Layers of Protection
A concept whereby several independent devices, systems, or actions are provided to reduce the likelihood and severity of an undesired event.
Layout
The relative location of equipment or buildings within a given site.
Leading Indicator
Process-oriented metrics, such as the degree of implementation or conformance to policies and procedures, that support the PSM program management system and has the capability of predicting performance.
Leading Metric
A forward-looking set of metrics that indicate the performance of the key work processes, operating discipline, or layers of protection that prevent incidents.
Less Vulnerable Object
In the Dutch regulatory context, an off-site population characterized by a lower population density of generally more fit and/or more easily evacuated persons, as contrasted with a vulnerable object.
Lessons Learned
Applying knowledge gained from past incidents in current practices.
Lethal Concentration (LC )
A concentration of a substance being tested which will kill a test animal.
Lethal Concentration 50 (LC50)
The concentration of a material in air which, on the basis of laboratory tests, is expected to kill 50% of a group of test animals when administered as a single exposure (usually 1 or 4 hours). The LC50 is expressed as parts of material per million parts of air, by volume (ppm) for gases and vapors, or as micrograms of material per liter of air (micro-g/L) or milligrams of material per cubic meter of air (mg/m3) for dusts and mists, as well as for gases and vapors. Importance: Both are measures of the toxicity of a substance.
Lethal Dose (LD)
A concentration of a substance being tested which will kill a test animal.
Lethal Dose 50 (LD50 )
A single dose of a material which on the basis of laboratory tests is expected to kill 50% of a group of test animals. The LD50 dose is usually expressed as milligrams or grams of material per kilogram of animal body weight (mg/kg or g/kg). Importance: Both are measures of the toxicity of a substance.
Lethal Service
Service utilizing poisonous gases or liquids of such a nature that a very small amount of the gas or the vapor of the liquid, mixed or unmixed with air, is dangerous to life when inhaled...this class includes substances of this nature that are stored under pressure, or may generate a pressure if stored in a closed vessel (ASME Boiler and Pressure Vessel Code, Section VIII, Div. I).
Level of Acceptable Practice
Good, successful, common, or best practices in PSM that have evolved, either through common and successful usage, interpretation by regulators, or in clear and measurable reductions in process safety risk, into informal criteria that are used by industry and by regulators to define acceptable practices in PSM.
Level of Concern
The concentration of an airborne chemical above which there may be adverse human health effects experience as a result of a short-term exposure during an episodic release.
Life Cycle
The stages that a physical process or a management system goes through as it proceeds from birth to death. These stages include conception, design, deployment, acquisition, operation, maintenance, decommissioning, and disposal.
Light cloud
A cloud with density less than that of the ambient air. Synonymous with buoyant cloud.
Lightning-like Discharge
Hypothetical discharge in large volumes containing clouds of charged, suspended material (such as silos being loaded with powder and supertanker tanks being water washed). Is observed in nature during dust storms, tornadoes and volcanic eruptions. Owing to limited size and/or charge density in practical containers, such discharges are highly improbable and have not been reported.
Likelihood
A measure of the expected probability or frequency of occurrence of an event. This may be expressed as an event frequency (e.g., events per year), a probability of occurrence during a time interval (e.g., annual probability) or a conditional probability (e.g., probability of occurrence, given that a precursor event has occurred).
Likelihood of Adversary Success (LAS)
The potential for causing a catastrophic event by defeating the countermeasures. LAS is an estimate of whether the security countermeasures will thwart or withstand the attempted attack or whether the attack will circumvent or exceed the existing security measures. This measure represents a surrogate for the conditional probability of success of the event.
Limit Dose
The upper limit dose used in testing (2000 <@150> 5000 mg/kg)
Limiting Conditions for Operation
Specifications for critical systems that must be operational and critical resources that must be available to start a process or continue normal operation. Critical systems often include fire protection, flares, scrubbers, emergency cooling, and thermal oxidizers; critical resources normally involve staffing levels for operations and other critical functions.
Limiting Oxidant Concentration (LOC)
The concentration of oxidant, in a fuel-oxidant-diluent mixture below which a deflagration cannot occur under specified conditions. The LOC is synonymous with the term Minimum Oxygen Concentration (MOC).
Liquefied Flammable Gas (LFG)
Any flammable gaseous material or mixture of materials that is in liquid form under pressure.
Liquefied Natural Gas (LNG)
A fluid in the liquid state composed predominantly of methane and that can contain minor quantities of ethane, propane, nitrogen, or other components normally found in natural gas. (NFPA 59A)
Liquefied Petroleum Gas (LPG)
Any material having a vapor pressure not exceeding that allowed for commercial propane composed predominantly of the following hydrocarbons, either by themselves or as mixtures: propane, propylene, butane (normal butane or isobutane), and butylenes. (NFPA 58)
Liquid Seal
A device for preventing the passage of flame by passing the gas mixture through a suitable liquid. See Hydraulic Flame Arrester.
Live-Vector Vaccine
a vaccine that stimulates an immune response by using a non-disease-causing bacterium or virus to transport foreign genes into the body.
