Use Elegant Design to Bolster Inherent Safety

Embrace a variety of strategies that can eliminate hazards from operations

By Kelly K. Keim and Scott W. Ostrowski, ExxonMobil Research and Engineering

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Trevor Kletz was able to simplify the concept of inherent safety in such a way that everyone “gets it.” His mantra “What you don’t have can’t leak” is so clear and powerful that it has grabbed the attention of all stakeholders, including owner/operators, labor, community members and regulators, who have an interest in safer processing facilities of all types. It expresses a vision that we all seek, one where no harm comes from the operation of process facilities that manufacture the materials that make our lives better every day.

Of course, the concept of inherent safety goes beyond simply not having materials that potentially could damage the pipes, vessels and equipment that make up manufacturing facilities. We must understand all the ways those materials can be involved in incidents that harm people, the environment and our facilities. Without a thorough understanding of those scenarios and how they can occur, we can’t properly evaluate the risks posed by different technological approaches and effectively apply inherently safer technologies.

For example, the lower annual corrosion rate of a stainless alloy compared to carbon steel in some processes may seem compelling. However, chloride exposure may cause stress corrosion cracking in the alloy; this damage is difficult to detect before a catastrophic component failure occurs. So, in fact, the inherently safer option may be to use carbon steel while implementing a strong inspection and replacement program that manages the hazard of corrosion effectively.

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Fundamental Strategies

Kletz in his groundbreaking 1984 paper [1] described four basic strategies for achieving inherently safer processes:

• intensification;
• substitution;
• attenuation; and
• limitation of effects.

In its 2007 book, “Inherently Safer Chemical Processes: A Life Cycle Approach” [2], the Center for Chemical Process Safety translated those terms into simpler ones readily understood by a wider audience than just safety professionals:

• substitute — replace a material with a less hazardous one;
• minimize — reduce the quantities of hazardous substances;
• moderate — use less hazardous conditions, a less hazardous form of a material or facilities that minimize the impact of a release of hazardous material or energy; and
• simplify — design facilities that eliminate unnecessary complexity and make operating errors less likely, and that accommodate errors that occur.
Let’s consider their application to the use of a chlorine cylinder:
• substitute — change from chlorine to a bromine tablet;
• minimize — keep only one cylinder on the site;
• moderate — connect a vacuum inductor to the cylinder; and
• simplify — adopt a distinct design with unique connections for chlorine hoses.

Other strategies can complement these simple ones. Here, we introduce the phrase “elegant design” to represent the selection of process technology, equipment, design or layout that makes higher-potential-consequence scenarios non-credible. Elegant design may take advantage of a number of Kletz’s strategies — and may even go beyond them to achieve risk reduction, minimization, or elimination.

Simply put, the concept of inherently safer design is: “What can’t happen can’t happen.”

Any number of design features can contribute to preventing something from happening. Substitution and some elegant design solutions can provide absolute certainty against an occurrence. Minimization, moderation and other elegant designs can afford a reasonable certainty. Instructions and procedures can help but offer the least degree of certainty. All are desirable steps toward a safer processing facility.

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  • <p>One of the flaws in the scrubber solution may be the need for a high-pressure spray nozzle to provide semi-atomized droplets to the face of the scrubber packing. I've tried troughs in this application but they are quickly plugged and replaced by spray nozzles. </p> <p>Typically, these spray nozzles operate at about 7.5 psig up to 12-15 psig. Much beyond 10 psig the typical nozzle produces atomized spray. The spray can carry over bring dissolved or partially dissolved chemicals. You would need to include a 17-23 ft elevation in the location of the water trough to compensate for the (unplugged) drop through the spray nozzle. This is quite do-able but it is something to be aware of.</p>

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