Tolerable Risk Level
The maximum level of risk of a particular technical process or condition that is regarded as tolerable in the context of the circumstances in questions. Ref. WHEP0.
Tolerance
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.
Toll Manufacturer
Contract manufacturer (external manufacturer).
Toller
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.
Tolling
Providing manufacturing services for a fee by a contractor (the toller), to a company issuing (letting) a contract for those services. Tolled services can include, reaction processes, formulation, blending, mixing or size reduction, separation, agglomeration, packaging/repackaging, and others or a combination of the above.
Top Event
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.
Top-Hat Dispersion Models
Dispersion models with no concentration gradients within the vapor cloud. The concentration is modeled as a well-mixed system within the vapor cloud and zero outside the cloud.
Toxic Dose
The combination of concentration and time for inhalation of a toxic gas that produces a specific harmful effect.
Toxic Gas (NFPA 55)
A gas having a health hazard rating of 3 or 4 as defined in NFPA 704.
Toxic Gases
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.
Toxic Hazard
In the context of these guidelines, a measure of the danger posed to living organisms by a toxic agent, determined not only by the toxicity of the agent itself, but also by the means by which it may be introduced into the subject organisms under prevailing conditions.
Toxic Material
An airborne agent that could result in acute adverse human health effects.
Toxicity
The quality, state, or degree to which a substance is poisonous and/or may chemically produce an injurious or deadly effect upon introduction into a living organism.
Trade Name
The trademark name or commercial trade name for a material. Importance: A MSDS will list trade name(s) to help identify specific materials.
Training
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.
Transect
A risk measure illustrating changes in individual risk with distance from the source of the risk such as a transportation route.
Transition Region
Any region marking the area where one formula is used at smaller values of the independent variable (say, x, or z) and another formula is used at larger values of the independent variable. In this book, transition or interpolation formulas are suggested so the solution varies smoothly across the transition region.
Transitional Brush Discharge
A partial discharge from a nonconductive layer on a conductive substrate. The effective energy is highly dependent on the breakdown voltage of the layer and is intermediate between brush discharges and propagating brush discharges. The more energetic of these discharges might ignite dust in air.
Transloading Operations
The act of transferring material from one transport container to another.
Transmissivity
The fraction of radiant energy that is transmitted from the radiating object through the atmosphere to a target. The transmissivity is reduced due to the absorption and scattering of energy by the atmosphere itself.
Transparency
Openness of an organization with regard to sharing information about how it operates.
Transportation Security Vulnerability Analysis (TSVA)
A modified SVA process specifically designed to account for specific threats, consequences, vulnerabilities, risks, and security countermeasures of transportation activities including hazardous materials in transit.
Transportation Supply Chain
Beginning with the raw material supplier, through the manufacturing and distribution process, to the final end-use customer, and any residuals management, including the transportation network in between.
Triac
A bidirectional rectifier (essentially two SCRs in parallel) that function as an electrically controlled switch for ac loads and having an npnpn structure that can be triggered into either forward or reverse conduction by a pulse applied to its gate electrode. A triac will pass an alternating current. (Formal name is bidirectional triode thyristor.) A thyristor that can be triggered into conduction in either direction. Terminals are called "main terminal 1", and "gate". Semiconductor switching element. Commonly used in a-c output modules for (PESs). A solid-state switch which, when turned on, will conduct electrical current in either direction.
Tribocharging
Charge separation caused by rubbing of surfaces, creating triboelectricity.
Trigger Event
Any change being made to an existing process, or any new facility being added to a process or facility, or any other activity that a facility designated as needing a pre-startup safety review. One example of a non-change-related trigger event is performing a PSSR before restart after an emergency shutdown.
Triple Bottom Line
Expanding the traditional business-reporting framework to take into account environmental and social performance in addition to financial performance.
Turbulence Intensity
The ratio of the turbulent velocity to the mean wind speed (e.g., su/u).
Turbulent Burning Velocity
See Burning Velocity
Turbulent Dispersion
Rate of spread in the vertical (z), lateral (y), or downwind (x) directions of the pollutant cloud about its center of mass due to atmospheric turbulence in the atmosphere.
Turbulent Velocities
Components of turbulent speed fluctuations. For example, the lateral, longitudinal, and vertical turbulent velocities in the boundary layer are equal to about σu (m/s) = 1.9u*, σv (m/s) = 2.4u*, and σw (m/s) = 1.25u*.
Turnaround
A scheduled shutdown period when planned inspection, testing, and preventive maintenance, as well as corrective maintenance such as modifications, replacements, or repairs is performed.
Two Guide Word Analysis
A procedure-based hazard evaluation technique, similar to a HAZOP Study, in which the adequacy of existing safeguards is evaluated by asking what would happen if each step in a procedure was 1) skipped or 2) performed incorrectly.
UN/NA Number
A four digit number used by both the United Nations and the U.S. Department of Transportation. This number is assigned to a hazardous chemical or group of hazardous chemicals.
Unavailability
The probability the fault event exists at a specified time.
Uncertainty
A measure, often quantitative, of the degree of doubt or lack of certainty associated with an estimate of the true value of a parameter.
Unconfined Vapor Cloud Explosion (UCVE)
When a flammable vapor is released, its mixture with air will form a flammable vapor cloud. If ignited, the flame speed may accelerate to high velocities and produce significant blast overpressure.
Undependability
The probability the fault event exists at a specified time or occurs during a specified time interval.
Underlying Causes
Actual root causes.
Undeveloped event
An event in a fault tree that is not developed because it is of no significance, because more detailed information is unavailable, or because its frequency or probability can be estimated without determining its basic events.
Unidirectional Flame Arrester
An in-line flame arrester that is designed to stop flame propagation approaching from only one direction.
Uniform Roughness Array, URA
Used at the Kit Fox experiments, consisting of rectangular plywood boards with width, W = 0.8 m, and with height, Hr = 0.2 m.
Uninterruptible Power Supply (UPS)
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).
Unprotected Side of Flame Arrester
The side of a flame arrester and system to which it is connected where flame is expected
Unreliability
The probability that the fault event occurs during a specified time interval.
Unrevealed Failure
A failure that may lie dormant in the system and only be discovered as a result of a thorough diagnostic testing procedure.
Unrevealed Faults
A failure may lie dormant in the system and only be discovered as a result of a thorough diagnostic testing procedure.
Unstable
Tending toward decomposition or other unwanted chemical change during normal handling or storage. Importance: A MSDS will list materials that are unstable and conditions to avoid to prevent decomposition or unwanted chemical changes.
Unstable Detonation
See Overdriven Detonation.
Unstable material
A material that, in the pure state or as commercially produced, will vigorously polymerize, decompose or condense, become self-reactive, or otherwise undergo a violent chemical change under conditions of shock, pressure, or temperature. (NFPA 704, 2001 edition)