CCPS Process Safety Glossary

A 15-minute, time -weighted average concentration to which workers may be exposed up to four times per day with at least 60 minutes between successive exposures with no ill effect if the TLV-TWA (see below) is not exceeded. The limit supplements the TLV-TWA where there are recognized acute effects from a substance with toxic effects that result primarily from chronic exposures. Developed by the ACGIH.

A failure that is primarily due to the degradation over the time that a device or system is in service. For example, the failure of a pump after 2000 hours of continuous service due to wear on the bearings would be considered a time-related failure.

An estimation of the time required for an exothermic reaction, in an adiabatic container, (that is, no heat gain or loss to the environment), to reach the point of thermal runaway. (ASTM E 1445)

Models of explosion characteristics relating vapor cloud explosions to the explosion of an equivalent mass of TNT (trinitrotoluene).

A measure of the uncertainty arising from the physical and the environmental differences between members of differing equipment populations when failure rate data is aggregated to produce a final generic data set.

A contracted company that manufactures, stores, uses, handles, or transports chemical components of a facility's final products. Sometimes called third party service provider, toll processor, supplier of outside services, external contract manufacturer, contract processor, contract manufacturer, custom chemical manufacturer.

The loss event or other undesired event at the top of a fault tree that is traced downward to more basic failures using Boolean logic gates to determine its possible causes.

The combination of concentration and time for inhalation of a toxic gas that produces a specific harmful effect.

Materials that can cause physiological harm other than asphyxiation and that are immediately dangerous to life and health and can be fatal at relatively low concentrations, such as phosgene or hydrogen sulfide.

An airborne agent that could result in acute adverse human health effects.

Practical instruction in job and task requirements and methods. Training may be provided in a classroom or at the workplace, and its objective is to enable workers to meet some minimum initial performance standards, to maintain their proficiency, or to qualify them for promotion to a more demanding position.

A risk measure illustrating changes in individual risk with distance from the source of the risk such as a transportation route.

A scheduled shutdown period when planned inspection, testing, and preventive maintenance, as well as corrective maintenance such as modifications, replacements, or repairs is performed.

The probability the fault event exists at a specified time.

CCPS RBPS Pillar II: Its two Elements provide hazard and risk information required to plan, develop, and implement stable, reduced-risk operations in the facility.

A power supply that employs automatic switching of main power supply from primary to secondary (usually battery and/or diesel generator) upon failure of the primary. A means of providing uninterrupted power to critical instrumentation for a limited period of time even in the event of a plant power failure. Usually accomplished by an inverter, transfer switch and batteries. Alternatives means may include motor generators. A type of power supply that can provide electrical power even when line power is lost. An inverter (AC from DC batteries) which provides a bumpless transition of power to the process control system in the event of plant power failure. The UPS will supply for a limited period (e.g., 15 minutes).

A failure may lie dormant in the system and only be discovered as a result of a thorough diagnostic testing procedure.

See Overdriven Detonation.

Substance or material which in a pure state or as normally produced decomposes either or not violently.

The highest concentration of a vapor or gas (the highest percentage of the substance in air) that will produce a flash of fire when an ignition source (heat, arc, or flame) is present. See also Lower Flammable Limit. At concentrations higher than the UFL, the mixture is too rich to burn. Also known as the Upper Explosive Limit (UEL)

The explosion resulting from the ignition of a cloud of flammable vapor, gas, or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure.

The weight of a vapor or gas compared to the weight of an equal volume of air; an expression of the density of the vapor or gas. Materials lighter than air have vapor densities less than 1.0 (example: acetylene, methane, hydrogen). Materials heavier than air (examples: propane, hydrogen sulfide, ethane, butane, chlorine, sulfur dioxide) have vapor densities greater than 1.0.

The hardware and software suppliers of a computerized system. There may be multiple vendors for a computerized system. The vendor may or may not be a manufacture of the computerized system. For example, the vendor may be a systems house who simply integrates subsystems or components manufactured by others or may be a computer store.

The process of supplying or removing an atmosphere to or from any space by natural or mechanical means.