Guidelines for Pressure Relief and Effluent Handling Systems by Center for Chemical Process Safety Copyright © 1998 American Institute of Chemical Engineers
Some of the terminology in the Glossary was taken directly from other CCPS publications, some created by the authors, while others are verbatim quotes from either ANSI B95.1,1977 [the same terminology is restated in ASME/ANSI PTC 25 (1994), Appendix 11, or API 520, Part I (1994), or API 521 (1997). If the source is other than a CCPS publication or the authors of this book, that source is indicated by a reference notation at the end of each definition as follows: (Ref. 1): ANSI B95.1(1977);ASME/ANSI PTC 25 (1994), Appendix 1. (Ref. 2): API 520, Part I (1994). (Ref. 3): API 521 (1997). Accumulation: The pressure increase over the maximum allowable working pressure of a vessel during discharge through the pressure relief device, expressed in pressure units or as a percent. Maximum allowable accumulations are established by applicable codes for operating and fire contingencies. (Ref. 3) Actual Discharge Area: The measured minimum net area which determines the flow through a valve. (Ref. 1) Administrative Controls: Procedural mechanisms, such as lockout/tagout procedures, for directing and/or checking human performance on plant tasks. Atmospheric Dispersion: The low momentum mixing of a gas or vapor with air. The mixing is the result of turbulent energy exchange, which is a function of wind (mechanical eddy formation) and atmospheric temperature profile (thermal eddy formation). 51 1
Autoignition Temperature: The autoignition temperature of a substance, whether solid, liquid, or gaseous, is the minimum temperature required to initiate or cause self-sustained combustion, in air, with no other source of ignition. Back Pressure: The static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system. (Ref. 1) The pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system. Back pressure can be either constant or variable. Back pressure is the sum of superimposed and built-up back pressure. (Ref. 3) Balanced Safety Relief Valve: A balanced safety relief valve incorporates means of minimizing the effect of back pressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity). (Ref. 1) Basic Process Control System (BPCS): The control equipment which is installed to support normal production hnctions. Best Estimate Flow Rate: The flow rate through a pressure relieving system calculated for a designated temperature, set pressure and overpressure, using a flow area and coefficient of discharge (or resistance factors for rupture disk device systems) determined in certification tests or otherwise specified in ASME BPVC; but without inclusion of the Codemandated 0.9factor, or use of overly conservative flow resistance factors. BLEW: See “Boiling Liquid Expanding Vapor Explosion.” Blowdown: The difference between actual popping pressure of a pressure relief valve and actual reseating pressure expressed as a percentage of set pressure or in pressure units. (Ref. 1) Boiling-Liquid-Expanding-Vapor Explosion (BLEW): A type of rapid phase transition in which a liquid contained above its atmospheric boiling point is rapidly depressurized, causing a nearly instantaneous transition from liquid to vapor with a corresponding energy release. A BLEVE is often accompanied by a large fireball if a flammable liquid is involved, since an external fire impinging on the vapor space of a pressure vessel is a common BLEVE scenario. However, it is not necessary for the liquid to be flammable to have a BLEVE occur. Bore Area: The minimum cross-sectional flow area of a nozzle. (Ref. 1)Note: Often termed the nozzle “throat” area. Breaking Pin Device:A nonreclosing pressure relief device actuated by inlet static pressure and designed to function by the breakage of a load-carrying section of a pin which supports a pressure containing member. (Ref. 1) Buckling Pin Device: Also known as “rupture pin devices.” A nonreclosing pressure relief device that is actuated by inlet static pressure causing a pin
to buckle under the compression load imposed by the pressure acting on a seat. The force under which buckling occurs is determined by the length and diameter of the pin (Euler’s Law).
Built-up Back Pressure: There are several definitions in use by various organizations as indicated below. When the term is used in CCPS’ Guidelines for Pressure Relief and EffIuent Handling Systems, the following meaning is intended: The increase in pressure at the outlet of the pressure relief device that develops as a result of flow after the pressure relief device opens.
