T = minimum thickness of tubes, that is, nominal thickness less the permissible negative tolerance in mm. (inch)
C = 0.75 mm for working pressure up to and including 70 kg/cm².

or

C = 0 for working pressure exceeding 70 kg/cm² (1000 lbs/sq. in.)
W. P.= Working pressure of boiler in kg/cm² (lbs/sq. inch)
D = External diameter of tube in mm (inch)
F = permissible stress for the material at the working metal temperature in kg/cm² (lbs/sq. Inch) to be determined on the basis given below:-

The working metal temperature shall be taken as :-

(a) For integral economiser tubes, the maximum water temperature for which the part of the element is designed plus 11°C (20°F).

(b) For furnace and boiler tubes, the saturation temperature corresponding to the working pressure plus 28°C (50^o F)

(c) For convection superheater tubes, the maximum steam temperature for which the part of the element is designed plus 39^oC (70^oF).

(d) For radiant superheater tubes the designed maximum steam temperature plus 50 °C (90^oF)

Permissible working stress for tubes:- For temperatures at or below 454 °C,

For temperature above 454 °C,

where,

T.S. = Minimum tensile strength of the material at room temperature.
Et = Yield point (0.2% proof stress) at working metal temperature 't'.
Sr = the average stress to produce rupture in 100,000 hours and in no case more than 1.33 times the lowest stress to produce rupture at the working metal temperature.
Sc = the average stress to produce an elongation of 1% (creep) in 100,000 hours, at the working metal temp.

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 material the allowable stress may be taken as the lower of

In no case, however, shall the thickness of tubes as supplied be less than those given in the table below (subject to tolerances specified in Regulations 36(d), 43(d), 48(c), 53(c), 56A (IV) and 58(d).

TABLE

In the case of coiled tubes for boilers of the forced flow or forced circulation type, the minimum thickness of tubes as supplied may, however, be as follows:-

(i) General: Tubes that are hot or cold bent for parts of boilers, including economizers, furnace walls, superheaters and reheaters, shall comply with this clause and suitably heat treated. 

Butt welds shall not be permitted within bends.

Thinning and departure from circularity limits shall be demonstrated by one of the following methods. 

(a) relevant and satisfactory service experience;

(b) a procedure test;.

(c) By measurement of 2% of the bends, including the first bend of each shift.

The method selected shall be at the option of the manufacturer.

(ii) Thinning on tube bends.  Thinning on tube bends shall comply with the following.

(a) At any location around the bend extrados, the reduction in thickness (in percent) below the calculated minimum permissible design thickness of the straight tube, except where permitted by (b), shall not exceed:

          __100___

             4R + 2                                    

              D

where

R is the mean radius of the bend to the centre line of the tube (in mm).

D is the ordered outside diameter of the tube (in mm).

               (b) Bend thickness below the minimum value required under (a) shall be permitted in cases where the manufacturer can demonstrate by bursting tests carried out on at least three bends that the strength of the bend is not less than that of the straight tube.

               (c) Where the amount of thinning at any location around the bend extrados on cold formed bends exceeds 25% of the actual thickness of the tube on the straight the bends shall be suitably heat treated.  The actual thickness of the same plans as the line of the extrados at each and of the bend.

(iii)  Departure from circularity in tube bends.  The departure from circularity at the bend apex shall not exceed 10% for bends performed in a single bending operation and 15% for bends which are hot pressed after the primary, bending operation.

The departure from circularity (in percent) shall be calculated from:

            D_max - D_min  ´ 100

                   D

where

D_max is the maximum outside diameter measured in the tube bend apex (mm),

D_min is the minimum outside diameter measured at the same cross section as D_max(mm)