Tackling The Issues Of Entrained Gas

Jon Lawson

Coriolis mass flowmeters are increasingly specified for entrained gas applications and although they have challenges, leading Coriolis manufacturers have developed technologies that enable their meters to work with two-phase flow. Frank Grunert explains more.

The oil and gas sector continues to face considerable challenges due to the falling and consistently low price of energy. Despite this, companies are expected to maintain high standards of quality and performance while improving efficiency. To meet these challenges, forward-looking firms are turning to cutting-edge technologies to stay competitive.

One proven way to improve efficiency is through accurate flow measurement – which is critical in upstream, midstream and downstream O&G processes. However, entrained gas in liquid (two-phase flow) has traditionally had a significant effect on measurement and performance in O&G applications and although systems are designed to prevent or remove entrained gas, this is not always successful or even practical.

What Are The Problems Of Two-Phase Flow?

Two-phase flow causes problems for most flow measurement technologies. Some are unable to measure at all when entrained gas is present, others will measure the gas as liquid – giving a measurement error proportional to the gas volume fraction (GVF), and others will stop measuring when the GVF reaches a certain level. Adding a further complication, changes in process conditions can cause the flow regime and GVF to shift, making it difficult to predict and manage.

The Challenges of Entrained Gas

Entrained gas presents a particular challenge for Coriolis flowmeters that calculate fluid density from the frequency of the measuring tube as the fluid passes through it. The lower the frequency of the tubes, the greater the density – and vice versa. During two-phase flow the gas and liquid ‘decouple’ and move at different speeds through the flow tubes, which dampens the tube vibration. Varying process conditions also cause the flow regime, GVF and tube frequency to change rapidly.

In the past, this rapid change in frequency caused Coriolis flowmeters to ‘lose’ the signal from the sensors mounted on the measuring tubes. As well as giving wildly erratic and non-repeatable measurement, the meters would often freeze at the last confirmed reading or go into reset mode having assumed that an internal error had occurred, resulting in no measurement of the process.

What Are The Solutions?

The leading Coriolis manufacturers have developed technologies that enable their meters to work with two-phase flow (albeit sometimes with limitations) but have come at the problem from very different angles.

Krohne first provided a two-phase flow solution in 2012, with the inclusion of Entrained Gas Management (EGM) as a standard feature on the MFC 400 transmitter that is fitted to all Optimass flowmeters.

Using high-speed digital signal processing and software-based algorithms, EGM makes constant and precise corrections to the tube driver level based on real-time frequency information received from the sensors and driver feedback. This allows the flowmeter to provide continuous and repeatable mass flow and density measurement across a wide range of gas volume fractions and complex flow conditions. Although Krohne does not specify an accuracy (in the company’s experience, the shifting nature of the flow regime and GVF make each application unique), it does point to the fact that EGM has been successfully proven in use for many years.

Frank Grunnert is with Krohne