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Reg. 272, Reg. 273
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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.
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| 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).
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| 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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