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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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.

Incorporating ISD considerations into the plant PHA follows a procedure similar to that used in an ISD-specific PHA. However, the team doesn't restrict its recommendations to ISD but considers ISD solutions as one of many options available for managing hazards and risks. (See the sidebar for some tips.) When the team identifies a danger, it first seeks an ISD solution, trying to eliminate or reduce the hazard. It also considers other alternatives, including active, passive and procedural risk-management strategies. If the facility is located in a jurisdiction that requires consideration of ISD, it's important to clearly document evaluation of ISD.

Understand Your Process!
Identifying and implementing ISD demands a thorough grasp of the manufacturing process. Obviously you must appreciate all the hazards of your current route and potential alternatives to eliminate or minimize them. But to identify inherently safer alternatives, you must have a fundamental understanding of how your process works and what physical and chemical factors are most important in controlling its behavior. Then you're in a position to properly determine process and equipment alternatives that optimize these important factors, minimizing the required size of equipment while improving control of the process and reducing or eliminating hazards. I can't overemphasize the importance of understanding what's important in controlling the process — in general a plant that's under control is safe and will produce the desired product quality and quantity, maximizing profitability.

As an example, consider a nitration process. Nitration chemistry can be very hazardous. The reaction usually is highly exothermic; loss of control can result in a runaway reaction and explosion. Products can be unstable and it's possible to get unstable byproducts if reactions are improperly controlled. For one particular product, a company developed a semi-batch process in which an organic substrate was mixed with an organic solvent and then a mixture of nitric acid and sulfuric acid catalyst was fed at a rate to maintain a specified batch temperature. Initial design called for a several-thousand-gallon reactor; reactant feed would take many hours. Because of the large reactor size, any runaway reaction posed major consequences. To consider ISD options, it was essential to fully understand what physical and chemical factors dominated this process. The actual chemical reaction was of little importance — the nitric acid and organic substrate reacted extremely rapidly once they contacted each other. Three things were really important in optimizing this process from both an inherent safety and economic viewpoint:

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