Upgrading Legacy Control Systems

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The day will eventually come when controllers and other equipment must be replaced. "Several years ago, I installed a PLC control system on an old Amsco Autoclave sterilizer," relates Jim Meils of Aventis Behring. "Prior to this, the system used timers and relays. The upgrade worked well for controlling the process, but the mechanical components were old and presented several operational problems because of their age. The lesson is that, even though you upgrade the control system, you need to look at the big picture and determine if the associated mechanical components are capable of sustaining the efficient operation of the machine."


Sometimes old controls just can't cut the mustard any more. Mark Richardson, senior engineering specialist at Hemlock Semiconductor, Hemlock, Mich., puts it more delicately, saying that sometimes new requirements exceed the capability of the old controllers. This is the case with Hemlock's Fischer & Porter DCI 5000 control systems.


"We replace legacy controllers with new controller boards and new I/O boards, but we re-use existing field instruments and terminations, and convert old control programs to current level," he explains. "This method is used when the reliability or control capability of the legacy controllers is inadequate, but the field instrumentation is capable."


When I/O can't keep up because the modules have corroded, or if the I/O network is suffering a high rate of failures, Hemlock replaces it. "We replace legacy distributed I/O with new fieldbus-based I/O modules and communication boards," he says. "We add software needed to address the new I/O modules, but leave the existing control schemes and programs alone."


Back in the 1990s, F&P developed adapters that allowed old DCI 5000 terminal boards to be connected to F&P's new System Six I/O boards. So far, Hemlock has replaced 16 of 18 legacy F&P controllers with "new" 1993 System Six controllers, but kept the existing field I/O.


F&P was acquired by Elsag Bailey in 1994, and Bailey was acquired by ABB in 1999, so the legacy DCI 5000 controls are now supported by ABB.


Updating control processors to a newer generation is a good way to upgrade a system for power or reliability. Wild says CRISP Automation can upgrade any CRISP system from an old DEC PDP-11 or VAX platform to the newer DEC Alpha platform, and soon will be able to upgrade those systems to the new Itanium platform running Open VMS. He can also do this for non-CRISP systems as well. "We can migrate any type of DEC VAX system running Open VMS to run on a DEC Alpha or Intel server, without any application changes," he claims. "We are currently developing a generic OPC server/client product for all VMS users, which will add enhanced connectivity to the many thousands of existing legacy DEC VAX systems."


Wild also claims that he has access to enough replacement DEC computer hardware and I/O boards to support all the legacy CRISP systems that the installed base is still running.


Wonderware's Steve Garbrecht says control engineers can find many listings on Ebay for "new old stock" or used PLC and DCS hardware. "If the online auction environment is not your cup of tea, then there are several companies that specialize in used and refurbished equipment, like Western Process Computers (www.westernprocess.com), located in Phoenix, Arizona, which specializes in refurbished Honeywell DCS hardware," he adds.


Step Four: Migrating to New Technology

So far, we've looked the first three steps of upgrade strategy. Step one, upgrade software, step two upgrade HMIs, and step three, update controller processors and networks. All are relatively easy and inexpensive solutions. It's when we get to step four, replacing controllers, that is where things can start to get expensive. Step four encompasses everything from simply replacing a PLC with a newer model to migrating over to an entirely new system.


PLCs can be one of the easier controllers to upgrade. Schneider Electric, for example, offers its customers its Validatable PLC service. The service provides users operating with prior-generation Modicon PLCs an upgrade path to new Modicon platforms and enables I/O cards installed 30 years ago to be connected to new PLCs. Through the Validatable service Schneider's pharmaceutical and critical batch process customers can upgrade their PLCs without losing FDA 21 CFR Part 11 certification because the system does not have to be re-validated.


Other DCS migrations allow plants to keep legacy I/O. For example, users running old Moore Products APACS+ systems can upgrade to Siemens PCS 7 without having to get rid of old I/O. This may not be as easy as it sounds, however.


Mississippi River Corp. (MRC), a 400 ton/day re-pulping and de-inking plant in Natchez, Miss., installed an APACS+ control system in the early 1990s. The system featured a Mycroadvantage HMI from TA Engineering, which ran under Windows 95.


After 10 years of upgrades, the system became unreliable. Because TA Engineering went out of business, MRC had no choice but to migrate, convert or up

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