1. Scope
1.1 This Practice covers the selection and design requirements for pressure relief and vapor depressuring
systems. It also provides guidance for alternative overpressure protection including High Integrity Process
Shutdown (HIPS) systems.. Systems which discharge vapor to the atmosphere and liquid blowdown
systems are outside the scope of this standard and are covered in SES S04-E01, Atmospheric Relief. This
standard does not cover systems for relieving explosion overpressure from buildings and other enclosures.
Disposal of toxic materials may require special design considerations, and therefore are outside the scope
of this standard. Guidance shall be sought from a specialist for these situations.
1.2 Air quality control is not included, as criteria shall be established for each installation or new project in
conformance with applicable environmental requirements.
2. References
American Petroleum Institute (API)
RP 520 PT I Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries Part I – Sizing
and Selection Sixth Edition; Errata – 1994
RP 520 PT II Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries
Part I – Installation Fourth Edition
RP 521 Guide for Pressure-Relieving and Depressuring Systems Fourth Edition
STD 526 Flanged Steel Pressure Relief Valves Fourth Edition
STD 527 Seat Tightness of Pressure Relief Valves Third Edition (R 1996)
STD 620 Design and Construction of Large, Welded, Low-Pressure Storage Tanks Ninth Edition;
Addendum 1-1996; Additional Pages for Addendum 1-1997
STD 2000 Venting Atmospheric and Low-Pressure Storage Tanks Nonrefrigerated and Refrigerated
Fourth Edition
American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE)
Standard 15 Safety Code for Mechanical Refrigeration (ANSI approved)
American Society of Mechanical Engineers(ASME)
SEC VIII D1 AB PVC SECTION VIII Rules for Construction of Pressure Vessels DIVISION 1 –
SUBSECTION A – General Requirements (Boiler and Pressure Vessel Codes)
SEC VIII D1 BB PVC SECTION VIII Rules for Construction of Pressure Vessels DIVISION 1 –
SUBSECTION B – Requirements Pertaining to Methods of Fabrication of Pressure Vessels (Boiler and
Pressure Vessel Codes)
B31.3 Process Piping
B31.4 Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia, and
Alcohols
National Association Corrosion Engineers International (NACE)
MR0175. Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment
National Fire Protection Association (NFPA)
30 Flammable and Combustible Liquids Code
68 Guide for Venting of Deflagrations (1988)
HCIS
Royal Commission Environmental Regulations (RCER)
3. Definitions
Accumulation. The pressure increase over the maximum allowable working pressure of the vessel
allowed during discharge through the pressure relief device, expressed in pressure units or as a
percentage of MAWP or design pressure. Maximum allowable accumulations are established by applicable
codes for emergency operating and fire contingencies.
Actual Discharge Area. The minimum net area that determines the flow through a valve.
Atmospheric Relief Vent. A vertical pipe on the outlet of a pressure relief valve, used for atmospheric
discharge of vapor or gas to a safe location.
Back Pressure. The pressure that exists at the outlet of a pressure relief device as a result of the
pressure in the discharge system. It is the sum of the superimposed and built-up back pressure.
Balanced Pressure Relief Valve. A spring loaded pressure relief valve that incorporates a bellows or
other means for minimizing the effect of back pressure on the operational characteristics of the valve.
Blowdown. The difference between the set pressure ant the closing pressure of a pressure relief valve,
expressed as a percentage of the set pressure or in pressure units.
Bore Area. The minimum cross-sectional flow area of a nozzle. Also referred to as nozzle area, nozzle
throat area and throat area.
Built Up Back Pressure. The increase in pressure at the outlet of a pressure relief device that develops
as a result of flow after the pressure relief device opens.
Burst Pressure. The pressure rating of a rupture disk at the specified temperature is the value of the
upstream static pressure minus the value of the downstream static pressure just prior to when the disk
bursts. When the downstream pressure is atmospheric, the burst pressure is the upstream static gage
pressure.
Burst Pressure Tolerance. The variation around the marked burst pressure at the specified disk
temperature in which a rupture disk shall burst.
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 as determined by seeing, feeling or hearing.
Coefficient of Discharge. The ratio of the mass flow rate in a valve to that of an ideal nozzle. It is used for
calculating flow through a pressure relief device.
a. The effective coefficient of discharge is a nominal value used with an effective discharge area to
calculate the minimum required relieving capacity of a pressure relief valve. This capacity is
determined in accordance with the applicable code or regulation and is used per the preliminary
sizing equations given in this Recommended Practice.
b. The rated coefficient of discharge is determined in accordance with the applicable code or
regulation and is used with the actual discharge area to calculate the rated flow capacity of a
pressure relief valve
Cold Differential Test Pressure (CDTP). The pressure at which a pressure relief valve is adjusted to
open on the test stand. The cold differential test pressure includes corrections for the service conditions of
back pressure or temperature or both.
