the number of times these compartments are
filled and emptied. The main types of PD flowmeters used for gas applications
are diaphragm and rotary. These types of meters differ according to the way they
trap the liquid into compartments with different shapes.
Diaphragm meters
have several diaphragms that capture the fluid as it passes through the meter.
Differential pressure across the meter causes one diaphragm to expand and one to
contract. A rotating crank mechanism helps produce a smooth flow of gas through
the meter. This mechanism is connected via gearing to the index, which registers
the amount of fluid that passes through the meter. Diaphragm meters are used for
gas applications.
Rotary flowmeters have one or more rotors that are used to trap the fluid. With
each rotation of the rotors, a specific amount of fluid is captured. Flowrate is
proportional to the rotational velocity of the rotors. Rotary meters are used
for gas applications.
One strength of PD meters is high
accuracy. They are also very good at measuring fluid at low fllowrates. Downsides of positive displacement meters include causing pressure drop and being essentially a
mechanical meter with moving parts that are subject to wear.
Applications
PD
meters are highly accurate meters used that are widely used
for custody transfer applications. They
are widely used for custody transfer of commercial and industrial water.
They are also used for custody transfer of hydrocarbon liquids to and
from delivery trucks. PD
meters have the advantage that they have been approved by a number of regulatory
bodies for this purpose, and they have not yet been displaced by other
applications. PD meters are also
used for custody transfer of natural gas.
Positive displacement meters do best in line sizes between 1½ inches and 10 inches. It is
unusual to find PD meters in line sizes above 10 inches.
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