“Built-up back pressure is pressure existing at the outlet of a pressure relief device occasioned by the flow through that particular device into a discharge system.” (Ref. 1) “The increase in pressure in the discharge header that develops as a result of flow after the pressure relief device opens.” (Ref. 2). “The increase in pressure in the discharge header that develops as a result of flow after the pressure relief device or devices open.” (Ref. 3) Note: The term “built-up back pressure” is used in some documents as the criterion for defining back pressure limitations on typical pressure reliefvalve styles (see part 220.127.116.11 of [ M I RP-520 PartI]). The term “builtu p back pressure’’ is not used in the ASME Code language for back pressure limitations (see Section 18.104.22.168.2 for Code wording). Calculated Relieving Capacity: The flow rate for a given fluid in a pressure relief system which uses a rupture disk device as the sole relief device at a designated relieving temperature and pressure, calculated using accepted engineering practices for determining fluid flow through piping, with the certified flow resistance K,, for the rupture disk device, and before application of the 0.9 factor specified in ASME BPVC, UG-127 (a)(Z)(b). (Also see 22.214.171.124.) Catastrophic Incident: An incident involving a major uncontrolled emission, fire or explosion with an outcome effect zone that might extend offsite into the surrounding community. Certified Capacity: The flow rate through a pressure relief valve calculated for a designated temperature, set pressure, and 10%overpressure (see UG 131(c)(2) for exception), using a flow area and coefficient of discharge determined in certification tests or otherwise specified in ASME BPVC, Section VIII. May be expressed as flow rate of either steam, water (if certified on water), air, and (optionally) other fluids. Chatter: Chatter is abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disk contacts the seat. (Ref. 1) Choked Flow:A condition that occurs when the flow of a compressible fluid through a pressure relief device, piping or other equipment does not respond to a decrease in downstream or back pressure on the device. The
mass flow rate then depends only on upstream conditions. This can occur with gashapor or with two-phase fluid systems.Also referred to as maximum or critical flow. Coefficient of Discharge: The ratio of the measured relieving capacity to the theoretical relieving capacity. (Ref. 1) Cold Differential Test Pressure: The inlet static pressure at which a pressure relief valve is adjusted to open on the test stand. This test pressure includes corrections for service conditions of back pressure and/or temperature. (Ref. 1) Note: The term “differential set pressure” is used in these Guidelines to denote the differential under service conditions rather than at test conditions. This differential set pressure is the dif€erence between the static pressure at the inlet and the pressure acting on the discharge side o f the disk at the opening condition. Combination Capacity Factor: The ratio of average flow capacity determined by tests of a pressure relief valve in combination with a rupture disk device to the flow capacity of the pressure relief valve alone. See 2.6.5. Combustible: A term used to classifi certain liquids that will bum on the basis of flash points. Both the National Fire Protection Association (NFPA 30) and the Department of Transportation (DOT) define “combustible liquids” as having a flash point of 100°F (37.8”C) or higher. See also, “Flammable.” Combustible liquids are subdivided as follows: Class 11: Those having flash points at o r above 100°F (37.8”C) and below 140°F (60°C).
Class IIIA: Those having flash points at or above 140°F (60°C). but below 200°F (93°C).
Class IIIB: Those having flash points at or above 200°F (93°C). Importance: Combustible liquid vapors d o not ignite as easily as flammable liquids; however, combustible vapors can be ignited when heated and
must be handled with caution. Common Mode Failure: An event having a single external cause with multiple failure effects which are not consequences of each other. Consequence Analysis: The analysis of the expected effects of incident outcome cases independent of frequency or probability.
