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Reg. 170
MANHOLES, MUDHOLES, ETC.

 

Compensation for cutting large holes in Shell Type Boilers

a) Where holes are cut in the cylindrical shell for the purpose of attaching a seating of frame or door, compensation shall be provided such that the added sectional area, including parts of the frame within 4 inches of the shell and excluding rivet holes, shall be not less than the sectional area of the plate removed which shall be the product of the diameter of the opening and the calculated thickness of the plate as found by Equation 1. Where holes are cut in the cylindrical shell for the purpose of fixing seating for mountings and the diameter of the holes is greater than 2½ times the thickness of the shell plate plus 2¾ inches, compensation shall be provided. Where a large opening is cut in a cylindrical shell to receive another part of the structure, the sides where cut away shall be efficiently cross-stayed or strengthened in some other effective manner.
b) Riveted construction.- Where frames, pads or branches are secured by riveting on shell for fixing of mountings or other pressure parts, the sectional area to be compensated shall be determined as follows:- (See Figure 25A).

i) The cross-sectional area of the frame or pad excluding rivet holes, or in the case of a branch the cross-sectional area of the wall of the branch and flange, excluding rivet holes, minus the sectional area of a branch of the same bore having thickness calculated by equation 1 for the same design pressure, using C=2.75, J=65 when butt welded, and J=95 when seamless, plus the thickness required to withstand external loads. The area shall be measured within the limit specified in Figure 25-A.

Due allowance shall be made in case where the material used for the branch and/or compensating ring differs from that of the plate to which it is attached.

ii) The area obtained by multiplying the difference between the actual shell

thickness and the calculated thickness, by a length

2(3 inches + Ts-d)

Ts = Thickness of the shell plate in inches.

d = diameter of rivet hole in inches.

In cases where the sum of (i) and (ii) is less than the sectional area to be compensated, a compensating plate shall be fitted having a total net cross-sectional area equal to the amount of the deficit.
 
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