Download the CCPS Glossary App
Degraded Failure
A failure which is gradual or partial; it does not cease all function but compromises that function. It may lower output below a designated point, raise output above a designated point or result in erratic output. A degraded mode might allow only one mode of operation. If left unattended, the degraded mode may result in a catastrophic failure.
Demand
A plant condition or event which requires a protective system or device to take appropriate action in order to prevent a hazard. (1) A signal or action that should change the state of a device, or (2) an opportunity to act, and thus, to fail.
Dense cloud
Cloud density is greater than ambient air density, due to the high molecular weight, the cold temperature, and/or the presence of aerosols in the emissions. Dense cloud and heavy cloud are synonymous.
Dense Gas
A gas with density exceeding that of air at ambient temperature. Sometimes the expression is used loosely to apply to aerosols such as flashing liquid ammonia. See also heavy gas and negatively buoyant vapors.
Dependent Failure
Failure whose probability cannot be expressed as the simple product of the unconditional probability of the individual events, which causes it.
Dermal
Used on or applied to the skin. Importance: Dermal exposure, as well as inhalation exposure, must be considered to prevent adverse health effects.
Dermal Toxicity
Adverse effects resulting from skin exposure to a substance. Also referred to as "Cutaneous toxicity". Importance: Ordinarily used to denote effects in experimental animals.
Design Case
Conditions to be used for the design of a vent system to ensure it will meet safety, health, environmental, and commercial objectives. For emergency vent headers, this will normally be based on the worst credible case scenario. For normal process vents, the design should take into consideration all non-emergency situations unless they will be addressed by some other means, such as by providing a temporary vent system for maintenance operations.
Design Institute for Emergency Relief Systems (DIERS)
Institute under the auspices of the American Institute of Chemical Engineers founded to study relief requirements for reactive chemical systems and two-phase flow systems.
Detonable Limits
The minimum and maximum concentrations of a combustible material, in a homogeneous mixture with a gaseous oxidizer, that will propagate a detonation
Detonation
A release of energy caused by the propagation of a chemical reaction in which the reaction front advances into the unreacted substance at greater than sonic velocity in the unreacted material.
Detonation Flame Arrester
A flame arrester used to prevent the transmission of a detonation.
Detonation Momentum Attenuator
A mechanical device inside of a detonation flame arrester whose purpose is to reduce both the high pressure and the dynamic energy of a detonation and to split the flame front before it reaches the actual flame arrester element, thus avoiding structural damage to the element. (This device is also called a shock absorber device by some manufacturers.)
Deviation
A process condition outside of established design limits, safe operating limits, or standard operating procedures.
Differential Scanning Calorimetry (DSC)
A technique in which the difference in energy inputs into a substance and a reference material is measured as a function of temperature, while the substance and the reference material are subjected to a controlled temperature program. (ASTM E 1445)
Dike
An embankment or wall built to act as a barrier blocking passage of liquids to surrounding areas. (Dictionary.com)
Dilution
Reduction in concentration due to effect of wind.
Directional Incident Outcome
An incident outcome whose consequences produce an effect zone determined by a given wind direction.
Directional Probability
Probability in a given wind direction.
Dispersion Coefficient
The standard deviation s in a specified direction of the Gaussian distribution model used in atmospheric dispersion. The dispersion coefficient is normally expressed as a function of distance for a given weather stability.
Dispersion Models
Mathematical models that characterize the transport of toxic/flammable materials released to the air and/or the water.
Disproportionation
A chemical reaction in which a single compound serves as both oxidizing and reducing agent and is thereby converted into a more oxidized and a more reduced derivative; e.g., a hypochlorite upon appropriate heating yields a chlorate and a chloride.
Distributed Control System (DCS)
A system which divides process control functions into specific areas interconnected by communications (normally data highways), to form a single entity. It is characterized by digital controllers and typically by central operation interfaces. Distributed control systems consist of subsystems that are functionally integrated but may be physically separated and remotely located from one another. Distributed control systems generally have at least one shared function within the system. This may be the controller, the communication link or the display device. All three of these functions maybe shared. A system of dividing plant or process control into several areas of responsibility, each managed by its own CPU, with the whole interconnected to form a single entity usually by communication buses of various kinds.
Diversity
The performance of the same overall protective function by a number of independent and different means.
Domino Effects
The triggering of secondary events, such as toxic releases, by a primary event, such as an explosion, such that the result is an increase in consequences or area of an effect zone. Generally only considered when a significant escalation of the original incident results.