Containment: A system condition in which under no condition reactants o r products are exchanged between the chemical system and its external environment. Conventional Safety Relief Valve: A conventional safety relief valve has its spring housing vented to the discharge side of the valve. The operating characteristics (opening pressure, closing pressure, and relieving capacity) are directly affected by changes of the back pressure on the valve. (Ref. 1)
Credible Event: A scenario or event that that has reasonable and sufficient likelihood of occurrence that it should be considered in selecting the design basis for an emergency relief system. This should be based on a risk analysis that includes a careful and thorough review of process characteristics, experience with similar systems, hazardous nature of the materials handled, and consequences of an incident. Curtain Area: The area of the cylindrical or conical discharge opening between the seating surfaces created by the lift of the disk above the seat. Refers to the flow area of a pressure relief valve. (Ref. 1) Deflagration: The chemical reaction of a substance in which the reaction front advances into the unreacted substance at less than sonic velocity. Where a blast wave is produced that has the potential to cause damage, the term explosive deflagration may be used. Design Basis: The basis for sizing equipment and associated piping used for process or emergency relief services. This might include the flow rate, composition, pressure, and temperature of streams handled under the most severe conditions reasonably expected to occur. Design Institute for Emergency Relief Systems (DIERS): Institute under the auspices of the American Institute of Chemical Engineers, founded to investigate design requirements for relief devices and vent lines for twophase venting of runaway chemical reactions. This activity has been carried forward under the DIERS Users Group, and the scope of activity has been broadened to include a variety of import technical topics related to pressure relief system design. Design Institute for Physical Property Data (DIPPR): Institute under the auspices of the American Institute of Chemical Engineers, founded to compile a database of physical, thermodynamic, and transport property data for most common chemicals. Design Pressure: The design pressure of a vessel is at least the most severe condition of coincident temperature and gauge pressure expected during operation. The design pressure is the pressure used in the design of a vessel to determine the minimum permissible thickness or other physical characteristics of the different parts of a vessel (see also maximum allowable working pressure). (Ref. 3) Note: For low pressure tanks (just above atmospheric), the design pressure may be expressed as a “pressure not to be exceeded.” Detonation:A release of energy caused by the extremely rapid chemical reaction of a substance in which the reaction front advances into the unreacted substance at equal to or greater than sonic velocity in the unreacted fluid. Differential Set Pressure: The difference between the set pressure and the superimposed back pressure in the valve bonnet (pressure acting on the top of the valve disk) at the time the valve is called upon to operate.
Disk (valve):A disk is the pressure containing movable element of a pressure relief valve which effects closure. (Ref. 1) Dow Fire and Explosion Index (FtkEI): A method (developed by Dow Chemical Company) for ranking the relative fire and explosion risk associated with a process. Analysts calculate various hazard and explosion indices using material characteristics and process data. Effective Discharge Area: A nominal or computed area of flow through a pressure relief valve, differing from the actual discharge area, for use in recognized flow formulas to determine the capacity of a pressure relief valve. (Ref. 1) Note: Also termed the “orifice”area. Emergency Shutdown (ESD) System: The safety control system which overrides the action of the basic control system when predetermined conditions are violated. Emergency Vent: Avent designed to respond to an overpressure or temperature excursion in protected equipment by opening and discharging material from the equipment to relieve excessive pressure. This is commonly a pressure relief device. Equipment Reliability: The probability that, when operating under stated environment conditions, process equipment will perform its intended function adequately for a specified exposure period. Equivalent Ideal Nozzle Area: The product of the nozzle area and the coefficient of discharge. Explosion: A release of energy that causes a pressure discontinuity or blast wave. Fail-safe: Design features which provide for the maintenance of safe operating conditions in the event of a malfunction of control devices or an interruption of an energy source (e.g., direction of failure of a motor operated valve on loss of motive power). Features incorporated for automatically counteracting the effect of an anticipated possible source of failure. A system is fail-safe if failure of a component, signal, or utility initiates action that return the system to a safe condition. Failure: An unacceptable difference between expected and observed performance. FlammabilityLimits: The range of gas or vapor amounts in air that will bum or explode if a flame or other ignition source is present. Importance: The range represents an unsafe gas or vapor mixture with air that may ignite or explode. Generally, the wider the range the greater the fire potential. See also Lower Explosive Limit/Lower Flammable Limit and Upper Explosive Limiwpper Flammable Limit. Flammable: A liquid that has a closed-cup flash point below 100°F (37.8”C), as determined by specified test procedures and apparatus. See NFPA 30
for procedures and apparatus. Flammable liquids may be classified as follows: Class 1A: Those having flash points below 73°F (22.8"C) and having a boiling point below 100°F (37.8"C). Class 1B: Those having flash points below 73°F (22.8"C) and having a boiling point at or above 100°F (37.8"C). Class 1C: Those having flash points at o r above 73 (22.8"C) and below 100°F (37.8"C).
