For a fixed differential pressure measurement (say 100 inches of water column) in a differential-pressure steam flow measurement system, increasing density causes the compensated steam mass flow measurement to:
A. Decrease proportional to the square root of steam density
B. Decrease proportional to the steam density
C. Increase proportional to the square root of steam density
D. Increase proportional to the steam density
Commentary
Differential pressure flow measurement devices are commonly applied to measure steam flow. In addition, pressure and temperature measurements are typically implemented using individual or multivariable transmitters. The compensated steam mass flowrate can be calculated using flow computer functionality that can be implemented in a flow computer or other computational device.
For differential-pressure flowmeters, the volumetric flow (Q) is proportional to the square root of the ratio of the differential pressure (ΔP) divided by the fluid density (ρ).
Q α (ΔP / ρ) ½
However, the mass flow (W) is the product of the volumetric flow and density:
W α ρ ● Q = ρ ● (ΔP / ρ) ½ = (ρ ● ΔP) ½
Therefore, for a given raw differential-pressure measurement, the compensated mass flow of steam is proportional to the square root of the steam density. Answer C is correct.
Additional Complicating Factors
Flow computers typically use steam tables to determine the steam density. Using other devices with more rudimentary density algorithms (such as the gas laws) may result in significant mass flow measurement error.
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 at 845 623-1830.
