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Standards demystify batch process control

Production plants benefit from new scheduling methods and control standards.

Independent standards for batch control and scheduling systems are saving money and effort for the chemical process industries (CPI) and control system suppliers alike. In the past eighteen months the first independent standard for batch control systems, ISA/ANSI S88.01, has proved its worth by giving engineers a clear set of terms to describe batch flowsheets and control schemes. Now new standards are promising to do the same for the way batch controllers share process data and communicate with higher-level systems such as schedulers, manufacturing execution systems (MESs) and enterprise-wide networks. Software vendors, meanwhile, are responding to their customers’ rising awareness of the subtleties of batch control with standards-based products that offer more features as well as more openness.

“S88.01 has been very successful in its fundamental task of explaining what batch control is all about, and in cutting the time needed to develop and configure software,” says Lynn Craig of consultancy Manufacturing Automation Associates (Medford, New Jersey) and chairman of ISA committee SP88. “We are beginning to see commercial products incorporating SP88 functionality, and these are certainly helpful to users,” he says. “But the most important benefit of S88.01 is independent of the commercial offerings: the standard gives users a way to say exactly what they expect from a batch control system. They no longer have to describe their processes in terms of vendors’ specifications.”

S88.01 (Batch Control Part 1: Models and Terminology) was developed by the International Society for Measurement and Control (ISA; Research Triangle Park, North Carolina) and published in 1995 by the ISA and the American National Standards Institute (ANSI; New York). It has now been approved by the national committees of the International Electrotechnical Commission (IEC; Geneva) and is to be published as international standard IEC 61512-01.

The related standard SP88.02 (Batch Control Part 2: Data Structures and Language Guidelines), currently in its fourth draft and scheduled for publication next year, describes how data needed for batch control can be stored and exchanged in ways that are independent of the actual hardware or software. It also provides more detail about what a batch recipe actually looks like, an issue that S88.01 did not cover fully. “At the moment people tend to use IEC 1131 sequential function charts to describe recipes,” says Craig. “IEC 1131 is an excellent standard for programmable controller languages, but was not designed for recipes. S88.02 will attempt to provide a more appropriate method.”

Many plants that are based on batch operations also incorporate some degree of continuous processing, and this too creates problems for S88.01. In April 1995 the European Batch Forum (EBF; Dublin) formed a group known as WG3 to address the issue. WG3 has developed two models — a milkshake production plant and a multi-product chemical plant — and is now analyzing them to produce a control requirements analysis consistent with S88.01 but which also handles the issues around the S88 “problem areas”, such as how to integrate a filling line and how to handle cleaning-in-place (CIP).

Software vendors follow suit

Standardization can benefit software vendors as well as users. “S88 has allowed us to get on with selling our products,” says Robert Hylton, director of marketing for batch control specialist PID (Phoenix, Ariz), “because we no longer have to spend so much time educating our customers about batch control terminology or the benefits of Windows NT.”

PID is one of the small, specialist companies currently leading the way in batch control. By concentrating on batch processes PID has been able to respond to the emergence of the new standards more quickly than many of the large control system manufacturers, using “open” technologies such as Microsoft’s Windows NT, OLE and ActiveX to develop products that work with many different control systems. This flexibility has helped PID’s OpenBatch batch control system to become a corporate standard for companies such as Procter and Gamble, Genentech and BASF, says Hylton, and PID also licenses OpenBatch to large control system vendors including Honeywell and Hartmann + Braun.

Wonderware (Irvine, Calif.) and Intellution (Norwood, Ma.) are other vendors whose backgrounds in Windows-based supervisory control (SCADA) systems have helped them develop an important presence in batch control. But the big companies are close behind and will probably be able to supply more complete products in the end, says Craig. “Most of the offerings to date concentrate on ‘batch engines’ that will work with any programmable controller or distributed control system, and I’m not criticizing those,” he says. “But they are not integrated solutions. Only the large companies can offer solutions that are integrated right down to the valve controller.” Among the major vendors working on such integration are GSE Systems (TK), ABB (Vasteras, Sweden), Moore Products, Honeywell (Phoenix, Ariz) and Yokogawa (TK).

