Coriolis Mass FlowmetersFilling Key Process NeedsOctober 1, 2005 By: Steven G. Pagano
Coriolis flowmeters, once limited to pure, clear liquids, are now satisfying the more difficult requirements of the process industries.
Measuring mass flow rate offers potential for improvements worth thousands of dollars in industries such as chemical, food and beverage, pulp and paper, petroleum, pharmaceuticals, and in water/wastewater treatment facilities. Although great strides have been made in the design of inferential and thermal mass flowmeters in recent years, refinements in Coriolis metering technology have made these instruments particularly effective in the process industries.
Increased accuracies of advanced instruments for measuring mass flow rate can offer significant savings in petroleum processing industries. Mass flow measurements account for such variables as temperature, pressure, density, and viscosity, which can otherwise degrade the accuracies of volumetric flowmeters.
Complementing Measurement and Control
Coriolis flowmeters are well-suited for industrial environments and readily tie in with complete process measurement and control systems. Today, the global market for these devices probably exceeds $480 milion—shared by more than a dozen suppliers.
One interesting development is that users have standardized these meters for practically all applications throughout a facility. Plant engineers can justify the higher unit cost of Coriolis mass flowmeters because of the improved accountability (accuracy), integration of process flow measurements into one unit (less hardware), and elimination of the need to correct flow profiles before the fluid enters the meter (pipe runs).
Coriolis mass flowmeters have two basic system configurations—a remote converter and an integral converter that is mounted directly on the primary housing (see Figure 1). The remote converter connects to the primary with shielded cable that can be up to 1000 ft. long. The converters receive the small electric measuring signal generated by the sensing system in the primary and electronically change it to usable outputs (current, pulse frequency, or digital). The converter can show the outputs on its display and transmit them to panel-mounted recording and control instrumentation or process-control computers in a centralized control room.
Figure 1. Coriolis flowmeters directly measure mass flow. The converter for the remote version (A), can be up to 1000 ft. from the primary flow element. Alternatively, the converter can be mounted integral with the primary, as in (B).
The primary mounts in the flow line and houses the essential sensing system components. The system adapts Coriolis technology to obtain the electrical signal that is a direct measure of mass flow rate. It also provides a measure of fluid density and temperature.
The main feature of the sensing system is the proprietary flow-tube assembly. Different manufacturers use different tube geometries for the flow path through the primary. Some use a single tube, and others use a parallel pair of flow tubes. Figure 2 shows a bent tube arrangement with dual tubes. There are, in fact 17 tube geometries that help determine performance of the flowmeter. Tube bore is sized to provide meter sizes from ~½–6 in. The tube has no obstructions to fluid flow.
Figure 2. Vibrating dual bent tubes represent one of many tube geometries used to create the Coriolis effect for measuring mass flow.
Other main components of the sensing system include detectors that precisely measure the Coriolis effect to detemine the mass flow rate, as well as a driver coil that vibrates the flow tube (see sidebar).
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