I heard about entrained gas, but it was never really clear to me exactly what it was until I read an article by Tim Patten, Handling Entrained Gas, in Flow Control magazine. Tim is the director of measurement technology for Emerson’s Micro Motion Coriolis flow and density measurement business.
In the article, he describes three categories of entrained gas: slug, bubble and empty-full-empty flow. Each poses unique challenges for flow and density measurements. Slug flow is large bubbles forming in liquids and usually found in improperly or incompletely filled, long-distance piping. Other sources of bubbles are leaks in pump suction or tank agitation, which cause air to be introduced into the line.
Slug flow is large bubbles forming in liquids and usually found in improperly or incompletely filled, long-distance piping. Other sources of bubbles are leaks in pump suction or tank agitation, which cause air to be introduced into the line.
Bubble flow as the name suggests is more of a continuous distribution of gas bubbles in a liquid process. These bubbles are commonly found in highly viscous liquids, like toothpaste or peanut butter, as Tim puts it. Other causes include high-speed agitation or pumping. Pumps with broken seals or ones that cavitate can also introduce bubble flow in the process.
Empty-full-empty (EFE) flow is common in batch-type processes. This batching is commonly found with railcar and tanker truck loading. A variation is multiphase flow, with multiple density phases with some degree of separation and a mixing layer between them. This type of flow is common for oil and gas producers with the pipelines containing gas, oil, and water before going through the separation process.
These three entrained gas scenarios and their impact on flow and density measurements led to research and development efforts to improve these measurements. Tim describes results from this R&D:
…that four key elements played a role in the entrained gas performance for Coriolis meters — the signal processing speed, processing algorithms, sensor design, and meter stability independent of environmental changes.
Entrained gas in slug flow and EFE caused frequent and large disturbances in the flow measurement. Digital signal processing at a very high rate in the Coriolis electronics allows many variables to be simultaneously measured and synchronized with the disturbances to allow these disturbances to be filtered out of the signals sent back to the automation system. Tim cites an improvement from 20% error rates with traditional measurement technologies to 1% with today’s Micro Motion Elite Coriolis flow and density meters.
For bubble flow conditions, Tim describes the importance of the sensor design and sensor stability. He describes why:
Sensor design is important because the critical bit of information is the relative difference in motion of the bubbles and fluid. Sensor stability is important because sensor vibration during bubble flow can be noisy and can cause the sensor to couple to the environment.
By getting this design and stability right, the noise introduced by the bubbles will cause minimal flow measurement errors.
These technologies allow measurements on applications once thought not possible or in applications where a problem has introduced entrained gas, such as a leaky pump seal.