Cut Your Sensor Count But Not Your Data

May 23, 2013
Take advantage of multivariable devices to gain operating and maintenance benefits.

You likely are spending more money than required on field sensors — not because you've selected the wrong sensors but because you have too many of them. "How can I have too many sensors when I barely have enough information now to monitor and control my process? What I really need is more not fewer sensors but I just can't afford them," you might argue. However, the fact is you're probably not fully using the capabilities of some of your sensors, and doing so would eliminate the need for other devices.


Figure 1. This vortex meter has an integral temperature sensor and an optional pressure sensor. Source: Honeywell.

The majority of transmitters today are "smart" in one way or another and support some form of digital communication. It's well known in the automation industry that more than 80% of the installations with these devices aren't using this communication capability. Yet, digital communication provides the ability to share status and diagnostic information useful for improved maintenance of the devices, and enables them to transmit more than one process variable or, in the case of a control valve, to give feedback on actual versus output position. Earlier articles [2, 3] discussed the maintenance aspects of smart sensors, so this article will look at the opportunity lost only from a signal perspective.The reason more facilities aren't using the multivariable capabilities of their installed equipment partly lies in the design process and partly stems from lack of knowledge by end-user engineers, something this article hopefully will start correcting.DEALING WITH DISINCENTIVESFor new projects, taking full advantage of multivariable devices requires a conscious decision early, preferably during front-end engineering design (FEED), to use digital communication and to address common roadblocks.The majority of projects today are designed and built on a "time and materials" basis using either an engineering procurement contractor (EPC) or main automation contractor (MAC). Because compensation is based on the number of engineering hours and, in the case of the MAC, which also likely supplies the field devices, the number of devices and point licenses sold, the contract doesn't provide an incentive to use fewer devices. For the EPC, each additional device means hours to design the necessary process connection, instrument specification and loop drawing, as well as the electrical work to connect it to the control system. So, even if the instrument discipline identifies an opportunity for a multivariable device, the mechanical and electrical disciplines will lose hours. For the MAC, selecting a multivariable device will cost it the sale of a field device and the associated configuration time (although the configuration time difference between a soft and hard tag won't be significant). Depending upon the pricing model of the control system supplier, the point count license actually may increase if you start using soft tags and data available from smart sensors.On the positive side for both EPCs and MACs, with staff time at a premium going this route provides the benefit of increasing productivity.A typical smart device has upwards of 300 parameters available, so how to select the right information to transmit and capture can pose a challenge. This likely contributes to plants staying with the traditional one-to-one relationship of field device to signal. (Consultants can provide guidance on parameter selection as well as training to engineers on when to use multivariable devices.)The best way to overcome some of the disincentives is to offer better incentives for shared success. For example, give the MAC incentives for every multivariable device it installs; because the savings to the project include the nozzle and connections, you're still ahead financially. This may not work as well for the EPC. So, consider framing the contract not as "time and materials" but as "cost plus." This provides an incentive to finish the project below a predetermined fixed cost and share the savings.Many companies now incorporate value engineering reviews as part of the work flow. Because automation and control typically amount to about 5% of the total project cost, not too much energy is spent on them. However, if you have the right people in the room or break up the scopes of work appropriately, you likely will identify more opportunities for multivariable devices. BROAD BENEFITSMaking the decision at FEED to use digital signals opens up new opportunities to minimize engineering and field construction costs. The level to which you incorporate fieldbus technology also will impact the physical plant layout in a number of ways, including potentially increasing the degree to which you use panel prefabrication and modular construction.Commissioning will be quicker — not only because there will be fewer physical devices to confirm but also because digital communications technology enables the work to be done faster and with fewer people. As we know, instrumentation and control groups always are the last ones finishing their commissioning, so any time saved here normally leads to an earlier startup and, hence, more production and faster return to positive cash flow.Once the plant is operating, the savings and opportunities to use smart field devices continue — although realizing the full benefits may require changes in traditional work practices. For example, the ability to communicate to field devices means that a change in the device, such as altering a range, now is propagated through to the control system and vice versa. As a result, the risk of a range mismatch between the field and control system decreases. However, it also necessitates implementing appropriate policies around who has access to what form of changes in the device and control system.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.LOW-HANGING FRUITYou 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.BE SMART ABOUT INSTRUMENTSVarious 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 [email protected].

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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