Multivariable Sensors Offer Better Accuracy and Insight

Multivariable measurements can help optimize processes and drive sustainability

With chemical manufacturers under pressure to gain a better understanding of their processes, improve operational efficiency and safety, control costs and provide accurate data to regulators, the use of multivariable process sensors is increasing. By delivering compensated flow or level, along with real-time pressure, temperature and other measurements from a single device, multivariable sensors support both operational and sustainability programs with data and insight that single-variable devices cannot offer.

“We are seeing a clear trend toward multivariable measurement, as chemical manufacturers seek to improve operational efficiency, energy management, sustainability performance and digitalization,” says Benedikt Schumann, industry marketing manager, chemicals, with Endress+Hauser (Greenwood, Ind.; us.endress.com). “Traditionally, plants relied on multiple discrete instruments that each measured a single process variable, but processors increasingly demand richer process information from fewer measurement points.”

He continues: “The need for better process visibility is driving this shift. Operators want to understand how variables interact and influence production performance, product quality and energy consumption. As a result, multivariable instruments are becoming an increasingly important component of modern chemical plants and digital transformation initiatives.”

Dietrich Wins, product portfolio manager at Siemens (Munich, Germany; siemens.com) agrees. “The growing role of digitalization in chemical plants means that plants are now expected to collect, transmit and act on process data in ways that were not common in the past,” he says. “Multivariable sensors fit these cases because they deliver more information out of a single measurement, while sustainability goals demand stronger process control that is realized by more data from the process.”

The data collected from multivariable instruments can be used to support three main workflows in a chemical plant: real-time process control; equipment reliability and predictive maintenance; and decarbonization and sustainability efforts, explains Michael Flesch, global product measurement engineer (pressure), with Emerson (St. Louis, Mo.; www.emerson.com). “The same accurate, compensated flow measurement that helps operators trim boilers and manage reactor feeds can also support carbon accounting, emissions reporting and energy-efficiency programs. At the same time, pressure and temperature insights can help identify equipment issues before they escalate.”

Multivariable sensor anatomy

The anatomy of a modern multivariable instrument extends far beyond sensing technology. In addition to measuring multiple process variables, these devices incorporate onboard processing, compensation algorithms, diagnostics and digital communications. “The result is a device that not only measures the process, but also converts data into actionable information for operations and maintenance teams,” explains Endress+Hauser’s Schumann.

For example, instead of installing separate transmitters for differential pressure, static pressure and temperature and then calculating the flow or level value via an additional processing unit, multivariable sensors offer an integrated solution because the devices provide the calculated output value, as well as multiple process variables from a single installation point, notes Carlos Ramos, global product specialist, pressure, with ABB (Zurich, Switzerland; abb.com). “Multivariable transmitters simplify engineering, reduce installation effort, improve measurement consistency and offer better process diagnostics,” he says. “These benefits are especially important in applications that require accurate measurements under challenging and changing operating conditions.”

However, the exact “anatomy” of a multivariable sensor depends upon the instrument, the desired measurement and the application. For example, Endress+Hauser’s Coriolis flowmeter simultaneously measures mass flow, density and temperature from a single instrument, enabling additional calculations, such as concentration and product quality indicators, while their vortex flowmeter combines flow, pressure and temperature measurements to calculate parameters such as compensated mass flow and energy consumption.

The VY Multivariable Vortex Flowmeter (Figure 1) from Yokogawa (Tokyo, Japan; www.yokogawa.com) measures flow using vortex shedding technology. As fluid passes around the shedding bar, vortices are generated at a frequency proportional to flow velocity. The integrated temperature sensor continuously measures process temperature, while onboard calculations use flow and temperature data to determine mass-flow and energy-flow values.

Multivariable pressure measurement devices have a different “anatomy.” For example, Emerson’s Rosemount 3051SMV multivariable transmitter (Figure 2) uses three individual sensors to measure differential pressure (DP), static line pressure and process temperature across a DP primary element and combines these measurements with process and piping information configured before commissioning to dynamically calculate density-compensated mass flow, energy flow or volumetric flow.