Importance: Flammable liquids provide ignitable vapor at room temperatures and must be handled with caution. Precautions such as bonding and grounding must be taken. Flash Point: The minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture with air, near the surface of the liquid or within the vessel used, as determined by the appropriate test procedure and apparatus. See NFPA 30 for test procedures appropriate for various liquids. There are several flash point test methods, and flash points may vary for the same material depending on the method used. Consequently, the test method is indicated when the flash point is given. A closed cup type test is used most frequently for regulatory purposes. Importance: The lower the flash point temperature of a liquid, the greater the chance of a fire hazard. Freeboard: The clear, vertical space above a liquid or a foam in a vessel. Frozen: A term to describe a mixture of vapor and liquid in which the ratio of phases does not change as the mixture moves through a pipe (i.e., neither flashing nor condensation occur). Fusible Link Device: A nonreclosing pressure relief device designed to function by the yielding or melting of a plug of suitable melting temperature material. (Ref. 1) Hazard: An inherent chemical or physical characteristic that has the potential for causing damage to people, property, or the environment. In this document it is typically the combination of a hazardous material, an operating environment, and certain unplanned events that could result in an accident. Hazard Analysis: The identification of undesired events that lead to the materialization of a hazard, the analysis of the mechanisms by which these undesired events could occur and usually the estimation of the consequences. Hazard and Operability Study (HAZOP): A systematic qualitative technique to identify process hazards and potential operating problems using a series of guide words to study process deviations. A HAZOP is used to discover what deviations from the intention of the design can occur and what their causes and consequences may be. This is a systematic detailed
review technique which can be applied to new or existing processes to identify hazards. Hazardous Material: In a broad sense, any substance or mixture of substances having properties capable of producing adverse effects on the health or safety of human beings. Material presenting dangers beyond the fire problems relating to flash point and boiling point. These dangers may arise from but are not limited to toxicity, reactivity, instability, or corrosivity. Homogeneous Equilibrium Flow: The flow of liquid and vapor phases under conditions where the phases are in equilibrium and flowing at the same velocity. Homogeneous Frozen Flow: The flow of liquid and vapor phases under conditions where the vapor and liquid phases are flowing at the same velocity, and where there is no flashing of the liquid or no condensation of the vapor. Homogeneous Nucleation: An explosive generation of vapor can occur when a cold volatile liquid contacts a warmer liquid. The cold liquid can superheat to such a degree that homogeneous nucleation results in very rapid generation of vapor. This is the phenomenon involved in LNGwater, hot oil-water, and molten metal-water explosions. Also referred to as rapid phase transition. Huddling Chamber: The annular pressure chamber located beyond the valve seat for the purposes of generating a popping characteristic. (Ref. 1) Human Error: Any human action (or lack thereof) that exceed some limit of acceptability (i.e., an out-of-tolerance action) where the limits of human performance are defined by the system. Includes actions by designers, operators, or managers that may contribute to or result in accidents. Human Factors: A discipline concerned with designing machines, operations, and work environments so that they match human capabilities, limitations, and needs. Includes any technical work (engineering, procedure writing, worker training, worker selection, etc.) related to the human factor in operator-machine systems. Inert Gas: A noncombustible, nonreactive gas that renders the combustible material in a system incapable of supporting combustion. Inherently Safe: A system is inherently safe if it remains in a nonhazardous situation after the occurrence of nonacceptable deviations from normal operating conditions. Inlet Size: The nominal pipe size of the inlet of a pressure relief valve, unless otherwise designated. (Ref. 1) Interlock System:A system that detects out-of-limitsor abnormal conditions or improper sequences and either halts further action or starts corrective action.
Intrinsically Safe: Equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture or hazardous layer. Isenthalpic Expansion: A reduction in pressure that occurs with no change in enthalpy of the system, which implies negligible change in kinetic energy effects, and no work or exchange of heat with the surroundings. This condition is approached in throttling processes or flow through piping, where friction effects usually occur, but where heat transfer with the surroundings and changes in kinetic energy effects can be neglected. Isentropic Expansion: A pressure reduction that occurs reversibly, with no friction, work, or exchange of heat with the surroundings. This condition is approached in flow of compressible gases through a smooth, well-rounded nozzle. Isothermal: A system condition in which the temperature remains constant. This implies that temperature increases and decreases are compensated by sufficient heat exchange with the environment of the system.
Lift: The actual travel of the disk away from closed position when a valve is relieving. (Ref. 1)
Likelihood: A measure of the expected frequency with which an event occurs. This may be expressed as a 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). Liquid Trim Valve: A safety relief valve designed for incompressible fluid service, typically with adjustable blowdown. The valve can be certified for use with compressible fluids. Lower Explosive Limit (LEL) or Lower Flammable Limit (LFL):The lowest concentration of a vapor or gas (the lowest 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 Upper Explosive Limit or Upper Flammable Limit. Importance: At concentration lower than the LEIJLFL, the mixture is too “lean” to burn. Maximum Allowable Concentration: The maximum concentration in air of a material that may be toxic or hazardous to health that the facility and the public authorities having jurisdiction are willing to tolerate at a populated downwind location in the event of a major accident. Maximum Allowable Venting (or Relieving) Pressure: The pressure corresponding to the Code allowable pressure rise (accumulation) over the maximum allowable working pressure (MAW) under relieving conditions.
