PRODUCTION • PROCESSING • HANDLING
T
he 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 effi ciency. To meet these challenges, forward- looking fi rms are turning to cutting-edge technologies to stay competitive. One proven way to improve effi ciency is through accurate fl ow measurement – which is critical in upstream, midstream and downstream O&G processes. However, entrained gas in liquid (two-phase fl ow) has traditionally had a signifi cant eff ect 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. Two-phase fl ow causes problems for most
fl ow 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 fl ow regime and GVF to shift, making it diffi cult to predict and manage.
A SPECIFIC CHALLENGE Entrained gas presents a particular challenge for Coriolis fl owmeters that calculate fl uid density from the frequency of the measuring tube as the fl uid passes through it. T e lower the frequency of the tubes, the greater the density – and vice versa. During two-phase fl ow the gas and liquid ‘decouple’ and move at diff erent speeds through the fl ow tubes, which dampens the tube vibration. Varying process conditions also cause the fl ow regime, GVF and tube frequency to change rapidly. In the past, this rapid change in
frequency caused Coriolis fl owmeters to ‘lose’ the signal from the sensors mounted on the measuring tubes. As well as
A bank of six DN400 straight tube Coriolis fl owmeters with Entrained Gas Management installed in a refi nery
TACKLINGENTRAINED GAS
Frank Grunert explains why Coriolis mass fl owmeters are increasingly specifi ed for entrained gas applications
giving wildly erratic and non-repeatable measurement, the meters would often freeze at the last confi rmed reading or go into reset mode having assumed that an internal error had occurred, resulting in no measurement of the process. T e leading Coriolis manufacturers have developed technologies that enable their meters to work with two-phase fl ow (albeit sometimes with limitations) but have come at the problem from very diff erent angles. Krohne fi rst provided a two-phase fl ow solution in 2012, with the inclusion of Entrained Gas Management (EGM) as a standard feature on the MFC 400 transmitter that is fi tted to all Optimass fl owmeters. 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. T is allows the fl owmeter to provide continuous and repeatable mass fl ow and density measurement across a wide range of gas volume fractions and complex fl ow conditions. Although Krohne does not specify an accuracy (in the company’s experience, the shifting nature of the fl ow regime and GVF make each application unique), it does point to the fact that EGM has been successfully proven in use for many years.
BELOW: Krohne’s Entrained Gas Management system identifi es two- phase fl ow and alerts the operator to check the process for entrained gas while continuing to measure
Frank Grunert is with Krohne.
www.krohne.com
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