CCPS Process Safety Glossary | AIChE

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CCPS Process Safety Glossary

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Abnormal Situation

A disturbance in an industrial process with which the basic process control system of the process cannot cope.

Note: In the context of a hazard evaluation, synonymous with deviation.


The process of contacting a vapor and gas stream with an absorbing liquid to remove specific materials from the gas stream.

Accelerating Rate Calorimetry (ARC)

A technique in which a substance is heated in stages until very slow decomposition [or other reaction] is detected. The substance is then held under adiabatic conditions and the course of the decomposition [or other reaction] is monitored. (Also the name of a commercial test apparatus.) [Barton and Rogers 1997]

Acceptable Risk

The average rate of loss that is considered tolerable for a given activity.

Acceptance criteria

Technical basis used to determine whether equipment is deficient (e.g., when analyzing inspection, testing, and preventive maintenance [ITPM] results).

For Laboratories: An event that can cause (or has caused) harm to workers and property within the laboratory facility.
For Industry: An event that can cause (or has caused) significant harm to workers, the environment, property, and the surrounding community. See Incident.
Accident Prevention Pillar

A group of mutually supporting RBPS elements. The RBPS management system is composed of four accident prevention pillars: (1) commit to process safety, (2) understand hazards and risk, (3) manage risk, and (4) learn from experience.

Accident-Initiated Event

An event (or the first event in an event sequence) that is caused by a movement-related transportation accident, such as a train derailment or a barge grounding.

Accidental Chemical Release

An unintended or sudden release of chemical(s) from manufacturing, processing, handling, or on-site storage facilities to the air, water, or land.


The obligation to explain and answer for one's actions that are related to expectations, objectives, and goals. In this context, those that are accountable for PSM activities are answerable to the one person who has the ultimate responsibility for the program. There may be multiple persons accountable for an activity but only one person with the ultimate responsibility. Accordingly, it is a powerful element of an effective process safety management system.

Action Tracking

A method of logging progress when implementing a task or set of tasks.

Activation Energy

The constant E in the exponential part of the Arrhenius equation, associated with the minimum energy difference between the reactants and an activated complex (transition state), which has a structure intermediate to those of the reactants and the products, or with the minimum collision energy between molecules that is required to enable a reaction to occur. It is a constant that defines the effect of temperature on reaction rate.

Active Equipment

Denotes physical motion or activity in the performance of the equipment's function, as with rotating machinery.

Active System

A system in which failures are immediately evident during normal operation.


Single, short-term exposure (less than 24 hr)

Acute Effect

An adverse effect on a human or animal body, with severe symptoms developing rapidly and coming quickly to a crisis. See also, "Chronic". Importance: How much and how long one is exposed to a chemical is the critical factor to how adverse the health effects will be.

Acute Exposure

A short-term or rare exposure to a toxic agent in a single episode that is unlikely to recur.

Acute Hazard

The potential for injury or damage to occur as a result of an instantaneous or short duration exposure to the effects of an incident.

Acute Risk

A risk arising from a short-term event such as a release causing a fire, explosion, or short-duration toxic exposure.

Acute Toxicity

The adverse (acute) effects resulting from a single dose or exposure to a substance. Importance: Ordinarily used to denote effects in experimental animals.

Ad Hoc Investigation

An incident investigation fashioned from the immediately available information and concerns. Typically, the ad hoc investigation is performed whenever there are no prior investigation procedures. A synonym to ad hoc is unsystematic.


A condiiton in which no heat transfer occurs to or from the environment surrounding the sample, including the sample container. (HSE 2000)

Adiabatic Decomposition Temperature Rise

An estimation of the computed temperature which aspecimen would attain if all of the enthalpy (heat) ofdecomposition reaction were to be absorbed by thesample itself. High values represent high hazard potential.

Adiabatic Flame Temperature

The temperature developed by the combustion of a fuel and oxidizer mixture in conditions where there are no heat losses. In practice this value is difficult to measure experimentally and most published figures are the results of theoretical calculations.

Adiabatic Induction Time

Induction period to event (spontaneous ignition, explosion, etc.) (ri) under adiabatic conditions. When log (ri) is plotted against 1/T a straight line is obtained.

Adiabatic Lapse Rate (ALR)

See Dry Adiabatic Lapse Rate.