Local Exhaust
A system for capturing and exhausting contaminants from the air at the point where the contaminants are produced (welding, grinding, sanding, dispersion operations). See also, "General Exhaust". Importance: Adequate ventilation is necessary to prevent adverse health effects from exposures to hazardous materials and prevent vapor accumulations that cen be a fire hazard.
Local Friction Velocity, u*local
Friction velocity measured at a local position (height) based on the local wind shear at some height, z, or the local square root of the average of (u'w').
Localized Incident
An incident whose effect zone is limited to a plant area (e.g., pump fire, small toxic release), and does not extend into the off-site surrounding community.
Location Risk
Individual risk calculated for a particular geographical location, independent of the nature of the population, or whether anyone is likely to be present at a particular location. The calculation assumes a receptor present 24 hours per day, 365 days per year, out of doors, with no protection.
Lockout/Tagout
A safe work practice in which energy sources are positively blocked away from a segment of a process with a locking mechanism and visibly tagged as such to help ensure worker safety during maintenance and some operations tasks.
Logic Control System
A control system in which definite output signal states are functions of the states of the Input signals in keeping with the rules of Boolean algebra.
Logic Gate
A logical relationship between input (lower) events and the single output (higher) event. These logical relationships are normally represented as AND or OR gates. AND gates combine input events, all of which must occur simultaneously for the output to occur. OR gates combine input events, any one of which is sufficient to cause the output. Other gate types which are variants of these are occasionally used.
Logic System
A group of interconnected logic elements that act in combination to perform a relatively complex logic function. Programming-recording system constructed of solid-state modules based on a series of binary logic (go/no go) components. A pneumatic or electronic system composed of relays, solid-state electronic logic modules, fluidic logic elements, PLCs or DCSs which solves complex problems of interlocking or sequencing through the repeated use of simple functions that define basic concepts such as OR and AND gates.
Lookup Table Approach
See "Spacing Table Approach"
Loop Reactors
Continuous flow reactors that are characterized by the fact that part of the effluent stream is re-supplied to the reactor, either directly or mixed with a reactant supply stream.
Loss Event
Point in time in an abnormal situation when an irreversible physical event occurs that has the potential for loss and harm impacts.
Note: Examples include release of a hazardous material, ignition of flammable vapors or ignitable dust cloud, and overpressurization rupture of a tank or vessel. An incident might involve more than one loss event, such as a flammable liquid spill (first loss event) followed by ignition of a flash fire and pool fire (second loss event) that heats up an adjacent vessel and its contents to the point of rupture (third loss event). Generally synonymous with hazardous event.
Loss of Primary Containment (LOPC)
An unplanned or uncontrolled release of material from primary containment, including non-toxic and non-flammable materials (e.g., steam, hot condensate, nitrogen, compressed CO2 or compressed air).
Low Pressure Tank
A storage tank designed to withstand an internal pressure above 0.5 psig but not more than 15 psig measured at the top of the tank.
Lower Flammable Limit (LFL)
That concentration of a combustible material in air below which ignition will not occur. It is often, interchangeably called Lower Explosive Limit (LEL) and for dusts, the Minimum Explosible Concentration (MEC).
Lowest-Observed-Adverse-Effect-Level (LOAEL)
The lowest exposure concentration at which there are statistically and biologically significant increases in the frequency or severity of adverse effects between the exposed population and its appropriate control group, i.e., lowest observed adverse effect level.
Lycopodium
Any of about 200 varieties of club moss. The commonest is the staghorn moss Lycopodium clavatum from which a resinous, reticulate-shaped pollen is obtained. This is commercially available and is characterized by a relatively spherical, uniformly sized particle (surface average diameter 29 mm, typically with 80 wt% contained within 25-35 mm) suitable for calibration purposes. When tested by non-inductive sparks in various dust ignition energy apparatus under the auspices of ASTM, its minimum ignition energy has been found to be about 20 mJ. This is believed to correspond to, or exceed, the maximum effective energy of bulking brush discharges.
Mach Number, Mw
The ratio of the speed of a flame or blast wave to the speed of sound at ambient temperature and pressure.
Macro-Assembler
An assembler that brings high-level language features to assembly language programming. It translates a single multi-argument source line o code into a sequence of machine instructions.
Main Processing Unit (MPU)
The portion of the PES which interprets or executes the (main part of the) Application Program. The MPU may include power supply, memory, and I/Os. .
Maintenance
Probability that a system survives a fault without erroneous output.
Major Incident
An incident whose effect zone, while significant, is still limited to site boundaries (e.g., major fire, spill).
Man Machine Interface (MMI)
The means by which human interaction with the control system is accomplished.
Man Machine System
A system in which the functions of the worker and the machine are interrelated and necessary for the operation of the system.
Man-Machine Interface (MMI)
A manufacturer's Catalogued Peripheral equipped with push buttons, lamps, keyboards, displays, or equivalent, intended as operator interface, such as AXIS or LOOP control/monitor panel, general purpose operator interface etc. MMIs may be part of the Permanent installation (e.g., mounted on front panels, doors, boards) or not.