And, ABB’s 266 MV multivariable pressure transmitters (Figure 3) can be applied to both DP-mass-flow or compensated level-measurement applications. “It is only a matter of parameterizing the device to the process data of the level measurement for use in applications such as heated reactors or vessels and tanks,” says Ramos.

Some solutions are engineered for specific markets or application demands. For example, Emerson’s Rosemount 4088, which offers a “dual-device architecture,” in that it measures DP, static line pressure and process temperature across a DP primary element and sends them digitally to an external flow computer, making it suitable for the upstream oil-and-gas industry, where regulatory requirements govern flow computer performance.

For flexible measurements or where corrosive chemicals or contamination of purified fluids must be avoided, Siemens offers SITRANS FS ultrasonic clamp-on flowmeters, which are non-invasive devices mounted around the process pipe with no direct contact with the process fluid. The SITRANS FS230 measures flow velocity and the ultrasonic speed of the media, which allows it to identify specific media types in a multi-product pipe.

The SITRANS FM electromagnetic flowmeter measures volume flow and conductivity simultaneously for monitoring mixing process quality to offer different verification levels across multiple parameters without interrupting the process.

Challenging applications

“Multivariable instruments are particularly valuable in applications where process performance depends on the interaction between several variables rather than a single measurement,” says Endress+Hauser’s Schumann. “Typical examples include steam networks, utility systems, distillation units, reactor feeds, blending operations, specialty-chemical production and solvent-recovery processes.”

In these applications, operators are often trying to understand process efficiency, product consistency or energy performance, he says. “A single measurement frequently provides only part of the story, but multivariable instruments help bridge that gap by combining related process information at the source. This empowers operators to make better decisions based on a more complete understanding of actual operating conditions.”

Multivariable instruments are also frequently applied when the goal is to reduce total flow-measurement error, says Emerson’s Flesch. “Accurate flow measurement can be difficult when process density, temperature, static pressure and primary element behavior change over time,” he says.

“A common example is steam mass-flow measurement in an energy-management system,” Flesch explains. “With changing process temperature and pressure, the Rosemount 3051SMV can typically maintain accuracy well within one percent of flow reading. Without dynamic compensation for process density and measurement bias, performance can be more than seven times worse than a compensated device at full scale, with error increasing further as flow decreases. Steam measurement uncertainty can affect the efficiency calculation of the entire steam system, which can have a significant financial impact.”

This ability to perform well where single-measurement instruments struggle to achieve accuracy in changing conditions is driving increased use of multivariable sensors in a myriad of applications throughout the chemical process industries (CPI).

A good example is flare-gas metering, notes Flesch. “Flare-gas composition, pressure and temperature vary constantly, and a single DP transmitter cannot compensate for the resulting density changes,” he says. “A multivariable transmitter delivers a compensated mass-flow measurement accurate enough to support emissions reporting, environmental compliance and flare-reduction efforts. This is something a single-variable device cannot reliably provide.”

Multivariable vortex flowmeters are being applied to boiler or steam-distribution systems where operators need to know the actual mass flowrate of steam to optimize boiler performance, energy efficiency and process control. “A traditional single-variable flowmeter measures only volumetric flow, which can become inaccurate when steam density changes due to fluctuations in temperature and pressure,” explains Liz Majestic, vortex and DP flow product manager with Yokogawa.

The VY Series multivariable vortex flowmeter addresses this challenge by combining volumetric flow measurement, integrated temperature measurement, embedded steam table calculations for density compensation, real-time mass flow calculation and optional analog pressure input functionality.

“As steam conditions change, the meter continuously compensates for density variations to calculate true mass flow. A single-variable flowmeter measuring only volume would not be able to provide this level of accuracy without additional instruments and external calculations,” Majestic says.

Endress+Hauser’s Proline Prowirl F 200 multivariable vortex flowmeter (Figure 4) is also widely used in steam and utility applications throughout the chemical industry. “The instrument combines flow, pressure and temperature measurement to deliver compensated mass flow and energy calculations from a single device. This helps chemical producers improve utility efficiency, optimize steam consumption and gain greater visibility into energy-intensive processes,” says Schumann.

