Much of the hot cutover was physically aided by hand jacks and bypasses. I’ve come to believe that every regulating valve should have a bypass if safety isn’t an issue. Hand jacks require too much care with settings to avoid the valve becoming fully opened or fully closed.
We accomplished the acrylonitrile unit cutover without a process trip. Unfortunately, failure of an associated ac power device due to a wiring error made after cutover did cause a trip, but this wasn’t attributable to the automation or its cutover.
Only one element of the former controls remained: a standalone PLC running truck and railcar loading. It was served by two HMIs in a setup that wouldn’t allow the operator to shift from one PC to the other once loading started. These controls were replaced at the end of the project with a more efficiently programmed HMI that communicates with the PLC over Modbus. This improvement will allow for loading to be controlled from anywhere on the system’s network.
The hot cutover provided substantial savings. At our plant, the cost per day in lost acrylonitrile production is roughly $100,000 at today’s product value. A simple trip requires a 1.5-day restart.
In addition, we’re gained a number of advantages from the new system:
- There haven’t been any control failures due to the automation since conversion.
- The plant turnaround in 2010 won’t be complicated or extended by the installation and startup of a new automation system.
- The new system’s scalability and flexibility allows us to relatively easily add, enhance and change automation hardware, software and strategies to suit operations. One cabinet has already been fitted with a Modbus TCP/IP card for the fastest communications with Modbus devices. All cabinets have plenty of room for expansion with 20% spare conventional I/O capacity (Figure 2). Every new cabinet has a Foundation Fieldbus card and power conditioner even though no fieldbus has been installed yet. The adjacent Ineos Catalysts plant, also under new automation, relies upon field devices very successfully served by Foundation Fieldbus, AS-Interface bus and Profibus DP segments. So, we aim to employ bus connections in the future in our units.
- We finally are making use of data from HART instrumentation that’s been in place for years. Especially helpful have been secondary variables from Coriolis mass flow meters, plus indications of oscillations in some valves, thanks to position feedback that couldn’t be sensed before.
- The DeltaV Analyze tool is helping to identify nuisance alarms, and the system is enabling us to quash such alarms by suppressing a particular parameter alone, not the entire tag. For instance, we temporarily quashed a high alarm that was a nuisance due to a field issue without suppressing a critical low alarm; this wasn’t possible with the old system where suppression took place at the tag level instead of at the individual alarm level. We’ve also added alarm suppress/re-enable when certain equipment is out of service. A pop-up faceplate allows operators to auto-suppress hand-chosen alarms via calculation blocks built into control modules. Hysteresis adjustments have permitted us to provide a larger dead band on some alarms, which also has eliminated some nuisance alarms.
The system promises additional benefits in the future. For instance, we’re considering eliminating an AspenTech DMC platform on the acetonitrile unit and incorporating its functions within a DeltaV PredictPro model-predictive-control package. This would cut the costs of five PCs and an annual maintenance contract.
We’re also considering opting for a safety instrumented system (SIS) integrated with the DCS for new construction. Our Triconex system works very well but is a separate platform that required substantial serial communications configuration into the new system during the recent conversion. A new SIS package is working well at the Ineos Catalyst plant for burner management; I’ve been told the added diagnostics are quite helpful.
A better approach
Looking back over our conversion experience, I don’t think I’d ever want to convert a process automation system for a critical unit as large as ours if it were not running. Trying to start up, troubleshoot and tune hundreds of loops all at once would be a nightmare.
There are other advantages to our approach as well. Separating a hot conversion effort into HMI and controller-I/O phases enables operations and maintenance staff to become comfortable with the replacement automation’s HMI without concern about controllers and I/O. Personnel prefer to adjust in small bites to many minor graphical differences. Typical might be the new system’s Auto and Cascade positions for switches versus the old system’s Auto and Manual. Later, personnel can focus on process logic changes while controllers and I/O are converted without, at the same time, being overwhelmed by a new HMI.
Eric Schnipke is a controls engineer for Ineos Nitriles, Lima, Ohio. E-mail him at email@example.com.