The large vendors will also be able to publicize batch control standards to an extent that is beyond their smaller competitors. “I’m disappointed that more people don’t know about SP88, in spite of the fact that three professional societies have sprung up to support it — the EBF, the World Batch Forum (WBF; Phoenix, Ariz.) and most recently the Japan Batch Forum,” says Craig. “Now that the big systems vendors are entering the market with S88-oriented products, however, we are getting close to ‘critical mass’. I hope so, because SP88 can make batch control projects a lot easier.”

Links to the wider world

Clever batch control is of limited use unless the batches themselves are processed at the right time and in the right order. The importance of scheduling was acknowledged in early drafts of SP88.01, notes independent batch control consultant Paul Sawyer (Chippenham, U.K.), but disappeared from the published version of the standard because developers of scheduling software regarded it as their exclusive territory. Now that MESs are successfully bridging the gap between control systems and enterprise-wide business systems, scheduling is back on the batch control agenda.

The ultimate challenge for scheduling is to do it in real time, says Sawyer. Superbatch 2 from the Centre for Process Systems Engineering at Imperial College (London) does just that. Developed in conjunction with APV (Crawley, U.K.), Superbatch 2 is claimed to be the first practical on-line scheduler, using novel batch production models and scheduling algorithms specially developed for process plants. “Most scheduling programs have their roots in discrete manufacturing,” says Sandro Macchietto, a professor of chemical engineering at Imperial College and Superbatch development manager. “That means they have problems with plant items such as tanks, and you end up having to bend reality to fit the model. Batch control systems, on the other hand, know all about process plant but not much about scheduling. We have bridged the gap with new scheduling algorithms that are much more suitable for the process industries.”

Superbatch 2 has proved its worth on a demanding application in a fromage frais and yoghurt plant at Minsterley, U.K., operated by Northern Foods/Eden Vale. The system has been running for nearly a year now, scheduling the production of 60 different product mixes in nine different pot types. Process times for the yoghurt cultures are unpredictable, and can be as long as 15 hours, while order lead times can be less than 24 hours. Traditional off-line scheduling struggled to keep up with changes in demand, says Macchietto, but Superbatch 2 is working well.

To simplify future developments of this kind ISA’s SP95 Enterprise/Control Integration Committee is developing a standard that will define the interface between control systems and MESs or Manufacturing/Enterprise Resource Planning (MRP/ERP) systems. The standard must define information exchange which is robust, safe, and cost-effective, with an exchange mechanism that preserves the integrity of each system’s information and span of control, notes SP95 chairman William Wray. The SP95 standard will build on the work already done by SP88 and on one of SP88’s precursors, the model developed by Ted Williams at Purdue University (West Lafayette, In.), says Wray. Publication is scheduled for mid-1998.

A related development, that has considerable crossover with SP95, is BATCIME (Batch Control and Integrated Manufacturing Execution), a two-year project started by the EBF in 1996 and funded by the European Union to standardize MESs in batch processing environments. Industrial partners include Novo Nordisk Engineering (Bagsvaerd, Denmark), Project Management Ltd. (Dublin), ICI (Northwich, U.K.) and Kvaerner John Brown BV (Zoetermeer, Netherlands).

The BATCIME participants are developing a model of the manufacturing processes in the business supply chain that need to be supported by the batch control, production planning and business management systems, and a manufacturing plant model that will be used to verify the business process model and the business requirements. The identified business processes will be mapped against the capabilities of available standard software of control system, SCADA, MES and ERP vendors.

“Standards relieve the control engineer from having to do everything from scratch,” says Craig. “S88, for instance, lets you generate a complete description of a control system before you even approach a computer. This can save half the time normally spent on programming the system — not because you write the actual software any faster, but because you get it right first time.”

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