Question 6 of 7: Hazard Elimination Examples, Limiting Hazard Levels, and the Hazards of a Pressurized Steel Tank

Question 6: Hazard Elimination Examples, Limiting Hazard Levels, and the Hazards of a Pressurized Steel Tank (20 marks: 7/6/7)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Examples of Hazard Elimination and Accident Avoidance

Elimination removes the hazard entirely rather than merely controlling it, and is the most effective (and most permanent) layer of the hierarchy of controls because it does not depend on continued correct human behaviour or equipment function. Concrete examples:

• Substituting a non-flammable or less toxic material for a flammable solvent or highly toxic chemical, removing the fire/toxic-release hazard rather than managing it with ventilation and PPE.
• Redesigning a process step to remove a pinch point entirely (e.g. replacing an in-running nip roller transfer with a non-contact conveyor), rather than guarding the existing pinch point.
• Automating a manual material-transfer task that previously required manual lifting or reaching into a hazardous zone, removing both the ergonomic and the point-of-operation hazard.
• Redesigning a vessel with external access/cleaning ports so a confined-space entry is no longer required for routine inspection or cleaning.
• Inherently safer design — reducing the on-site inventory of a hazardous chemical (e.g. just-in-time delivery instead of large bulk storage) so a release, if it occurs, is smaller by design.
• Interlocking a machine guard so the machine physically cannot run while the guard is open, removing the possibility of operating with the hazard exposed (rather than relying on the operator to keep the guard closed).
• Relocating an elevated work task to ground level (e.g. pre-assembling a structure at grade before lifting it into place), eliminating the fall hazard for that task entirely.

(ii) Means by Which Hazard Levels May Be Limited

Where a hazard cannot be eliminated outright, its level — the magnitude, duration, or extent of exposure — can still be reduced:

• Reducing the quantity/inventory of hazardous material present at any one time, limiting the maximum credible consequence of a release.
• Reducing exposure time or frequency — job rotation, shorter task durations, or scheduling high-exposure tasks to minimize the number of workers and the duration of exposure.
• Reducing the energy magnitude — lower operating pressure, voltage, speed, or temperature than the process could otherwise use, so a failure releases less energy.
• Distance and shielding — increasing separation between the hazard and personnel, or interposing a barrier, so the same hazard magnitude produces a smaller effect on an exposed worker.
• Engineering controls that reduce concentration — ventilation, dilution, and capture-at-source systems that keep an airborne contaminant below its exposure limit even though the source itself remains.
• Administrative limits — permit systems that cap the allowable duration or frequency of a hazardous task, and redundancy/backup systems (relief devices, backup power) that limit how far a single failure can escalate.

(iii) Initiating and Contributing Hazards for a Pressurized Steel Tank

A pressurized steel tank stores energy in its compressed/pressurized contents; injury and property damage occur when that stored energy is released suddenly through an uncontrolled rupture rather than through the tank's designed relief path. The hazards separate into two categories:

Initiating hazards — the direct trigger of failure:

• Overpressure from a process upset, a blocked or undersized pressure relief device, or external fire exposure that heats the contents and raises internal pressure beyond the vessel's rated capacity.
• Loss of wall strength from corrosion/erosion thinning, mechanical damage (impact, dents, gouges), fatigue cracking from repeated pressure cycling, or a pre-existing weld defect.
• Brittle fracture if the steel is operated below its ductile-to-brittle transition temperature for an unforeseen low-temperature condition.

Contributing hazards — management/operational gaps that allow an initiating hazard to develop undetected or unchecked:

• Absent or lapsed inspection and maintenance program, so corrosion, cracking, or a degraded relief valve is never caught before it becomes critical.
• Inadequately sized, improperly maintained, or blocked-in relief device, so the vessel's designed overpressure protection is not actually available when needed.
• Operator error during filling (over-pressurizing, overfilling, or wrong-material charging), and lack of pressure/temperature monitoring and alarms to catch the deviation before it becomes critical.
• Exposure to external fire without adequate fireproofing or emergency depressurizing/venting, which can drive the classic catastrophic failure mode for a pressurized vessel — a sudden, violent rupture releasing the stored energy essentially all at once, generating blast overpressure, vessel fragments as projectiles, and (if the contents are flammable) a fireball.

Injury and damage therefore result from a chain: a design/degradation condition (initiating hazard) combines with a management/operational gap (contributing hazard) that allows it to go undetected or unmitigated until the vessel fails catastrophically rather than through its intended relief path.