Cut Your Sensor Count But Not Your Data

Take advantage of multivariable devices to gain operating and maintenance benefits.

By Ian Verhappen, Industrial Automation Networks

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Additional operating savings result because fewer nozzles also mean fewer possible leak points for EPA monitoring.

Being able to read the actual position of a final control element and compare it against the target output value will assist in confirming proper process operation while also giving an indication of the actual loop response time and health of the control element as well.

You likely already have a number of HART transmitters — and can take fuller advantage of them relatively easily and at minimal cost. An offline option is to capture information via your handheld or laptop-based HART communicators and then putting those data into a central database. An online option is to transmit the data directly if you have a HART modem connected to each device. Check that your control system has cards that contain HART communications capability; modern analog input and analog output cards do. If need be, you can change out old cards to new ones during a plant shutdown. A second option for online data delivery is to install third-party HART "strippers" in your input/output cabinets to remove and retransmit the HART signal along a parallel system. A third option is to equip devices with WirelessHART so they can send data via a plantwide wireless network.

The HART data then can be used to confirm work history, allowing you to identify root cause problems and potentially modify your maintenance practices accordingly.

If you're restricted to staging your migration to using digital signals, the biggest return will come from diagnostic data on your control valves and, hence, your analog output cards. Because control valves have moving parts and contact process fluids that often are aggressive, they should get constant monitoring. Smart positioners not only can confirm the signal feedback but also can provide data on a suite of valve diagnostic parameters. The most commonly used parameter is "cycles," which indicates how many times the valve has changed direction and the distance traveled, i.e., how far the valve stem has moved (e.g., 0.25 in. in a down stroke and 0.5 in. in an upward direction for a total of 0.75 in.). A change in the ratio of cycles to distance traveled usually indicates packing or tuning problems, or some other actuator-related difficulty.

Myriad other diagnostic applications exist. Many microprocessor-based devices continuously check not only the health of their own electronics but also the sensor used to measure the process variables. For instance, differential-pressure-based flow meters monitor the frequency and amplitude of the pressure impulses in both legs and compare these over time to determine if one or both of the pressure taps are deteriorating (plugging). These data can identify when a problem is developing, allowing action to be taken at an opportune time before the measurement and, hence, control loop are affected.

Besides diagnostics, differential-pressure-based and other flow meters now are capable of measuring and reporting digitally multiple variables. Therefore, it's possible to use a differential pressure meter to measure both the flow and the bulk line pressure to calculate a pressure-compensated flow, or with a vortex meter having an integral thermocouple to provide a temperature-compensated flow — in both cases increasing the accuracy of the measurement and, thus, enabling tighter control of the process.

There also are integrated differential pressure meters with the orifice directly connected to the pressure sensors, and a vortex meter with embedded pressure- and temperature-compensation sensors (Figure 1) allowing calculation of mass flow of vapor or steam from one device at a much lower cost than that of a Coriolis meter.

Various initiatives, such as the "Smart Manufacturing Leadership Coalition" and the "Advanced Manufacturing Partnership" [1] that bring together government, academia, suppliers and major end user companies, should foster increasing use of smart instruments. In addition, the International Society of Automation (ISA) recently formed a committee, ISA108, to develop standards and practices to assist in integrating smart device parameters into distributed control and maintenance systems and, hence, improving day-to-day operations.

Getting more value from your multivariable devices requires taking a different approach and understanding what those sensors can tell you. This can result in fewer instruments, better availability and improved return on your investment.

IAN VERHAPPEN, P.E., is director of Industrial Automation Networks, Calgary, Alta. E-mail him at

1. M. Bryner, "Smart Manufacturing: The Next Revolution," p. 4, Chemical Engineering Progress (Oct. 2012).
2. R. K. Pihlaja, "Is Your Process Whispering to You?," p. 30, Chemical Processing (Sept. 2011),
3. I. Verhappen, "Control Your Maintenance," p. 40, Chemical Processing (July 2006),

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