

Reg. 563, Reg. 564
Classification of FusionWelded Boilers


The boilers covered by this Chapter shall be classified
in accordance with Table XII/3.
TABLE
XII/3
Classification of FusionWelded 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 diameter5/16
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 diameter3/8 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 (PD) 
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). 


