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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 two-and-a-half 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.
Ts 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.
Tn is actual thickness of branch in inches.
T2 is minimum thickness body of branch calculated by Equation XII/1 (with C=35 if seamless) in inches.
T3 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.

Tc is minimum thickness of compensating plate in inches.
S1 is minimum tensile strength of shell plate in tons per square inch.
S2 is minimum tensile strength of branch in tons per square inch.
S3 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 T1

Compensating area
(i) Portion of shell available for compensation:-

B =2(3+Ts) (Ts-T1)
(ii) Portion of branch available for compensation:-

C=2(4+Ts)[Tn-(T2+T3)]x S2
S4
(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 Tc x S3
S1

 
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