Measuring Flow Below Atmospheric Pressure

For the past few installments of “Applications Corner,” we’ve been considering some unique applications that I’ve run into over the years. Last month we examined instrument selection for below absolute zero pressure. This month we’ll consider how processes that operate at pressures below atmospheric pressure impact flow measurement.

In gas applications, the gas tends to expand when its pressure is reduced so its density decreases. Stated differently, for a given gas volume, a smaller number of gas molecules pass through the flowmeter. This phenomenon can affect different flowmeters in different ways. For example, many vortex-shedding flowmeters require a certain amount of energy to operate their sensors. This constraint typically manifests itself as a minimum velocity constraint that increases with decreasing gas density. Therefore, the minimum velocity for gases operating below (and even at) atmospheric pressure can be so high relative to process constraints that there is insufficient energy to operate the flowmeter. Consider also that operating near a full vacuum means that almost no molecules are going through the flowmeter.

Liquid applications can be a bit more insidious, in that multiple physical properties may need to be considered. In most liquid applications, the amount of pressure drop available for the flowmeter is limited so the size of the flowmeter may have to increase in order to pass the flow without violating process constraints. This typically reduces measurement accuracy because the flowmeter will tend to operate lower in its flow range where it is typically less accurate.

Pressures below atmospheric pressure tend to be closer to the liquid’s vapor pressure, which is the pressure at which the liquid will boil (flash). The formation of bubbles upstream of the flowmeter results in two-phase flow that generally causes flowmeters to measure inaccurately and, in some applications, causes the flowmeter to cease operating.

Formation of bubbles (flashing) within the flowmeter can have similar results. However, cavitation can occur if the pressure recovers and increases above the vapor pressure of the liquid. When this occurs, the increased pressure causes the bubbles to implode and reform liquid. Not only can this adversely affect the performance of the flowmeter, but it can also damage the flowmeter.

The presence of pressure below atmospheric pressure can even damage flowmeters. For example, some magnetic flowmeter liners cannot withstand pressures below atmospheric pressure because their liners can be sucked into the pipe. It should be noted that some processes that appear to operate under positive pressure can operate at pressures below atmospheric pressure if vapor in the pipe condenses.

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 845 623-1830.

www.spitzerandboyes.com