Pressure Temperature Rating (PT-Rating)
Among other methods of classification by using pressure temperature ratings is ‘Pression Nominal’ or ‘Pressure Nominal’ or PN number method. This PN number is a rough indicator of pressure rating in bar.
Pressure rating or pound rating of a pipe is dependent on the pipe material and design temperature. The pipe pressure rating for the same material changes at different temperatures. For the same material and constant design pressure, different pressure ratings are applicable over different ranges of design temperatures. As the design temperature increases the pressure rating requirement for the pipe also increases for the same design pressure.
It should be noted that the pound rating for the whole piping system is equal to the pound rating of the weakest part, having the lowest pressure rating in the system. The weakest part may be any piping component or fitting which contains the pressure in the system and has the lowest pound rating due to any possible reasons.
All the piping components are not designated by Pressure class, only flanges and items related to flanges like gaskets(not bolts) are designated as class.Also socket welded components are designated by Pressure class eg 3000, 6000 & 9000 AND SCREWED COMPONENTS Eg 2000, 3000 & 6000 based on the thickness as per ASME 16.11.Rest all components are designated by thickness(either sch or thickness)
So we can broadly divide the piping components in two groups
The basic principle of any design is to make the weakest component strong.A flange joint is the weakest in piping system. (Note: joint is weak not the flange.) this is the reason why we establish rating first for flanges(group-1). And then based on the corresponding pressure and temperature .Allowable stresses are noted down and pipe thickness are computed.
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.
We get design condition from process and base material from metallurgist, we need to choose relevant material group based on base material. The material group in B16.5 is only flange material not pipe/fitting material.
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 at the 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)
There are two methods of calculating pipe thickness.
1. Based on exact design conditions provided by process.
2. Flange rated method.(sometimes called P/S ratio method)
Based on exact design conditions provided by process.
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.
Based on design condition pipe thickness calculations are straight and simple. Put the design pressure and corresponding allowable for temp and calculates as per 31.3. this calculation is normally not used unless governed by economical reasons. Eg exotic/costly materials.
Flange rated method.(sometimes called P/S ratio method)
Flange rated method is to calculate the thickness based temp-pressure combination which will give you the max thickness for the particular rating and material. This is the most conservative approach and most of the time used.We get more then required thickness here.one more important reason to follow this method is to generalize a piping class(spec) for the various design temperature-pressure with same rating and material. This will not lead to various thickness and difficult to procure handle and maintain inventory.
To calculate PT-Rating of any piping component, predefined class(predefined means ,values present in the standard ASME B16.5 were derived theoretically and substantiated by investigative work) of flange is considered because flange joints are considered to be the weakest joint assembly in a piping system. If any failure of piping system should occur unfortunately, then it’s supposed to occur at flange joint assembly.
By knowing PT-Rating or class of a component it’s 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.
Again one more thing we need to take care while selecting the allowable stress based on exact design conditions.
We need to see all the three material’s allowable and select the one which will give you max thickness. And apply this thickness to all the components.
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