From CheicalProcessing.com, an excellent article on Inherent Safety Design (ISD). Here are a few important highlights for ISD. This article has been condensed for space, for the full article, click on the link above.
Consider Inherent Safety at Your Plant
Many sites can benefit -- but confusion about how to identify options stymies efforts.By Dennis C. Hendershot, process safety consultant.
Inherently safer design (ISD) is a philosophy for designing and operating a safe process plant [1,2]. ISD aims to eliminate or significantly reduce hazards, rather than managing them with hardware and procedures. When feasible, ISD provides more robust and reliable risk management and, by eliminating costs associated with safety equipment and procedures, may make processes simpler and more economical.
Levels of Inherent Safety
Used during detailed equipment configuration and design, it can eliminate or significantly reduce many risks within a process that still contains major hazards.
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.
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.
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.
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.
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.
Independent ISD PHA. This type of a review - also a team activity - focuses on specific hazards associated with the process and applies ISD strategies (substitute, minimize, moderate, simplify) to identify ways of eliminating or minimizing them. It uses one of the standard PHA tools (e.g., What If, Hazop) to pinpoint hazards but team discussion centers on ISD considerations.
CCPS has published another useful tool for consideration of ISD . This book provides a series of tables of potential failure mechanisms for a wide range of process equipment and identifies potential design solutions, including inherent, passive, active and procedural approaches to managing risk.
Plant PHA incorporating ISD. My personal preference is to minimize (an ISD strategy!) the proliferation of process reviews that seem to be required by the many demands being made on plant designers and operators. Plants are asked to do PHA, reliability and maintenance evaluations, ISO certification reviews, and now it's suggested (or required in some jurisdictions) ISD studies. Many of these use similar techniques. Combining them as much as possible increases efficiency and yields a better review. All reviews aim to accomplish the same thing — excellence in manufacturing, which includes best possible safety, environmental performance, product quality, productivity, plant reliability and profitability. These multiple demands often result in design or operational changes that improve performance in several areas simultaneously - e.g., a change boosting reliability and profitability also may enhance safety. But this isn't necessarily always true. So, it makes sense to consider as many of the competing performance demands as possible with a team having a broad understanding of the benefits and costs in all important performance areas.