Optimation, a systems integrator in Rochester, N.Y., designs and installs automation systems for process plants and also builds process skids.
On an acid purification skid, the automation systems had to meet Class I, Div. 1 requirements (Figure 1). After a chemical pretreatment step is completed on the skid, the system runs automatically to create purified acid, minimizing human interaction with the process.
"The fluids recovered from the production process are very corrosive and can eat through stainless steel, so the skid was fabricated with specialized materials," says Dan Curry, manager of automation and integration, Optimation. "The fluids are also flammable, so the electrical systems had to be designed to meet electrical hazard classifications."
The immediate area around the skid is Class I, Div. 1, and at a distance from the skid the classification changes to Class I, Div. 2.
"A combination of explosion-proof wiring and intrinsically safe barriers were used," explains Curry. "Some pumps are powered by air rather than electricity, and some valves were controlled pneumatically rather than with electrical signals. The I/Ps and solenoid valves were installed outside the classified areas."
The combination of explosion-proof wiring and intrinsically safe barriers was necessary because IS barriers can only be used for low-voltage applications, and the skid had some high-voltage pump controls. "IS systems are generally always 24 Vdc," notes Curry. "I'm not aware of any 120-Vac IS barriers due to the available power level being too high, and power to the pump motors is in excess of IS limits."
Cost was analyzed for each component to determine which could accomplish safe operation while staying within the budget, and the combination of explosion-proof wiring and intrinsically safe barriers proved to be workable and affordable.