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| David W. Spitzer, P.E.
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A flow control loop
consisting of a flowmeter, controller, and control valve is used to
control the flow of a gas to a process. Measurements from a pressure
transmitter installed some time ago indicate the incoming air pressure
is typically two bar, but that it can vary from one to 2.5 bar due to
process upsets that occur upstream. How will these pressure variations
affect the flow measurement, assuming the flowmeter is calibrated to
operate at two bar?
Commentary
First, it is the
absolute pressure of the gas that is important when measuring flow. The
pressure of a standard atmosphere is 1.01325 bar absolute, but to make
the mathematics simpler here, we will approximate an atmosphere to be
one bar absolute. Making this approximation, the typical operating
pressure is three bar absolute with variations that can reach from two
to 3.5 bar absolute.
Second, many gas
flow measurements are inferential mass measurements in units expressed
as a standard cubic volume per unit time. However, regardless of the
gas pressure, raw flowmeter measurements reflect the mass, volume,
velocity, or velocity head of the gas.
If the flowmeter
measures mass flow, there will (in theory) be no effect on the flow
measurement. That is, a mass meter should not be affected by process
pressure variations.
However, if the
flowmeter measures volume or velocity, the effects are inversely
proportional to the variation in absolute pressure. For example, if the
gas were operating at a pressure of 3.5 bar absolute, its volume would
be approximately 3/3.5, or 85.7 percent of what it would be if the
operating pressure were three bar absolute. As a result, the flowmeter
would measure approximately 14.3 percent lower than it would if the
same mass of gas were flowing at three bar absolute.
The output of
flowmeters that infer mass flow by measuring the differential pressure
across a restriction is also inversely affected by variations in
operating pressure. However, the relationship is not one-to-one as in
the case of flowmeters that measure volume or velocity, but rather
approximately 0.5-to-one. In the above example, the effect of operating
at 3.5 bar absolute is approximately 0.5*14.3, or 7.2 percent. A more
accurate approximation can be calculated as one-sqrt (3/3.5), or 7.4
percent.
David W. Spitzer,
P.E., is a regular contributor to Flow Control. He has more than 30
years of experience in specifying, building, installing, startup, and
troubleshooting process control instrumentation. He has developed and
taught seminars for over 20 years and is a member of ISA and belongs to
the ASME MFC and ISO TC30 committees. Mr. Spitzer is currently a
principal in Spitzer and Boyes LLC, offering engineering, product
development, marketing, and distribution consulting for manufacturing
and automation companies. He can be reached at 845 623-1830.
www.spitzerandboyes.com
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