Maximum Allowable Working Pressure: The maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature. The pressure is based on calculations for each element in a vessel using nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure. The maximum allowable working pressure is the basis for the pressure setting of the pressure relief devices that protect the vessel. (Ref. 3) Note: In general, M A W is the basis for the maximum permissible settings of the relief devices (may be set lower). Minimum Required Relief Capacity:Relieving capacity required t o prevent the pressure in the protected vessel from exceeding the maximum allowable working pressure by more than allowed by the ASME BPVC or other applicable codes or practices.. Mitigation: Lessening the risk of an accident event sequence by acting o n the source in a preventive way by reducing the likelihood of occurrence of the event, or in a protective way by reducing the magnitude of the event and/or the exposure of local persons or property. Nameplate Capacity: See Certified Capacity. Net Flow Area: The area which determines the flow after a nonreclosing pressure relief device has operated. The (minimum) net flow area of a rupture disk is the calculated net area after a complete burst of the disk, with appropriate allowance for any structural members which may reduce the net flow area through the rupture disk device. (Ref. 1) Net Positive Suction Head (NPSH): Equivalent total head of liquid at the pump center line, less the vapor pressure, required to avoid cavitation in a pump.
Noncredible Event: A scenario or event that has very low and unreasonable likelihood of occurrence, and need not be considered in selecting the design basis for an emergency relief system. This should be based o n a risk analysis that includes a careful and thorough review of process characteristics, experience with similar systems, hazardous nature of the materials handled, and consequences of an incident. Nonreclosing Pressure Relief Device: A pressure relief device designed t o remain open after operation. A manual resetting means may be provided. (Ref. 1) Nozzle: A pressure containing element of a pressure reliefvalve which constitutes the smooth inlet flow passage and includes the fured portion of the seat closure. The downstream end of the nozzle is often termed the valve “orifice” or nozzle “throat.” (Ref. 1) Note: Also can refer to the inlet or outlet connection on a vessel.
Opening Pressure: The value of increasing inlet static pressure of a pressure relief valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling, or hearing. (Ref. 1) Operating Margin: The difference between the highest pressure attained in normal operation and the set pressure, expressed either in pressure units or as a percentage of the set pressure. Note that the Code term is “pressure differential”, Appendix M-7 [ASME VIII]. Outlet Size: The nominal pipe size of the outlet of a pressure relief valve, unless otherwise designated. (Ref. 1) Overpressure:A pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure. (Ref. 1) Phi-factor: A correction factor which is based on the ratio of the total heat capacity of a vessel (mvesCp.ves) and the total heat capacity of the vessel contents (mC,). The phi-factor [ = 1 (m,,C,,,&zC,,)] approaches the value of one for large vessels, for extremely light vessels, or at genuine adiabatic conditions. Pilot Operated Safety Relief Valve: A pressure relief valve in which the major relieving device is combined with and is controlled by a selfactuating auxiliary pressure relief valve. (Ref. 1) Popping Pressure: The pressure at which a safety (relief) valve passing compressible fluid “pops” from partially open to full open with no further increase in the inlet pressure. Pressure Relief Valve: A pressure relief device which is designed to reclose and prevent the further flow of fluid after normal conditions have been restored. (Ref. 1) Note: A typical pressure relief valve is shown in Figure 2.4-1, along with a legend to identify the various valve components. Process Hazard Analysis: An organized effort to identify and evaluate hazards associated with chemical processes and operations to enable their control. This review normally involves the use of qualitative techniques to identify and assess the significance of hazards. Conclusions and appropriate recommendations are developed. Occasionally, quantitative methods are used to help prioritized risk reduction. Process Safety: A discipline that focuses on the prevention of fires, explosions, and accidental chemical releases at chemical process facilities. Excludes classic worker health and safety issues involving working surfaces, ladders, protective equipment, etc. Process Safety Management:A program or activity involving the application of management principles and analytical techniques to ensure the safety of chemical process facilities. Sometimes called process hazard management. Purge Gas: A gas that is continuously or intermittently added to a system to render the atmosphere nonignitable. The purge gas may be inert or combustible.
