The Power Is In Your Hands

Looking for a Digital Field Network? With HART, It’s Right in Front of You

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T BRITISH PETROLEUM’S PTA (Purified Terephthalic Acid) PLANT in Wando, S.C., managers are facing a dilemma familiar to their counterparts at thousands of manufacturing facilities throughout the country: do more with less.

“BP is investing heavily in plant construction in China, so our objective is to lower costs and increase our availability. We need to use what we have and use it better,” says A.J. Lambert, an instrument and electrical reliability specialist with BP.

One way plant personnel are achieving that goal is by using a previously underutilized technology that is providing them with a remarkably detailed view of conditions throughout their plant and the ability to reduce asset management/maintenance costs.

The “silver bullet” that’s generating these benefits: the HART Field cCommunications protocolProtocol. “Through the use of HART diagnostic information, this BP plant is saving approximately $300,000 per year in just maintenance and production costs alone,” says Lambert.

Feel the Power
Of course, not every plant will attain such dramatic results, but most users can achieve all, or nearly all, of the functionality they seek by tapping into the proven power of the HART-enabled field devices they may already own. As a result, most of the benefits they achieve through the use of HART drop right to the bottom line. Few automation projects are as easily justified economically as that.

HART’s exceptional value to process manufacturers lies in its ability to simultaneously communicate an instrument’s primary variable via a standard 4-20 mA analog signal and and up to three secondary valuesadditional process variables and plus diagnostic information via digital signals on the same wire. Initially, HART was initially embraced by maintenance staffs who were delighted to be able to use handheld communicators and calibrators, such as those made by Meriam Process Technologies or Emerson Process Management and , and take them into the field. All they had to do was attach them to any HART-compliant instrument--regardless of manufacturer–and they were immediately able to retrieve critical information about an instrument’s condition, its health and its range, as well as perform calibrations, document them and obtain much more digital information besides.

Since then, manufacturers have developed a wide range of new intelligent products and added HART capabilities to existing ones, providing even greater benefits to a broader base of users. For example, field device makers such as Siemens differentiate themselves by communicating additional HART data that are unique to the company’s instruments. Siemens pressure transmitters have the ability to monitor and store data on the highest and lowest pressures experienced by the entire instrument, the instrument’s pressure capsule and its electronics, says Lou DiNapoli, marketing manager for the company’s pressure and temperature transmitters.

“All users have to do,” says DiNapoli, “is use a HART handheld device or our Device Description Language-enabled Process Device Manager (PDM) software to get that information, which is highly useful in diagnosing process problems. If you use someone else’s equipment and software, you just need the Device Description for our instrument.”

Sophisticated Asset Management
In recent years vendors have developed sophisticated asset management software packages, including Yokogawa’s Plant Resource Manager (PRM), Siemens’ PDM, Emerson Process Management’s AMS and Honeywell’s Asset Manager PKS, that enable maintenance engineers to view, track and analyze the condition of instruments remotely. In this situation, data from a device is split into separate analog and digital signals. The analog signal, carrying the primary variable, is routed to the control system, while the digital secondary values and diagnostic data are conveyed to the asset management systems.

Asset management systems and their ability to use HART data are proving enormously beneficial as numerous users will attest.

BP’s Wando plant started tapping the full potential of HART about five years ago, when it installed Emerson Process Management’s ValveLink software to gain diagnostic information from its approximately 125 most critical control valves. Among the areas in which the plant has cut costs is in valve maintenance during planned shutdowns.

“Before using HART, we would pull out 35-50 valves for maintenance during shutdowns every two years,” says Lambert. “There might have been a work order or some concern about a particular valve, but we really didn’t know what might be wrong with it. As a result, we’d spend a lot of money and time. Now, with more information from ValveLink and HART, we pull only five or six valves during a shutdown. And we have a lot more information about why we’re pulling it. Our diagnostic system can show us when a problem assumed to be in a valve really isn’t, but is perhaps somewhere else in the process. In other cases, we’re able to see potential problems in valves before they become serious.”

A senior engineer for field instrumentation with another  major oil company refiner says that digital HART instrument data, routed back to the refinery’s asset management system, has enabled it to improve its processes as well as realize savings on maintenance.

According to this petroleum industry source, “Right after we started capturing this data, we were picking up diagnostic alerts for valve travel deviations, air pressure problems on valve actuators and other variances.,” he says. “We were skeptical at first, but when we investigated those alerts, we discovered that we had actual problems–bent positioner feedback rods, filter regulators that were either failing or malfunctioning, leaks in diaphragms and other problems. So what we learned from our HART-enabled asset management system was that our system was still operating, but it was not controlling the process as well as it could, and that we might be facing pending failure because of these problems. We were able to correct these problems before they had an impact on the process.”

