Section
5 Specific requirements for pressure piping
5.1 Scope
5.1.1 Fabrication of piping is to be carried out in accordance with the
requirements of this Section and the general requirements given in Ch 13, 1 General welding requirements, unless more stringent requirements have been specified.
5.1.2 Piping systems
are to be constructed in accordance with the approved plans and specifications.
5.1.3 Fabricated piping will be accepted only if manufactured by firms that have
demonstrated that they have the facilities and equipment and are competent to undertake
the quality of welding required for the Class of pipework proposed.
5.2 Manufacture and workmanship
5.2.1 Pipe welding may be performed using manual, semi-automatic or fully
automatic electric arc processes. The use of oxy-acetylene welding will be limited to
Class 3 piping in carbon steel or carbon/manganese material that is not for carrying
flammable fluids and limited to butt joints in pipes not exceeding 100 mm diameter or
9,5 mm thickness.
5.2.2 Welding of piping, including attachment welds directly to pressure
retaining parts is to be performed in accordance with approved welding procedures that
have been qualified in accordance with Ch 12 Welding Qualifications.
5.2.3 Where the
work involves a significant number of branch connections, tests will
be required to demonstrate that the type of joint(s) and welding techniques
employed are capable of achieving the required quality.
5.2.4 Where pressure piping is assembled and butt welded in situ, the piping is
to be arranged well clear of adjacent structures to allow sufficient access for
preheating, welding, heat-treatment and non-destructive examination of the joints.
5.2.5 Alignment
of pipe butt welds is to be in accordance with Table 13.5.1 Pipe butt weld alignment
tolerances unless more stringent requirements
have been agreed. Where fusible inserts are used, the alignment is
to be within 0,5 mm in all cases.
Table 13.5.1 Pipe butt weld alignment
tolerances
| Pipe size
|
Maximum permitted
misalignment
|
|
D < 150 mm and t ≤ 6 mm
|
1,0 mm or 25% of t, whichever is the
lesser
|
|
D < 300 mm and t ≤ 9,5 mm
|
1,5 mm or 25% of t, whichever is the
lesser
|
|
D ≥ 300 and t > 9,5 mm
|
2,0 mm or 25% of t, whichever is the
lesser
|
| where
D =
pipe internal diameter
t = pipe wall thickness
|
5.2.6 The number
of welds is to be kept to a minimum. The minimum separation between
welds, measured toe-to-toe, is to be not be less than 75 mm. Where
it is not possible to achieve this, adjacent welds are to be subjected
to surface crack detection NDE.
5.2.7 Welding
consumables and fusible root inserts, where used, are to be suitable
for the materials being joined.
5.2.8 Acceptable
methods of flange attachment are to be used, see
Figure 12.2.2 Typical welded-on flanges in Pt 5, Ch 12 Piping Design Requirementsof the Rules for Ships. Where
backing rings are used with flange type (a) they are to fit closely
to the bore of the pipe and be removed after welding. The rings are
to be made of the same material as the pipes. The use of flange types
(b) and (c) with alloy steel pipes is limited to pipes up to and including
168,3 mm outside diameter.
5.2.9 Where socket
welded fittings are employed, the diametrical clearance between the
outside diameter of the pipe and the base of the fitting is not to
exceed 0,8 mm, and a gap of approximately 1,5 mm is to be provided
between the end of the pipe and the internal step at the bottom of
the socket.
5.2.10 For welding
of carbon, carbon/manganese and low alloy steels, the preheat to be
applied will be dependent on the material grade, thickness and hydrogen
grading of the welding consumable in accordance with Table 13.5.2 Welding preheat levels for
piping, unless welding procedure
testing indicates that a higher level is required.
Table 13.5.2 Welding preheat levels for
piping
| Material Grade
|
Thickness,t (mm) see Note 4
|
Minimum preheat temperature (°C) See Note 1
|
| Non-low
H2
|
Low H2
see Note 2
|
| Carbon and carbon/manganese
grades:
|
t ≤ 15
|
50
|
10
|
| 320 and 360
|
t ≥ 15
|
100
|
50
|
| Carbon and carbon/manganese
grades:
|
t ≤ 15
|
75
|
20
|
| 410, 460 and 490
|
t ≥ 15
|
150
|
100
|
| 1Cr ½Mo
|
t < 13
|
|
100
|
| t ≥ 13
|
See Note 3
|
150
|
| 2¼Cr 1Mo
|
t < 13
|
See Note 3
|
150
|
| t ≥ 13
|
200
|
| ½Cr ½Mo ¼V
|
t < 13
|
See Note 3
|
150
|
| t ≥ 13
|
200
|
Note
1. Where the ambient temperature is 0°C
or below, pre-warming of the weld joint is required in all cases.
|
|
|
Note
3. Low hydrogen welding process is
required for these materials.
|
Note
4.
t = the thickness of the thinner member for butt welds, and the
thicker member for fillet and branch welds.
|
5.2.11 Welding
without filler metal is generally not permitted for welding of duplex
stainless steel materials.
