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DAVID W SPITZER'S E-ZINE (January 2021)

TECHNICAL AND MARKETING SERVICES
FOR INSTRUMENTATION SUPPLIERS AND END-USERS

Coriolis Mass Flowmeters (Part 2 of 3) by David W Spitzer

Typical Coriolis mass flowmeter construction is such that the only wetted parts are the tubes and manifolds that can be made from materials that can withstand corrosion and do not contaminate the fluid. Therefore, these flowmeters can be applied to measure corrosive fluids and fluids where contamination is an issue, such as in sanitary applications. However, some Coriolis mass flowmeter designs do incorporate internal seals that should be considered for compatibility with the process. All welded designs are preferable for applications where corrosive fluids are present.

In addition, the straight-through (obstruction-less) nature of some designs reduces the loss of hydraulic energy across the flowmeter (pressure drop). Reducing the pressure drop across the flowmeter can conserve hydraulic energy in some applications, such as when a pump or fan is controlled with a variable speed drive. Note that installing a flowmeter with a lower pressure drop in place of a flowmeter with a higher pressure drop can cause the pressure drop to be transferred from the flowmeter to the control valve, and result in no energy savings.

Suppliers generally state that Coriolis mass flowmeters do not require straight run upstream or downstream of the flowmeter. However, examination of their mounting requirements reveals that some designs require supports located in straight piping upstream and downstream of the flowmeter. While these designs may not require straight run to improve the hydraulic velocity profile entering the flowmeter, their piping support requirements effectively resemble a straight run. Piping support and straight run requirements can reduce the usability of some Coriolis mass flowmeters in some applications. The supplier's detailed mounting recommendations should be examined to evaluate suitability for a particular application.

Coriolis mass flowmeter technology has no Reynolds number constraints, so it can be applied where the liquid exhibits high or varying viscosity. However, it should be noted that as fluid viscosity increases, the pressure drop across the flowmeter can become excessive and may limit the applicability of the flowmeter. This can usually remedied by increasing the size of the flowmeter, however performance can be significantly degraded when the flowmeter operates in the lower portion of its range.

Coriolis mass flowmeters with fast response times can measure liquids that flow for relatively short periods of time, such as in batch and fill operations. Response time and reproducibility become important parameters to quantify in these applications.

Coriolis mass flowmeters can be used to measure the mass flow of gases and vapors. The advantage of this technology is that this technology measures mass flow directly, and does not infer mass flow from other measurements, such as pressure, temperature and compressibility. The caveat is the Coriolis mass flowmeter should be operated within a range of mass flow rates where its accuracy is acceptable. Because the density of gases is generally significantly lower than that of liquids, the hydraulic ability to flow a sufficient mass of gas through the flowmeter can be limited. Therefore, Coriolis mass flowmeters in gas applications are usually sized larger than would be required for an equivalent liquid mass flow rate. As a result, they tend to operate in the lower portion of their range where accuracy may be degraded.

Coriolis mass flowmeters measure mass flow rate of the fluid. The measurement is linear with mass flow and exhibits a relatively large turndown. In addition, the range of flow measurement is relatively large and easy to change after installation. The measurement is virtually independent of density and the same flowmeter can be used for multiple products. This allows potential cost savings through the installation of one flowmeter to measure multiple flow streams when the flow streams are not required to operate at the same time.

Coriolis mass flowmeters can be used to measure fluid density in addition to flow rate. In some applications, density measurement can be used to determine fluid composition or potential process problems. Coriolis mass flowmeters that measure flow and density can offer potential cost savings over the purchase and installation of a flowmeter and density transmitter.

Excerpted from The Consumer Guide to Coriolis Mass Flowmeters

Measuring Gravity Flow (Part 2) by David W Spitzer

Three situations quickly come to mind as to where most technical people should consider stopping, taking a deep breath and thinking slowly --- operation under vacuum, measuring downward liquid flow and gravity flow. The previous article discussed flowmeter sizing in gravity flow applications. In particular, the pressure drop across the flowmeter often needs to be low in order to pass the desired flow lest the liquid accumulate upstream. This can be accomplished by increasing the flowmeter size which tends to degrade accuracy because the larger flowmeter is operated lower in its flow range.

Consider a one meter high liquid separator fed by a pump operated with a variable speed drive with a tangential inlet, gravity underflow and gravity overflow with flow rates of 100, 98 and 2 flow units respectively where it is desired to measure the inlet and underflow flow rates for operational purposes. A 1-1/2 inch inlet flowmeter was selected based upon operation at a reasonable flow velocity and pressure drop. As a side note, the variable speed drive will increase the inlet pump speed to generate more pressure as needed to provide the desired flow to the separator.

These inlet and underflow flow rates are effectively the same (100 and 98 respectively) so one might expect to use identical flowmeters in both applications. Not so fast! The pressure drop across the 1-1/2 inch inlet flowmeter was approximately one meter of water column at the design flow. However the liquid must also pass through a control valve that develops an additional pressure drop. Therefore the total pressure drop associated with the flowmeter and control valve is greater than the available head in the separator so the underflow flowmeter and control valve cannot pass the entire design flow without backing up liquid in the separator and causing an inappropriately high overflow of improper composition.

Increasing the size of the underflow flowmeter to 2 inches reduced the pressure drop sufficiently to allow the flowmeter and control valve to operate properly without backing up liquid into the separator. Note that different size flowmeters were used to measure effectively the same flow rates.

Be careful and remember to install flowmeters and control valves in piping that ensures that both remain full of liquid at all times.

This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.

Orifice Plate Flow Measurement System Performance by David W Spitzer

A differential pressure transmitter with an accuracy specification of +/- 0.05 percent of span calibrated in the shop and set for 0-100 differential pressure units will be used to measure the flow through an orifice plate. What is the approximate performance of the orifice plate flow measurement system at 20 percent of flow?

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.