Spontaneously Combustible
Capable of igniting and burning in air without the presence of an ignition source. Pyrophoric materials are spontaneously combustible, although some pyrophorics require the presence of a minimum amount of moisture (humidity) to spontaneously ignite. Other spontaneously combustible substances and mixtures may require more time or an insulating environment to self-heat to the point of ignition.
Stability
Refers to the ratio of the suppression of turbulence by thermal effects to the generation of turbulence by mechanical effects such as wind shears. Transport and dispersion models characterize the effect of stability through use of one or more dimensionless stability parameters, such as the Richardson number, Ri, the Monin-Obukhov length, L, or the Pasquill stability class.
Stability Class
The Pasquill stability class scheme is based on time of day, wind speed, cloudiness, and sun's intensity. The six stability classes are denoted by the letters A through F, with A being very unstable, D being neutral, and F being very stable
Stabilized Burning
Steady burning of a flame, stabilized at, or close to the flame arrester element.
Stable Detonation
A detonation that progresses through a confined system without significant variation of velocity and pressure characteristics. For atmospheric conditions, typical velocities range between 1600 and 2200 m/s for standard test mixtures and test procedures.
Stable Materials
Those materials that normally have the capacity to resist changes in their chemical composition, despite exposure to air, water, and heat as encountered in fire emergencies. (NFPA 704 2001)
Staging
Assembling, testing, and operating a process control system in a controlled environment prior to plant implementation according to a test plan to ensure that it performs properly and meets all requirements of the functional specification. Staging is performed after the standard vendor system acceptance test has been conducted and is done under the end user's supervision to thoroughly exercise all essential control system function. Testing often includes some type of simulation of the actual plan operation with real or simulated inputs to test key application-related functions, such as program interaction.
Stakeholder
Individuals or organizations that can (or believe they can) be affected by the facility's operations, or who are involved with assisting or monitoring facility operation.
Stakeholder Outreach
A PSM program element associated with efforts to (1) seek out and engage stakeholders in a dialogue about process safety; (2) establish a relationship with community organizations, other companies and professional groups, and local, state, and federal authorities; and (3) provide accurate information about company/facility operations, products, plans, hazards, and risks.
Stand-Off Distance, R
Distance from the center of an explosion
Standard Deviation (cloud concentration), s
The square root of the sum of squared values of concentration minus centerline concentration. The cloud half-width can be defined as the distance corresponding to a concentration of two or three standard deviations from the centerline.
Standards
The PSM program element, Compliance with Standards, that helps identify, develop, acquire, evaluate, disseminate, and provide access to applicable standards, codes, regulations, and laws that affect a facility and/or the process safety requirements applicable to a facility. More generally, standards also refers to requirements promulgated by regulators, professional or industry-sponsored organizations, companies, or other groups that apply to the design and implementation of management systems, design and operation of process equipment, or similar activities.
Standby Mode
Hardware operation that is normally not running but must be ready run, e.g., an emergency diesel generator.
Statement
A programming language construct that represents a set of declarations or a step in a sequence of actions.
Static Activation Pressure
Pressure that actuated a vent closure when the pressure is increased slowly (with a rate of pressure rise less than 0.1 bar/min (0.15 psi/min).
Static Electricity
The class of phenomena recognized by the presence of electrical charges, either stationary or moving, and the interaction of these charges, this interaction being solely by reason of the charges themselves and their position, and not by reason of their motion.
Static Flame Arrester
A flame arrester designed to prevent flame transmission by quenching gaps (apertures). These are usually dry type flame arresters with elements such as crimped metal ribbon, parallel plates, wire gauze (mesh), and sintered metal.
Stationary Conditions
Conditions that are characterized by constant concentrations and temperatures as a function of time, i.e. the time derivatives of all variables are zero.
Statistical Dependence
Two events (A and B) are statistically dependent if the probability that A occurs given B has already occurred is different than the separate probability of A occurring. That is, P(A|B) - P(A).
Statistical Independence
Two events (A and B) are statistically independent if the probability that they both occur is the product of their separate probabilities: P(A and B) = P(A) * P(B).
Statistical Process Control (SPC)
The use of statistical techniques (such as control charts) to analyze a process and take appropriate action to maintain statistical control and improve process capability.
Stoichiometric Concentration
A term used to describe a balanced ratio of chemical reactants that would result in all of the chemical reactants becoming products if a chemical reaction were to occur.
Stoichiometric Mixture
A balanced mixture of fuel and oxidizer such that no excess of either remains after combustion.