Quantitative Risk Assessment: The systematic development of numerical estimates of the expected frequency and/or consequence of potential accidents associated with a facility or operation based on engineering evaluation and mathematical techniques. Quench Pool: A vessel containing liquid and a sparger for quenching an effluent stream. Commonly used for cooling or condensing vapor or vapor-liquid mixtures, or for reacting effluent with a neutralizing agent, or for absorbing hamrdous components from an effluent. Quenching: Rapid cooling from an elevated temperature, for example, severe cooling of the reaction system in a short time (almost instantaneously), “freezes” the status of a reaction and prevents further decomposition. Rain Out: When a superheated liquid is released to the atmosphere, a fraction of it will flash into vapor. Another fraction may remain suspended as an aerosol. The remaining liquid, as well as portions of aerosol, may “rain out” on the ground. Rapid Phase Transition: An explosive generation of vapor that can occur when a cold volatile liquid contacts a warmer liquid. The cold liquid can superheat to such a degree that homogeneous nucleation can occur resulting in very rapid generation of vapor. This is the phenomenon involved in LNG-water, hot oil-water, and molten-metal-water explosions. Also referred to as homogenous nucleation. Redundancy: The employment of several devices, each performing the same function, in order to improve the reliability of a particular function. Reliability: The probability that an item is able to perform a required function under stated conditions for a stated period of time or for a stated demand. Relief Valve: A pressure relief valve actuated by inlet static pressure having a gradual lift dependent on the increase in pressure over the opening pressure. I t may be provided with an enclosed spring housing suitable for closed discharge system application and is primarily used for liquid service. (Ref. 1) Relieving Capacity:The flow rate through a pressure relieving system calculated for a designated temperature, set pressure and overpressure, using a flow area and coefficient of discharge (or resistance factors for rupture disk device systems) determined in certification tests or otherwise specified in ASME BPVC. For pressure relief devices covered under the ASME BPVC but normally not capacity-certified, and for low pressure relief devices not covered by the ASME BPVC, the relieving capacity is based on the manufacturer’s calibration data. Relieving Pressure: Relieving pressure is the set pressure plus overpressure. (Ref. 1)
Remote-Sensing Pressure Relief Valve: A pilot operated safety relief valve in which the pressure signal is obtained remotely from the valve body (e.g., from the vessel being protected). Risk Analysis: The development of a quantitative estimate of risk based on engineering evaluation and mathematical techniques for combining estimates of incident consequences and frequencies. Risk Assessment: The process by which the results of a risk analysis (i.e., risk estimates) are used to make decisions, either through relative ranking of risk reduction strategies or through comparison with risk targets. Risk Evaluation: The assessment of risk, coupled with an appraisal of the significance of the results, both overall and from individual events. Runaway Reaction: A thermally unstable chemical reaction system which shows an accelerating increase of temperature and reaction rate. The runaway reaction can finally result in vessel failure. Rupture Disk Device: A nonreclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disk. Also called a frangible disk device or a bursting disk device. (Ref. 1) A nonreclosing differential pressure relief device actuated by inlet static pressure and designed to function by bursting the pressurecontaining rupture disk. A rupture disk device includes a rupture disk and a rupture disk holder. (Ref. 3) Safety Layer: A system or subsystem that is considered adequate to protect against a specific hazard. The safety layer is totally independent of any other protective layers cannot be compromised by the failure of another safety layer must have acceptable reliability must be approved according to company policy and procedures must meet proper equipment classification may be a noncontrol alternative (i.e., chemical, mechanical) may require diverse hardware and software packages may be an administrative procedure Safety Relief Valve: A safety relief valve is a pressure relief valve characterized by a rapid opening pop action or by opening generally proportional to the increase in pressure over the opening pressure. It may be used for either compressible or incompressible fluids, depending in design, adjustments or application. Safety System: Equipment and/or procedures designed to respond to an accident event sequence by preventing accident propagation, thereby preventing the accident and its consequences. Safety Valve:A pressure reliefvalve actuated by inlet static pressure and characterized by rapid opening or pop action. It is normally used for steam
and air services. (Ref. 1) Note; A distinction is made between “low lift” safety valves (discharge area is determined by position of disk) or “full lift ”safetyvalves (discharge area is not determined by position of disk). Used also for gases and vapors in process services. Seat: The pressure containing contact between the fmed and moving portions of the pressure containing elements of a valve. (Ref. 1) Secondary Pressure: Secondary pressure in a safety, safety relief, or relief valve is the pressure existing in the passage between the actual discharge area and the valve outlet. (Ref. 1) Set Pressure: There are several definitions in use by various organizations as indicated below. When the term is used in this book, the following meaning is intended: The value of increasing inlet static pressure at which a pressure relief device displays one of the operational characteristics as defined by opening pressure, popping pressure, start-to-leak pressure, burst pressure, o r breaking pressure under service conditions. (The applicable operating characteristic for ;I specific device design is specified by the manufacturer.) Set pressure is usually expressed as the gauge pressure at which the device opens under service conditions. Popping pressure pertains only to safety o r safety relief valves in compressible fluid service; start-toleak pertains only to a safety relief valve of the resilient disk type (soft seat).
