The post How to Choose Wired vs. Wireless for New Industrial Installations first appeared on the ISA Interchange blog site.
This article is from the January/February 2015 issue of InTech magazine and was written by Moazzam Shamsi, global director of wireless consulting and execution solutions at Emerson Process Management.
Fieldbus technology has been available for more than 20 years. Initially, FOUNDATION Fieldbus and other digital fieldbus technologies were developed in the 1990s as a replacement for the 4–20 mA standard. Today, virtually every manufacturer of flowmeters, pressure transmitters, and similar 
instrumentation offers FOUNDATION Fieldbus and other popular fieldbus interfaces, and every major automation system vendor supports one or more fieldbus standards.
More recently, ISA-100 and WirelessHART were also developed. The International Electrotechnical Commission (IEC) approved WirelessHART in March 2010 as IEC 62591, and ISA-100 was approved in September 2014 as IEC 62734. For the purposes of this article, wireless refers only to wireless sensor systems, and not to other wireless technologies such as 802.11 Wi-Fi.
Today, automation professionals in a process plant have a choice to make for new installations: wired or wireless? For plants with existing wired and wireless infrastructure, the choice hinges on a straight comparison of the two technologies and the application of the solution that makes sense. For plants without an existing wireless infrastructure, the cost of installing one must be considered.
This article shows the advantages of each technology, recognizing that almost every plant will end up with a mix of wired and wireless. To simplify comparisons, FOUNDATION Fieldbus and WirelessHART will be used as leading examples of fieldbus and wireless technologies. Each has significant competitors, but comparison of competing technologies in each area is outside the scope of this discussion.
Fieldbus details
Compared to traditional 4–20 mA wiring, fieldbus technologies save wiring costs, simplify expansion, and are easier to make redundant because they allow multiple instruments to use a single cable called a trunk or segment. A trunk or segment begins at an interface device at the automation system. On a FOUNDATION Fieldbus system, the interface is called an H1 card.
The DC power needed for instruments on a FOUNDATION Fieldbus segment is provided by a power supply rated up to 500 mA, enough to theoretically power more than 32 instruments. In practice, however, 12 to 16 instruments are typically installed on a segment. Some instruments require more than 20 mA; available power diminishes over long cable lengths; and engineers like to allow capacity for adding instruments. Typically, up to 12 devices can be installed on a fieldbus segment up to 120 m long. If the process unit has more than 12 instruments, a second or third segment can be installed.
If a problem occurs in any instrument on the segment—such as a short circuit—it can disable the entire segment. Therefore, many plants install a segment protector or device coupler, allowing multiple instruments to connect at one location. The device coupler is installed in an enclosure near the process unit. Connections to the individual instruments are called spurs.
A typical segment includes an interface card, a fieldbus power supply, a device coupler, and individual spur cables from the coupler to the instruments. With FOUNDATION Fieldbus, the H1 card communicates to the plant’s distributed control system (DCS) via a high-speed Ethernet (HSE) connection. Other fieldbus technologies use similar architectures.
For critical process units, a redundant segment can be installed, using duplicate segment cables and power supplies. In hazardous areas, intrinsic safety barriers provide protection.
Where do you use Fieldbus?
The ideal application for wired fieldbus is a process unit containing many flow, pressure, temperature, level, multivariable, and other instruments, all within a reasonable distance of each other. The more instruments in a relatively small area, particularly complex multivariable units, the more fieldbus makes sense
By using device couplers and marshalling cabinets strategically located around the unit, wiring from instruments to device couplers can be minimized. Most fieldbus instrument suppliers offer automated design tools, making it easy to design a segment, calculate maximum distances, and determine the wire types. When instruments on a segment are far apart, repeaters allow segment distances up to 300 m.
Instrumenting such an application is fairly easy, because multiple vendors make components with various fieldbus interfaces. If a plant wants to modernize its legacy control system and install fieldbus-based instrumentation, HART can use existing 4–20 mA wiring from older instruments to carry digital information to the device couplers. The device couplers can be installed in the old marshalling cabinet, saving a considerable amount in wiring and labor costs. Although HART does not have performance levels comparable to newer fieldbus technologies, it is the least costly wired digital option, and often sufficient from an operational standpoint.
Valve and pump controls are also available with fieldbus, so it is possible to set up local control loops within the fieldbus array operating independently from the DCS. This is accomplished with function blocks allowing, for example, local proportional, integral, derivative (PID) control of a digital valve controller based on signals from a nearby level transmitter. If the DCS or the HSE go down, the control loop will continue to operate.
Shanghai Wujing Chemical, an acetic acid plant in Shanghai, China, upgraded its controls and instrumentation to increase its capacity from 300,000 tons/yr to 530,000 tons/yr. The new system included an Emerson DeltaV DCS and FOUNDATION Fieldbus instrumentation. Shanghai Wujing used local control for 114 PID loops. This saved 74 percent of the DCS controller process time. FOUNDATION Fieldbus diagnostics and communications are now used during calibration of control valves, saving 80 percent of the time previously needed for maintenance and operations.
Typically, wired fieldbus devices and segments have more power available than wireless transmitters. This makes wired fieldbus suitable when working with loop-powered devices such as two-wire level transmitters with continuous wave modulation, eight-channel process temperature transmitters, tank gauging multispot temperature transmitters, intelligent on/off valves, and field indicators.
Wired fieldbus is also suitable for real-time process control. The response speed of a wired system—from the spur to the segment to the H1 to HSE to the DCS and back again to a control component, such as a control valve—can be significantly faster than a wireless system.
Because of the cost of the hardware (H1 interface, power supply, cable, etc.), wired fieldbus is not suitable for a few devices located far away from other instrumentation. Such applications are better handled by wireless transmitters, along with other scenarios such as adding instruments to existing plants without installed extra capacity in the wired infrastructure.
Click here to continue reading Moazzam Shamsi’s article on wired vs. wireless at InTech magazine.
About the Author
Moazzam Shamsi, M.Sc., B.Eng., C.Eng., MinstMC, has been an automation professional for 25 years, and his career spans a broad range of industries and roles from technical leader to project management. He presently works for Emerson Process Management where he globally directs Emerson’s wireless consulting and execution solutions on large capital projects. Moazzam specializes in working with clients and contractors to implement technology solutions for capital and operational efficiency projects.
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