Adiabatic Temperature Rise

Maximum increase in temperature that can be achieved. This increase only occurs when the substance or reaction mixture decomposes completely and at adiabatic conditions.

Administrative control

Procedures that will hold human and/or equipment performance within established limits.

Administrative Controls

Procedural mechanism for controlling, monitoring, or auditing human performance, such as lock out/tagout procedures, bypass approval processes, car seals, and permit systems.


The process of contacting a vapor and gas stream with the surface of a solid adsorbent material.


The transport of material by and in the wind

Adventitious agents

viruses and toxins, often infectious agents, that can accidentally contaminate a cell line.


Any individual, group, organization, or government that conducts activities, or has the intention and capability to conduct activities, detrimental to critical assets. An adversary can include intelligence services of host nations, or third-party nations, political and terrorist groups, criminals, disgruntled employees or contractors, and private interests. Adversaries also can include insiders, outsiders, or the two acting in collusion.

Adverse reaction

undesirable effect of a drug, vaccine, or medical device; it can be as mild as a short-term injection-site irritation or as serious as a life-threatening acute onset of anaphylaxis; also referred to as adverse event.

Aerodynamic Diameter

The diameter of a sphere of the same particle density having the same terminal velocity in air or some other relevant fluid.

Aerodynamic Equivalent Diameter

The diameter of a unit density sphere having the same settling velocity (due to gravity) as the particle of interest of whatever shape and density.

Aerosol Fraction

The fraction of liquid phase, 1 - x, which, after flashing to the atmosphere, remains suspended as an aerosol.


The principal object, substance, or material inflicting the physical harm or property damage in an accident.


a microorganism or chemical substance, the presence or absence of which triggers a particular disease or infection.

Aggregate Risk

Societal risk for on-site workers in occupied buildings (API 752).


The statistical combination of several data points to form a single data point and confidence interval.


Sea level concentrations of the six principal constituents of dry and staurated air are given below. Dry air is often obtained by drying compressed atmospheric air using a suitable drying agent. In some cases dry air is made up in cylinders using 20.95 mol% oxygen with the balance being exclusively nitrogen. Care should be taken that argon is not analyzed as oxygen, as can occur in GC analysis. Dry air contains a higher concentration of oxygen than atmospheric air, which contains moisture. The absolute humidity of saturated air is found using steam tables. For example, saturated air at 100 degrees F contains 6.46 mol% water vapor. Relative humidity at any temperature is the fraction of the water vapor concentration corresponding to saturation. Constituent Gases [Dry Air (mol%), Sat Air @ 100 degrees F(mol%)]; Nitrogen (78.09, 73.04), Oxygen (20.95, 19.60) Water (0.00, 6.46), Argon (0.93, 0.87), Carbon Dioxide (0.030, 0.028), Neon (0.0018, 0.0017), Helium (0.000524, 0.00049)

Air Quality Control

The control of the level of pollutants prescribed by regulations that may not be exceeded during a specified time in a defined area. (Association of Engineering Geologists online dictionary)

Alarm Management

Procedures, schematic, software, maintenance, documentation, hardware, logic, prioritization, characterization, etc., pertaining to the management of process alarm system.


As low as reasonably practicable; the concept that efforts to reduce risk should be continued until the incremental sacrifice (in terms of cost, time, effort, or other expenditure of resources) is grossly disproportionate to the incremental risk reduction achieved. The term as low as reasonably achievable (ALARA) is often used synonymously.

Alert Levels

Describes a progressive, qualitative measure of the likelihood of terrorist actions, from negligible to imminent, based on government or company intelligence information. Different security measures may be implemented at each alert level based on the level of threat.


The act of a moving vessel striking against, or colliding with, a stationary object.

Along-Wind Distance, x

Distance in the direction the vapor cloud is traveling, i.e. the wind direction. Since the wind direction may change, the along-wind distance may change in direction and time as well.

Alpha Test Unit

A device or system that is comprised of components that meet both the functional and form prototype requirements. The scale of test is limited, usually to a laboratory, for operational and life cycle tests. The initial preproduction device.

Alternating Mode

Hardware operation that alternates between standby and running, e.g., a pump with an installed spare, each of which operates for a comparable amount of time.