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Reg. 563, Reg. 564

Classification of Fusion-Welded Boilers

 

The boilers covered by this Chapter shall be classified in accordance with Table XII/3.

TABLE XII/3

Classification of Fusion-Welded Boilers
Classification Limits of application Minimum thickness
Class I No. limit ¼ inch.
Class II When the following limits are not exceed:

(a) Working pressure105 pounds per sq. in.
(b) Working pressure in pounds per sq. in. multiplied by internal diameter in inches = 5250


Boilers up to and including 36 inches internal diameter-5/16 inch.
Class III When the following limits are not exceeded:

(a) Working presure 30 lbs. per sq. in.
(b) Working pressure multiplied by internal diameter in inches = 3000.
Boiler over 36 inches internal diameter-3/8 inch.



Reg. 564

Shells

a)

The working pressure of cylindrical shells for class I boilers shall be determined in accordance with regulations 270 and 271.
b) The working pressure of cylindrical shells for Class II and Class III boilers shall be determined by the following formula:-

WP = (t – 1.5)SC Equation XII/1
0.7 D

Where,

t is minimum thickness of shell plate in mm.
WP is working pressure in kg/cm².
D is maximum internal diameter in mm.
S is minimum tensile strength of plate in kg/mm².
C is a constant, as given below.

In no case, however, shall the factor of safety of the shell be less than 4 or the plate thickness be less than specified in Table XII/3 given under Regulation 563.

C=27 where Class II requirements are complied with.
C=23 for Class III boilers when stress relieved.
C=21 for Class III boilers when not stress relieved.

Where boilers have a nest or nests of horizontal tubes, so that there is a direct tension on the tube plates due to the vertical load on the boiler ends or to tube plates acting as horizontal ties across the shell:

i) each alternate tube in the outer vertical row of tubes shall be a stay tube.
ii) The thickness of tube plates and the spacing of the tubes shall be such that the section of metal taking the load is sufficient to keep the stress within that allowed on the shell plate, as determined by the following formula:-

WP = (t – 2) SJ Equation XII/2
2D

Where,

t is the thickness of the tube plate in mm.
WP is the working pressure in kg/cm².
S is the minimum tensile strength in kg/mm².
D is twice the radial distance of the centre of the outer row of tube holes from the axis of the shell in mm.
J is the percentage strength of the plate through the tube holes,

i.e. = 100 (P-D) Eqn. XII/3
P

P is the vertical pitch of tubes in mm.
D is the diameter of the tube holes in mm.

NOTE:- The tube plates between the stay tubes shall comply with the requirements for tubes plates (Regulation 577).
 
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