1 June 2002
When disaster ERUPTS
Plant safety measures notwithstanding, accidents happen.
By Ellen Fussell
"You can put safety measures in equipment, hold courses, or add interlocks and safeguards into your system to avoid catastrophes, but no matter how many you put in, they won't work unless people using the equipment follow the procedures," said Andy Mobley, senior systems engineer at 3M Co. in St. Paul, Minn.
Trying to speed up processes, bypass safety devices, or fool the system are several ways operators have inadvertently caused explosions or harmed themselves, Mobley said. A simple thing such as misapplying a component may seriously hurt or kill someone. "I've heard of someone jamming a stick into a switch to make it look like it was closed (made up) or in safe position, but in actuality it wasn't. In doing this, they physically disabled the safety devices. So if anything goes wrong, the safety device isn't there to prevent it," he said. "When a tank's being filled and someone bypasses an upper level cutoff, something could spill over, and someone could be hit with acid."
While 3M has many people working to minimize risks-engineers, plant workers, and equipment operators-Mobley deals specifically with making sure the electrical equipment the company purchases won't cause a spark or get hot enough to ignite flammable gases and vapors. One way to avoid incidents is to implement procedures stemming from the ISA SP84 standards committee, he said. The standard ANSI/ISA-84.01-1996, Application of Safety Instrumented Systems, focuses on safety instrumented systems (SISs). "The procedures say you assess the system for its integrity in relationship to safety," Mobley said. "It's fairly new, and we have some systems in place, but we're constantly learning how to implement them, so it will take some time."
PREVENTING AND RESPONDING
Most manufacturers have ample procedures in place to prevent and respond to catastrophes. A typical management system failure is loss of containment-a chemical spill-said George King, process safety associate at Dow Chemical in Freeport, Texas. Then, of course, there are explosions and fires that could result from a spill. "When a flammable material finds an ignition source, you have a fire in addition to a spill," King said. The goal is to prevent such catastrophes.
One of the newest protective systems Dow uses is a methodology called layers of protection analysis (LOPA). "This can help us look at our process and identify potential scenarios for spills or a reactive chemical incident. We pair the consequence [fire or explosion] with what might cause it," King said. "Then we document all the protective features, or layers of protection, we have in place to prevent it."
Through the analysis, King's team determines when enough layers of protection are in place to guard against a system failure. "With the LOPA analysis, we can determine if safety instrumented systems procedures [integrating the necessary levels of safety into systems used for process control] are needed," he said. "And if so, what safety integrity level do we need to achieve an adequate level of protection?"
SIS could be one layer or, if it has a high level of reliability, make up two or three layers. Another layer might be a safety relief device (all these layers need to be independent). A safety relief device could be trying to protect the same thing, such as preventing a vessel from overpressuring.
"LOPA can help us recognize where we have deficiencies and improve them, and it brings a discipline to that so it doesn't have to be intuitive to an operator of a facility," King said. "Bringing in an outside discipline makes the application consistent across the various processes on site."
To respond to disasters, Dow has an integrated contingency plan. It starts at the highest levels in the company and is then implemented at all the sites, King said. "If we had a spill but it hadn't turned into a fire, we'd restrict access to the plant so we wouldn't be introducing more people who could become part of the incident as a victim or make things worse by introducing an ignition source."
Dow also practices simulated emergencies to keep responses fresh. "We involve outside emergency responders who we might call on in a real incident to participate in a drill," King said. The company uses local sheriff or police departments (who would normally block roads or facilitate access to emergency vehicles) to practice with the response team as part of the emergency drill.
EXPERTS PRESENT NEW IDEAS
Walt Pastorius of LMI Technologies in Delta, British Columbia, will present an overview of in-process monitoring applications for vision sensors, particularly in hostile manufacturing environments such as arc and spot welding, molten metal pouring, wood processing, and electronics, at the ISA Western Regional Conference, Training & Exhibition on 18-19 June 2002 in Las Vegas (www.isa.org/westernregional).
"Each in-process monitoring environment has its own specific needs regarding sensor performance specifications, environmental factors, operator interface requirements, and different industrial practices," Pastorius said in his abstract. He'll explore sensor environmental protection for hostile in-process applications and describe the development effort in packaging vision sensors to survive harsh environments such as high-temperature and electromagnetic fields, as well as metal splatter found near weld guns, dirty environments, and saw mill applications operating in high-ambient light levels.
Instrumented safety systems should reduce the likelihood or severity of impact when things go wrong in safety systems, protecting employees from flammable and toxic chemicals, said Riyaz Ali of Fisher Controls in Marshalltown, Iowa. Because the systems use final control elements, which stay in one position and move only in an emergency, they may not operate successfully on demand, he said. These systems require frequent testing, with the system shut down.
Ali will describe at the conference a smart positioner in an emergency shutdown application that lowers equipment costs and reduces testing time. It enables the safety valve to be tested while the process is operating. Operators can actually move the valve (about 10%) to confirm it's working without disrupting production, Ali said. The new positioner will reduce the need for scheduled maintenance, he said. IT
Behind the byline
Ellen Fussell is Assistant Editor for InTech.