By David W. Spitzer, CONTROL Contributor
One of the cardinal rules of liquid flow measurement is to ensure that the flowmeter and its associated piping are completely full of liquid and that no noncondensable gas, such as air, remains in the metering run. Locating the flowmeter in a vertical pipe, configuring the piping so that the flowmeter remains submersed and orienting the flowmeter so it remains full of liquid can often eliminate noncondensable gases from accumulating in the metering run.
But what if the process itself has practical limitations that prohibit the elimination of noncondensable gases? Perhaps more perplexing—what if the gas is a necessary and desirable component of the product whose flow you want to measure? In such cases, removing the gas is simply not an option. Doing so may make the product unsaleable. Yet the flow must be measured.
Jim Reizner, a flow specialist at Procter & Gamble, Cincinnati, Ohio, confronts this problem on a regular basis. “Measuring the flow of shampoo is not that difficult. However, recent shampoo formulations include bubbles of air that enhance the appearance and hence salability of the final product in its translucent container,” he explains.
Traditionally, ignoring the issue was the approach to measuring two-phase flows. This typically entailed installing a single-phase flowmeter that may or may not have had a correction factor to account for two-phase flow. Single-phase flowmeter suppliers often fostered this paradigm by citing theoretical performance without the results of testing under two-phase flow conditions. Other suppliers tested their flowmeters, but did not release detailed information describing the tests they did perform.
Measurement of two-phase flow involves the measurement of the mass or volumetric flow of the liquid phase, the gas phase or total flow. Applications exist for all permutations of these parameters. However, the total mass flow and the volumetric flow of the liquid phase where the gas-phase volume is relatively small are the most common two-phase flow measurements encountered.
Reizner adds, “We are interested in assuring that the consumer receives the proper amount of product that is sold in volumetric units. Therefore, we need to measure the volume of the liquid phase accurately, so that we do not charge the consumer for bubbles.”
One approach to solving the two-phase flow measurement problem is to remove the gas phase and measure the liquid phase that remains with a single-phase flowmeter. This approach adds cost and size to the installation and creates the secondary problem of how and where to vent the gas phase.
Two-Phase Coriolis Mass Flowmeter
Many two-phase Coriolis mass flowmeter problems involve accuracy degradation in certain processes.
The phase distribution and velocity profile present yet another challenge to measuring two-phase flows containing liquid and gas. Flow in horizontal pipes can exhibit stratified, wavy, semi-slug, slug and annular phase distributions. Slug, churn, annular and bubble phase distributions are more common in vertical piping. The gas-void fraction, liquid viscosity, fluid velocity and orientation of the piping influence which of these phase distributions is present. Two-phase flowmeters typically identify these phase distributions and compensate for their presence.
In this article we’ll limit the discussion to liquid flows containing entrained gas with phase distributions that exhibit bubble flow and slug flow. These phase distributions tend to be present in fluid flows with a relatively low gas-void fraction that travel at relatively high velocities. Identifying the phase distribution is not necessarily easy because the piping affects the type of phase distribution present. For example, bubble flow in a vertical pipe can change to stratified flow in horizontal piping because the bubbles tend to rise and form a large bubble at the top of the pipe as the fluid travels.
Coriolis Mass Flowmeter Issues
Single-phase flowmeters applied to two-phase flow include Coriolis mass, differential pressure, turbine, ultrasonic and vortex-shedding flowmeters. In theory, turbine, ultrasonic and vortex-shedding flowmeters measure the velocity of the entire flow stream. The velocity measurement generated by these flowmeters poses a fundamental measurement problem when the liquid-phase flow is desired because varying amounts of gas affect their measurements. Pressure effects create additional measurement problems. Coriolis mass flowmeters exhibit similar problems when they are used to measure the volume of two-phase flow. Differential pressure flowmeters are affected by density so that varying amounts of gas and its pressure also create measurement problems.
Coriolis mass flowmeters can accurately measure the mass flow of liquids or gases. In principle, Coriolis mass flowmeters to measure the total mass flow and fluid density to allow calculation of the flow of all of the permutations cited above. However Reizner notes that “approximately 80% of flow measurement problems involve Coriolis mass flowmeters. Further, approximately two-thirds of Coriolis mass flowmeter problems involve some aspect of gas entrainment.” In particular, higher frequency straight-tube designs tend to be more affected by two-phase flow conditions.
Nitrogen Padded Tank Causes Entrained Gas
A two-phase flowmeter sometimes can handle entrained gas in a padded tank.
Jerry Stevens, senior marketing manager for flow at Endress+Hauser
adds, “approximately 90% of two-phase Coriolis mass flowmeter problems we encounter involve accuracy degradation during a process that involves filling an empty flowmeter, flowing for a period of time and then emptying the flowmeter.”