5.2.12 All welds
in high pressure, high temperature pipelines are to have a smooth
surface finish and even contour; and where necessary, made smooth
by grinding.
5.2.13 Check
tests of the quality of the welding are to be carried out periodically.
5.3 Heat treatment after bending of pipes
5.3.1 After forming
or bending of pipes, the heat treatments specified in this Section
are to be applied unless the pipe material manufacturer specifies
or recommends other requirements.
5.3.2 Generally, hot forming is to be carried out within the normalising
temperature range. When carried out within this temperature range, no subsequent heat
treatment is required for carbon and carbon/manganese steels. For alloy steels, 1Cr ½Mo,
2¼Cr 1Mo and ½Cr ½Mo ¼V, a subsequent stress relieving heat treatment in accordance
with Ch 13, 1.16 Post-weld heat treatment at the
temperatures and times specified in Table 13.5.4 Post-weld heat treatment
requirements for piping is
required, irrespective of material thickness.
Table 13.5.3 Heat treatment after bending of
pipes
| Type of steel
|
Heat treatment required
|
Carbon and
carbon/manganese:
Grades 320, 360, 410, 460 and 490
|
Normalise at 880 to 940°C
|
| 1Cr ½Mo
|
Normalise at 900 to
940°C, followed by tempering at 640 to 720°C
|
| 2¼Cr 1Mo
|
Normalise at 900 to
960°C, followed by tempering at 650 to 780°C
|
| ½Cr ½Mo
¼V
|
Normalise at
930 to 980°C, followed by tempering at 670 to 720°C
|
| Other alloy steels
|
Subject to special
consideration
|
5.3.5 Bending
procedures and subsequent heat treatment for other alloy steels will
be subject to special consideration.
5.4 Post-weld heat treatment
5.4.2 The thickness
limits, the recommended soaking temperatures and periods, for application
of post-weld heat treatment are given in Table 13.5.4 Post-weld heat treatment
requirements for piping.
Table 13.5.4 Post-weld heat treatment
requirements for piping
| Material Grade
|
Thickness for which post-weld heat
treatment is required
|
Soak temperature (°C) see
Note 2
|
Soak period
|
| Carbon and carbon/manganese grades: 320, 360,
410, 460, 490
|
Over 30 mm
|
580–620°C
|
1 hour per 25 mm
of thickness, 320, 360, 410, 460, 490 minimum of 1 hour
|
| 1Cr ½Mo
|
Over 8 mm
|
620–660°C
|
1 hour per 25 mm
of thickness, minimum of 1 hour
|
| 2¼Cr 1Mo
|
All
|
650–690°C
|
1 hour per 25 mm
of thickness, minimum of 1 hour
|
| ½Cr ½Mo ¼V
|
All, see
Note 1
|
670–720°C
|
1 hour per 25 mm
of thickness, minimum of 1 hour
|
Note
1. Heat treatment may be omitted for
thicknesses up to 8 mm and diameters not exceeding 100mm and minimum
service temperature 450°C provided that welding procedure tests have
demonstrated acceptable properties in the as welded condition.
Note
2. For materials supplied in the tempered
condition, the post weld heat treatment temperature is to be at least
20°C less than the material tempering temperature.
|
5.4.3 Where the
use of oxy-acetylene welding is proposed, due consideration is to
be given to the need for normalising and tempering after such welding.
5.5 Non-destructive examination
5.5.2 Butt welds
in Class 1 pipes with an outside diameter greater or equal to 75 mm
are to be subject to 100 per cent volumetric and visual inspections.
Consideration is to be given to the extent and method of testing applied
to butt welds in Class 1 pipes with an outside diameter less than
75 mm.
5.5.3 Butt welds in Class II pipes are to be subjected to at least 10 per cent
random volumetric inspections when the outside diameter is 100 mm and greater.
5.5.4 NDE for
Class II pipes with a diameter less than 100 mm is to be at the discretion
of the Surveyor.
5.5.6 Fillet welds
on flange pipe connections of Class I pipes are to be examined by
surface crack detection methods.
5.6 Repairs to pipe welds
5.6.1 Where non-destructive examinations reveal unacceptable defects in a weld,
the defects are to be removed and repaired in accordance with Ch 13, 1.15 Rectification of welds defects. Completed repairs are to be shown by further
non-destructive examination to have eliminated the defects.
5.6.2 For pipes
with diameter less than 88 mm and where unacceptable defects have
been found during nondestructive examination, consideration is to
be given to cutting the weld out completely, re-making the weld preparation
and re-welding as a new joint (because of the difficulty of making
small repairs).
5.6.3 Where repeated
weld repairs have to be made to a weld, only two such attempts are
to be permitted, thereafter the weld is to be cut apart and removed,
and re-welded as a new joint.
5.6.4 Where piping requires post-weld heat treatment weld, repairs to the
pressure retaining parts are to be subjected to a subsequent heat treatment. Similarly,
where welding is conducted after pressure testing, a further pressure test is to be
required unless specific exemption has been agreed.
|