The inlet gauge pressure at which the pressure relief valve is set to open under service conditions. (Ref. 2 and 3) Shear Pin Device: A nonreclosing pressure relief device actuated by inlet static pressure and designed to function by the shearing of a load carrying pin which supports a pressure containing member. (Ref. 1) Sonic Flow:A condition that occurs when flow of gashapor through a pressure relief device, piping or other equipment does not respond to a decrease in downstream or back pressure on the device. This usually occurs when the fluid velocity in the device equals the velocity of sound. The mass flow rate then depends only on upstream conditions. Specified Burst Pressure: The specified burst pressure of a rupture disk device is the value of increasing inlet static pressure, at a specified temperature, at which a rupture disk device is designed to function. (Ref. 1) Stagnation Pressure: Stagnation pressure is the pressure that would be observed if a flowing fluid were brought to rest along an isentropic path. Standard Trim Valve: A safety relief valve designed for compressible fluid service. With adjustable blowdown, the valve can be set up for use with incompressible fluids. Superimposed Back Pressure: The static pressure existing at the outlet of a safety relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources. (Ref. 1)
Temperature-Actuated Pressure Relief Valve: A pressure relief valve which is actuated by external or internal temperature. (Ref. 1) Theoretical Relieving Capacity: The computed capacity expressed in gravimetric or volumetric units of a theoretically perfect nozzle having a minimum cross sectional flow area equal to the actual discharge area of a pressure relief valve or relief area of a nonreclosing pressure relief device. The flow path in a perfect nozzle is conventionally taken as isentropic. (Ref. 1) Toxic Hazard: In the context of these guidelines, a measure of the danger posed t o 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. Turndown: An expression of the range of minimum and maximum flow rates expected under commonly encountered operating conditions. Commonly expressed as a ratio of minimum-to-maximum flow rate. Unconfrned Vapor Cloud Explosion (UCVE): Explosive oxidation of a vapor cloud in a nonconfined space (i.e., not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in densely packed plant areas (pipe lanes, units, etc.) may show accelerations in flame speeds and intensification of blast. Upper Explosive Limit (UEL) or 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 Explosive Limit or Lower Flammable Limit. Importance: At concentrations higher then the UEL, the mixture is too “rich” to burn. Vacuum Relief Valve: A pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is designed to reclose and prevent and prevent further flow of fluid after normal conditions have been restored. (Ref. 1) Vacuum Support: An auxiliary element of a rupture disk device designed to prevent rupture or deformation of the disk d u e to vacuum or reversal of pressure. (Ref. 1) Valve bonnet: The housing around the spring of an enclosed-spring pressure relief valve. Venting: Flow of vessel contents out the vessel. The pressure is reduced by adequate venting, thus avoiding a failure of the vessel by overpressurization. The emergency flow can be one-phase o r multiphase, each ofwhich results in different flow and pressure characteristics. Vessel Neck: A piping connection o n a vessel, also referred to as a “vessel nozzle.’’ Commonly constructed of a short section of pipe welded to the
vessel, and with either a flanged or threaded end for connecting piping o r instrumentation. Worst Case Consequence: A conservative (high) estimate of the consequences of the most severe accident identified. For example, the assumption that the entire contents of a contained volume of toxic material is released to the most vulnerable area in such a way (all at once or continuous) as to have the maximum effect on the public or employees in that area. The contained volume could be chosen as the containers and pipes between shutoff valves or the entire process unit but probably not the entire plant. Worst Credible Incident: Hypothesized incident requiring the largest pressure relief device, that is considered plausible or reasonably believable.