
Reg. 220, Reg. 221, Reg. 222
FURNACES
Plain Furnaces of Horizontal Boilers
For plain furnaces or furnaces strengthened by Adamson or other
joints or stiffeners of sufficient strength and for the semicylindrical
tops of fireboxes and bottom of combustion chambers where the
sides are securely stayed, the working pressure shall not exceed
the smaller of the values obtained from the following formulae
:
W. P. = 
C 
X 
(t  1)2 
Eqn.(57) 
D 
( L + 24 ) 
W. P. = 
C1 
X 
{10 ( t  1)
 L ) 
Eqn.(58) 
D 
Where
d = the external diameter of the furnace or chamber top or bottom
in inches
t = the thickness of the furnace plate in 32nds of an inch.
L = the length of the furnace or other part in inches measured
between points of substantial support, i.e. centres of rows
of rivets in end seams or commencement of curvature of flange,
whichever is applicable.
C = 1450 where the longitudinal seams of steel furnaces are
welded and 1300 where they are riveted,
C = 725 where the longitudinal seams of circular copper fireboxes
or furnaces are fitted with double butt straps and 650 where
they are lapped,
C1 = 50 where the longitudinal seams of steel furnaces are welded
and 45 where they are riveted,
C1 = 25 where the longitudinal seams of circular copper fireboxes
or furnaces are fitted with double butt straps and 22.5 where
they are lapped.
Reg. 221
FURNACES
Corrugated Furnaces of Horizontal Boilers
The maximum working pressure to be allowed on corrugated
furnaces shall be determined by the following formula 
W. P. = 
C 
( t  1 ) 
Eqn.(59) 
D 
where
D is the least external diameter in inches measured at the bottom
of the corrugations on the water side.
t is the thickness of the furnace plate in 32nds of an inch
measured at the bottom of the corrugation or chamber,
C = 480 for the Fox, Morrison, Deighton, Purves,, and other
similar furnaces and 510 for the leeds Forge Bulb Suspension
Furnace.
Reg. 222
FURNACES
Plain Furnaces of Vertical Boilers
The same formulae as for the plain furnaces of horizontal
boilers shall be used, but where the furnaces are tapered the
diameter to be taken for calculation purposes shall be the mean
of that at the top and of that at the bottom where it meets
the substantial support from flange or ring. The length for
the same purpose shall be the distance from the centre of the
row of rivets connecting the crown and the body of the furnace
to the substantial support at the bottom of the furnace, or
to a row of screwed stays connecting the furnace to the shell,
provided that the pitch of stays at the furnace does not exceed
14 times the thickness of the furnace plate when the stays are
riveted at their ends or 16 times when the stays are fitted
with nuts. Such screwed stays shall be in diameter over the
threads not less than twice the thickness of the furnace plate
and in no case less than 3/4" diameter {See Reg.135
(d)}. 
