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Coriolis Mass Flowmeters (Part 3) by David W Spitzer
Variation in certain characteristics of Coriolis mass flowmeters has allowed this technology to be applied to many processes. The following sections define flowmeter design categories and how they fit the needs of their intended applications.
Liquid Coriolis Mass Flowmeters
Most Coriolis mass flowmeters measure liquid flows, however some designs were developed specifically and exclusively for gas flow measurement.
Gas Coriolis Mass Flowmeters
Although the majority of Coriolis mass flowmeters were designed to measure liquid flow, many were found to measure the mass flow of gases with acceptable accuracy. Applying Coriolis mass flowmeters to gas flows eliminates the need for pressure and temperature instrumentation that is used to compensate flowmeters that measure volume, velocity or inferentially. By measuring mass directly and eliminating the additional measurements, the installation is more straightforward and can be more economical.
High Pressure Coriolis Mass Flowmeters
High pressure Coriolis mass flowmeters include flowmeters that can measure fluids at pressures over 125 bar. The flow tubes in these flowmeters usually have thicker walls, so they may not perform as well as Coriolis mass flowmeters designed for lower pressures.
High Temperature Coriolis Mass Flowmeters
Most Coriolis mass flowmeters are limited to approximately temperatures of approximately 200 degC. High temperature Coriolis mass flowmeters are capable of measuring over 225 degC. These flowmeters are often used in high temperature heating systems.
Metal (other than 316SS) Coriolis Mass Flowmeters
Due to the cost and general applicability of 316SS, most Coriolis mass flowmeters were designed and are constructed of 316SS wetted parts. However, to increase the applicability of the technology, Coriolis mass flowmeters can be constructed using other metals. Note that some Coriolis mass flowmeters have seals that are constructed from non-metallic materials. Therefore, the availability of a Coriolis mass flowmeter made of materials other than 316SS does not necessarily imply that the seals are (or can be) constructed of a material that is compatible with the process.
Plastic/Polymer Coriolis Mass Flowmeters
Most Coriolis mass flowmeters are metal however, Coriolis mass flowmeters constructed of plastics and polymers are available. These flowmeters are used for low flows of high purity fluids.
Sanitary Coriolis Mass Flowmeters
Sanitary Coriolis mass flowmeters are designed and fabricated with materials and finishes that allow application to the food and pharmaceutical industries where they may be cleaned and/or steamed in place. In addition, they can usually be oriented to be self-filling and self-draining for easier cleaning.
Single Path Coriolis Mass Flowmeters
Single path Coriolis mass flowmeters do not split the flow so as allow the fluid to take multiple paths through the flowmeter. This feature is important in applications where it is desirable to ensure that the path through the flowmeter is free of trapped material, such as in applications where the process fluid can "freeze", or solidify.
Straight Path Coriolis Mass Flowmeters
Straight path Coriolis mass flowmeters exhibit the benefits of single path flowmeters and are obstructionless. Their geometry generally reduces the pressure drop at a given operating condition and allows them to be installed in orientations that self-drain and self-fill.
Excerpted from The Consumer Guide to Coriolis Mass Flowmeters
Flowmeter Installation by David W Spitzer
Physically examining flow and level measurement installations can be quite interesting because the range of issues found can vary widely from incorrect tubing, inappropriate materials of construction, attack from the environment, improper calibration techniques and the like. It is rewarding to sort things out and solve client problems. However there is one situation that makes me stop in my tracks and check everything - and I mean everything.
Most flowmeters require a good velocity profile upstream of the flowmeter to achieve accurate measurement. The velocity profile downstream of a control valve is distorted due to the fluid passing through a relatively small and often tortuous flow path that varies with valve position. There is also the possibility of flashing and cavitation in some applications. In other words, the velocity profile downstream of a control valve is the antithesis of a good velocity profile.
Knowing this, why then would anyone install a flowmeter downstream of a control valve? In 45 years of practice, I have never run into an application where a flowmeter should be installed downstream of a control valve. Maybe there are a handful of applications somewhere with this requirement but finding them has been elusive.
Seeing a flowmeter installed or designed to be installed downstream of a control valve prompts me to stop all work and investigate everything (and I mean everything) because such installations are indicative of one or more voids in knowledge at some point in the design cycle. Brushing aside political correctness, whoever did this or was party to it did not understand what they were doing. No one having stopped them is a likely indication that there are additional (hidden) issues elsewhere.
Stop and tread carefully when you stumble upon a telltale sign.
This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.
Flowmeter Error (Reynolds Number) by David W Spitzer
A process requires the batch addition of 100 liters of a liquid that has a specific gravity of 1.10 and a viscosity of 10 to 20 cP. The rate of addition is adjusted manually in a 2-inch pipe and takes from one and two minutes. An investigation into a quality problem using a weigh scale to verify flowmeter operation revealed actual additions in error by up to 15 liters --- despite repeated totalized flow measurements of 100 liters. The flowmeter was removed, wet calibrated and correctly returned to service. Is Reynolds number a concern?
Most commonly applied flowmeters operate accurately in the turbulent flow regime and usually exhibit nonlinearity and/or turn off in the other flow regimes.
In this application, the flow rate is adjusted manually between 50 and 100 liters per minute so the flowmeter operates at Reynolds numbers between approximately 1150 and 4600 so the flowmeter could be in the laminar, transitional or turbulent flow regime depending upon where the manual valve is set. Therefore nonlinearities of the nature described could be introduced.
Additional Complicating Factors
Cold weather in the winter could cause the liquid to further cool which would increase its viscosity, reduce Reynolds number and tend to operate more often in the laminar flow regime where it is likely less accurate.
This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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