You can classify levels of ISD as follows:
• First-order inherent safety — eliminating hazards from the process altogether;
• Second-order inherent safety — reducing the magnitude of a hazard, or making it extremely unlikely, perhaps nearly impossible, for an accident to occur; and
• Layers of protection — making passive, active and procedural risk-management safeguards inherently more reliable and robust.
An ISD "strict constructionist" might consider only first-order ISD to be truly inherent — you have entirely eliminated a particular hazard. However, this often is impossible to achieve. In contrast, many opportunities exist to design a more robustly safe plant by applying second-order strategies and even by using ISD thought processes during design of safety hardware and procedures that manage risk of major inherent hazards. Unfortunately, several myths have kept sites from seriously considering ISD (see sidebar).
In an ideal world, plant designers and operators would think about ISD throughout the process design and operational lifecycle; specific ISD review tools wouldn't be needed. But, in the real world, most facilities already exist and ISD wasn't considered during their design, or companies and engineers aren't familiar with ISD and don't look for opportunities in process design. Specific ISD review tools can help overcome these problems. Chemical engineers have used three approaches for implementing ISD in new and existing plants:
1. An inherent safety analysis of a process using an ISD checklist;
2. An independent process hazard analysis (PHA) for a plant focusing on ISD; and
3. A complete PHA of the plant with ISD considerations fully incorporated into the PHA discussions.
ISD checklist analysis. A checklist is a common PHA technique and can serve to identify ISD options. The checklist is best used in a team setting, with a variety of people familiar with all aspects of the plant and process considering the questions on the checklist. Treat it as as a "creative checklist" — in other words, use it to prompt creative thinking by the team, not just "yes" or "no" responses.
Reference 3 includes an extensive checklist of practical inherent safety considerations. It's organized around four major ISD strategies as well as plant geography:
• Simplify; and
• Location, siting and transportation.
The book gives more than 40 specific questions, many with additional considerations and sub-questions, providing hundreds of ISD ideas to consider for your process. Table 1 shows some examples. It's important to make sure use of checklists doesn't limit team creativity. No general checklist can identify every potential ISD option for a specific process — the review team itself will have more knowledge about the plant and should use the checklist as a tool to facilitate creative thinking about how to eliminate or reduce hazards.