Section 1 General requirements

1.1 Application

1.1.1 The requirements of this Chapter are applicable to fusion welded pressure vessels and their mountings and fittings, where plans have to be submitted in accordance with Pt 15, Ch 4, 1.2 Details to be submitted.

1.1.2 Seamless pressure vessels are to be manufactured in accordance with the requirements of the Rules for the Manufacture, Testing and Certification of Materials, July 2022 (hereinafter referred to as the Rules for Materials) where applicable.

1.1.3 Steam raising plant and associated pressure vessels should be designed and constructed in accordance with Pt 5, Ch 10 Steam Raising Plant and Associated Pressure Vessels of the Rules and Regulations for the Classification of Ships (hereinafter referred to as the Rules for Ships).

1.2 Details to be submitted

1.2.1 Plans of pressure vessels are to be submitted in triplicate for consideration where all the conditions in (a) or (b) are satisfied:

The vessel contains vapours or gases, e.g. air receivers, hydrophore or similar vessels and gaseous CO₂ vessels for fire-fighting, and

pV > 60
p > 0,1
V > 100

volume (litres) of gas or vapour space

design pressure (MPa)

The vessel contains liquefied gases, or flammable liquids, and

p > 0,7
V > 100

volume (litres)

design pressure (MPa).

1.3 Materials

1.3.1 Materials used in the construction of Class 1, 2/1 and 2/2 pressure vessels are to be manufactured, tested and certified in accordance with the requirements of the Rules for Materials. Materials used in the construction of Class 3 pressure vessels may be in accordance with the requirements of an acceptable National or International Standard. The manufacturer’s certificate will be accepted in lieu of LR’s material certificate for such materials.

1.3.2 The specified minimum tensile strength of carbon and carbon-manganese steel plates, pipes, forgings and castings is to be within the following general limits:

For seamless and Class 1 and Class 2/1 fusion welded pressure vessels:

340 to 520 N/mm².
For Class 2/2 and where required Class 3 fusion welded pressure vessels:

340 to 430 N/mm².

1.3.3 Where it is proposed to use materials other than those specified in the Rules for the Manufacture, Testing and Certification of Materials, July 2022, details of the chemical compositions, heat treatment and mechanical properties are to be submitted for approval. In such cases, the values of the mechanical properties used for deriving the allowable stress are to be subject to agreement by Clasifications Register (hereinafter referred to as 'LR').

1.4 Classification of fusion welded pressure vessels

1.4.1 Fusion welded pressure vessels are graded as Class 1 where the shell thickness exceeds 38 mm.

1.4.2 Fusion welded pressure vessels are graded as Class 2/1 and Class 2/2 if they comply with the following conditions:

where the design pressure exceeds 1,72 MPa, or
where the metal temperature exceeds 150°C, or
where the design pressure, in MPa, multiplied by the actual thickness of the shell, in mm exceeds 15,7, or
where the shell thickness does not exceed 38 mm.

1.4.3 For Rule purposes, Class 3 pressure vessels are to have a maximum shell thickness of 16 mm, and are pressure vessels not included in Classes 1, 2/1 or 2/2.

1.5 Design pressure

1.5.1 The design pressure is the maximum permissible working pressure and is to be not less than the highest set pressure of any safety valve.

1.6 Metal temperature

1.6.1 The metal temperature, T, used to evaluate the allowable stress, σ, is to be taken as the actual mean wall metal temperature expected under operating conditions for the pressure part concerned, and is to be stated by the manufacturer when plans of the pressure parts are submitted for consideration.

1.6.2 For fusion welded pressure vessels the minimum design temperature, T, is not to be less than 50^oC.

1.7 Definition of symbols

1.7.1 The symbols used in the various formulae in Pt 15, Ch 4, 2 Cylindrical shells subject to internal pressure, unless otherwise stated, are defined as follows and are applicable to the specific part of the pressure vessel under consideration:

design pressure, in MPa, see Pt 15, Ch 4, 1.5 Design pressure

r _i

inside knuckle radius, in mm

r _o

outside knuckle radius, in mm

minimum thickness, in mm

D _i

inside diameter, in mm

D _o

outside diameter, in mm

joint factor applicable to welded seams

R _i

inside radius, in mm

R _o

outside radius, in mm

design temperature, ^oC

allowable stress in N/mm², see Pt 15, Ch 4, 1.8 Allowable stress.

1.7.2 Where reference is made to calculated or actual plate thickness for the derivation of other values, these thicknesses are to be minus the standard Rule corrosion allowance of 0,75 mm, if not so stated.

1.8 Allowable stress

1.8.1 The term `allowable stress', σ, is the stress to be used in the formulae for the calculation of scantlings of pressure parts.

1.8.2 The allowable stress, σ, is to be the lowest of the following values:

where

E _t	=	specified minimum lower yield stress or 0,2 per cent proof stress at temperature, T, for carbon and carbon-manganese steels. In the case of austenitic steels, the 1,0 per cent proof stress at temperature, T, is to be used.
R ₂₀	=	specified minimum tensile strength at room temperature
S _R	=	average stress to produce rupture in 100 000 hours at temperature, T
T	=	metal temperature, see Pt 15, Ch 4, 1.6 Metal temperature.

1.8.3 The allowable stress for steel castings is to be taken as 80 per cent of the value determined by the method indicated in Pt 15, Ch 4, 1.8 Allowable stress 1.8.2, using the appropriate values for cast steel.

1.8.4 Where steels castings, which have been tested in accordance with Rules for the Manufacture, Testing and Certification of Materials, July 2022, are also subjected to non-destructive tests, consideration will be given to increasing the allowable stress using a factor up to 90 per cent in lieu of the 80 per cent referred to in Pt 15, Ch 4, 1.8 Allowable stress 1.8.3. Particulars of the non-destructive test proposals are to be submitted for consideration.

1.9 Joint factors

1.9.1 The following joint factors are to be used in the equations in Pt 15, Ch 4, 2 Cylindrical shells subject to internal pressure, where applicable. Fusion welded pressure parts are to be made in accordance with Pt 15, Ch 4, 8 Requirements for fusion welded pressure vessels.

Class of pressure vessel	Joint factor
Class 1	1,0
Class 2/1	0,85
Class 2/2	0,75
Class 3	0,6

1.9.2 The longitudinal joints for all Classes of vessels are to be butt joints. Circumferential joints for Class 1 vessels and all classes of vessel for the production and storage of steam are also to be butt welds. Circumferential joints for Class 2/1, 2/2 and 3 vessels should also be butt joints with the following exceptions:

Circumferential joints for Class 2/1, 2/2 and 3 vessels may be of the joggle type provided neither plate at the joints exceeds 16 mm thickness.
Circumferential joints for Class 3 vessels may be of the lap type provided neither plate at the joint exceeds 16 mm thickness nor the internal diameter of the vessel exceeds 610 mm.

For typical acceptable methods of attaching dished ends, see Figure 4.6.1 Typical methods of attachment of unstayed circular flat end plates.

1.10 Pressure parts of irregular shape

1.10.1 Where pressure parts are of such irregular shape that it is impracticable to design their scantlings by the application of formulae in this Chapter, the suitability of their construction is to be determined by hydraulic proof test of a prototype or by agreed alternative method.