Terminology for Pressure Relief Devices

A. General

A.1 Pressure Relief Devices

A pressure relief device is a device designed to prevent internal fluid pressure from rising above a predetermined maximum pressure in a pressure vessel exposed to emergency or abnormal conditions.

A.2 Flow Capacity Testing

Testing of a pressure relief device to determine its operating characteristics including measured relieving capacity.

A.3 In-Service Testing

Testing of a pressure relief device while protecting the system on which it is installed to determine some or all of its operating characteristics using system pressure solely or in conjunction with an auxiliary lift device or other pressure source.

A.4 Bench Testing

Testing of a pressure relief device on a pressurized system to determine set pressure and seat tightness.

B. Types of Devices

B.1 Reclosing Pressure Relief Devices

(a) Pressure Relief ValveA pressure relief valve is a spring loaded pressure relief device which is designed to open to relieve excess pressure and to reclose and prevent the further flow of fluid after normal conditions have been restored. It is characterized by rapid opening pop action or by opening generally proportional to the increase in pressure over the open-ing pressure. It may be used for either compressible or incompressible fluids, depending on design, adjustment or application.

(b) Safety Valve. A safety valve is a pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop action. (It is normally used for steam and air services.)

(1) Low-Lift Safety Valve. A low-lift safety valve is a safety valve in which the disc lifts automatically such that the actual discharge area is determined by the position of the disc.

(2) Full-Lift Safety Valve. A full-lift safety valve is a safety valve in which the disc lifts automatically such that the actual discharge area is not determined by the position of the disc.

(c) Relief Valve. A relief valve is a pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure. It may be provided with an enclosed spring housing suitable for closed discharge system application and is primarily used for liquid service.

(d) Safety Relief ValveA safety relief valve is a pressure relief valve characterized by rapid opening or pop action or by opening in proportion to the increase in pressure over the opening pressure, depending on the application and may be used either for liquid or compressible fluid.

(1) Conventional Safety Relief Valve. A conventional safety relief valve is a pressure relief valve which has its spring housing vented to the discharge side of the valve. The operational characteristics (opening pressure, closing pressure and relieving capacity) are directly affected by changes of the back pressure on the valve.

(2) Balanced Safety Relief Valve. A balanced safety relief valve is a pressure relief valve which incorporates means of minimizing the effect of back pressure on the operational characteristics (opening pressure, closing pressure and relieving capacity).

(e) Pilot-Operated Pressure Relief ValveA pilot-operated pressure relief valve is a pressure relief valve in which the major relieving device is combined with and is controlled by a self-actuated auxiliary pressure relief valve.

(f) Power-Actuated Pressure Relief Valve. A power-actuated pressure relief valve is a pressure relief valve in which the major relieving device is combined with and controlled by a device requiring an external source of energy.

(g) Temperature-Actuated Pressure Relief Valve. A temperature-actuated pressure relief valve is a pressure relief valve which may be actuated by external or internal temperature or by pressure on the inlet side.

(h) Vacuum Relief Valve. A vacuum relief valve is a pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is designed to reclose and prevent further flow of fluid after normal conditions have been restored.

B.2 Non-Reclosing Pressure Relief DevicesA non-reclosing pressure relief device is a pressure relief device designed to remain open after operation. A manual resetting means may be provided.

(a) Rupture Disc DeviceA rupture disc device is a non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disc.

(b) Breaking Pin Device. A breaking pin device is a non-reclosing 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.

C. Parts of Pressure Relief Devices

Approach channel – the passage through which the fluid must pass to reach the operating parts of a pressure relief device

Breaking pin – the load-carrying element of a breaking pin device

Breaking pin housing – the structure which encloses the breaking pin mechanism

Discharge channel – the passage through which the fluid must pass between the operating parts of a pressure relief device and its outlet

Disc – the pressure containing movable element of a pressure relief valve which effects closure

Huddling chamber – the annular pressure chamber located beyond the valve seat for the purpose of generating a popping characteristic

Lifting device – a device for manually opening a pres-sure relief valve by the application of external force to lessen the spring loading which holds the valve closed

Lifting lever – see lifting device

Nozzle – a pressure containing element which constitutes the inlet flow passage and includes the fixed portion of the seat closure

