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| David W. Spitzer, P.E.
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In gas
applications, gas-pressure fluctuations can affect the velocity of the
gas, thus dramatically impacting gas flow measurement instruments. For
example, the line pressure at the flowmeter in a gas stream may
normally be 1.3 bar. However, under certain operating conditions, the
pressure may be 2.6 bar for extended periods of time. If a flowmeter
measures the gas velocity of a given mass of gas, approximately how
much will the flow measurement be affected by the increased gas
pressure?
A. 50 percent decrease
B. 35 percent decrease
C. No effect
D. 55 percent increase
E. 100 percent increase.
Commentary
Increasing the
pressure causes the gas to become compressed, so the gas density is
higher, and the gas will occupy a smaller volume. In other words, if a
given mass of gas is flowing, its volume is smaller. Because the same
mass of the gas flows through the same pipe, the velocity of the
compressed gas will decrease. Therefore, if the flowmeter measures gas
velocity, for the given mass flow, the flow measurement will also
measure lower at 2.6 bar. Therefore, no effect (Answer C), a 55 percent
increase (Answer D), and a 100 percent increase (Answer E) are not
correct.
At 1.3 bar, the absolute pressure of the gas is approximately 2.3
bar absolute. At 2.6 bar, the absolute pressure of the gas is
approximately 3.6 bar absolute. Using the Ideal Gas Law, the gas volume
at 2.6 bar is (2.3/3.6) or approximately 64 percent of the volume at
1.3 bar. This means that the gas velocity at 2.6 bar is approximately
64 percent of the gas velocity at 1.3 bar, and the gas flow measurement
at 2.6 bar is approximately 64 percent of the gas flow measurement at
1.3 bar. This represents a (100-64), or approximately 36 percent change
from the flow measurement of the gas at 1.3 bar, so Answer B is the
most reasonable response.
Pressure compensation of the flowmeter should be considered due
to the relatively large effect of pressure fluctuations in this
measurement.
Additional Complicating Factors
The gas under
consideration here was assumed to be an ideal gas that obeys the Ideal
Gas Law. If the gas were non-ideal, the gas flow measurement would
likely be in error by another few percent.
If the flow measurement were the primary measurement in a flow
control loop, the flow controller would modulate the valve to achieve
the desired flow measurement. Because the flowmeter measures
approximately 36 percent low, the flow controller would increase the
control valve position to make the flowmeter measure the desired flow.
The flow-control loop would increase the flow measurement by
approximately (100/64) or 56 percent. In this case, the flow-control
loop would deliver approximately 56 percent more gas at a pressure of
2.6 bar than at a pressure of 1.3 bar.
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 has written a number
of books concerning the application and use of fluid handling
technology, including the popular "Consumer Guide" series, which
compares flowmeters by supplier. 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 845 623-1830.
www.spitzerandboyes.com
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