pressure temperature rating - piping study

7/29/2019 Pressure Temperature Rating - Piping Study

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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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