Conventional Pressure Relief Valve. A spring loaded pressure relief valve whose operational
characteristics are directly affected by changes in the back pressure.
Curtain Area. The area of the cylindrical or conical discharge opening between the seating surfaces
above the nozzle seat created by the lift of the disc.
Design Pressure. The pressure rating of the vessel along with the design temperature that is used to
determine the minimum permissible thickness or physical characteristic of each vessel component as
determined by the vessel design rules. The design pressure is selected by the user to provide a suitable
margin above the most severe pressure expected during normal operation at a coincident temperature. It
is the pressure specified on the purchase order. This pressure may be used in place of the maximum
allowable working pressure in all cases where the MAWP has not been established. The design pressure
is equal to or less than the MAWP
Effective Discharge Area. A nominal or computed area used with an effective discharge coefficient to
calculate the minimum required relieving capacity for a pressure relief valve per the preliminary sizing
equations contained in this practice. API Standard 526 provides effective discharge areas for a range of
sizes in terms of letter designations, “D” through `T’.
Huddling Chamber. An annular chamber located downstream of the seat of a pressure relief valve for the
purpose of assisting the valve to achieve lift.
Inlet Size. The nominal pipe size (NPS) of the valve at the inlet connection, unless otherwise designated.
Isokinetic Sampling. A term describing a condition of sampling, in which the flow of gas into the sampling
device (at the opening or face of the inlet) has the same flow rate as and direction, as the ambient
atmosphere being sampled.
Knock Out Drum. An onsite liquid knockout (KO) drum is a horizontal or vertical drum that retains the
unflashed liquid that is discharged from a unit liquid header. The drum discharges vapor to the closed
pressure relief vapor system.
Leak-Test Pressure. The specified inlet static pressure at which a seat leak test is performed.
Lift. The actual travel of the disc from the closed position when a valve is relieving.
Lot of Rupture Disks. Those disks manufactured at the same time and of the same size, material,
thickness, type, heat and manufacturing process, including the heat treatment.
Manufacturing Range. The pressure range in which the rupture disk shall be marked. Manufacturing
ranges are usually catalogued by the manufacturer as a percentage of the specified burst pressure.
Catalogued manufacturing ranges may be modified by agreement between the user and the manufacturer.
Marked Burst Pressure. Or rate burst pressure of a rupture disk, is the burst pressure established by tests
for the specified temperature and marked on the disk tag by the manufacturer. The marked burst pressure
may be any pressure within the manufacturing range unless otherwise specified by the customer. The
marked burst pressure is applied to all of the rupture disks of the same lot.
Maximum Allowable Working Pressure (MAWP). The maximum gauge pressure permissible at the top
of a completed vessel in its normal operating position at the designated coincident temperature specified
for that pressure. The pressure is the least of the values for the internal or external pressure as determined
by the vessel design rules for each element of the vessel using actual 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.
The MAWP is normally greater than the design pressure but must be equal to the design pressure when
the design rules are used only to calculate the minimum thickness for each element and calculations are
not made to determine the value of the MAWP
Maximum Operating Pressure. The maximum pressure expected during normal system operation.
Minimum Net Flow Area. The calculated net area after a complete burst of a rupture disc with appropriate
allowance for any structural members which may reduce the net flow area through the rupture disk device.
The net flow area for sizing purposes shall not exceed the nominal pipe size area of the rupture disk
device.
Non-Reclosing Pressure Relief Device. A pressure relief device which remains open after operation. A
manual resetting means may be provided.
Opening Pressure. The value of increasing inlet static pressure at which there is a measurable lift of the
disc or at which discharge of the fluid becomes continuous, as determined by seeing, feeling or hearing.
Operating Ratio. For a pressure relief valve it is the ratio of maximum system operating pressure to the set
pressure.
Operating Ratio. For a rupture disk it is the ratio of maximum system operating pressure to a pressure
associated with a rupture disk as follows:
a. For marked burst pressure above 40 psi: The operating ratio is the ratio of maximum system
operating pressure to the disk marked burst pressure.
b. For marked burst pressure of 40 psi and below: The operating ratio is the ratio of maximum
system operating pressure to the marked burst pressure minus 2 psi.
Outlet Size. The nominal pipe size (NPS) of the valve at the discharge connection, unless otherwise
designated.
Overpressure. The pressure increase over the set pressure of the relieving device allowed to achieve
rated flow. Overpressure is expressed in pressure units or as a percentage of set pressure. It is the same as accumulation only when the relieving device is set to open at the maximum allowable working pressure
of the vessel.
