HART Plant of the Year: Digital Output

AT A GLANCE

Award-winning savings

Sidestep signal conversion

Eliminate errors

HART beyond maintenance

Sidebars: Real-time recognition

Just as all roads once led to Rome, today all pipelines lead to Fort Saskatchewan– at least those carrying hydrocarbons in the middle of Canada’s oil-rich Alberta province. The pipeline hub is the BP Canada plant located in the city. “We have just about every pipeline in central Alberta coming in and out of Ft. Saskatchewan,” says Marcel Boisvert, a senior instrument/electrical craftsman with BP Canada Energy.

The material in the pipelines not only moves through the area but also changes owners. BP Canada Ft. Saskatchewan does the custody transfer measurement, and getting those readings right presents a challenge. However, thanks to hard work and HART Communication technology, Boisvert reports, “Our measurement balances are fantastic. Our accuracies are just phenomenal.” .

The plant’s engineering staff put the digital process variable output of HART-enabled transmitters to use, thereby improving accuracy, saving up to a quarter million dollars annually, and earning HART Plant of the Year honors for 2006. Theirs is a tale interest for anyone who wants to know how HART technologies can help improve processes.

Balancing the books

The Ft. Saskatchewan plant makes propane, butane, natural gas liquid (NGL) condensate and ethane with carbon dioxide removed. In a given year, 1.5 million cubic meters of propane leaves the plant, bound for other BP facilities. The other products produced also ship in large volumes. The plant staff employs temperature and pressure readings to compensate the flow measurement and quantify what’s in the pipes.

A decade ago BP Canada started installing HART-enabled temperature and pressure transmitters in Ft. Saskatchewan. Today a majority of the 600 transmitters in the plant are HART-enabled. As part of the upgrade package, the company also installed flow computers, up to four transmitters per computer.

In the initial configuration, Boisvert says the 4-20 mA analog signal output from a transmitter was fed into a flow computer. The digital signal superimposed atop the analog signal as part of the HART standard functioned as a second communication channel to and from the transmitter but was not being used by the flow computer.

For audit and other purposes, flow-computer readings were and are compared to that of a standard, certified gauge. One plant staff person handled transmitter calibration while another did the meter proving. The accuracy in the mass delivery demanded by contract was one or two tenths of a percent, a figure that translated into a need for highly accurate temperature and pressure readings. For example, a temperature error of 0.25 °C results in up to a 0.07% net flow error. Multiplied by the amount of product Ft. Saskatchewan ships, that error meant a potential loss of $250,000 a year. For NGL, the corresponding figure was $350,000, creating strong incentives for accurate measurements. In addition, those receiving the product would make their own readings and take action if a large enough difference was found. “There will be a correction, if you have somebody at the other end able to do the measurement as well as you do or better,” says Boisvert in explaining what would happen.

Speaking one language

Problems, however, cropped up when Boisvert and coworkers tried to reduce the measurement error. Despite diligent transmitter calibration efforts that should have kept the devices operating within a tenth degree, meter proving continued to indicate an inaccuracy of up to half a degree. The employee proving the meter would then adjust the transmitters or ask for them to be calibrated. Calibration showed that the transmitters were working well within tolerances. The sequence would then repeat for each employee’s area until various team members witnessed each he team finally found the problem to be within the flow computer, specifically in the transformation of the analog signal into its digital equivalent. “The analog to digital conversion was not as accurate as what we were looking for,” notes Boisvert.

The errors weren’t great, perhaps 0.2 to 0.5 °C on a range of -18 to +65.5 °C. On the pressure side, the discrepancy might be up to 50 kiloPascals on a range of 10,000 kPa. Those small errors, though, were enough to eat up a significant part of the overall allowable error budget.

Faced with this problem, the team started looking for a solution. A series of tests convinced them the readout at the transmitter agreed with analog and digital signals. That was when the group decided to sidestep conversion, notes Boisvert. “If the flow computer speaks HART and the transmitter speaks HART, let’s read the digital value rather than the analog value,” he says, thinking through the process.

The simple-sounding idea required a bit of work. For one thing, the flow computer’s model didn’t natively read or write HART commands. However, it was programmable at a low level. The vendor provided what was essentially a translator, allowing Ft. Saskatchewan’s team to worry only about the higher level application software.

Part of the solution also involved upgrading the transmitters. A typical specification for a transmitter of 0.5-degree accuracy wouldn’t work. “We were looking for that 0.2 degree accuracy,” says Boisvert. Working with vendors, they found transmitters that would hit the more stringent spec. Such a tightening only made sense, though, because the conversion-induced error had been eliminated.

Highly accurate results

With transmitters and flow computers talking without an analog step in the middle and with more accurate transmitters, the Ft. Saskatchewan plant achieved the desired performance. It’s hard to pinpoint an exact dollar figure saved due to the improvement because, as Boisvert notes, the error could have been positive or negative. The total amount of the possible adjustment, however, runs in the hundreds of thousands of dollars a year. Since the setup has been in place for a decade, potential savings are in the millions of dollars.

Another benefit even harder to quantify has been removal of internal inconsistencies. That has contributed to creation of a well- deserved reputation that helps in certifying measurements to external parties. “There’s never any issues about our accuracy,” says Boisvert.

There’s also been some savings of engineering and technician time. To take one example, the old meter proving required one employee to take a reading from a certified device placed next to the transmitter. The staffer would then have to go to where the flow computer was located and see what it measured. Finally, the employee would have to travel back to the certified instrument and hope there hadn’t been a process change during this back and forth. Now the task is much easier because the transmitter’s digital display shows what the flow computer sees. “He can look at the certified device sitting right next to the digital display on the transmitter and compare his readings,” notes Boisvert.

There are also time savings because instruments aren’t being tweaked, and other adjustments aren’t being made only to be undone later. BP Canada Energy estimates total savings of at least an hour a week of engineering and technician time.

As for lessons learned by the whole process, Boisvert cites one that could apply to a wide variety of situations. “HART is not just a maintenance tool. It is a process improvement tool as well,” he says.

Real-time recognition

HART Plant of the Year is an international award presented annually by the HART Communication Foundation (HCF) to recognize innovative use of the HART Field Communications Protocol in real-time industrial process plant applications.

HFC says this is the only public award presented to an end-user company in the process automation industry to recognize ingenuity in the application of HART Communication technology. The award recognizes “people, companies, and plant sites around the globe that are using the full capabilities of HART Communication in real-time applications to improve operations, lower costs, and increase plant availability. Recipients are plants that have taken the capabilities of HART instruments beyond configuration and calibration, or are using the real-time diagnostics and process variables of HART-enabled devices integrated with control, information, and safety systems.”

Previous HART Plant of the Year recipients are Sasol Solvents SMG Operations plant in Sasolburg, South Africa (2005); the BP Cooper River plant in Wando, SC, and the Clariant Antioxidants Plant in Gersthofen, Germany (2004); the Detroit Water and Sewerage Dept. in Detroit, MI (2003); and the DuPont DeLisle manufacturing facility in Pass Christian, MS (2002).

The HART Communication Foundation is an independent not-for-profit organization providing worldwide support for application of HART technology. Established in 1993, HFC is the technology owner and standards setting body for the HART Communication Protocol, with support from major instrumentation manufacturers and users around the world. Membership is open.

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