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Coriolis Mass Flowmeters

2014/3/17      view:
How Coriolis Mass Flowmeters Work
Coriolis mass flowmeters measure the force resulting from the acceleration caused by mass moving toward (or away from) a center of rotation. This effect can be experienced when riding a merry-go-round, where moving toward the center will cause a person to have to “lean into” the rotation so as to maintain balance. As related to flowmeters, the effect can be demonstrated by flowing water in a loop of flexible hose that is “swung” back and forth in front of the body with both hands. Because the water is flowing toward and away from the hands, opposite forces are generated and cause the hose to twist. 

In a Coriolis mass flowmeter, the “swinging” is generated by vibrating the tube(s) in which the fluid flows. The amount of twist is proportional to the mass flow rate of fluid passing through the tube(s). Sensors and a Coriolis mass flowmeter transmitter are used to measure the twist and generate a linear flow signal.

Early Coriolis mass flowmeters were single-tube designs with a U-tube shape similar to the garden hose (described above). These flowmeters could be significantly affected by vibration in all but the smallest sizes. Subsequent designs split the flow into two flow streams and measured the relative twist between the two tubes. These designs were much less susceptible to vibration and allowed Coriolis mass flowmeters to be installed in “normal” piping systems without extensive mechanical supports. Electronic and mechanical improvements allow more recent designs to include single-tube Coriolis mass flowmeters with straight tubes that reduce pressure drop and reduce the probability of plugging. 

Most Coriolis mass flowmeters can measure fluid flow, fluid density, and the temperature of the outside wall of the pipe in the flowmeter. Some Coriolis mass flowmeters can also measure the fluid viscosity in some applications while other Coriolis mass flowmeters can measure two-phase flow accurately. 

How to Use Coriolis Mass Flowmeters
Coriolis mass flowmeters measure the mass flow of liquids, such as water, acids, caustic, chemicals, and gases/vapors. Because mass flow is measured, the measurement is not affected by fluid density changes. Coriolis mass flowmeters can measure flow extremely accurately so they are often used to measure high value products or the introduction of fluids that affect the production of high value products. 

Be particularly careful when using Coriolis mass flowmeters to measure gas/vapor flows because flow rates tend to be low in the flow range (where accuracy is degraded). Also, in gas/vapor applications, large pressure drops across the flowmeter and its associated piping can occur so special care should be taken to design the piping immediately upstream and downstream of the flowmeter to reduce its pressure drop in the system.

This flowmeter can be applied to sanitary, cryogenic, relatively clean, and corrosive liquids and gases/vapors in pipes smaller than 6-12 inches. General applications are found in the water, wastewater, mining, mineral processing, power, pulp and paper, petroleum, chemical, and petrochemical industries. Coriolis mass flowmeters have also been developed for specialized applications such as for the custody transfer of high-pressure natural gas. While Coriolis mass flowmeters are versatile and can handle many applications, Coriolis mass flowmeters are not a universal flowmeter that can handle all applications. 

Materials of construction are generally limited to stainless steel and Hastelloy C. Straight-tube designs are available to measure some dirty and/or abrasive liquids. Single-path designs are available to measure fluids where plugging can occur. 

Many applications for Coriolis mass flowmeters are found in chemical processes where fluids can be corrosive and otherwise difficult to measure. In addition, the relative insensitivity to density allows Coriolis mass flowmeters to be applied in applications where the physical properties of the fluid are not well known. These flowmeters can also be used in chemical feed systems found in most industries. 

Application Cautions for Coriolis Mass Flowmeters
If the pressure drop is acceptable, operate a Coriolis mass flowmeter in the upper part of its flow range because operation at low flow rates can degrade accuracy. Note that high viscosity fluids increase the pressure drop across the flowmeter so size of the flowmeter may increase due to accommodate pressure drop constraints, in turn causing flowmeter operation in the lower part of its range and degraded accuracy. Note that even though the flowmeter specification may be (say) 0.1% of rate, the flow measurement accuracy in a particular application could be as high as (say) 2% of rate or worse.

For liquid flows, make sure that the flowmeter is completely full of liquid and contains no gas or bubbles. Conversely, for gas applications, be sure that the flowmeter is completely full of gas and contains no liquid. 

Be especially careful when measuring gas/vapor flow with Coriolis mass flowmeters due to the pressure drops that can occur due to entrance and exit affects. Pay special attention to the piping installation because pipe vibration can cause problems during operation. 
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