Pilot valve – an auxiliary valve which actuates a major relieving device

Pressure containing member (of a pressure relief device) – a part which is in actual contact with the pressure media in the protected vessel

Pressure retaining member (of a pressure relief device) – a part which is stressed due to its function in holding one or more pressure containing members in position

Rupture disc  the pressure containing and pressure sensitive element of a rupture disc device

Rupture disc holder – the structure which encloses and clamps the rupture disc in position

Seat – the pressure containing contact between the fixed and moving portions of the pressure containing elements of a valve

Vacuum support – an auxiliary element of a rupture disc device designed to prevent rupture or deformation of the disc due to vacuum or back pressure

D. Pressure Relief Valve Dimensional Characteristics

Actual discharge area – the measured minimum net area which determines the flow through a valve.

Bore area – the minimum cross-sectional flow area of a nozzle

Bore diameter – the minimum diameter of a nozzle

Curtain area – the area of the cylindrical or conical discharge opening between the seating surfaces created by the lift of the disc above the seat

Developed lift – the actual travel of the disc from closed position to the position reached when the valve is at flow-rating pressure

Discharge area – see actual discharge area

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

Inlet size – the nominal pipe size of the inlet of a pressure relief valve, unless otherwise designated

Lift – the actual travel of the disc away from closed position when a valve is relieving

Nozzle area, nozzle throat area – see bore area

Nozzle diameter – see bore diameter

Orifice area – see effective discharge area

Outlet size – the nominal pipe size of the outlet of a pressure relief valve, unless otherwise designated

Rated lift – the design lift at which a valve attains its rated relieving capacity

Seat angle – the angle between the axis of a valve and the seating surface. A flat-seated valve has a seat angle of 90 degrees.

Seat area – the area determined by the seat diameter

Seat diameter – the smallest diameter of contact between the fixed and moving portions of the pressure containing elements of a valve

Seat flow area – see curtain area

Throat area – see bore area

Throat diamete– see bore diameter

E. Operational Characteristics of Pressure Relief Devices

Back pressure – the static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system

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

Blowdown pressure – the value of decreasing inlet static pressure at which no further discharge is detected at the outlet of a pressure relief valve after the valve has been subjected to a pressure equal to or above the popping pressure

Breaking pressure – the value of inlet static pressure at which a breaking pin or shear pin device functions

Built-up back pressure – pressure existing at the outlet of a pressure relief device caused by the flow through that particular device into a discharge system

Burst pressure – the value of inlet static pressure at which a rupture disc device functions

Chatter – abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc contacts the seat

Closing pressure – the value of decreasing inlet static pressure at which the valve disc reestablishes contact with the seat or at which lift becomes zero

Coefficient of discharge – the ratio of the measured relieving capacity to the theoretical relieving capacity

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 superimposed back pressure and/or temperature.

Constant back pressure – a superimposed back pressure which is constant with time

Cracking pressure – see opening pressure

Flow capacity – see measured relieving capacity

Flow-rating pressure – the inlet static pressure at which the relieving capacity of a pressure relief device is measured

Flutter – abnormal, rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc does not contact the seat

Leak pressure – see start-to-leak pressure

Leak test pressure – the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure

Marked breaking pressure – the value of pressure marked on a breaking pin device or its nameplate

Marked burst pressure – the value of pressure marked on the rupture disc device or its nameplate or on the tag of the rupture disc and indicates the burst pressure at the coincident disc temperature

Marked pressure – the value or values of pressure marked on a pressure relief device

Marked relieving capacity – see rated relieving capacity

Measured relieving capacity – the relieving capacity of a pressure relief device measured at the flow-rating pressure, expressed in gravimetric or volumetric units

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

Start-to-discharge pressure – see opening pressure

Start-to-leak pressure – the value of increasing inlet static pressure at which the first bubble occurs when a pressure relief valve is tested by means of air under a specified water seal on the outlet

Superimposed back pressure – the static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources.

Test pressure – see relieving pressure

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 non-reclosing pressure relief device

Vapor-tight pressure – see resealing pressure

Variable back pressure – a superimposed back pressure that will vary with time

Warn – see simmer