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.

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.