6.1.1 The minimum wall thickness of standpipes and branches is to be not less
than that determined by Pt 5, Ch 10, 7.1 Minimum thickness increased by the addition of a corrosion allowance
(c as defined in Pt 5, Ch 10, 1.2 Definition of symbols 1.2.1), making
such additions as may be necessary on account of bending, static loads and vibration.
The wall thickness, however, is to be not less than:
This thickness need only be maintained for a length, L, from the outside surface of the vessel, but need not extend
past the first connection, butt weld or flange, where:
where t and D
o are
as defined in Pt 5, Ch 10, 1.2 Definition of symbols.
6.1.2 For boilers having a working pressure exceeding 5 MPa and safety valves of
full lift or full bore type, the thickness of the branch pipe carrying the superheater
or drum safety valves is to be not less than:
|
|
= |
where t and σ are as defined in Pt 5, Ch 10, 1.2 Definition of symbols
|
|
d
|
= |
inside
diameter of branch, in mm |
|
D
|
= |
inside
diameter of safety valve discharge, in mm |
|
K
|
= |
2
for superheater safety valves |
|
|
= |
1 for drum safety
valves |
|
W
|
= |
total
valve throughput, in kg/h. |
6.1.3 The
offset from the centreline of the waste steam pipe to the centreline
of the safety valve is not to exceed four times the outside diameter
of the safety valve discharge pipe. The waste steam pipe system is
to be supported and arrangements made for expansion such that no direct
loading is imposed on the safety valve chests and the effects of vibration
are to be minimised.
6.1.4 The
pipe or header which carries the superheater safety valve is to be
suitably thickened but is to be not less than the thickness required
for the branch for a distance of 
on either side of the opening,
where
|
t
|
= |
thickness
required for the branch |
|
D
2
|
= |
inside diameter of the pipe or header. |
6.1.5 Except
as required by Pt 5, Ch 10, 6.1 Minimum thickness 6.1.4, in
no case need the wall thickness exceed the minimum shell thickness
as required by Pt 5, Ch 10, 2.1 Minimum thickness, Pt 5, Ch 10, 3.1 Minimum thickness or Pt 5, Ch 10, 4.1 Minimum thickness, as applicable.
6.1.6 Where
a standpipe or branch is connected by screwing, the thickness is to
be measured at the root of the thread.
6.1.7 For
boiler, superheater or economiser tubes, the minimum thickness of
the drum or the header connection or tube stub is to be calculated
as part of the tube in accordance with Pt 5, Ch 10, 7.1 Minimum thickness.