Unstable Substance/Material
Substance or material which in a pure state or as normally produced decomposes either or not violently.
Update and Revalidate
To revise a prior PHA, as required, to reflect any changes that have occurred since the prior PHA; new learnings about the hazards of the process; changes in risk management requirements; etc.
Upper Flammable Limit (UFL)
The highest concentration of a vapor or gas (the highest percentage of the substance in air) that will produce a flash of fire when an ignition source (heat, arc, or flame) is present. See also Lower Flammable Limit. At concentrations higher than the UFL, the mixture is too rich to burn. Also known as the Upper Explosive Limit (UEL)
Urban/industrial heat island
Because of heat generated by man's activities and by industrial processes, an urban or industrial area is often several degrees warmer than its surroundings. This is called a heat island. The thermal input from home heating or from industrial processes are typically of order 10 to 100 W/m2, or the same order as the natural boundary layer daytime heat flux, Hs
User Program
Synonymous with Application Program.
User Program Memory
The portion of the PES memory reserved for the storage of Application Programs.
Valve Failure Positions
In the event of instrument air or electrical power failure, valves Fail Closed (FC), Fail Open (FO), or Fail in the last position (FL). The position of failure must be carefully selected so as to bring the system to, or leave the system in a safe operating state.
Vapor
The gaseous phase formed by a material that is liquid at ambient temperature and pressure.
Vapor Cloud Explosion (VCE)
The explosion resulting from the ignition of a cloud of flammable vapor, gas, or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure.
Vapor Collection System
A piping system to which vessels are connected, that collects vapors from these vessels and directs them to environmental control equipment such as flares, incinerators, scrubbers, and activated carbon adsorbers.
Vapor Density
The weight of a vapor or gas compared to the weight of an equal volume of air; an expression of the density of the vapor or gas. Materials lighter than air have vapor densities less than 1.0 (example: acetylene, methane, hydrogen). Materials heavier than air (examples: propane, hydrogen sulfide, ethane, butane, chlorine, sulfur dioxide) have vapor densities greater than 1.0.
Vapor Pressure
The pressure exerted by a vapor above its own liquid. The higher the vapor pressure, the easier it is for a liquid to evaporate and fill the work area with vapors which can cause health or fire hazards.
Variable
A quantity or condition whose value is subject to change and can usually be measured. A language object that may take different values , one at a time. Note: the values of a variable are usually restricted to a certain data type.
Variation
A change in data, process parameter, or human behavior. Within prescribed limits, changes in data, process parameters, and human behavior are anticipated and acceptable. Variation outside established limits is called deviation.
Vector
A plasmid, virus, or other vehicle for carrying a DNA sequence into the cells of another species; also a method (such as genetically engineered viruses or bacteria) of delivering genetic material to cells.
Velocity Flame Stopper
See Flow Controlled Aperture.
Vendor
The hardware and software suppliers of a computerized system. There may be multiple vendors for a computerized system. The vendor may or may not be a manufacture of the computerized system. For example, the vendor may be a systems house who simply integrates subsystems or components manufactured by others or may be a computer store.
Vent
An opening for the passage of, or dissipation of, fluids, such as gases, fumes, smoke, and the like.
Vent Duct
Piping attached to a vent on indoor equipment to direct the fireball and products of combustion to the outside of a building
Vent Manifold
See Vapor Collection System
Vent models
A group of specialized models for expressing the concentration on the roof or side of a building resulting from emissions from a short vent on the building.
Ventilation
The process of supplying or removing an atmosphere to or from any space by natural or mechanical means.
Venting
Emergency flow of vessel contents out of a vessel. The pressure is controlled or reduced by venting, thus avoiding a failure of the vessel by overpressurization. The emergency flow can be one-phase or multi-phase, each of which results in different flow characteristics.
Verification
A wide variety of activities that can be employed to increase confidence in the audit data, including evaluating the application of, and adherence to laws, regulations, policies and procedures, standards, and management directives; certifying the validity of data and reports; and evaluating the effectiveness of management systems.
Verification (Instruments and Controls)
Confirming by examination and provision of objective evidence that the requirements have been fulfilled.
Verification Activity
A test, field observation, or other activity used to ensure that personnel have acquired necessary skills and knowledge following training.
Verify
To confirm the truth, accuracy, or correctness of, by competent examination; to substantiate.