Stoichiometry
Calculations about masses or volumes of reactants and products involved in a chemical reaction.
Strain
a genetic variant within a species.
Stratification
As used here, any concentration profile that develops in an enclosure. The flammable mass will be between the LFL and UFL concentrations along such a concentration profile.
Stray Capacitance
Capacitance unessential for the function of a circuit and of relatively small value that nevertheless may introduce errors if not eliminated or unaccounted for. Of particular importance in high voltage, low capacitance spark ignition tests and in the measurement of small charges and currents.
Stray Currents
Electric currents which wholly or in part do not follow the original circuits designed for them.
Streaming Current
The charging current introduced to a system by liquid flow, expressed in Amperes.
Street Canyons and Tunnel Entrances
Special situations for which models are developed based on empirical analysis of field data, simplified scaling models, or Computational Fluid Dynamics (CFD) models.
Study Cube
A geometrical representation of the variety of depths of study possible for a CPQRA using the three major factors that define the scope of a CPQRA: risk estimation technique, complexity of analysis, and number of incidents selected for study.
Subchronic Exposure
Multiple or continuous exposures occurring for approximately 10% of a species lifetime, usually over 3 months.
Subcontractor
A company or individual performing work at a PSM-covered facility whose business relationship is with a third party (i.e., a general or specialty contractor) and not with the host facility directly. Subcontractors are subject to the Contractor Management element of PSM programs.
Substrate (in cell culture)
surface on which a cell or organism grows or is attached such as the use of microcarriers in cell culture; most eukaryotic cell types require attachment to a substrate for survival; also called extracellular matrices; in the body they are composed mainly of proteins and provide chemical cues that affect or guide the behavior of cells.
Subsystem
A portion of a system.
Superheated Liquids
Liquids at a temperature above their normal boiling point.
Suppressant
A chemical agent used in a deflagration suppressant system to extinguish the deflagration.
Suppressant Barrier
An isolation system using a suppressant.
Surface boundary layer
Layer of depth 50 to 100 m where the friction velocity u* can be assumed constant.
Surface Charge Density (Qs or s).
The charge per unit area of solid surface expressed in Coulomb/square meter. For a unidirectional, uniform field in air the maximum surface charge density before air breakdown occurs is 2.65 x10-5 C/m2. Only a fraction of this value is achieved in most practical situations. However, for non-uniform fields around a charged particle much greater charge densities can be supported in air, depending on the radius of the particle. Also, much greater surface charge densities can be supported when nonconductive charged surfaces have a metal substrate so as to form a capacitor. Since the electric field is primarily exerted through the nonconductor to the substrate the maximum charge density may be limited by the breakdown strength of the nonconductor.
Surface Heat Flux, Hs
Turbulent heat flux upward from the surface, measured by fast response turbulence instruments using the identity Hs = cpρ(w'T'), where w' is the fluctuation in vertical wind speed and T' is the fluctuation in temperature, and the average is taken over about one hour. A typical value of Hs in the daytime is about 200 w/m2 (J/sm2) and at night is about -20 W/m2.
Surface Resistivity (rs)
The resistance across a surface element having unit length and unit width, expressed in ohm/square or ohm (Method : ASTM D257). In EOS/ESD S11.11-1993, a static dissipative packaging material is defined as having a surface resistivity equal to or greater than 1.0x104 W and less than 1.0x1011 W, while an insulating packaging material has a surface resistivity equal to or greater than 1.0x1011 W.
Surface roughness length, zo
A measure of the amount of mechanical mixing introduced by the surface roughness elements and, as a rough rule of thumb, is equal to about 0.1 times the average height, Hr, of the roughness elements.
Surface Streamer
A wall-to-surface discharge observed on charged liquids during tanker filling, appearing as a flash up to 2 ft long and accompanied by crackling sound. Effective energy unknown but possibly up to about 10 mJ. Also observed on tribo-charged solid surfaces such as between aircraft windshields and metal airframe, where puncturing does not occur.
Sustainability
Meeting the needs of the present without compromising the ability of future generations to meet their own needs.
Synchronous
Pertaining to two or more processes that depend upon the occurrence of a specific event such as a common timing signal.
System
A collection of people, machines and methods organized to accomplish a set of specific functions.
System Designer
The individual responsible for designing the complete computerized system.
System Documentation
The accumulated documents relating to a computerized system that are prepared as the development project proceeds. System documentation is not a separated phase of development.
System Evaluation
A phase in the development of computerized systems in which the system is compared with its requirements to determine possibilities for growth and improvement and to preserve the lessons of that development project in preparation for the next one.