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

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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. If, for example, the team finds a runaway exothermic reaction caused by water contamination in a batch reactor to be a hazard, it would look for opportunities to eliminate or reduce this risk. Some considerations might include:

• Substitute — using a non-reactive coolant in reactor coils instead of water;
• Minimize — removing all direct water connections to the inside of the reactor (for example, those to add water for reactor cleaning during shutdowns);
• Moderate — evaluating chemistry or solvent alternatives that might reduce sensitivity of the reaction mixture to water contamination; and
• Simplify — eliminating complex piping in the raw-material supply headers that increases potential for accidentally connecting water to the reactor.

CCPS has published another useful tool for consideration of ISD [4]. 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 ISDMy 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. For example, collecting contaminated process vent gas from various pieces of equipment for treatment by a thermal oxidizer before discharge to the atmosphere may bolster environmental performance but introduce a safety hazard — a potential explosion in the vent gas collection system if organic material concentration is within flammable limits and an ignition source is present. 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.

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