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API RP 576 (2017) Terms/Definitions

Terms in this set (29)

3.1 Accumulation
The pressure increase over the maximum allowable working pressure (MAWP) of the vessel or piping system, expressed in pressure units or as a percentage of MAWP or design pressure if a MAWP has not been established. Maximum allowable accumulations are established by applicable codes for emergency, operating, and fire contingencies.
3.2 Backpressure
The pressure that exists at the outlet of a pressure-relief device as a result of the pressure in the discharge system. Backpressure is the sum of the superimposed and built-up backpressures.
3.3 Blowdown
The difference between the set pressure and the closing pressure of a pressure-relief valve, expressed as a percentage of the set pressure or in pressure units.
3.4 Built-up Backpressure
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.
3.5 Burst Pressure
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 gauge pressure.
3.6 Burst Pressure Tolerance
The variation around the marked burst pressure at the specified disk temperature in which a rupture disk will burst.
3.7 Car Seal
A device installed on a valve to secure it in a specified position (open or closed). When properly installed, the associated valve cannot be operated unless the car seal is physically removed.
3.8 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.
3.9 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 superimposed backpressure or temperature or both.
3.10 Design Pressure
The pressure, together with the design temperature, 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 MAWP in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.
3.11 Galling
A condition whereby excessive friction between high spots results in localized welding with subsequent splitting and a further roughening of rubbing surfaces of one or both of two mating parts.
3.12 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.
3.13 Lift
The actual travel of the disc away from the closed position when a pressure-relief valve is relieving.
3.14 Lifting Lever
A device on the relief valve that applies external force to the stem of the relief valve, which can be used to manually operate the valve.
3.15 Manufacturing Design Range
The pressure range at which the rupture disk shall be marked. Manufacturing design 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.
3.16 Marked Burst Pressure
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 design range unless otherwise specified by the customer. The marked burst pressure is applied to all of the rupture disks of the same lot.
3.17 Maximum Allowable Working Pressure - MAWP
The maximum gauge pressure permissible at the top of a vessel in its 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 MAWP 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 can 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.
3.18 Nonreclosing Pressure-Relief Device
A pressure-relief device that remains open after operation. A manual resetting means may be provided.
3.19 Opening Pressure
The value of increasing inlet static pressure whereby there is a measurable lift of the disc or at which discharge of the fluid becomes continuous, as determined by seeing, feeling, or hearing.
3.20 Overpressure
The pressure increase over the set pressure of the relieving device. Overpressure is expressed in pressure units or as a percentage of set pressure. Overpressure is the same as accumulation only when the relieving device is set to open at the MAWP of the vessel.
3.21 Pin-Actuated Device
A nonreclosing pressure-relief device actuated by static pressure and designed to function by buckling or breaking a pin that 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.
3.22 Pop Pressure
The value of increasing inlet static pressure at which the disc moves in the opening direction at a faster rate as compared with corresponding movement at higher or lower pressures.
3.23 Qualified Person
A competent person who has met the knowledge, skill requirements and expectations of the owner-user.
3.24 Set Pressure
The inlet gauge pressure at which a pressure-relief valve is set to open under service conditions.
3.25 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.
3.26 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 design range. The user is cautioned to consider manufacturing design range, superimposed backpressure, and specified temperature when determining a specified burst pressure.
3.27 Superimposed Backpressure
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.
3.28 Telltale Indicator
An assembly installed in the space between a rupture disk and another relief device (in series) to detect or prevent the accumulation of pressure between the rupture disk and the other device.
3.29 Trim
Refers to internal parts of a pressure-relief valve that are manufactured using materials that are resistant to degradation from the associated process. At a minimum, it refers to the nozzle and disc, but may also include other components that are in contact with the process fluids (disc holder, blowdown ring, guide, spindle, etc.) or are required for proper valve performance.
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