Which Flowmeter Shows Lowest Pressure Drop in Gas Service?    
David W. Spitzer, P.E.
Many processes can tolerate only a small pressure drop across a flowmeter. For example, many ventilation ducts operate at a pressure of only a few millimeters of water column. In these applications, flowmeter selection can be predominately predicated on the pressure drop across the flowmeter. Which of the following flowmeter technologies exhibit the lowest pressure drop for gas service?

A. Coriolis
B. Target
C. Thermal
D. Ultrasonic
E. Vortex shedding

Flowmeters that restrict the flow typically tend to exhibit more pressure drop than those that do not restrict flow. Target and vortex shedding flowmeters both contain restrictions to flow so Answers B and E are not correct.
Ultrasonic flowmeters tend to be full-bore flowmeters with no restrictions to flow, so Answer D is a candidate to exhibit the lowest pressure drop.
Small thermal flowmeters (Answer C) can present a restriction to the flow, but larger thermal flowmeters tend to have internal probe(s). The pressure drop across the probe(s) in generally small, but it will be larger than that of a full-bore ultrasonic flowmeter with no restrictions.
Coriolis mass flowmeters (Answer A) are available with straight single-tube designs that are full-bore flowmeters and do not restrict flow. Using this analysis, one might conclude that Coriolis mass flowmeters also exhibit the lowest pressure drop. However, Coriolis mass flowmeter installations should be considered in their entirety. In order to operate accurately, Coriolis mass flowmeters should not be operated at relatively low mass flowrates. Maintaining sufficiently high mass flowrates typically entails increasing the gas velocity. This is typically accomplished by reducing the size of the flowmeter — often by as many as three pipe sizes. While a straight single-tube flowmeter may appear to exhibit almost no loss of pressure, pipe reductions can cause significant pressure drop.
Therefore, full-bore ultrasonic flowmeters with no restrictions (Answer D) typically exhibit the lowest pressure drop of the listed flowmeters.

Additional Complicating Factors
Full-bore thermal flowmeters can exhibit almost the same (low) pressure drop as full-bore ultrasonic flowmeters. 

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 services. He can be reached at 845 623-1830.



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