Endress + Hauser’s Stevens agrees that, “Coriolis mass flowmeters need to recognize the presence of two-phase flow conditions and take action to ensure that the flowmeter does not stall. Coriolis mass flowmeters designed for two-phase flow typically measure bubble effect and plug flow. Each application is individually reviewed for applicability with regard to the fluid temperature, viscosity, velocity, bubble distribution and installation effects to determine the anticipated measurement accuracy. In particular, a more uniform phase distribution occurs in low-viscosity liquids flowing in vertical piping as compared to the same flow in a horizontal pipe where stratification might occur. Higher-viscosity fluids tend to exhibit measurement problems due to relatively poor bubble distribution at low velocities. Fluids flowing at higher velocity that use a backpressure element have better bubble distribution and exhibit less measurement error.”
Stevens adds that uncompensated flow measurement errors can approach 40% of the measured flow in some services and installations. “Overall, addressing two-phase flow issues means that the flowmeter design needs to be able to recognize flow changes. This is largely accomplished by using high sampling rates and robust sensor drive circuits to increase measurement stability. Digital electronics are designed to identify and accurately compensate for these changes.”
Micromotion’s Patten agrees with the potential magnitude of the error, but states that, “Coriolis mass flowmeters are relatively unaffected by two-phase flow when the presence of entrained air is approached as a transient condition. As such, the mass flow and density can be accurately determined from first principles when measurements are performed sufficiently fast.”
“Previous Coriolis mass flowmeter designs alarmed and turned the flowmeter off when air was present. This reinforced the notion that Coriolis mass flowmeters were not able to measure two-phase flows. However, current designs account for two-phase conditions so flow errors have been reduced from approximately 40% to less than 2%. Similarly, current designs allow the measurement errors associated with filling applications that start and stop with empty pipes have been reduced from approximately 2% to negligible.”
Sonar-based correlation flowmeters can measure the flow rate and gas void fraction for continuous mixtures with up to 20% entrained gas (by volume). Dan Gysling, chief technology officer at flowmeter manufacturer CiDRA explains, “Sonar-based correlation flowmeters use an array of passive pressure sensors clamped to the outside of the pipe to measure the mixture velocity to determine the flow rate and speed of sound through the mixture to determine the gas void fraction. Combining these measurements provides the liquid flow rate and gas-void fraction for two-phase gas/liquid mixtures operating in the bubbly regime. This eliminates many of the measurement challenges associated with applying single-phase flowmeters to two-phase flow.”
Measuring two-phase flow in industrial applications is a perplexing problem. Using single-phase flowmeters to measure these flows has often been far from effective and has created additional problems and increased installed costs in many applications. Fortunately, off-the-shelf “non-custom” flowmeters designed to measure two-phase flow are available and can solve many of these problem applications.
David W. Spitzer is a principal in Spitzer and Boyes, LLC, and can be reached at 1.845.623.1830 or www.spitzerandboyes.com.