Expert guidance for measuring the flow of aerated liquids
Measuring flow in a liquid hydrocarbon application becomes considerably more challenging when aeration complicates the picture. Aeration in a pipeline has the potential to wreak havoc on many flow devices by wearing them down and negatively impacting accuracy. In the worst-case scenario, it can prevent a meter from functioning at all.
Oil and gas operators pay a very high price for unreliable flow data and unplanned process downtime resulting from this type of meter failure. If not properly addressed, aeration threatens not only profitability, but worker and environmental safety as well. Today’s hydrocarbon industry relies on several different technologies to measure the flow of aerated liquids, but all flow systems are not created equal.
Differential pressure (DP) technology has been used widely throughout the industry for decades. DP meters measure flow by way of a primary element such as an orifice plate, which creates a pressure-reducing restriction in the liquid and relays measurements to a transmitter through impulse lines. DP is a tried-and-tested method available at a comparatively low cost, but it is generally not the most suitable choice when aeration is involved.
If impulse lines are not precisely oriented to compensate for the frequency of bubbles, slugs and settled solids within a particular liquid flow, the lines can easily become blocked or develop gas voids, impeding the measurement process and reducing accuracy. Additionally, should aeration occur as liquid passes through an orifice plate, the pressure drop will force bubbles to expand and lead to cavitation. In a process characterised by high-pressure or high-velocity flow, constant cavitation can wear down the orifice plate and over time may even cause the plate to fail entirely.
Clamp-on ultrasonic flow technology is another alternative, and it offers the cost-effective benefit of externally mounted sensors that do not require pipe alterations or regular maintenance. The sensors send and receive acoustic signals through the pipe wall and the flowing liquid. A transmitter then calculates flow velocity by measuring the differences in transit time between sound waves transmitted with and against flow, or the frequency shift of the signal as it encounters bubbles or other sonically reflective materials (known as the Doppler effect).
Although some transit-time sensors can measure liquids with minimal levels of aeration, too much will interrupt the ultrasonic signal and send the meter into a fault condition. Doppler mode is capable of measuring flow with higher concentrations of aeration; however, its lower accuracy and repeatability precludes it from being used in most hydrocarbon applications.
Coriolis measurement
Coriolis measurement has long been favoured as the best-performing flow technology available to the industry, particularly for custody-transfer applications. As Coriolis flowmeters calculate mass flow directly, they can achieve very high levels of precision even in rapidly fluctuating process conditions – a crucial advantage when valuable crude is changing hands.
But even Coriolis technology is often hampered by aeration. Most traditional Coriolis meters are able to measure liquids with an aeration content up to approximately 6%. Beyond this range, flow tube damping is likely to occur, and the meter will no longer be able to supply the drive energy and gain necessary to maintain tube oscillation. Many transmitters also cannot respond quickly enough to the constant sensor signal changes stemming from aeration, potentially triggering a stall.
In recent years, automation companies such as Siemens have ramped up investments in Coriolis R&D to better address the needs of the hydrocarbon market. As Siemens embarked on design work for its new digital Coriolis flow systems, the Sitrans FC430 and FC410, a major goal was to improve performance in the presence of aeration – while also reducing operational downtime from aeration-induced meter failure. Both models feature an advanced driver circuit with a gain higher than 20 and a high-speed signal update rate of 100Hz. As a result, the FC430 and FC410 can reliably measure a variety of liquid hydrocarbon applications containing up to 10% aeration, including separators.
A novel approach
Coriolis technology has come a long way, but even the most sophisticated systems have their limitations. In applications where aeration levels exceed 10% (e.g. high-pressure separators and wellheads), it is virtually impossible for a flowmeter to measure with any acceptable level of accuracy and reliability. It is with these particularly challenging installations in mind that Siemens introduced the Gas Void Fraction Eliminator (GVFE).
Easily integrated into existing liquid pipelines, the GVFE improves the performance of any flow metering technology by removing and diverting all varieties of undesirable aeration, including bubbly, annular, slug and stratified flow. The GVFE is capable of extracting gas under full-flow and full-velocity conditions, making it possible to maintain designated flow rates and efficiency. Its compact footprint requires only minimal usage of space in skids or pads, and by removing aeration from the pipeline, the system also offers added protection against corrosion.
Ultimately, whether operators choose to meter the extracted gas separately for alternative use, reintroduce it downstream of the liquid flow or divert it to flare, the innovative GVFE will empower them to create and maintain full control over their value streams.
