Of course, procedures, personal protective equipment and other safety-management systems can control the risk to the operator, but these are not inherent. Both the neighbors and the operators are correct in their perceptions of the ISD characteristics of the chlorine supply options but they are concerned about different kinds of incidents. To make an intelligent choice, the designer not only should understand these conflicting requirements but also the potential role of all risk-management systems (inherent, passive, active and procedural).
It also is important to consider whether an ISD option just transfers risk somewhere else. For instance, a plant might reduce its risk by decreasing the size of a hazardous material storage tank on the site and thus its inventory of the substance. However, the smaller tank may require a switch from getting the material via railroad tank cars (typically about 300,000-lb. shipments) to trucks (typically about 30,000 lb.). There will be ten times as many shipments, and they will go by road rather than by rail — depending upon the particular location, road shipment may be inherently more hazardous. So, while the site risk is reduced, the overall risk to society actually may be increased.
The chemical industry will benefit if designers and operators consider ISD options throughout a process’s life cycle, from initial conception through R & D, plant design (including detailed design of equipment and operating procedures), construction, operation, modification and eventual shutdown.
ISD is not a specific program or design technique but a philosophy and mindset. It challenges you to rethink how to approach hazards — not just to accept them and concentrate on control, but to seek to eliminate or minimize them. It can lead to choices that better reflect differences in particular situations and environments, while taking into account other factors such as economics, resource allocation, and the feasibility, reliability and effectiveness of conventional process risk-management features.
A designer should ask the following questions, in this order, once a hazard has been identified:
- Can the hazard be eliminated?
- If not, can the magnitude of the hazard be reduced?
- Do the process options identified from the first two questions increase the scale of any other hazards or create new ones? If so, consider all hazards in selecting the best alternative.
- What passive, active and procedural protective systems are required to manage the hazards that remain?
Too often we skip directly to the fourth question and focus on hazard management without striving to eliminate or reduce hazards. We accept the hazards, believing they are unavoidable. This may be true but we can never eliminate or reduce hazards if we never challenge ourselves to do so.
- “Inherently Safer Chemical Processes: A Life Cycle Approach,” D. A. Crowl, ed., Center for Chemical Process Safety, American Institute of Chemical Engineers, New York (1996).
Updated ISD reference
The Center for Chemical Process Safety in currently developing the 2nd Edition of “Inherently Safer Chemical Processes: A Life Cycle Approach” , with publication anticipated around the end of 2007. It will incorporate the latest developments and literature on ISD and will greatly enhance the checklists and tools for helping identify ISD options for new and existing processes and plants.
Dennis C. Hendershot is principal process safety specialist for Chilworth Technology, Plainsboro, N.J. E-mail him at DHendershot@chilworth.com.