Hot cutover boosts control system migration

Keeping critical unit running provides significant economic and implementation benefits

By Eric Schnipke, Ineos Nitriles

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Component failures in a 1980s–1990s-era distributed control system (DCS) at Ineos Nitriles’ Lima, Ohio, complex were jeopardizing production of acrylonitrile and its marketable acetonitrile and cyanide byproducts early in 2005. Shutdowns of the plant’s two continuous acrylonitrile reactors are intolerable. Production has been running at 98% reliability for several years; lost sales can’t be made up.

Most hardware failures in the Bailey Network 90/Infi 90 DCS were in the DEC Alpha-based, OSI Conductor VMS consoles that had replaced original Bailey MCS consoles. DEC parts are out of production; used parts are difficult to locate. We also were seeing occasional failures in DCS controller input/output (I/O). In addition, an associated programmable logic controller (PLC) running the Lima site’s deep-well injection system was suffering spurious failures of unknown origin that tended to be in attention-getting critical logic and EPA-required recordkeeping. Opinion was divided on whether the failures were random or symptomatic of equipment age and possible failure acceleration. We initiated an analysis to find out but results were inconclusive.

The next scheduled turnaround for the acrylonitrile unit isn’t until 2010, so management had to grapple with whether the plant could wait four years for an automation upgrade or replacement. The decision was that it couldn’t.

Upgrade or replace?
We evaluated three potential solutions:

  • Refurbish the VMS console computers, replace the lower CRT monitors with LCDs (the top CRTs already had been swapped out as a stopgap), and hope for the best from the controller hardware until 2010. This option posed two major downsides: the cost and questionable quality of refurbished computers to gain only a three-to-four-year reprieve; and, of course, potential controller and I/O unreliability.
  • Replace the old console computers with new HMI hardware and software from the same company. The downsides: the difficulty in reconfiguring from Conductor VMS to Windows; time-consuming graphics conversion because no automated utilities were available; doubt about whether the consoles could be replaced without shutting down acrylonitrile production; and, again, questionable controller and I/O reliability.
  • Install a new DCS, plus perhaps a new PLC, as soon as possible, with cutover of the acrylonitrile operation accomplished hot. The major downside here was a possible cutover-induced process trip.

None of these options involved changing field devices or altering control strategies.
We found that the last alternative was the only practical choice. First, the cost of configuring and switching from existing consoles to new workstations was lower than that for retaining the old HMIs. The new DCS also offered the best avenue for hot cutover of both consoles and reactors — one reason is that it incorporates advanced digital automation, so we could avoid the need for add-on technology and the attendant chance of mistakes. Last, Ineos has had excellent experience with this automation equipment at other sites, including an adjacent catalysts plant.

However, management was very concerned about a hot cutover; no one at Ineos to our knowledge had ever retrofitted controls for such a large and important process while it ran. Management also questioned whether enough real estate existed in electrical equipment rooms for both new and old control cabinets to run simultaneously and thereby allow a hot cutover. We won over management by developing a two-phase, six-month plan. This involved replacing the HMIs first, then the controllers, and finally the I/O.

The plan
The first phase employed Emerson’s DeltaV Connect Solution for Bailey Systems, a transition package that replaces the existing HMIs and provides modern functions, while retaining the original system’s controllers, I/O and field devices. The new PC workstations were mounted in semicircular console furniture (Figure 1) and connected in parallel with the existing consoles, which were moved against the back wall of the control room and remained live and operable until the new workstations were tested and proven to work with the existing controllers. To operators, the transition was essentially seamless; I don’t believe any of them ever used the old HMIs after the new workstations were up and running.

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