Section 1 General requirements

1.1.1 The requirements of this Chapter are applicable to fusion welded pressure vessels and their mountings and fittings, for the following uses:

1. Production or storage of steam.

2. Heating of pressurised hot water above 120°C.

3. Heating of pressurised thermal liquid.

The formulae in 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).

1.1.2 The scantlings of coil type heaters with pumped circulation, which are fired or heated by exhaust gas, are to comply with the appropriate requirements of this Chapter.

1.1.3 Where exhaust gas emissions abatement equipment is fitted to steam raising plant, it is to meet the requirements of Pt 5, Ch 24 Emissions Abatement Plant for Combustion Machinery.

1.2.1 The symbols used in the various formulae in Pt 5, Ch 10, 2 Cylindrical shells and drums subject to internal pressure to Pt 5, Ch 10, 8 Headers, 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 corrosion allowance (c).

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 safety 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 boiler or pressure vessel operates and the lowest pressure at which any safety valve is set to lift, to prevent unnecessary lifting of the safety 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 following values are to be regarded as the minimum:

1. For fired steam boilers, T, is to be taken as not less than 250°C.

2. For steam heated generators, secondary drums of double evaporation boilers, steam receivers and pressure parts of fired pressure vessels, not heated by hot gases and adequately protected by insulation, T, is to be taken as the maximum temperature of the internal fluid.

3. For pressure parts heated by hot gases, T, is to be taken as not less than 25°C in excess of the maximum temperature of the internal fluid.

4. For boiler, superheater, reheater and economiser tubes, T, is to be taken as indicated in Pt 5, Ch 10, 7.1 Minimum thickness 7.1.2.

5. For combustion chambers of the type used in horizontal wet-back boilers, T, is to be taken as not less than 50°C in excess of the maximum temperature of the internal fluid.

6. For furnaces, fireboxes, rear tube plates of dry-back boilers and pressure parts subject to similar rates of heat transfer, T, is to be taken as not less than 90°C in excess of the maximum temperature of the internal fluid.

1.4.3 In general any parts of boiler drums or headers not protected by tubes, and exposed to radiation from the fire or to the impact of hot gases, are to be protected by a shield of good refractory material or by other approved means.

1.4.4 Drums and headers of thickness greater than 35 mm are not to be exposed to combustion gases having an anticipated temperature in excess of 650°C unless they are efficiently cooled by closely arranged tubes.

1.5.1 For Rule purposes, pressure vessels with fusion welded seams are graded as Class 1 if they comply with the following conditions:

1. For pressure parts of fired steam boilers, fired thermal liquid heaters and exhaust gas heated shell type steam boilers where the design pressure exceeds 0,34 MPa.

2. For pressure parts of steam heated steam generators and separate steam receivers where the design pressure exceeds 1,13 MPa, or where the pressure, in MPa, multiplied by the internal diameter of the shell, in mm, exceeds 1442.

1.5.2 For Rule purposes, pressure vessels with fusion welded seams, used for the production or storage of steam, the heating of pressurised hot water above 120°C or the heating of pressurised thermal liquid not included in Class 1 are graded as Class 2/1 and 2/2.

1.5.3 Pressure vessels which are constructed in accordance with Class 2/1 or Class 2/2 standards (as indicated above) will, if manufactured in accordance with requirements of a superior class, be approved with the scantlings appropriate to that class.

1.5.4 Pressure vessels which have only 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 10, 1.5 Classification of fusion welded pressure vessels 1.5.1 and Pt 5, Ch 10, 1.5 Classification of fusion welded pressure vessels 1.5.2.

1.5.5 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.

1.5.6 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.

1.5.7 Hydraulic testing is required for pressure vessels of Class 1, 2/1 and 2/2.

1.6.1 Plans of boilers, superheaters and economisers are to be submitted in triplicate for consideration. When plans of water tube boilers are submitted for approval, particulars of the safety valves and their disposition on boilers and superheaters, together with the estimated pressure drop through the superheaters, are to be stated. The pressures proposed for the settings of boiler and superheater safety valves are to be indicated on the boiler plan.

1.6.2 Plans, in triplicate, showing full constructional features of fusion welded pressure vessels and dimensional details of the weld preparation 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.

