a) 
Shells:– The maximum working pressure of
shells shall be determined by the following formula:
W. P. = 
2 f E (T .03) 
D + T  .03 
where,
T is thickness in inches.
D is maximum internal diameter in inches.
W. P. is Working pressure in lbs./sq. inch.
F is maximum permissible working stress as prescribed
in regulations 271 or 350, whichever is applicable.
E is Efficiency of longitudinal riveted seam as given
in Regulation 177.
is Efficiency factor for fusion welded shells as given
in table below.
is 1.00 for seamless shells or shells made from seamless
tubes.
is Efficiency of ligaments between holes or opening
in shell expressed as a fraction
Class 
Efficiency factor
E 
I. 
0.90 
II. 
0.75 if
welded from both side
0.50 if welded from one side only. 
III. 
0.45 
Minimum thickness of shells shall be as given in table
below:
Classification 
Internal diameter
inches. 
Minimum thickness
inch 
Fusion welded Class I 

1 / 4 
Fusion welded
Class II and Class III, and shells other than fusion
welded shells. 
Upto and
including 24 
1 / 4 
Over 24
and upto & including 36 
5/16 
Over 36 
3/8 
Where steels are intended for service at temperatures
in excess of 700°F this shall be so stated and silicon
contents shall be not less than 0.10 per cent or alternatively,
the material must pass the ‘Proof test for creep
quality of carbon steel plate of boiler quality as in
Appendix D.
Compensation for openings in shells: Where the
major axis or diameter of any hole cut in cylindrical
part of the shell is greater than 2 ½ times the
thickness of the shell plate plus 2 ¾ inches,
compensation shall be provided.
The sectional area to be compensated measured in the
plane parallel to the longitudinal axis of the shell,
which makes this area a maximum, shall be the product
of the length of the opening (including any rivet holes
in the plane) and the thickness of a seamless shell
of similar material calculated in accordance with Equation
72 (Regulation 270)
for the same conditions of pressure and temperature.
When frames, pads or branches are secured by rivets,
the compensating area shall be calculated by the method
given in Regulation
170.
Where frames, pads or branches are secured by welding,
the compensating area shall be calculated by the method
given in Regulation
279.
