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Pressure Temperature Rating (PT-Rating)
Rating of a piping component is designated by class followed by a dimensionless number(for example
Class150 ). This is the very important term used to differentiate piping components in a specifications.
Understanding of pressure temperature rating is very essential and mandatory for the study of piping
engineering.
Pressure temperature ratings are defined on the basis of definitions of design pressure and designtemparature stated in code ASME B31.3
Design Pressure as per code ASME B31.3-2008(Revision of ASME B31.3-2006)states that
as per para 301.2.1
"(a)The design pressure of each component in a piping system shall be not less than the pressure atthe most severe condition of coincident internal or external pressure and temperature (minimum or
maximum) expected during service, except as provided in para. 302.2.4.(b)The most severe condition is that which results in the greatest required component thickness and
the highest component rating.
(refer ASME B31.3 for full definition,points (c) and (d) will complete the definition of code)
Henceforth ,the maximum withstanding pressure of a piping component below its respective
allowable stress depends on temperature and the greatest thickness computed.
Co-incidental maximum pressure and temperature computed for the greatest thickness required,
below its respective allowable stresses are selected(interpolated) below the range of pre-defined
piping class of flange for the respective material in the standard.This range of series of co-
incidental pressure and temperature are tabulated in specification document for every material
sheet for reference.
Notes:
To calculate PT-Rating of any piping component, predefined class(predefined means ,values present in thestandard ASME B16.5 were derived theorotically and substantiated by investigative work) of flange is
considered because flanges are considered to be the weakest component in a piping system. If any
failure of piping system should occur unfortunately, then its supposed to occur at flange.
By knowing PT-Rating or class of a component its impossible to identify its design pressure or
temperature.Hence PT-Rating for a particular material are therefore specified on each specification sheet to
identify the range of co-incidental pressure and temperatures that the particular material and class can
withstand.
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Difference Between Design Pressure & Design Temperature in ASME B31.3 ,B31.3,B31.4,B31.8
ASME Pressure Piping Codes
Code Design Pressure Design Temperature
B31.1
The internal design pressure shall be not less thanthe maximum sustained operating pressure
(MSOP) within the piping system including the
effects of static head.
The piping shall be designed for a metal
temperature representing the maximum
sustained condition expected. The design
temperature shall be assumed to be thesame as the fluid temperature unless
calculations or tests support the use of
other data, in which case the design
temperature shall not be less than the
average of the fluid temperature and the
outside wall temperature.
B31.3
The design pressure of each component in a piping
system shall be not less than the pressure at the
most severe condition of coincident internal or
external pressure and temperature expected
during service, except as provided in par. 302.2.4.
The design temperature of each
component in a piping system is the
temperature at which, under the coincident
pressure, the greatest thickness or highest
component rating is required in accordance
with par. 301.2
B31.4
The piping component at any point in the piping
system shall be designed for an internal design
pressure which shall not be less than the maximum
steady state operating pressure at that point, or
less than the static head pressure at that point
with the line in a static condition. The maximum
steady state operating pressure shall be the sum
of the static head pressure, pressure required to
overcome friction losses and any required back
pressure.
The design temperature is the metal
temperature expected in normal
operation. It is not necessary to vary the
design stress for metal temperatures
between 20 F and 250 F.
B31.8
Design pressure is the maximum operating
pressure permitted by the Code, as determined by
the design procedures applicable to the materials
and locations involved.
No design temperature. The Code
mentions only ambient temperature and
ground temperature. (1975)
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