The maximum permissible stress for drum shells and headers shall be taken as available in governing Boiler codes of the country of the material to which it belongs.  In case of non-availability of the value, the following procedure for evaluating shall be adopted. 

(i)                  For temperatures at or below 454^o C, the smaller of the two values:-

                Et                  R

f  =         ------      or  ------

               1.5                   2.7

 (ii)              For temperatures above 454^o C, the least of the following three values:-

                                            Et                    SR        

   (a)   -----      (b)       -----     and      (c)   Sc

            1.5                  1.5

where,

                t    =   Working metal temperature,

                R   =  Minimum specified tensile strength of the steel at room temperature

                E   =  Minimum specified Yield point at room temperature

                Et  =  Yield Point (0.2% proof stress) at the temperature 't'.

                Sc = The average stress to produce an elongation of 1% (creep) in 100,000 hours at temperature 't'.

SR =  The average stress to produce rupture in 100,000 hours at the temperature 't' and in no case more than 1.33 times the lowest stress to produce rupture at the temperature.

Note: In case Sc values are not available in Material Standard and such materials are known to have been used in boilers in India or abroad, then for such materials the allowable stress may be taken as the lower of

                                                Et                or                Sr

1.5                                                 1.5

For fusion welded drums, when the wall thickness exceed 60mm (2-3/8"), a deduction of 1 per cent in the value of "f" so determined shall be made for each increase of 5mm (3/16") in the thickness.  Such deduction need not be made provided the minimum specified values of R and E at any part of the section of the steel plate used in the manufacture of the Boiler drum are guaranteed by a "Well known Steel Maker" or certified by an Inspecting Authority.

The working metal temperature shall be taken as:-

(a)                 For saturated steam, water and mud drums, the saturation temperature corresponding to the pressure WP plus 50^oF.

 (b)                For superheated steam the designed maximum steam temperature for that drum plus 50^oF.

Where the drums are adequately protected from the gases of combustion or swept by such gasses in the third or subsequent pass of a boiler, the working metal temperature shall be taken as the saturation or designed maximum steam temperature as defined above, whichever applies.  A covering of refractory or insulating material which may be liable to become dislodged shall not be deemed adequate protection.

NOTE.- Where steels are for service at temperatures in excess of 370^oC (700^o F) it shall be so stated as the silicon content shall be 0.10 per cent minimum or alternatively the material must pass the "Proof Test for Creep Quality of Carbon Steel Plate of Boiler Plate Quality" as in the Appendix D