1.6.3 Plans, in triplicate, showing details of the air flow through the combustion chamber, boiler furnace and boiler uptake spaces, including measures taken to assure effective purging in all of the spaces, are to be submitted for consideration. See also Pt 5, Ch 10, 18.3 Main, auxiliary and other boilers and Pt 5, Ch 14, 12.2 Thermal fluid heaters.

1.6.4 Plans, in triplicate, showing all areas of refractory material in the combustion chamber and boiler furnace spaces, are to be submitted for consideration. See Pt 5, Ch 10, 1.12 Furnace explosion prevention 1.12.1.

1.6.5 Calculations, in triplicate, showing that a minimum of 4 air changes of the combustion chamber, boiler furnace and boiler uptake spaces will be achieved during automatic purging operations, with details of the forced draft fans and arrangements of air flow from fan intake to flue outlet, are to be submitted for consideration, See Pt 5, Ch 10, 1.12 Furnace explosion prevention 1.12.1.

1.6.6 Calculations, in triplicate, are to be submitted showing that the ventilation of machinery spaces containing boilers is adequate for the air consumers within the space with an unimpaired air supply, in accordance with the equipment manufacturer’s recommendations, under operating conditions as defined in Ch 1, Pt 5, Ch 1, 4.5 Ventilation 4.5.2.

1.7.1 Materials used in the construction are to be manufactured and tested in accordance with the requirements of the Rules for the Manufacture, Testing and Certification of Materials, July 2022.

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

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

   340 to 520 N/mm².

2. For boiler furnaces, combustion chambers and flanged plates:

   400 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 The specified minimum tensile strength of boiler and superheater tubes is to be within the following general limits:

1. Carbon and carbon-manganese steels: 320 to 460 N/mm².

2. Low alloy steels: 400 to 500 N/mm².

1.7.5 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 Lloyd’s Register (hereinafter referred to as ‘LR’).

1.7.6 Where a fusion welded pressure vessel is to be made of alloy steel, and approval of the scantlings is required on the basis of the high temperature properties of the material, 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.

1.8.1 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 10, 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 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 5, Ch 10, 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 10, 2 Cylindrical shells and drums subject to internal pressure to Pt 5, Ch 10, 8 Headers, where applicable. Fusion welded pressure parts are to be made in accordance with Pt 5, Ch 17 Requirements for Fusion Welding of Pressure Vessels and Piping.

1.9.2 The longitudinal and circumferential joints for all classes of pressure vessels for the purposes of this Chapter are to be butt joints. For typical acceptable methods of attaching dished ends, see Figure 10.14.1 Typical attachments of dished ends to cylindrical shells.

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 Pt 5, Ch 10, 2 Cylindrical shells and drums subject to internal pressure to Pt 5, Ch 10, 8 Headers, the suitability of their construction is to be determined by hydraulic proof test of a prototype or by agreed alternative method.

1.11.1 Where working conditions are adverse, special consideration may be required to be given to increasing the scantlings derived from the formulae. In this connection, where necessary, account should also be taken of any excess of 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.

1.12.1 The design of combustion chamber and furnace arrangements is to incorporate measures to minimise the risk of explosion as far as practicable. Measures are to be taken to prevent the accumulation of flammable gases in spaces which may not effectively be reached by purging air. Measures are to be taken to minimise heat retaining surfaces e.g. refractory which can become sources of ignition in the furnace and uptakes.

1.13.1 The design of exhaust gas economisers/boilers of the plain or extended surface fin tube types is to be compatible with the installed engine design parameters. The parameters which influence the build up of soot deposits and overheating such as fuel, exhaust gas temperature and efflux velocity are to be considered in the design of the exhaust gas economiser/boiler for use with the installed engine, in order to minimise the risk of fire and breakdown during operation.

1.13.2 A Design Statement demonstrating compliance with the requirements of Pt 5, Ch 10, 1.13 Exhaust gas economiser/boiler arrangements 1.13.1 or alternative means of preventing the accumulation of soot or overheating, such as the use of exhaust gas bypass ducting with automatic flap valve arrangements and/or effective soot prevention and cleaning systems, is to be submitted for approval.