Dose
Time-integrated concentration
Dow Chemical Exposure Index (CEI)
A method, developed by The Dow Chemical Company, used to identify and rank the relative acute health hazards associated with potential chemical releases. The CEI is calculated from five factors: a measure of toxicity; the quantity of volatile material available for a release; the distance to each area of concern; the molecular weight of the material being evaluated; and process variables that can affect the conditions of a release such as temperature, pressure, and reactivity.
Dow Fire and Explosion Index (F&EI)
A method (developed by Dow Chemical Company) for ranking the relative fire and explosion risk associated with a process. Analysts calculate various hazard and explosion indexes using material characteristics and process data.
Dry Type Flame Arrester
A flame arrester that uses an element consisting of small gaps or apertures to quench and extinguish the flame, as distinguished from hydraulic flame arresters.
Dump Tank
A separate collection and containment vessel intended to receive an emergency discharge of liquids, liquid reaction mass, or slurries originating from the bottom of a process vessel or reactor. The dump is usually automatically triggered by a process safety interlock, but may also be initiated manually. An uncontrolled exothermic or other runaway reaction can be controlled by discharging the process vessel contents to the dump tank. This may allow the process vessel to be returned to service in a shorter time.
Dust
Any finely divided solid, 420 microns or 0.017 inches, or less in diameter (that is, material that can pass through a U.S. No. 40 standard sieve).
Dust Hazard Analysis (DHA)
A systematic review to identify and evaluate the potential fire, flash fire, or explosion hazards associated with the presence of one or more combustible particulate solids in a process or facility.
Effect Models
Models that predict effects of incident outcomes usually with respect to human injury or fatality or property damage.
Effect Zone
For an incident that produces an incident outcome of toxic release, the area over which the airborne concentration equals or exceeds some level of concern. For a flammable release, the area over which a particular incident outcome case produces an effect based on a specified criterion. For a loss of containment incident producing thermal effects, the area over which a particular incident outcome case produces an effect based on a specified radiative heat stress limit.
Effectiveness
The combination of process safety management performance and process safety management efficiency. An effective process safety management program produces the required work products of sufficient quality while consuming the minimum amount of resources.
Element
Basic division in a process safety management system that correlates to the type of work that must be done (e.g., management of change [MOC]).
Element Owner
the person charged with overall responsibility for overseeing a particular RBPS element. This role is normally assigned to someone who has management or technical oversight of the bulk of the work activities associated with the element, not necessarily someone who performs the work activities on a day-to-day basis.
Emergency Management
CCPS RBPS Element 16: This Element includes emergency planning and training, responding safely, performing drills, and communicating with all affected personnel and agencies.
Emergency Management
A PSM program element involving work activities to plan for and respond to emergencies.
Emergency Operations
Process changes initiated by the operations staff to place the process into a safe condition (back to normal operations or shutdown) in response to any abnormal situation that could cause a release, explosion, or other significant event.
Emergency Relief Device
A device that is designed to open during emergency or abnormal conditions to prevent rise of internal fluid pressure in excess of a specified value. The device also may be designed to prevent excessive internal vacuum. The device may be a pressure relief valve, a non-reclosing pressure relief device, or a vacuum relief valve.
Emergency Response Plan
A written plan which addresses actions to take in case of plant fire, explosion or accidental chemical release.
Emergency Response Planning Guidelines (ERPG)
A system of guidelines for airborne concentrations of toxic materials prepared by the AIHA. For example, ERPG-2 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair an individual's ability to take protective action.
Emergency Shutdown Device
A device that is designed to shutdown the system to a safe condition on command from the emergency shutdown system.
Emergency Shutdown System
The safety system which overrides the action of the basic control system when predetermined conditions are violated.
Emergency Trip Button
A hard-wired push button that, when pushed, will maintain the trip position and place selected final control elements in a safety position.
Enabling Condition
A condition that is not a failure, error or a protection layer but makes it possible for an incident sequence to proceed to a consequence of concern. It consists of a condition or operating phase that does not directly cause the scenario, but that must be present or active in order for the scenario to proceed to a loss event; expressed as a dimensionless probability.
Enabling Event
An event that makes another event possible. Sometimes used for enabling condition. The term enabling condition is preferred, since enabling conditions are not generally events but rather conditional states.
End-of-Line Flame Arrester
A deflagration flame arrester that is installed on atmospheric pressure tank vent nozzles, having only one pipe connection. It is also called a tank vent deflagration flame arrester.
Endothermic
A physical or chemical change that requires or is accompanied by the absorption of heat.