Petro-Canada, which is implementing projects at its Montreal and Edmonton refineries to reduce sulfur in its gasoline and diesel fuels, also will use HART data from for troubleshooting on valves and field instruments, says Pat Castellino, an engineer with the company’s process technology and reliability group.

“I expect a manpower reduction in the startup phase of the project and then on a lifecycle basis,” he says Castelino, when asked about potential benefits stemming from the use of HART data. “We feel that the additional information we incorporate into our asset management solutions will help us establish more effective predictive maintenance practices.”

Several factors make HART solutions relatively easy and cost-effective to implement. First, HART does not require a costly “rip and replace” strategy. The HART protocol runs on standard wiring that most plants already have in place and, as mentioned, there are millions of HART-enabled devices already in use. Often, field instruments that are not HART-ready can be easily and inexpensively upgraded.

Because HART represents an enhancement of the standard 4-20 mA analog field communications standard, there is no need for companies to undertake major retraining programs. Courses offered by HCF, vendors and other sources generally provide all the background information plant personnel need to use HART Effectivelyeffectively.

In addition, HART network architecture is simpler than that of most all-digital fieldbus protocols, making it the ideal option for many plants.

“If I’m putting in a 4-20 mA system with HART, the engineering is very simplified; it’s basically a matter of routing wires from the control system to the final elements,” says Petro-Canada’s Castellinothe major refiner’s senior engineer. “Designing a HART system doesn’t require a great deal of specialized knowledge concerning matters such as the number of devices to assign per segment, to which segment should each device be assigned, failure modes on loss of communication, surge protection, signal reflections, power and signal-attenuation calculations.

"When I increase the amount of engineering that has to go into a system, I’m increasing my opportunity for human error, and those are things I’d like to avoid.,” he adds.

Easy and Effective
In addition to being relatively easy and cost-effective to implement, plants can adopt HART very gradually and realize benefits, even on a small scale. For example, many users get their first experience with HART by commissioning instruments on the “bench” in maintenance shops. All that’s required is a PC equipped with DDL-enabled asset management software and an interface between the computer and the field device–typically a HART modem.

Tom Holmes, president of HART modem manufacturer MACTek Corporation, says there are significant advantages to using a desktop or notebook computer for bench or field configuration of devices rather than using handheld configurators.

What is HART?

In 1993, the HART Communication Foundation (HCF) was established to provide worldwide support for application of the Highway Addressable Remote Transducer technology – or HART Field Communications Protocol. HCF owns the HART technology, manages the protocol standards, and ensures that the technology is openly available for the benefit of the industry. The ARC Advisory Group, Dedham, Mass., estimates that, of the approximately 40 million field devices installed worldwide, 26% percent are HART-enabled, making it the mostly widely used protocol for smart field instrumentation.

HART is a hybrid protocol communications technology in which a modulated, two-way digital signal is imposed on the industry-standard 4-20 mA analog signal carrying the primary process variable. The digital signal conveys up to three additional process variables, device status and diagnostics information that can be routed to asset management, process control and safety systems. This means that HART provides two simultaneous communication channels on the same wire – the industry standard 4-20 mA channel for fast, reliable and robust control (PV) and a digital channel signal for real-time communication of additional process/device information.

HART includes a standardized application layer addressing device status and diagnostics; cyclical process data, including floating-point digital value; engineering units; data quality; and status. The protocol also enables field devices to continuously publish their process data and standardized operating procedures (e.g., loop test, current loop re-ranging and transducer calibration.)

Every HART device includes 35-40 standard pieces of information, which are easily accessible by all HART-enabled systems. These include device identification, basic calibration data, process variables (measured and calculated) and diagnostic alerts. All HART smart field devices continuously assess and monitor their own performance, and all return diagnostic status information with every message.

A HART innovation was the creation of Device Description Language (DDL) – an object-oriented, text-based language for modeling the characteristics and real-time capabilities of intelligent field devices. Instrumentation suppliers use DDL to create Device Description (DD) files, which is similar to an electronic data sheet describing all capabilities of the smart field device so that the DD-enabled and host systems can communicate with all device features. In early 2004, the International Electrotechnical Commission (IEC) approved DDL as an international standard.

