Flowmeters, which measure the amount of liquid, gas, or vapor flowing through pipes or troughs, are used in virtually every type of industrial process. While flow measurement is an ancient practice, dating back to at least 5,000 B.C., the past few decades has been a particularly exciting time, as vendors and users have adopted new technologies to improve the state of the art.
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Flowmeters, which measure the amount of liquid, gas, or vapor flowing through pipes or troughs, are used in virtually every type of industrial process. While flow measurement is an ancient practice, dating back to at least 5,000 B.C., the past few decades has been a particularly exciting time, as vendors and users have adopted new technologies to improve the state of the art.
Though traditional devices, such as turbine meters, are still widely applied, higher-technology product families are available to more applications, offering capabilities beyond basic flow measurement at lower cost. Coriolis flowmeters make use of a fluid’s inertial force in vibrating tubes. The meter senses a twisting motion that is in direct proportion to mass flow. In magnetic flowmeters, a conductive fluid passes through a magnetic field, generating a voltage proportional to the velocity of the conductive medium. Ultrasonic meters use ultrasonic signals sent from opposite sides of a pipe and measure flow based on signal transit times.
With the variety of flowmeters available, it is essential that buyers select the most appropriate meters for their applications. In general, Coriolis meters are well suited for applications involving clean liquids and gases, flowing fast enough to operate the meter. They are also a good choice for high-accuracy applications, such as custody transfer. Magnetic meters are appropriate only for applications involving conductive liquids, which makes them a poor choice for hydrocarbon applications. Vortex meters are not a good choice for applications involving low flow rates.
Once buyers identify suitable types of meters, they should consider whether they want to purchase older types, which generally have moving parts (as is the case with turbine meters) and are more likely to require maintenance, or newer technologies (such as Coriolis, vortex), which have few—if any—moving parts, and which may be more expensive in capital cost, but less expensive over time.
A major consideration is to not buy more meter than needed for the application. For example, if an application requires only a moderate degree of accuracy, and meter readings are taken visually by field personnel, it is not necessary to purchase an instrument with sophisticated networking capabilities and high accuracy.
As is the case with many field devices and applications, vendors and users alike recommend that a flowmeter, first and foremost, be easy to use.
Users who need to integrate flowmeter data directly into control and/or asset management systems should consider the networking capabilities of the devices they purchase. While virtually any instrument connected to a control architecture can transmit a primary variable via a standard, analog 4-20 mA signal, many instruments are capable of communicating via digital standards, such as FOUNDATION fieldbus, HART, and Profibus, as well as proprietary digital protocols. Since digital protocols can transmit multiple variables, users also should consider if they want or need instruments that can calculate and transmit other data, such as pressure and temperature. Using a single instrument for multiple purposes limits the number of pipeline intrusions.
Numerous small meter companies are active in the market, and many manufacture excellent products. Users should consider the reputations of the companies from which they purchase instruments and those companies’ abilities to provide support and spare parts in a timely manner. Buyers also should consider a company’s breadth of offerings, since multiple applications in a plant will require different types of instruments, and businesses often seek to limit the number of vendors with which they transact.
Buyers also should consider how easily a device can be maintained when or if it malfunctions. For example, if electronic elements within the meter fail, can you replace them without having to shut down the process?
Flowmeter prices vary widely, based on technologies, degree of accuracy, and pipe size. At the low end of the price spectrum, users can purchase basic turbine meters or variable area meters for as little as $500. These are not top-of-the-line devices, as they lack network capability and a high degree of accuracy, but they are well-suited for many typical plant applications.
Starting at about $2,000, users can purchase flowmeters that deliver a higher degree of accuracy and which are capable of analog 4-20 mA communications.
Many mid-range instruments, which cost from $3,500 to $6,000, offer multivariable capabilities, multiple outputs, local displays and local interfaces for changing settings and checking diagnostics.
At the high end of the price range ($10,000 and up) instruments can accommodate large line sizes and have a high degree of accuracy.
Vendors note that an increasing number of plants are centralizing purchasing authority, leaving some process engineers to be more directly involved in specification and recommendation process rather than the actual purchase of the flowmeter. In such cases, it is imperative for the process engineer to be well versed in the long-term needs of the facility and the capabilities and costs of available instrumentation.
“Before (purchasing centralization), a process engineer might have had $10,000 or $20,000 in signing authority, but now, he or she might not have any,” says Tom O’Banion, director of chemical industry marketing for Micromotion. “As a result, we’ve seen companies shift away from using a lifecycle cost model, which seems very short-sighted to us. Some of the decisions customers have been making are committing them to a stream of ongoing expenses that could have been reduced.”
Flowmeter checklist
Select a meter based on technology best-suited for the application
Balance the lower cost of older technologies and their moving parts against the future cost of maintenance
Do not overspend on unnecessary levels of accuracy, network capabilities, etc.
Consider a flowmeter’s ease of use and maintenance (lifecycle costs)
Make sure the required level of long-term support is available from vendors under consideration
