Section 4 Pressure vessels and bulk storage

4.1.1 The Rules in this Section are applicable to fired and unfired pressure vessels associated with process plant, and drilling plant defined in Pt 3, Ch 7 Drilling Plant Facility.

4.1.2 Pressure vessels are to be designed in accordance with Pt 5, Ch 10 Steam Raising Plant and Associated Pressure Vessels and Pt 5, Ch 11 Other Pressure Vessels or with internationally recognised and agreed Codes and Standards and in accordance with the requirements of Pt 3, Ch 8, 1 General.

4.1.3 The list in Pt 3, Ch 17, 1.2 Recognised Codes and Standards 1.2.10 gives reference to some generally recognised Codes and Standards frequently specified for drilling and production equipment. These Codes and Standards may be used for certification but the additional requirements given in the Rules apply and the Rules will take precedence over the Codes and Standards wherever conflict occurs.

4.1.4 Portable gas cylinders and other pressure vessels used to transport liquids or gases under pressure are to comply with an acceptable National or International Standard.

4.1.5 Where pressure parts are of such an irregular shape that it is impracticable to design their scantlings by the application of recognised formulae, the acceptability of their construction is to be determined by hydraulic proof testing and strain gauging or by an agreed alternative method.

4.2.1 Design documentation is to be submitted for all pressure vessels.

4.3.1 Equipment certification is to be carried out in accordance with Pt 3, Ch 8, 1 General and equipment categories are to comply with Pt 3, Ch 17, 2.3 Production equipment 2.3.1 in Pt 3, Ch 17 Appendix A Codes, Standards and Equipment Categories.

4.4.1 Materials for pressure vessels are to comply with Pt 3, Ch 1, 4 Materials and the Rules for the Manufacture, Testing and Certification of Materials (hereinafter referred to as the Rules for Materials), except where modified by this Section.

4.4.2 Welded carbon/manganese (C–Mn) steels used for major pressure containing parts should have a chemical composition limited by the carbon content and the carbon equivalent:

Carbon content C ≤ 0,25

When the elements in the following formula are known, this formula is to be used:

Carbon Equivalent:

Symbols are as defined in the Rules for Materials.

4.4.3 The use of material not meeting these limitations is subject to special consideration in each case. The welding of such materials normally requires more stringent fabrication procedures regarding the selection of consumables, preheating and post weld heat treatment.

4.4.4 Materials for pressure containing parts are to be tested at the temperature specified in Ch 13, 4.2 Cutting and forming of shells and heads 4.2.5 in Ch 13 Requirements for Welded Construction of the Rules for Materials and shall achieve a minimum energy of 27J for materials with specified minimum yield strength less than or equal to 360 MPa and 42J for higher strength materials.

4.4.5 Equipment and components required for hydrogen sulphide sour service shall meet the property requirements of NACE MR0175/ISO15156 – Petroleum and Natural Gas Industries – Materials for use in -containing Environments in Oil and Gas Production.

4.4.6 Materials employed in liquefied natural gas pressure vessels are to be impact tested in accordance with Pt 4, Ch 2 Materials.

4.5.1 The design pressure is the maximum permissible working pressure and is not to be less than the highest set pressure of the safety valve. If the design of the system is such that it may be possible for it to see a vacuum, the design pressure shall also consider the minimum working pressure which the system may see.

4.5.2 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 operating conditions.

4.5.3 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 safety valve is set to lift, to prevent unnecessary lifting of the safety valve.

4.5.4 The design 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 the plans of the pressure part are being considered. For fired steam boilers, T is to be taken as not less than 250°C.

4.6.1 The term ‘allowable stress’, σ, is the stress to be used in the formulae for calculating the scantlings of pressure vessels.

4.6.2 The allowable stress used for the design of a pressure vessel is to be in accordance with the Code or Standard being used to design that vessel.

4.6.3 Pressure vessels are to be designed for the emergency conditions referred to in Pt 3, Ch 8, 1.4 Plant design characteristics.

4.6.4 It is not permissible to use the allowable stress levels of one Code or Standard to determine the scantlings using the formulae from a different Code or Standard.

4.6.5 The yield strength used in the determination of allowable stress or in calculations is not to exceed 0,85 of the specified minimum tensile strength of the material in question.

