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Reg. 272, Reg. 273

Reg. 272
The ligament efficiency of drum shells shall comply with Reg. 215.


Reg. 273

Longitudinal Stress


Notwithstanding the working pressure as calculated by equation 72, the thickness of drum or cylindrical header shells shall be such that in no case does the longitudinal stress resulting from the combination of stress arising from internal steam pressure, the self-weight of the drum or header and its contents and all externally applied loads, exceed the permissible working stress corresponding to the working metal temperature as prescribed in Regulation 271.

a) The maximum direct longitudinal stress due to the internal steam pressure acting on the drum ends shall be calculated as follows:-

fd = P D² Equation (72a)
1.273 A

where,

fd = Maximum direct longitudinal stress in pounds per square inch.
P = Design Pressure in pounds per square inch.
D = Internal diameter of the drum or header in inches.
A = Net cross-sectional area of the drum of header in square inches taken through the tube holes in a plane at right angle to its axis.
b) The resultant bending moment M at any section shall be the algebraic sum of the bending moments due to the eccentricity of the end pressure and that due to the externally applied loads.

MR = Me + Mw Equation (72b)
The bending moment due to the eccentricity of end pressure shall be calculated as follows:-
M = P D2 e Equation (72c)
1.273


where,

M = Resultant bending moment due to eccentricity in pound-inches.
P = Design pressure in pounds per square inch.
D = Internal diameter of drum or header in inches.
e = Eccentricity of the nett cross section i.e., the distance from the neutral axis of the nett section to the drum or header axis in inches.

The bending moment (Mw) due to externally applied loads shall be calculated by treating the drum or header as a beam carrying the externally applied loads, including the self-weight of the drum or header and its contents under working conditions.

c) The stress due to bending shall be calculated as follows:-

fb = MR Y Equation (72d)
I

Where,

fb = Stress due to bending in pounds per square inch.
M = Resultant bending moment at the section in pound-inches.
Y = Distance from the neutral axis of the nett cross section to the extreme fibre of the drum of header shell in inches.

I = moment of inertia of the nett cross section taken about its neutral axis in (inches4).

The resultant longitudinal stress is the algebraic sum of the stresses given under (a) and (c).

d) in calculating the longitudinal stress due to bending in a drum supported at or near its ends and connected to a lower drum by a bank of tubes (so arranged as to form substantial struts between the drums) the value of the moment of inertia Ia used in the formula in sub-regulation (c) shall be:

Moment of inertia of upper drum (Ia) plus a proportion (S) of the moment of inertia of lower drum (Ic).

where S = 1 - a² Equation (72e)
240

where,

a = The angle in degrees between the vertical and the line joining the centres of the upper and lower drums. Where a is equal to or greater than 15½°; S shall be taken as O. In no case shall the actual value of Ia used in Equation 72d be taken as more than 1.33 times the moment of inertia of the upper drum (Ib),

In the foregoing, unless otherwise agreed the term “bank of tubes” shall be defined as consisting of four or more rows of tubes extending over at least three quarters of the drum length between supports, and pitched longitudinally at not greater than an average pitch of four tube diameters.

 
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