Very near-field
The area very close to the source where the specific geometry of one or two individual obstacles influence the flow and dispersion.
Virtual Source
The offset in distance to the specified source of a gas or vapor release that results in a maximum concentration of 100% at the source using a Gaussian dispersion model.
Virtual source method
This method is applied when there is a change in dispersion conditions at some point along a plume trajectory due to changes in underlying surface, stability, wind speed, or other effects. In order that the calculated plume dispersion coefficient has no discontinuities, a virtual distance is calculated upwind of the position of interest so that the dispersion coefficient is the same at that point for the upstream conditions and the downstream conditions.
Virus
a microorganism that grows and reproduces in living cells of a host (bacteria, plant, or animal); the simplest form of life, more than 200 viruses are known to produce human disease.
Viscous Nonconductive Liquid
Nonconductive liquid having a kinematic viscosity above 30-100 cS and a very slow rate of charge dissipation, equivalent to a conductivity of about 0.02 pS/m and a relaxation time of about 1000 s.
Voltage (V)
See Potential.
Voltage-Common Mode
The voltage common to all conductors of a group as measured between that group at a given location and an arbitrary reference (usually Earth).
Voltage-Transverse Mode (Differential)
The voltage at a given location between two conductors of a group.
Volume Blockage Ratio (VBR):
The ratio of the volume occupied by congestion elements such as pipes, beams, plates, etc. to the volume of the portion of the plant under consideration.
Volume-Source Explosion Models
Models that predict explosion characteristics based on the volumetric portion of the flammable cloud involved in the explosion (that portion of the flammable cloud that is influenced by congested and/or confined volumes in a plant).
Voluntary Consensus PSM program
A PSM program developed in response to a consensus program that is not required by law or regulation, but is specified by an industry trade or professional organization, such as ACC, ISO, or another organization that has developed EHS consensus standards containing PSM provisions and either has recommended them to their membership or requires them to be implemented as a condition of membership.
Voluntary Risk
Risk that is consciously tolerated by someone seeking to obtain the benefits of the activity that poses the risk.
Voting System
An m out of n redundant system which requires at least m of the n channels to be working to ensure the system is working.
Vulnerability
Any weakness that can be exploited by an adversary to gain access to an asset.
Vulnerable object
In the Dutch regulatory context, an off-site population that may be more significantly impacted by a risk source, as contrasted with a less vulnerable object. This greater susceptibility may be due to a variety of considerations such as the numbers of persons present, the physical characteristics of the population (e.g., the young or infirm), occupancy patterns (e.g., continuously present versus transient), or constraints on the ability to evacuate. Individual risk criteria for vulnerable objects are more rigidly enforced than those for less vulnerable objects.
Watch Dog Timer (WDT)
A timer implemented to prevent the system from looping endlessly or becoming idle because or program errors or equipment faults.
Watchdog
A manufacturer provided means which independently monitors the Duration of internal hardware functions, and/or Application Program functions, and/or Operating System Software functions, and which will cause specific actions to be performed if not periodically reset at a predetermined interval. In control systems, a combination of hardware and software which acts as an interlock scheme, disconnecting the system's output from the process in event of system malfunction.
Watchdog Timer (External)
An external watchdog timer is preferred to an internal watchdog timer to assure that outputs go to the Off state of the watchdog's logic is not properly executed every scan. Same as an "internal" WDT except the WDT performance cannot be compromised by any failure in the system it is monitoring (e.g., no common mode fault within the PES). As a result many WDTs provided with PESs are not suitable as safety WDTs.
Watchdog Timer (Internal)
A time internal to the CPU used to ensure that a scan is completed at least once each 200 milliseconds. Internal WDTs are subject to misoperaiton due to faults in the PES they are monitoring (e.g., common mode faults).
Water for Injection (WFI)
Very pure water used for medical purposes.
Water-Compatible
A material that is neither reactive with water nor incompatible with water and consequently can be extinguished with a water-based extinguishing system.
Water-Incompatible
A material that does not chemically react with water but undergoes a change of phase or state upon mixture with water that renders it permanently changed or incompatible with the remainder of the process.
Water-Reactive
A material that will react upon contact with water under normal ambient conditions. Includes materials that react violently with water and other materials that react slower but can generate heat or gases that can result in elevated pressure if contained.