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  Reg. 366

Determination of Working Pressure

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.

 
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