|Reg. 175, Reg. 176
REGULATIONS FOR DETERMINING THE WORKING PRESSURE
TO BE ALLOWED ON VARIOUS PARTS OF BOILERS OTHER THAN FUSION
WELDED AND SEAMLESS FORGED DRUMS
The maximum pressure at which a boiler may be used shall be
determined in accordance with the provisions of this Chapter.
The regulations in this chapter refer to material subjected
to steam temperature not exceeding 500° F.
Formula for Working Pressure of Shell
||For cylindrical shells, barrels,
steam and water drums, and domes of boilers for maximum
working pressure per square inch to be allowed shall be
calculated from the following formula :-
(Note - Where parts of drums of water tube boilers are
perforated for tubes, the working pressure shall be calculated
by equation (53) under 'Tube Plates')
|W. P .=
||(t - 2) x S x J
|C x D
W. P. = the working pressure in lbs. per square inch;
t = the thickness of shell plates in 32nds of an inch;
S = the minimum tensile breaking strength of the shell
plates in tons per square inch or whatever strength is
allowed under Regulation 5;
J = the percentage of strength of the longitudinal seams
of shell or of a line of holes in the shell for stays,
or rivets, or of an opening in the shell not fully compensated,
whichever is least calculated by the methods hereafter
C is a co-efficient as follows :-
2.75 when the longitudinal seams are made with double
butt straps and when small shells are formed from solid
2.83 when the longitudinal seams are made with lap joints
and are treble riveted.
2.9 when the longitudinal seams are made with lap joints
and are double riveted
3.0 when the longitudinal seams are welded and are fitted
with a single butt strap.
3.3 when the longitudinal seams are made with lap joints
and are single riveted.
D = the inside diameter of the outer strake of plating
of the cylindrical shell measured in inches.
|The factor of Safety shall
in no case be less than 4.
The actual factor of Safety in each case may be found
from the equation ;
With the best form of joint and least co-efficient (c)
the Factor of Safety for shell plates, 1/4 inch to 1-3/4
inches in thickness varies from 5.13 to 3.99.
An addition of 10 per cent, shall be made to the appropriate
co-efficient for shells of boilers in which the
longitudinal seams are lap jointed and are not
accessible to close visual inspection and for shell
plates of externally fired boilers exposed to the direct
impact of furnace flame; provided that this addition
shall not apply to the steam and water drums of water
tube boilers where the drums are not close to the
fire-grate and there are tubes between the fire-grate
and the drum.
The above co-efficient are standards and shall be
used only for boiler open to inspection by an inspecting
officer during the whole period of construction and
which are certified by him as having constructed in
accordance with the standard condition laid down in