The boiler has a longitudinal joint as well as
circumferential joint. The longitudinal joint is used to join the
ends of the plate to get the required diameter of a boiler. For this purpose, a
butt joint with two cover plates is used. The circumferential joint is
used to get the required length of the boiler. For this purpose, a lap joint
with one ring overlapping the other alternately is used. Since a boiler is made
up of number of rings, therefore the longitudinal joints are staggered for convenience
of connecting rings at places where both longitudinal and circumferential
joints occur.
Assumptions
in Designing Boiler Joints
The
following assumptions are made while designing a joint for boilers:
1. The load on the joint is equally shared by all the
rivets. The assumption implies that the shell and plate are rigid and that all
the deformation of the joint takes place in the rivets themselves.
2. The tensile
stress is equally distributed over the section of metal between the rivets.
3. The
shearing stress in all the rivets is uniform.
4. The
crushing stress is uniform.
5. There is no
bending stress in the rivets.
6. The holes
into which the rivets are driven do not weaken the member.
7. The rivet
fills the hole after it is driven.
8. The
friction between the surfaces of the plate is neglected.
DESIGN OF LONGITUDINAL BUTT JOINT FOR A BOILER
According to Indian Boiler Regulations
(I.B.R), the following procedure should be adopted for the design of
longitudinal butt joint for a boiler.
2. Diameter of rivets.
After finding out the
thickness of the boiler shell (t), the diameter of the rivet hole (d)
may be determined by using Unwin's empirical formula, i.e.
d = 6
(when t is
greater than 8 mm)
But
if the thickness of plate is less than 8 mm, then the diameter of the rivet
hole may be calculated by equating the shearing resistance of the rivets to
crushing resistance.
Ps = Pc
In no case, the diameter of rivet hole should not
be less than the thickness of the plate, because there will be danger of punch
crushing.
3. Pitch of rivets.
The pitch
of the rivets is obtained by equating the tearing resistance of the plate to
the shearing resistance of the rivets.
Pt = Ps
It may noted that
(a) The pitch
of the rivets should not be less than 2d, which is necessary for the
formation of head.
(b) The maximum
value of the pitch of rivets for a longitudinal joint of a boiler as per I.B.R.
is
pmax = C ×
t + 41.28 mm
where t =
Thickness of the shell plate in mm, and
C = Constant.
The value
of the constant C is given in Table
Note : If the pitch of rivets as obtained by equating the
tearing resistance to the shearing resistance is more than pmax,
then the value of pmax is taken.
4. Distance between the rows of rivets.
The
distance between the rows of rivets as specified by Indian Boiler Regulations
is as follows :
(a) For equal
number of rivets in more than one row for lap joint or butt joint, the distance
between the rows of rivets ( pb) should not be less than
0.33 p + 0.67 d, for
zig-zig riveting, and
2 d, for chain riveting.
(b) For joints
in which the number of rivets in outer rows is half the number of
rivets in inner rows and if the inner rows are chain riveted, the distance
between the outer rows and the next rows should not be less than
0.33 p +
0.67 or 2 d,
whichever is greater.
The distance between the
rows in which there are full number of rivets shall not be less than 2d.
(c) For joints
in which the number of rivets in outer rows is half the number of
rivets in inner rows and if the inner rows are zig-zig riveted, the distance
between the outer rows and the next rows shall not be less than 0.2 p +
1.15 d. The distance between the rows in which there are full number of
rivets (zig-zag) shall not be less than 0.165 p + 0.67 d.
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