Section 1 General requirements

1.1.1 The requirements of this Chapter are applicable to fusion welded pressure vessels and plate heat exchangers, intended for marine purposes but not for steam raising plant. The equations of this Chapter may be used for determining the thickness of seamless pressure vessels using a joint factor of 1,0. Seamless pressure vessels are to be manufactured and tested in accordance with the requirements of Ch 5 Steel Forgings of the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials). For design requirements of pressure vessels intended for the carriage of liquefied gases, see Pt 5, Ch 13 Piping Systems for Ships Intended for the Carriage of Liquids in Bulk.

1.1.2 Where the required design criteria for pressure vessels are not indicated within this Chapter, the relevant Sections of 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) are applicable.

1.1.3 Seamless pressure vessels are to be manufactured in accordance with the requirements of the Rules for Materials where applicable.

1.2.1 The symbols used in the various formulae in Pt 5, Ch 9, 2 Cylindrical shells and drums subject to internal pressure inclusive, unless otherwise stated, are defined as follows, and are applicable to the specific part of the pressure vessel under consideration:

1.2.2 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.

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

1.3.2 The calculations made to determine the scantlings of the pressure parts are to be based on the design pressure, adjusted where necessary to take account of pressure variations corresponding to the most severe operational conditions.

1.3.3 It is desirable that there should be a margin between the normal pressure at which the pressure vessel operates and the lowest pressure at which any relief valve is set to lift, to prevent unnecessary lifting of the relief valve.

1.4.1 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.

1.4.2 The design temperature, T, for calculation purposes is to be not less than 50°C.

1.5.1 For Rule purposes, pressure vessels are graded as Class 1 where the shell thickness exceeds 38 mm.

1.5.2 For Rule purposes, pressure vessels are graded as Class 2/1 and Class 2/2 if they comply with the following conditions:

1. where the design pressure exceeds 1,72 MPa; or

2. where the metal temperature exceeds 150°C; or

3. where the design pressure, in MPa, multiplied by the actual thickness of the shell, in mm, exceeds 15,7; or

4. where the shell thickness does not exceed 38 mm.

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

1.5.4 Pressure vessels which are constructed in accordance with Class 2/1, 2/2 or 3 standards (as indicated in Pt 5, Ch 9, 1.5 Classification of fusion welded pressure vessels 1.5.2 and Pt 5, Ch 9, 1.5 Classification of fusion welded pressure vessels 1.5.3) will, if manufactured in accordance with the requirements of a superior Class, be approved with the scantlings appropriate to that Class.

1.5.5 Pressure vessels which only have circumferential fusion welded seams, will be considered as seamless with no Class being assigned. Preliminary weld procedure tests and non-destructive examination for the circumferential seam welds should be carried out for the equivalent Class as determined by Pt 5, Ch 9, 1.5 Classification of fusion welded pressure vessels 1.5.1, Pt 5, Ch 9, 1.5 Classification of fusion welded pressure vessels 1.5.2 and Pt 5, Ch 9, 1.5 Classification of fusion welded pressure vessels 1.5.3.

1.5.6 In special circumstances relating to service conditions, materials, operating temperature, the carriage of dangerous gases and liquids, etc. it may be required that certain pressure vessels be manufactured in accordance with the requirements of a superior Class.

1.5.7 Details of heat treatment, non-destructive examination and routine tests (where required) are given in Ch 13 Requirements for Welded Construction of the Rules for Materials.

1.5.8 Hydraulic testing is required for all Classes of pressure vessels.

1.5.9 For a full definition of Classes of pressure vessels relating to boilers and associated pressure vessels, see Pt 5, Ch 10, 1 General requirements of the Rules for Ships.

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

1. 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

   V = volume (litres) of gas or vapour space

2. The vessel contains liquefied gases for fire-fighting or flammable liquids, and

   p > 0,7

   V > 100

   V = volume (litres)

   p is as defined in Pt 5, Ch 9, 1.2 Definition of symbols 1.2.1.

1.6.2 Plans of full constructional features of the vessel and dimensional details of the weld preparations for longitudinal and circumferential seams and attachments, together with particulars of the welding consumables and of the mechanical properties of the materials, are to be submitted before construction is commenced.

1.7.1 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 specification. The manufacturer's certificate will be accepted in lieu of Lloyd’s Register’s (hereinafter referred to as LR’s) material certificate for such materials.

1.7.2 The specified minimum tensile strength of carbon and carbon-manganese steel plates, pipes, forgings and castings is to be within the general limits of 340 to 520 N/mm².

1.7.3 The specified minimum tensile strength of low alloy steel plates, pipes, forgings and castings is to be within the general limits of 400 to 500 N/mm², and pressure vessels made in these steels are to be either seamless or Class 1 fusion welded.

1.7.4 Where it is proposed to use materials other than those specified in the Rules for Materials, 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 LR.

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:

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 5, Ch 9, 1.8 Allowable stress 1.8.2 using the appropriate values for cast steel.

1.8.4 Where steel castings, which have been tested in accordance with the Rules for Materials 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 5, Ch 9, 1.8 Allowable stress 1.8.3. Particulars of the non-destructive test proposals are to be submitted for consideration.

1.9.1 The following joint factors are to be used in the equations in Pt 5, Ch 9, 2 Cylindrical shells and drums subject to internal pressure and Pt 5, Ch 9, 3 Dished ends subject to internal pressure. Fusion welded pressure parts are to be made in accordance with Pt 5, Ch 14 Requirements for Fusion Welding of Pressure Vessels and Piping.

1.9.2 The longitudinal joints for all Classes of vessels are to be butt joints. Circumferential joints for Class 1 vessels 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:

1. 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.

2. 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 flat ends, see Figure 9.6.1 Typical methods of attachment of un-stayed circular flat end plates.

For typical acceptable methods of attaching dished ends, see Figure 9.7.1 Typical attachments of dished ends to cylindrical shells.

1.9.3 Where a pressure vessel is to be made of alloy steel, particulars of the welding consumables to be used, including typical mechanical properties and chemical composition of the deposited weld metal, are to be submitted for approval.

1.10.1 Where working conditions are adverse, special consideration may require to be given to increasing the scantlings derived from the formulæ. In this connection, where necessary, account should also be taken of any excess loading resulting from:

1. Impact loads, including rapidly fluctuating pressures.

2. Weight of the vessel and normal contents under operating and test conditions.

3. Superimposed loads, such as other pressure vessels, operating equipment, insulation, corrosion-resistant or erosion-resistant linings and piping.

4. Reactions of supporting lugs, rings, saddles or other types of supports, or

5. The effect of temperature gradients on maximum stress.

1.11.1 Where pressure parts are of such irregular shape that it is impracticable to design their scantlings by the application of the formulae in Pt 5, Ch 9, 2 Cylindrical shells and drums subject to internal pressure, the suitability of their construction is to be determined by hydraulic proof test of a prototype or by an agreed alternative method.