David Matherly

David Matherly is a product manager of Advanced Product Solutions with Yokogawa Corporation of America. He holds a bachelor’s degree in Applied Science from Arizona State University in Engineering Technology and an associate’s degree in Applied Science from New River Community College in Instrumentation Technology. He has over 28 years experience in instrumentation and controls and has held engineering, product management, marketing, and technical positions with Hercules Chemical, ITT Corporation, Marsh Bellofram, Fairchild Industrial Products, Circor Instrumentation Technologies, and Yokogawa. Mr. Matherly can be contacted via email at david.matherly@us.yokogawa.com.

Q: Why is it important for end-users to enter the flowmeter selection process with a gameplan?

A: A gameplan is always important to ensure all bases are covered and to avoid problems after installation. In many instances, process data or instrument sheets will dictate the type of flowmeter that is used in a particular installation. When new or retrofit projects are planned and the correct measurement technology is chosen and fits the application, the end-user can enjoy many years of proper, accurate flow measurement of their process fluid.

Q: When selecting a flowmeter, what are the key application parameters the end-user should be considering?

A: Process compatibility along with process attributes such as density, viscosity, temperature, line pressure, connection size and type, desired minimum and maximum flowrates, process disturbance, straight piping runs, volumetric or mass indication, turndown, environmental, and, of course, overall accuracy, are all key application parameters to consider. All of these components along with installation costs will ultimately determine the best type of technology to use.

Q: In your experience, what are the most common pitfalls end-users encounter during the flowmeter selection process? How can end-users best avoid such pitfalls?

A: Selecting or specifying the flowmeter based upon price, pipe, or connection type only are the most common pitfalls I see. The best and easiest way for the end-user to avoid making mistakes during the selection stage is to use sizing and selection tools supplied by flowmeter manufacturers. Most manufacturers offer either Web or PC-based selection software that makes specifying the correct meter relatively easy. The other recommended way of avoiding mistakes is to speak with the manufacturer’s sales representative or company sales personnel. They are usually well-trained and experienced in the process.

Q: Generally speaking, the higher performing flowmeters also carry a higher price. How can end-users best balance the price vs. performance to ensure the chosen solution isn’t too much meter and/or too little meter for the application?

A: The application should almost always dictate the type of flowmeter and associated technology that is applied to the process. What many end-users do not consider during the specifying period are the long-term consequences of selecting a meter based upon initial investment price only. For example, if the accumulative measurement error of utilizing a +/-2 percent accurate device as compared to utilizing a +/-0.5 percent accurate device over an extended period of time is considered, the differences can be quite dramatic. The savings generated over time due to product measurement accuracy (or inaccuracies) can far outweigh the initial investment in a more highly accurate flowmeter.

A cost-benefit analysis should always be performed before deciding on and investing in a specific flowmeter technology. The results can sometimes be surprising.

Q: When considering performance statements, what are the key metrics end-users should be monitoring to ensure the flowmeter is going to provide a level of performance consistent with the needs of their application?

A: Flowmeter performance is usually expressed as a percentage accuracy related to flowrate (reading), calibration span, upper range limit (URL), or full-scale value. These all have very different meanings and can skew the customer’s expectations and view of meter performance. So the end-user needs to beware – flowmeter manufacturers express performance accuracy differently. But the required performance accuracy, just like other parameters, should be dictated by the application requirements. The end-user just needs to make sure that flowmeter accuracy is expressed in the same terms when comparing manufacturers’ products.

Another metric worth mentioning when considering performance statements is repeatability. Consistency of performance of any measured variable allows the end-user to “calibrate out” system inaccuracy as long as that error is repeatable.

Q: Why is straight-run piping such a key requirement when it comes to flow measurement applications? How might the availability of straight-run piping affect the flowmeter selection process?

A: The two predominant types of fluid disturbance within a flowing pipe are distortion and swirl. Distortion is usually caused by obstructions within the pipe, such as a partially closed valve or a flange gasket protruding into the pipe. (Obstructions can cause errors of 50 percent or more and greatly affect the flowmeter’s flow coeffiecients.) Swirl is usually caused by pipe bends of various planes. Swirl, of course, is difficult to correct because it is determined by the piping design and installation. Distortion can often be corrected by removing obstructions that are upstream and/or close to the flowmeter.

The location where the flow measurement needs to be made is usually nonnegotiable when determining the type of flowmeter selected. If a flow straightener or flow conditioner cannot be used, generally the flow technology will dictate how many straight-runs upstream and downstream are required for a particular type of flowmeter. The flowmeter manufacturer usually indicates what those requirements are and the application dictates whether the meter needs to be a magnetic, vortex, Coriolis, target, DP, ultrasonic, variable-area, thermal mass, or other type.

Q: With so many flowmeters now offering digital communication options, such as HART and fieldbus, how should these features be factored into the flowmeter selection process? How can an end-user best determine if such features are and/or will be relevant for their application now or in the future?

A: Practically all manufacturers of electronic flowmetering instruments offer some type of digital communications in addition to their standard analog outputs. For end-users that have not yet embraced the use of digital communications, a wise first step is to begin utilizing HART communications because of its availability and proven legacy within the process control industry. HART communications is easily understood and applied and actually operates over the same two wires that are already carrying the current (milliamp) analog signal to the end-user’s computer/DCS/PLC/controller. With this additional digital signal, manufacturer’s flowmeters now offer so much more information than the traditional, analog process variable signal provides. Additional process data, meter health, and diagnostic information are just a few additional variables available through digital communications. Adding HART communications to a control system can be relatively easy. A HART module can usually be added to a PLC’s I/O bay or utilized within most controllers so that the additional information offered can be realized.

Experienced HART communication users sometimes decide to migrate to purely digital forms of communication and control, particularly when upgrades or new installations take place within their process plants. Not all flowmeter manufacturers offer these types of digital communication platforms instead of the traditional analog signals. The end-user needs to be careful when deciding to upgrade to a digital protocol (for many reasons). The future needs of the process should dictate this migration path.

While purely digital forms of communications such as Modbus, DeviceNet, and Ethernet have been around for many years, control topologies such as Foundation Fieldbus and Profibus are relatively new (15 yrs). While adoption of Fieldbus in particular has been slow and industry-focused at times, wider acceptance is assured as manufacturers make their digital flowmeters easier to use. Both Fieldbus and HART technologies are still evolving and improving and will be around for a long time. There is no danger of either technology becoming obsolete in the near future.

Q: How much weight should calibration be given in the flowmeter selection process? Should the end-user be looking at the calibration requirement when selecting a given flowmeter for their application?

A: Calibration requirements should definitely be considered when selecting a flowmeter. On the extreme end of the requirement, meter calibration and verification is of particular concern within industries such as natural gas transmission, pharmaceutical, power, and the food industry. Anywhere any type of metering is used for billing purposes of the product (gas, steam, electricity) or when exacting amounts of additives to a process are required. In those instances, preventative maintenance and meter calibration must be performed and documented at regular intervals. Therefore, expensive calibration equipment must be kept on-hand and technicians continually trained, or the meters must be removed and sent to calibration facilities (or back to the manufacturer).

As such, equipment, people, and training requirements should be considered. Smaller companies sometimes do not employ experienced instrument technicians. In many cases, plant maintenance mechanics or electricians are expected to perform maintenance and calibration on flowmetering equipment. They are not given the tools to be successful because much of the equipment needed is very expensive and accuracy and traceability requirements can make purchasing the needed calibration equipment cost-prohibitive. Utilizing outside contractors to perform those duties can also be very expensive. Some industries have contractors that visit installation sites with proving meters just to test, calibrate, and repair flowmeters.