QUIZ CORNER: Meas. Forward & Reverse Flow

Regardless of technology, this discussion assumes the flowmeter is installed in a manner consistent with bidirectional measurement. In particular, upstream straight-run requirements should be maintained both upstream and downstream of the flowmeter element so as to ensure that a good velocity profile enters the flow element from both directions.

Most differential-pressure flow elements are designed to accurately measure flow in one direction. However, some differential flow elements, such as segmental wedges, certain orifice plates, and laminar flow elements are often symmetrical and can accurately measure flow in both directions with appropriate secondary instrumentation.

Magnetic flowmeters are largely symmetrical, such that many designs allow flow measurement in both directions. Magnetic flowmeters often have independent totalizers for forward and reverse flow. Ultrasonic flowmeters are similar to magnetic flowmeters in that they are typically symmetrical.

Turbine flowmeters are not necessarily symmetrical, however their rotors can spin in both directions so reverse flow can be measured.Accurate flow measurement in the reverse direction is typically predicated on calibrating the turbine flowmeter in the reverse direction also.

Additional Complicating Factors

While not recommended, differential-pressure flow elements can measure flow in both directions — but not necessarily accurately. In extenuating circumstances, such as when the primary element is mistakenly installed backwards, any measurement (even if in error) can be overwhelmingly better than no measurement at all.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup,

troubleshooting and teaching process control instrumentation. Mr. Spitzer

has written over 10 books and 150 technical articles about instrumentation

and process control, including the popular “Consumer Guide” series that

compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.