

Reg. 571, Reg. 571 a, Reg. 571 b, Reg.
571 c, Reg. 571 d
Compensation for openings in shells


Where the width of any opening (measured in a direction
parallel to the longitudinal axis of the boiler) exceeds
twoandahalf times the thickness of the shell plate
in inches plus 2 ¾ inches, compensation shall
be provided.

(a)
Area to be compensated
The area to be
compensated shall be the product of the maximum width
of the opening cut in the shell and the thickness of an
unpierced seamless shell of similar material calculated
by Equation XII/1 with constant C=35 for the same working
pressure. 
(b)
Compensating area
Compensation shall
be considered adequate when the sum of the following areas
is not less than the area to be compensated:
(i) 
The area obtained by
multiplying the difference between the actual thickness
of the shell and the thickness required for an equivalent
unpierced seamless shell by a length 2(3 inch +
Ts), where Ts is the actual thickness of the shell
plate in inches. 
(ii) 
The net cross sectional
area of the frame or pad, or in the case of a branch
the cross sectional area of the wall of the branch
minus the sectional area of a branch of the same
bore having thickness equal to that calculated by
Equation XII/1 for the same working pressure (with
constant C=35 if seamless), plus the thickness required
to withstand external loads. The areas shall be
measured within limits such that (a+b) in Figure
XII/66 does not exceed 4 inches.
Where the material of a frame, pad or branch has
a tensile strength which differs from that of the
shell to which it is attached, the compensating
area shall be multiplied by the ratio:
Tensile strength
of compensating part. 
Tensile strength
of shell plate. 

(iii) 
The cross sectional areas
of the welding fillets. 

(c)
Compensating plates
In cases where
the sum of (I), (ii) and (iii) is less than the area to
be compensated a compensating plate shall be welded to
the shell, its total cross sectional area being equal
to the amount of the deficit and due allowance being made
as above when the tensile strength of the material of
the plate differs from that of the shell.
Notwithstanding compliance with the requirements of paragraph
(b) however, a reinforcing plate shall be fitted around
all branches of 4 inches bore and over welded to shell
plates having a thickness of ½ inch or less.
The compensating plate may be fitted on the inside or
outside of the shell plate as most convenient and its
diameter shall not exceed twice the longitudinal width
of the opening in the shell.

(d) Figure
XII/66 shows a welded branch to which foregoing
requirements apply
where,
W its width of the opening in the shell in inches.
T_{s} is actual thickness of shell in inches.
T1 is thickness of an equivalent unpierced seamless
shell calculated by Equation XII/1 with C=35, in inches.
T_{n} is actual thickness of branch in inches.
T_{2} is minimum thickness body of branch calculated by
Equation XII/1 (with C=35 if seamless) in inches.
T_{3} is thickness required to withstand external loads
in inches=1/8 inches unless the loading is known to
be such as to require a greater thickness and is = 0 where
there is no external loading.
T_{c} is minimum thickness of compensating plate in inches.
S_{1} is minimum tensile strength of shell plate in tons
per square inch.
S_{2} is minimum tensile strength of branch in tons per
square inch.
S_{3} is minimum tensile strength of compensating plate
in tons per square inch.
W is width of compensating plate in inches measured
in the same plate as the shell opening.
Area to be compensated.
A=W x T_{1}
Compensating area 
(i) 
Portion of shell available
for compensation:
B =2(3+T_{s}) (T_{s}T_{1}) 
(ii) 
Portion of branch available
for compensation:
C=2(4+T_{s})[T_{n}(T_{2}+T_{3})]x 
S_{2} 
S_{4} 

(iii) 
Portion of welds available
for compensation:
D=Total cross sectional area of the internal and
external welding fillets.
If A is greater than (B+C+D), additional compensation
is required to make up the difference. In the
example shown in Fig.
XII/66 this is provided by means
of a compensating plate, the total compensating
area being such that (B+C+D)+Z is not less than A where
Z=2
x W x T_{c} x 
S_{3} 
S_{1} 