Pilot Operated Pressure Relief Valve. A pressure relief valve in which the major relieving device or main
valve is combined with and controlled by a self actuated auxiliary pressure relief valve (pilot).
Pin-Actuated Device. A non-reclosing pressure relief device actuated by static pressure and designed to
function by buckling or breaking a pin which holds a piston or a plug in place. Upon buckling or breaking of
the pin, the piston or plug instantly moves to the full open position.
Pressure Relief Device. Actuated by inlet static pressure and designed to open during emergency or
abnormal conditions to prevent a rise of internal fluid pressure in excess of a specified design value. The
device also may be designed to prevent excessive internal vacuum. The device may be a pressure relief
valve, a non-reclosing pressure relief device, or a vacuum relief valve.
Pressure Relief Valve. A pressure relief device designed to open and relieve excess pressure and to
reclose and prevent the further flow of fluid after normal conditions have been restored.
Rated Relieving Capacity. The relieving capacity used as the basis for the application of a pressure relief
device code or regulation and is provided by the manufacturer. Note: The capacity marked on the device is
the rated capacity on steam, air, gas or water as required by the applicable code.
Relief Valve. A spring loaded pressure relief valve actuated by the static pressure upstream of the valve.
The valve opens normally in proportion to the pressure increase over the opening pressure. A relief valve
is used primarily with incompressible fluids.
Relieving Conditions. The inlet pressure and temperature on a pressure relief device during and
overpressure condition. The relieving pressure is equal to the valve set pressure (or rupture disk burst
pressure) plus the overpressure (The temperature of the flowing fluid at relieving conditions may be higher
or lower than the operating temperature.)
Rupture Disk Device. A non-reclosing pressure relief device actuated by static differential pressure
between the inlet and outlet of the device and designed to function by the bursting of a rupture disk. A
rupture disk device includes a rupture disk and a rupture disk holder.
a. A rupture disk is a pressure containing, pressure and temperature sensitive element of a rupture
disk device.
b. A rupture disk holder is the structure which encloses and clamps the rupture disk in position.
(Some disks are designed to be installed between standard flanges without holders.)
c. A nonfragmenting rupture disk is a rupture disk designed and manufactured to be installed
upstream of other piping components, for example, pressure relief valves, and will not impair the
function of those components when the disk ruptures.
Safety Relief Valve. A spring loaded pressure relief valve that may be used as either a safety or relief
valve depending on the application.
Safety Valve. A spring loaded pressure relief valve actuated by the static pressure upstream of the valve
and characterized by rapid opening or pop action. A safety valve is normally used with compressible fluids.
Set Pressure. The inlet gage pressure at which the pressure relief device is set to open under service
conditions.
Simmer. The audible or visible escape of compressible fluid between the seat and disc which may occur
at an inlet static pressure below the set pressure prior to opening.
Specified Burst Pressure. The burst pressure specified by the user. The marked burst pressure may be
greater than or less than the specified burst pressure but shall be within the manufacturing range. The
user is cautioned to consider manufacturing range, superimposed back pressure and specified
temperature when determining a specified bust pressure.
Specified Disk Temperature. The temperature of the rupture disk when the disk is expected to burst. It is
the temperature the manufacturer uses to establish the marked burst pressure. The specified disk temperature is rarely ever the design temperature of the vessel and may not even be the operating
temperature or relief temperature, depending on the relief system configuration.
Superimposed Back Pressure. The static pressure that exists 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 coming from
other sources and may be constant or variable.
Two-Phase Flow. As a liquid boils or a vapor- or gas-generating chemical reaction takes place, each new
bubble formed in a vessel occupies volume and, so long as it remains below the liquid surface, displaces
that surface upward. Bubbles rise through the liquid at a velocity that is dependent on relative density
(buoyancy), surface tension, and viscosity of the liquid. If the bubbles below the liquid surface occupy
sufficient volume, the liquid/gas surface can reach the level of the relief device (liquid swell) and two-phase
flow can occur. The onset of two-phase flow is thus exacerbated by high liquid viscosity and tendency to
foam, although nonviscous, non-foamy liquids can swell appreciably at high vapor- or gas-generation
rates. If the inlet of the relief device is located close to the liquid surface during normal operation, a
relatively small amount of liquid swell is required for the liquid/gas surface to reach the inlet. Two-phase
venting, in this case, might occur under less severe upset conditions than would be the case if a greater
vapor space were available to allow for vapor/liquid disengagement.
Vent Stack. A vertical pipe that transfers discharged vapor and/or gas, from a main collection header to a
safe, elevated location for discharge to the atmosphere.