Pressure Temperature Rating (PTRating)
(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. Coincidental maximum pressure and temperature computed for the greatest thickness required, below its respective allowable stresses are selected(interpolated) below the range of predefined piping class of flange for the respective material in the standard.This range of series of coincidental 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 temppressure 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 temperaturepressure with same rating and material. This will not lead to various thickness and difficult to procure handle and maintain inventory. Notes: One Question. Which component material’s allowable stress to be considered for thickness calculation. 1 pipe, 2. Flange, 3. Fitting. Which one??? 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. 

Code  Design Pressure  Design Temperature 

B31.1  The internal design pressure shall be not less than the 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 the same 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) 