David W. Spitzer, P.E.

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

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.