How would a decrease in temperature of a liquid with a viscosity of 1000 cP tend to affect the pressure drop and size of a positive-displacement flowmeter?
A: Pressure drop increases and size increases
B: Pressure drop increases and size decreases
C: No pressure drop or size changes
D: Pressure drop decreases and size increases
E: Pressure drop decreases and size decreases
Decreasing the liquid temperature tends to increase its viscosity. Increasing viscosity tends to increase the pressure drop across the flowmeter. Therefore Answers C, D and E are not correct.
The pressure drop across positive-displacement flowmeters is limited because operating with a high pressure drop across the flowmeter tends to wear and potentially damage its bearings. In a given application, a larger flowmeter exhibits a lower pressure drop.
Sizing positive-displacement flowmeters involves selecting a flowmeter large enough that its pressure drop is within the pressure drop limits of the flowmeter. Therefore, increasing viscosity will tend to increase the pressure drop across the flowmeter, which can potentially increase the size of the flowmeter. Answer A is correct.
Additional complicating factors
Lower temperature tends to increase viscosity, increase the pressure drop across the flowmeter, and potentially increase its size. Whether or not an existing flowmeter is still applicable or requires replacement requires more detailed investigation.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic, marketing consulting, distribution consulting, and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. David has more than 40 years of experience and has written over 10 books and 300 articles about flow measurement, instrumentation and process control. He can be reached at 845 623-1830 or via spitzerandboyes.com. Click on the “Products” tab to find his Consumer Guides to various flow and level measurement technologies.