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1-6 The Nature of the Accident Process

Chemical plant accidents follow typical patterns. It is important to study these patterns in order to anticipate the types of accidents that will occur. As shown in Table 1-6, fires are the most common, followed by explosion and toxic release. With respect to fatalities, the order reverses, with toxic release having the greatest potential for fatalities.

Table 1-6. Three Types of Chemical Plant Accidents

Type of accident

Probability of occurrence

Potential for fatalities

Potential for economic loss









Toxic release




Economic loss is consistently high for accidents involving explosions. The most damaging type of explosion is an unconfined vapor cloud explosion, where a large cloud of volatile and flammable vapor is released and dispersed throughout the plant site followed by ignition and explosion of the cloud. An analysis of the largest chemical plant accidents (based on worldwide accidents and 1998 dollars) is provided in Figure 1-6. As illustrated, vapor cloud explosions account for the largest percentage of these large losses. The "other" category of Figure 1-6 includes losses resulting from floods and windstorms.

Figure 1-6

Figure 1-6 Types of loss for large hydrocarbon-chemical plant accidents. Data from The 100 Largest Losses, 1972-2001.

Toxic release typically results in little damage to capital equipment. Personnel injuries, employee losses, legal compensation, and cleanup liabilities can be significant.

Figure 1-7 presents the causes of losses for these largest accidents. By far the most frequent cause is mechanical failures, such as pipe failures due to corrosion, erosion, and high pressures, and seal/gasket failures. Failures of this type are usually due to poor maintenance or the poor utilization of the principles of inherent safety (Section 1-7) and process safety management (Section 3-1). Pumps, valves, and control equipment will fail if not properly maintained. The second largest cause is operator error. For example, valves are not opened or closed in the proper sequence or reactants are not charged to a reactor in the correct order. Process upsets caused by, for example, power or cooling water failures account for 3% of the losses.

Figure 1-7

Figure 1-7 Causes of losses for largest hydrocarbon-chemical plant accidents. Data from The 100 Largest Losses, 1972-2001.

Human error is frequently used to describe a cause of losses. Almost all accidents, except those caused by natural hazards, can be attributed to human error. For instance, mechanical failures could all be due to human error as a result of improper maintenance or inspection. The term "operator error," used in Figure 1-7, includes human errors made on-site that led directly to the loss.

Figure 1-8 presents a survey of the type of hardware associated with large accidents. Piping system failure represents the bulk of the accidents, followed by storage tanks and reactors. An interesting result of this study is that the most complicated mechanical components (pumps and compressors) are minimally responsible for large losses.

Figure 1-8

Figure 1-8 Hardware associated with the largest hydrocarbon-chemical plant accidents. Data from The 100 Largest Losses, 1972-2001.

The loss distribution for the hydrocarbon and chemical industry over 5-year intervals is shown in Figure 1-9. The number and magnitude of the losses increase over each consecutive 10-year period for the past 30 years. This increase corresponds to the trend of building larger and more complex plants.

Figure 1-9

Figure 1-9 Loss distribution for the largest hydrocarbon-chemical plant accidents over a 30-year period. Data from The 100 Largest Losses, 1972-2001.

The lower losses between 1992 and 1996 are likely the temporary result of governmental regulations that were implemented in the United States during this time; that is, on February 24, 1992, OSHA published its final rule "Process Safety Management of Highly Hazardous Chemicals (PSM)." This rule became effective on May 26, 1992. As shown, however, the lower losses between 1992 and 1996 were probably a start-up benefit of PSM because in the last 5-year period (1997-01) the losses went up again.

Accidents follow a three-step process. The following chemical plant accident illustrates these steps.

A worker walking across a high walkway in a process plant stumbles and falls toward the edge. To prevent the fall, he grabs a nearby valve stem. Unfortunately, the valve stem shears off and flammable liquid begins to spew out. A cloud of flammable vapor rapidly forms and is ignited by a nearby truck. The explosion and fire quickly spread to nearby equipment. The resulting fire lasts for six days until all flammable materials in the plant are consumed, and the plant is completely destroyed.

This disaster occurred in 19699 and led to an economic loss of $4,161,000. It demonstrates an important point: Even the simplest accident can result in a major catastrophe.

Most accidents follow a three-step sequence:

  • Initiation (the event that starts the accident),
  • Propagation (the event or events that maintain or expand the accident), and
  • Termination (the event or events that stop the accident or diminish it in size).

In the example the worker tripped to initiate the accident. The accident was propagated by the shearing of the valve and the resulting explosion and growing fire. The event was terminated by consumption of all flammable materials.

Safety engineering involves eliminating the initiating step and replacing the propagation steps with termination events. Table 1-7 presents a few ways to accomplish this. In theory, accidents can be stopped by eliminating the initiating step. In practice this is not effective: It is unrealistic to expect elimination of all initiations. A much more effective approach is to work on all three areas to ensure that accidents, once initiated, do not propagate and will terminate as quickly as possible.

Table 1-7. Defeating the Accident Process


Desired effect




Grounding and bonding


Explosion proof electrical

Guardrails and guards

Maintenance procedures

Hot work permits

Human factors design

Process design

Awareness of dangerous properties of chemicals



Emergency material transfer

Reduce inventories of flammable materials

Equipment spacing and layout

Nonflammable construction materials

Installation of check and emergency shutoff valves



Fire-fighting equipment and procedures

Relief systems

Sprinkler systems

Installation of check and emergency shutoff valves

Example 1-6.

The following accident report has been filed10.

  • Failure of a threaded 1 1/2" drain connection on a rich oil line at the base of an absorber tower in a large (1.35 MCF/D) gas producing plant allowed the release of rich oil and gas at 850 psi and –40°F. The resulting vapor cloud probably ignited from the ignition system of engine-driven recompressors. The 75' high x 10' diameter absorber tower eventually collapsed across the pipe rack and on two exchanger trains. Breaking pipelines added more fuel to the fire. Severe flame impingement on an 11,000-horsepower gas turbine–driven compressor, waste heat recovery, and super-heater train resulted in its near total destruction.

Identify the initiation, propagation, and termination steps for this accident.



Failure of threaded 1 1/2" drain connection


Release of rich oil and gas, formation of vapor cloud, ignition of vapor cloud by recompressors, collapse of absorber tower across pipe rack


Consumption of combustible materials in process

As mentioned previously, the study of case histories is an especially important step in the process of accident prevention. To understand these histories, it is helpful to know the definitions of terms that are commonly used in the descriptions (see Table 1-8).

Table 1-8. Definitions for Case Historiesa




The occurrence of a sequence of events that produce unintended injury, death, or property damage. "Accident" refers to the event, not the result of the event.


A chemical or physical condition that has the potential for causing damage to people, property, or the environment.


The loss of containment of material or energy; not all events propagate into incidents; not all incidents propagate into accidents.


A measure of the expected effects of the results of an incident.


A measure of the expected probability or frequency of occurrence of an event. This may be expressed as a frequency, a probability of occurrence during some time interval, or a conditional probability.


A measure of human injury, environmental damage, or economic loss in terms of both the incident likelihood and the magnitude of the loss or injury.

Risk analysis

The development of a quantitative estimate of risk based on an engineering evaluation and mathematical techniques for combining estimates of incident consequences and frequencies.

Risk assessment

The process by which the results of a risk analysis are used to make decisions, either through a relative ranking of risk reduction strategies or through comparison with risk targets.


A description of the events that result in an accident or incident. The description should contain information relevant to defining the root causes.

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