Process Safety Glossary | Aetso Technical

By the Aetso Technical engineering team · Reviewed November 2026 · Reference

Process safety has a vocabulary of its own. These definitions are written plainly for managers, operators and engineers new to the field. Where a term comes from a regulation or standard, that document's own definition governs.

AEGL: Acute Exposure Guideline Levels — airborne concentrations above which the general population could experience defined effects, used in toxic consequence analysis.
ALARP: As Low As Reasonably Practicable — risk reduced until the cost of further reduction is grossly disproportionate to the benefit.
BLEVE: Boiling Liquid Expanding Vapor Explosion — the sudden rupture of a vessel containing a liquid above its boiling point, often caused by fire exposure.
Bowtie: A diagram showing threats, prevention barriers, a central top event, mitigation barriers and consequences.
BPCS: Basic Process Control System — the control system that keeps a process within its normal operating limits.
CCPS: Center for Chemical Process Safety, part of the American Institute of Chemical Engineers (AIChE).
DHA: Dust Hazard Analysis — a systematic review of fire, flash fire and explosion hazards from combustible particulate solids, required by NFPA standards.
ERPG: Emergency Response Planning Guidelines — airborne concentration guidelines used in emergency planning and consequence analysis.
FMEA: Failure Modes and Effects Analysis — a method examining how each component can fail and the effect on the system.
FSA: Functional Safety Assessment — an independent judgment of the functional safety achieved by a safety instrumented system at defined lifecycle stages.
HAZID: Hazard Identification — a broad, early-stage method for identifying major hazards.
HAZOP: Hazard and Operability study — a structured method using guidewords and parameters to identify deviations from design intent.
HIPS: High-Integrity Protective System — an instrumented system used to protect against overpressure in place of, or to reduce, relief capacity.
Initiating event: The failure or error that starts a hazardous scenario.
IPL: Independent Protection Layer — a safeguard that is independent, effective, auditable and dependable enough to be credited in LOPA.
KSt: Deflagration index of a dust cloud — a measure of explosion severity used to size venting and suppression.
LOPA: Layer of Protection Analysis — a semi-quantitative method for judging whether safeguards provide sufficient risk reduction.
MAWP: Maximum Allowable Working Pressure — the maximum pressure permitted at the top of a vessel at a designated temperature.
MI: Mechanical Integrity — the program that keeps equipment and safeguards fit for service.
MIE: Minimum Ignition Energy — the lowest spark energy that can ignite a dust cloud or flammable mixture.
MOC: Management of Change — the process for reviewing and authorizing changes before implementation.
PFD: Probability of Failure on Demand — the probability that a protective function will fail to act when required.
PHA: Process Hazard Analysis — a systematic evaluation of the hazards of a process.
PSI: Process Safety Information — written information on chemical hazards, process technology and equipment.
PSM: Process Safety Management — OSHA standard 29 CFR 1910.119, and more generally the management of process hazards.
PSSR: Pre-Startup Safety Review — the check performed before hazardous chemicals are introduced to a new or modified process.
QRA: Quantitative Risk Assessment — combining frequencies and consequences of many scenarios to estimate risk numerically.
RAGAGEP: Recognized and Generally Accepted Good Engineering Practices — codes, standards and practices that PSM relies on.
RBPS: Risk Based Process Safety — the CCPS framework of 20 process safety elements in four pillars.
RMP: Risk Management Program — EPA regulation 40 CFR Part 68, and the Risk Management Plan submitted under it.
RRF: Risk Reduction Factor — the factor by which a protective function reduces the frequency of a scenario; the inverse of PFD.
SIF: Safety Instrumented Function — a specific function, implemented by an SIS, that brings a process to a safe state.
SIL: Safety Integrity Level — a discrete level (1 to 4) specifying the required performance of a safety instrumented function.
SIS: Safety Instrumented System — sensors, logic solvers and final elements implementing one or more SIFs.
SRS: Safety Requirements Specification — the document defining the functional and integrity requirements of each SIF.
STAA: Safer Technology and Alternatives Analysis — an evaluation of inherently safer options, introduced for certain RMP processes in the 2024 amendments.
TQ: Threshold Quantity — the amount of a listed chemical above which a process is covered by PSM or RMP.
VCE: Vapor Cloud Explosion — the explosion of a flammable vapor cloud, whose severity depends strongly on congestion and confinement.