Analyze Process Hazards by Developing Accident Scenarios

DOE-HDBK-1100-2004

List all obvious hazards. Most processes include a number of hazards that

are already fully recognized, such as the flammability of propane or the

inhalation toxicity of chlorine.

Examine the hazardous characteristics of each process chemical. Review the

MSDSs, which should have information on the toxicity, flammability, and

reactivity of process chemicals and on their incompatibilities with other

materials.

Examine all process parameters. Parameters (e.g., pressure, temperature,

flow rate, level, pH) that are controlled or measured in a process are good

indicators of possible process hazards. Process parameters should be

examined for all modes of operation, independent of process chemicals,

because some hazards exist that do not involve the chemicals. For example,

if a process uses high-pressure steam, then both thermal energy and

pressure-volume energy hazards exist even though steam is non-toxic, non-

flammable, and non-reactive with most materials.

Examine material interactions for incompatibilities. Even if process

chemicals are relatively non-hazardous when considered independently,

some potentially dangerous interactions may occur when materials are

combined. Interactions between process chemicals, containment materials,

or other materials with which the chemicals come in contact can be

examined in pairs by using an interaction matrix. A sample matrix is shown

in Figure 3.2.

Document the identified hazards. The PrHA report should list identified

hazards in tabular form and/or discuss each hazard briefly in the text. Doing

both is preferred. New or previously unidentified hazards should receive

particular attention and discussion.

3.2.2 Analyze Process Hazards by Developing Accident Scenarios

The parts of an accident event involving a process operation are shown in Figure 3.3.

Each sequence of failures and conditions leading to an accident is a unique scenario.

Every accident scenario starts with an initiating event or cause, which is a mechanical

failure, operational error, external event, or other condition that causes normal operation

to be interrupted or changed. Initiating events can lead to process deviations. For

example, failure of a cooling water pump (initiating event) may result in loss of cooling

to a process involving an exothermic reaction. A deviation occurs when the process

temperature exceeds the upper limit of the normal operating temperature for the reaction