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Accurate measurement of liquids is important for all oil and gas production or consumption sites. This is especially true for bulk-transfer devices where large volumes of products are being moved and must be monitored, including crude oil depots, gasoline and jet fuel tank farms, refineries, and even cruise line terminals.
In the past, mass transfer was measured in batches with weigh scales or load cells. However, installation, calibration and maintenance of a scale or load cell are costly, difficult to do, time consuming, and don’t work for continuous processes. For these processes, such methods as orifice plates and magnetic flow tubes can measure volumetric flow, but 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 percent.
Now several measurement standards are moving toward use of Coriolis mass flowmeters, which can measure mass flow directly at the same time as they measure temperature and density. What’s more, transfer measurement by mass is the most accurate method, since 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 being touted by some as 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, a definite plus for space-challenged sites.
The Coriolis Effect – What Is It and Why Does it Help with Bulk Measurement?
In physics, 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. 
Figure 1 provides a simple overview of a Coriolis measuring
system, which makes use of the effect by using a symmetrical design
consisting of one or two measuring tubes, either straight or curved. 
A driver sets the measuring tube (AB) into a uniform fundamental
When the flow velocity (v) equals 0 meters/second / 0 feet/second (0m/s / 0ft/s), the Coriolis force (Fc) is also 0. At flowing conditions, when the flow velocity (v) > 0 m/s / 0 ft./s, the fluid particles in the product are accelerated between points AC and decelerated between points CB.
The Coriolis force (Fc) is generated by the inertia of the fluid particles accelerated between points AC and of those decelerated between points CB.
This force causes an extremely 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 special 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. The two vibrating tubes rotate around the two fixed endpoints, creating a Coriolis effect when mass flows through.??
Moving from Mechanical Meters to Coriolis Mass Flowmeters
Measurement of petroleum fluids has been around for hundreds of years and has been performed fairly reliably with the use of a wide range of mechanical meters.
One significant drawback to mechanical meters is that they experience serious wear and tear and can be plagued by high maintenance costs and the need for frequent replacement parts.
Also, mechanical meters have to be calibrated on a single grade, making recalibration, also known as “meter proving,” necessary each time a different product is measured. For example, suppose a facility is handling crude oil from Venezuela one day and from Texas on another day. 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.
All of this comes with a time and money cost. Usually the facility operator outsources these activities, which adds to the cost of running the facility.
Advances in manufacturing technology have led to the recent ability to make Coriolis meters in larger sizes, making them viable for oil and gas applications that have heretofore been the domain of mechanical meters. Coriolis meters have no moving parts, so they have fewer parts to maintain. Also, unlike mechanical meter measurements that are 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. It also reduces overall costs, because there is only a single instrument to buy and maintain, cables and power consumption are reduced, and controls are simplified. Figure 2 shows a graphic portrayal of a system that could be used for volumetric flow measurement.
|Figure 2 shows a graphic portrayal of a system that could be used for volumetric flow measurement.|
|Figure 3 shows how the meter would fit into the measurement skid.
 www.krohne.com/Measuring_Principle_Coriolis_Mass_Flowmeters_en.730.0.html, retrieved June 20, 2011