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By Andre Verdone
Accurate liquids measurement is important for the oil and gas industry. But it’s essential for bulk-transfer devices where large product volumes are moved and monitored, including crude oil depots, gasoline and jet fuel tank farms, refineries and even cruise line terminals.
Mass transfer has been measured in batches with weigh scales or load cells. But their use is not suitable for continuous flow measurement.
For continuous processes, orifice plate and magnetic flow tubes can measure volumetric flow, but other instruments are needed to measure temperature and pressure to compensate for fluid density changes. Additional instruments also introduce errors, resulting in an overall measurement error rate as high as 3.0%.
Several measurement standards are moving toward use of Coriolis mass flowmeters, which can measure mass flow directly and at the same time measure temperature and density. What’s more, mass transfer measurement is independent of, and unaffected by, changing process fluid characteristics, including pressure, temperature, viscosity, conductivity and gravity.
Among Coriolis devices available, the straight-tube design is the most accurate and easiest to install and maintain. Especially for measurement skids, widely used in the oil and gas industry, the straight-tube Coriolis meter can be a factor in minimizing skid size.
From mechanical to Coriolis
Petroleum fluid flow has been measured fairly reliably using a wide range of mechanical meters. One significant drawback is that mechanical meters experience serious wear and tear, which leads to high maintenance costs and need for frequent replacement parts.
Also, mechanical meters must be calibrated on a single grade, making recalibration, also known as meter proving, necessary each time a different product is measured. Custody system operators often need to use a prover for measuring one batch to another, reproving for every different fluid transaction.
Advances in manufacturing technology have led to larger Coriolis meter sizes, making them viable for oil and gas applications that have heretofore been the domain of mechanical meters. Coriolis meters have no moving parts. Unlike mechanical meter measurements used for custody transfer, Coriolis meters measure product mass directly, independent of pressure and temperature.
Direct mass flow measurement with Coriolis flowmeters means that one flowmeter with a single point of measurement can obtain multiple measured values, including mass flow and mass total, density and concentration, volume flow, volume total and temperature. It also reduces overall costs, because only a single instrument is bought and maintained. Cable laying and power consumption are reduced, and controls simplified.
Reducing pressure drop
Pushing fluids past a mechanical meter always creates a pressure drop, which is not recoverable. Every time pressure drop is imposed, it must be overcome with pump use, which adds equipment, cabling and energy costs.
No extra pressure drop is experienced across a Coriolis mass flowmeter. Most meters have twin tubes, and the only pressure drop is caused by splitting the flow into the two pipes. A straight-pipe Coriolis meter creates less permanent pressure drop than the bent-tube variety.
Almost all operators using mechanical flowmeters also run separate pressure and temperature meters. Say the mechanical flowmeter exhibits a 0.5% accuracy error; the pressure meter also poses a 0.5% accuracy error; and the temperature meter a further 1.0%. Combined in a flow corrector or computer, the overall calculated error rate could be 2.0% or higher. Plus, since each instrument independently drifts over time, accuracy is further degraded, perhaps to as high as 3.0%.
The Coriolis meter’s error rate is only 0.1% to 0.2% of rate, making it nominally five to 10 times more accurate than mechanical measurement. Its long-term drift is minimal compared to devices with moving parts.
The straight-tube Coriolis meter takes up little space with simple process piping. The straight-tube design is easier to clean than the bent-tube version, which tends to get deposit buildup on the tube, and much simpler than equivalent mechanical meters, which also require strainers and upstream filters.
With a typical Coriolis mass flowmeter, operators view all fluid-related parameters, including volumetric flow, mass flow, density and temperature. Most have options for multiple outputs, including digital converters that allow for direct MODBUS to all parameters. Available controls ensure the correct product is going through the meter; verify product is drawn from the right tank; and tell operators if product is hotter or cooler than expected.
Ontario refinery experience
At one Ontario petroleum refinery site that produces refined gasoline, jet fuel and other petroleum products, operators ship and meter product through a 5-kilometer long, 8-inch diameter pipeline that connects to a major pipeline delivering product to the greater Toronto area. Product is measured leaving the plant site and going to the shipping pipeline, and a density reading was wanted at the same time. Different products are shipped through one line and the site needs to confirm correct product shipment.
Management did not want to do a large-scale modification to install additional structural supports for meters. Pressure drop through the meter needed to be minimal to avoid increasing the cost of running the pipeline.
The facility looked at ultrasonic, vortex shedding and orifice plate meters, but ultimately chose a Coriolis mass flowmeter.
The straight-tube design offered lower pressure drop and was less subject to plugging. Most importantly, it was simpler to install and took up less space than other available options.
Managers initially had reservations about using a straight-tube design. Bent tube is the more traditional choice and has a larger installed base. However, they chose a straight-tube OPTIMASS 2000 Coriolis meter from KROHNE, based on lower installation costs, combined with high accuracy and linearity.
The refinery can monitor the meter remotely from a laptop. The meter sensor has its own smart-sensor interface and on-board memory, so the electronic converter/transmitter “talks” digitally and directly to the sensor.
While sharing device configuration data, the two installed devices are remote and segregated, so if there is a problem with either sensor or transmitter, technicians can replace one or the other independently. Sensor calibration files or converter parameters can be uploaded from either device, adding to the meter’s reliability.
Custody transfer has a significant impact on the bottom line, so precision and reliability are key requirements. Today’s Coriolis mass flowmeters can compete with traditional mechanical meters. For the bulk measurement of any fluid with a significant value, large diameter Coriolis mass flowmeters offer more accurate and repeatable bulk fluid transfer measurement. Further, straight-tube Coriolis meters offer the lowest pressure drop and highest accuracy.
Andre Verdone is a vice president and technical expert with Krohne, Inc., a global technology leader in the development, manufacture and distribution of accurate, reliable and cost-effective measuring instruments for the process industries. He has spent the last 35 years working in the water industry and has written and presented several papers around the world on key issues related to improving the operation, conservation and management of water systems.