Multivariable sensors also bring higher levels of accuracy to dosing, mixing and additive control applications, says Siemens’ Wins. “Even a small accuracy difference in the measured quantity of an additive can significantly impact the final product quality, so having mass flow, density and temperature from a single measurement point can make a real difference,” he says.

“For corrosive media, the non-invasive clamp-on flowmeter SITRANS FS230 (Figure 5) eliminates the chemical compatibility question entirely, because the sensor never contacts the process fluid and can indicate the type of media currently running through the pipe,” Wins says. For mixing processes, measuring media conductivity and volume flow together provides the opportunity to better monitor the mixing quality.

Wins continues to explain that mass-flow and dosing applications for specialty chemicals and formulation plants are other good examples of how multivariable sensors can bring greater levels of accuracy. “Specialty chemical plants produce high-value, low-volume chemicals tailored for specific applications. These processes are recipe-driven and often involve hazardous or sensitive materials that require exact formulations.

“The SITRANS FC Coriolis multivariable flowmeter provides the required accuracy and ensures measurement integrity that contributes to correct dosing and liquid transfer, even with density fluctuations,” Wins says. “Their dynamic adaptability helps to maintain process performance even in challenging conditions.”

And, in applications involving level measurement in open tanks or vessels with varying fluid density; drum-level measurement in boiler drums, reactors and storage vessels; and high-temperature heated reactor and tank level measurements, multivariable measurements provide an ideal solution, says ABB’s Ramos.

“In these applications, SIL-certified multivariable transmitters provide accurate density-compensated level measurement with enhanced safety, improved boiler safety and reliability, and by measuring various process variables, the possibility for the user to recognize early warning of process anomalies, all with reduced instrumentation requirements and engineering complexity,” he says.

In these applications, accurate measurement of steam, energy, utilities and material flows provide the foundation for identifying efficiency improvements and quantifying emissions-reduction initiatives. And, as chemical producers place greater emphasis on energy performance and sustainability reporting, reliable measurement data are now critical business assets.

Offering greater insight

“As with any instrumentation, the key to reaping the benefits of enhanced accuracy in challenging chemical processing conditions is choosing the appropriate measuring principle for the application,” says Endress+Hauser’s Schumann. “When applied correctly, multivariable technologies deliver both high measurement performance and enhanced process understanding.”

In many cases, multivariable sensors support overall system performance with greater levels of accuracy than single-measurement devices because measurements are generated from a common sensing architecture and calculations are performed directly within the device. “This reduces uncertainty associated with combining measurements from multiple locations and instruments.”

But the true advantage of multivariable sensors is enhanced insight into the process. “A single variable tells operators what is happening, while multiple correlated variables help explain why it is happening,” says Schumann. “Understanding those relationships is increasingly important as chemical manufacturers implement advanced process control, digital analytics and optimization initiatives.

“However, the next level of process insight is measurement confidence,” he says. “Operators need to know not only what the process is doing, but also whether they can trust the underlying measurement. This is where intelligent instrumentation and advanced diagnostics are becoming increasingly important.”

Yokogawa’s Majestic agrees: “For chemical processors, this enhanced visibility supports better process control, improved product consistency, reduced energy consumption and more effective asset management.

“Integrated diagnostics can also help maintenance teams identify potential issues before they impact production, supporting predictive maintenance initiatives and reducing unplanned downtime,” Majestic says. “Ultimately, multivariable measurement empowers chemical-plant staff to improve operational efficiency, increase reliability, support sustainability objectives and lower overall operating costs by extracting more value from a single field instrument.”

Improved visibility also enables better decision making that contributes to improved facility safety, regulatory compliance and productivity, as well as cost reductions, making multivariable measurement data critical business assets.

For example, ABB’s Ramos notes that operators can use the data to maintain process stability, optimize yields and improve product quality, while accurate steam, gas and utility measurement support energy accounting and efficiency improvements.

“Additionally, pressure, flow and temperature trends combined with the diagnosis capability of the ABB multivariable transmitter, such as the always on-board Plugged Impulse Line Detection (PILD) diagnosis, can indicate changes in conditions, or abnormal operating conditions, that result from plugging, leakages, fouling or other issues,” says Ramos. “More accurate measurements are the basis for process optimizations that reduce consumption in energy-intensive processes.” 

Joy LePree