06 December 2011
Accurate measurement of liquids is a critical consideration for all oil and gas industry production or consumption sites. This is especially true for bulk transfer devices where large volumes of product is being moved and must be monitored. Traditionally, mass transfer was measured in batches with weigh scales or load cells. However installation, calibration, and maintenance is costly, difficult, time consuming, and is not suited to continuous processes, where methods such as orifice plates and magnetic flow tubes can measure volumetric flow. However, additional instruments are needed to measure temperature and pressure to compensate for fluid density changes. Introducing additional instruments also introduces errors, which can result in an overall measurement error rate as high as 3%.Several measurement standards are now moving towards the use of Coriolis mass flowmeters, which can measure mass flow directly, at the same time as they measure temperature and density. Transfer measurement by mass is also an accurate method, as mass is independent of, and unaffected by, changing process fluid characteristics, including pressure, temperature, viscosity, conductivity, and gravity. Among the Coriolis devices available, the straight tube design is considered to be 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 minimising skid size. The Coriolis effect The Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right. The mathematical expression for the Coriolis force appeared in an 1835 paper by the French scientist Gaspard Gustave Coriolis, in connection with the theory of water wheels. The Coriolis force is generated by the inertia of the fluid particles accelerated between two points and of then decelerated. This force causes a slight distortion of the measuring tube that is superimposed on the fundamental component and is directly proportional to the mass flowrate. This distortion is picked up by sensors. Since the oscillatory characteristics of the measuring tube are dependent on temperature, the temperature is measured continuously and the measured values corrected accordingly. In a dual-tube Coriolis meter, a manifold splits the flow through each of the two tubes. The full flow always goes through the sensor, while two vibrating tubes rotate around two fixed end points, creating a Coriolis effect when mass flows through.Mechanical meters need to be calibrated on a single grade of petroleum making recalibration, also known as meter proving, necessary each time a different product is measured. Suppose, for example, a facility is handling crude oil from Venezuela one day and from Texas the next. Since these have a different fluid basis, the meter would have to be re-proved since the meter’s calibration factor is affected by a variety of fluid conditions, especially density. Custody system operators often need to use a prover for measuring one batch to another, reproving for every different fluid transaction. This often leads to these activities being outsourced, adding to the cost of running the facility. Coriolis meters are now being produced in larger sizes, making them viable for oil and gas applications. Because they have no moving parts they have few maintenance requirements. Also, unlike mechanical meter measurements used for custody transfer, which have to be corrected using pressure and temperature compensation, the Coriolis meter measures product mass directly, independent of pressure and temperature.Direct mass flow measurement with Coriolis mass 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 and volume total, and temperature. Reducing pressure drop Reduction in pressure drop is one of the important differentiating factors between mechanical meters and Coriolis mass flowmeters. Pushing fluids past a mechanical meter and creates a pressure drop, which is not recoverable and must be overcome with the use of a pump –adding equipment, cabling, and energy costs to the system. With a Coriolis mass flowmeter, there will be no extra pressure drop across the meter. Most meters include are twin tubes, and the only pressure drop is caused by splitting the flow into the two pipes inside the meter. Using a straight pipe Coriolis meter design creates less permanent pressure drop than the bent tube variety.Accuracy is another benefit offered by Coriolis meters. The majority of operators using mechanical flowmeters are also running separate pressure and temperature meters. If 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 percent. When those values are combined in a flow corrector or computer, the overall calculated error rate could be 2% or higher. As each instrument also drifts over time independently, accuracy would be further degraded, perhaps to as high as 3%. The Coriolis meter’s error rate is only 0.1 to 0.2% of rate, making it nominally five to ten times more accurate and its long term drift is minimal as compared to devices with moving parts.The Coriolis meter is also easier to install and maintain. Especially for measurement skids that may have to fit into a relatively small footprint, the straight tube Coriolis meter takes up the smallest space with simple process piping. In addition, the straight tube design is considerably easier to clean than the bent tube version, which can suffer from deposit build up on the tube. A final benefit is the electronic controls, which provide more flexibility in terms of operational performance. Operators are able to view everything that is going on with the fluid – including volumetric flow, mass flow, density, and temperature. Most have options for multiple outputs, including digital converters that allow for direct MODBUS to see all the parameters. Refinery choice One petroleum refinery site in Canada, which produces refined gasoline, jet fuel and other petroleum products, ship and meter product through a 5km 8in pipeline that connects to a major Canadian pipeline. It needed to measure the product leaving the plant site and the product going to the shipping pipeline, and wanted a density reading at the same time. The refinery ships different products through one line and had to confirm that they were shipping the correct product.The pipeline was large, the available real estate in the existing location was limited. The company did not want to undertake a large scale modification to install additional structural support for meters and also wanted to limit pressure drop through the meter to avoid wasting energy, reducing energy costs to run the pipeline. Various types of metering technologies were considered, including ultrasonic, vortex shedding, and orifice plate meter. A Coriolis mass flowmeter was chosen, in part because it would allow flow and density measurements from a single device, making it possible to fit the device into the restricted space. Facility operators evaluated the use of both straight tube and bent tube Coriolis. The cost of the meters were equal, but installation costs for the straight tube meter were lower. A straight tube OPTIMASS 2000 Coriolis meter from KROHNE was specified, based on the meter’s lower installation costs, combined with its high accuracy and linearity. In addition, the cost of the unit included onsite startup support. The refinery also makes use of the communication options of the flowmeter, monitoring it remotely from a laptop. The meter’s sensor has its own built-in smart sensor interface and on board memory, so the electronic converter/transmitter can talk directly to the sensor digitally. The two installed devices are remote and segregated while sharing device configuration data, so if there is a problem with either the sensor or the transmitter, technicians can replace one or the other independently and sensor calibration files or converter parameters can be uploaded from either device, adding to the meter’s reliability.
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