4.7.1 Fabrication documentation is to be compiled by the manufacturer simultaneously with the fabrication in a systematic and traceable manner so that all the information regarding the design specification, materials, fabrication processes, inspection, heat treatment, etc., can be readily examined by the Surveyor.

4.7.2 Welding procedures and construction requirements for welding shall be in accordance with those specified in Ch 12 Welding Qualifications and Ch 13 Requirements for Welded Construction of the Rules for Materials.

4.7.3 Procedures for performing non-destructive examination and the acceptance criteria to be applied shall be in accordance with Ch 13 Requirements for Welded Construction of the Rules for Materials.

4.8.1 Pressure vessels are to be subject to a hydrostatic test in accordance with the applied Code, Standard, or specification before being taken into service.

4.8.2 The hydrostatic test pressure is to be a minimum of 1,5 x design pressure if not specified in the Code or Standard.

4.8.3 The pressure and holding time are to be recorded.

4.8.4 Primary general membrane stresses are in no case to exceed 90 per cent of the minimum yield strength of the material.

4.9.1 Pressure vessels are to be provided with protective devices so that they remain safe under all foreseeable conditions.

4.9.2 Where pumps and pressure surges are capable of developing pressures exceeding the design conditions of the system, effective means of protection such as pressure relief devices or equivalent are to be provided.

4.9.3 Pressure relief valves are to be sized such that any accumulation of pressure from any source will not exceed 121 per cent of the design pressure. For specific fire contingencies where accumulated pressure could exceed 121 per cent, design proposals will be specially considered.

4.9.4 Bursting discs fitted in place of or in series with safety valves are to be rated to burst at a maximum pressure not exceeding the design pressure of the vessel at the operating temperature. Bursting discs are only to be used for pressure vessels located in open areas or if fitted in conjunction with a relief line led to an open area.

4.9.5 Where a bursting disc is fitted downstream of a safety valve, the maximum bursting pressure is also to be compatible with the pressure rating of the discharge system.

4.9.6 In the case of bursting discs fitted in parallel with relief valves to protect a vessel against rapid increase of pressure, the bursting disc is to be rated to burst at a maximum pressure not exceeding 1,3 times the design pressure of the vessel at operating temperature.

4.9.7 Pressure relief devices are to be type tested to establish their discharge capacities at their maximum rated design pressures and temperatures in accordance with an approved Code or Standard.

4.9.8 Where pressure relief devices can be isolated from the pressure vessel whilst in service, there is to be an alternative independent pressure relief device. The system pressure relief valve set pressure and bursting disc rupture pressure should be displayed at the respective operating position.

4.9.9 Any isolating valves used in conjunction with pressure relief devices are to be the full flow type capable of being locked in the full open position. Where isolating valves are arranged downstream and upstream of a relief device they are to be interlocked with each other.

4.9.10 Where pressure relief devices are duplicated on the same vessel or system and fitted with isolating valves, these valves are to be so interlocked as to ensure that before one relief device is isolated the other relief device is fully open and the required discharge capacity is maintained. The interlocking system is to be submitted for approval.

4.9.11 The design of the pressure-relieving system is to take into account the characteristics of the fluid handled and any extreme environmental condition recorded for the geographical zone of operation. The vent and pressure relieving systems are to be self-draining.

4.9.12 The rated discharge capacity of any pressure relief device is to take into account the back pressure in the vent systems. Where hazardous vapours are discharged directly to the atmosphere, the outlets are to be arranged to vent to a safe location.

4.10.1 Bulk storage vessels are to be designed in accordance with the general requirements of this Section and with one of the internationally recognised Codes or Standards for fusion welded pressure vessels quoted in Pt 3, Ch 17, 1.2 Recognised Codes and Standards 1.2.10, and in accordance with the design requirements given in Pt 3, Ch 8, 1 General, see also Pt 3, Ch 7, 3 Drilling plant systems

4.10.2 For bulk storage vessels in enclosed areas, testable safety valves are to be used, which can be vented out of the area. Such enclosed areas are to be ventilated so that a pressure build-up will not occur in the event of a break or a leak in the air supply system.

4.10.3 Bulk storage vessels are normally to be supported by suitable skirts in order to distribute the loads into the supporting structure.

4.10.4 Bulk storage vessels which penetrate watertight decks or flats are to be suitably reinforced, see Pt 3, Ch 3, 2.10 Watertight and weathertight integrity.