“All of the information you need about all of the devices you use is right there on your computer’s hard drive. You can do signal processing, a host of higher-level diagnostic applications; data storage; downloading of archived configurations that are right on the computer. You don’t have to go through the trouble of downloading a device configuration from a server, loading it into the handheld configurator and then reversing the process when you get out to the instrument,” he says.

In addition to producing modems for RS-232 serial ports, MACTek recently began producing models that use USB ports. In addition to being simpler to use than the serial port versions, many computer vendors no longer include serial ports on their computers, making the USB version a necessity for many users. Also, multiple devices can be attached to a single USB port through the use of USB hubs. As a result, says Holmes, some customers have been able to set up small monitoring/data acquisition systems using several USB modems.

“There’s one customer doing a fiscal metering application involving custody transfers of product. He needs high accuracy and reasonable sampling rates,” offers Holmes. “The application required him to monitor eight HART loops in multidrop mode. They used eight USB modems, connected to two four-port hubs, which, in turn were plugged into two PC USB ports. They’re using Emerson Process Management AMS Device Manager to monitor the equipment.”

Moore Industries’ HART Interface Monitor (HIM) is another product particularly well suited to small and medium-sized installations or those that lack an asset management system, says John Emmett, Moore’s London-based HART specialist.

The Moore Industries HIM passes the 4-20 mA signal directly into a DCS or a PLC, but it also breaks out the digital signals, converts them to analog and makes them available to the control system as well for alarming and other functions.

Using the two alarm relay contacts in each HIM, users can set limits around process variables.

“For example, with a smart valve positioner, you have the control system using a 4-20 mA signal driving it open and closed. Using an HIM , you can read the valve position feedback over the same pair of wires and return it to the control system. In addition to there being a cable savings, the alarm relay contacts act as ‘soft’ limit switches. So you can set a trip limit within the HIM and cause a relay to close when the valve reaches its limit,” says Emmett.

The digital feedback on the valve position also can be used in safety system applications, a burgeoning application for HART technology. (see sidebar).

Other components that enable users to implement and expand their HART-based field networks are smart distributed I/O modules, such as those provided by Rockwell Automation.

“Instead of running all of the wiring back to the control room from each individual device, our FLEX I/O is out near the field and brings the data back via a network. This reduces long home-run wiring runs from multiple field devices, and it reduces the number of terminations you have to do per point for field devices,” says Pat Moyer, Rockwell Automation’s marketing manager for distributed I/O.”

Like a multiplexer, smart, distributed I/O separates the analog and digital signals and routes them to the appropriate systems.

Multiplexers, including those manufactured by MTL Instruments, enable users to simplify their wiring and scale up their systems incrementally. For example, by using MTL’s MTL4840 HART connection system, users can connect nearly 8,000 loops to a single PC communications port and easily configure a scan list and get LED indications of the loop being scanned, says Tess Thonger, a MTL marketing manager.

Because HART delivers many of the same benefits as all-digital fieldbus protocols, it’s unsurprising that there’s confusion in some potential users’ minds about them. Part of the problem stems from users’ mindset based on their initial exposure to HART, says Jim Cobb, Emerson Process Management’s Plantweb marketing manager.

“It’s a little bit of a case of HART having been around long enough that people think they know it, but in reality, they only know the surface,” says Cobb. “Now, when you see some of the systems that are coming out today, they have pretty good HART support. As the systems vendors start educating their customers, that’s going to be the most important way the message gets out to the users.”

Joe Serafin, Honeywell’s product manager for HART integration, says Honeywell tells its customers that their choice of field communications protocol should be dependent on what they want to achieve. “If they’re interested in peer-to-peer control on the wire, we tell them to go with Foundation fieldbus. But if they don’t want to do that, they can probably get everything else they want out of HART,” says Serafin.

“But we don’t really care which they use,” adds Serafin. You can actually buy both HART and fieldbus devices and have them in functioning in the same control module. That’s how well they’re integrated.”

In for the Long Haul
Few industry observers question that, in the long run, the process industries will eventually migrate to fieldbus protocols. But don’t even think about mothballing your HART devices anytime soon. HART is probably the most economical and best solution for modernizing existing plants.

HART combines the reliability and robustness of today’s analog signal with the power of tomorrow’s digital field network. Considering the millions of HART devices already installed, the most responsible action for the industry is using HART to its full potential.
“After all, people are still using pneumatic devices today,” says Moore Industries’ Emmett. “And if you think about all of the millions of HART instruments already installed around the world, you can be sure that HART is going to be with us for at least the next decade or two.”
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