Section
4 Pressure vessels and bulk storage
4.1 General
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.
4.1.4 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 Plans and data submissions
4.2.1 Design documentation is to be submitted for all pressure vessels.
4.2.2 The submitted information is to include the following:
- Design specification, including data of working medium and
pressures.
- Minimum/maximum temperatures, corrosion allowance, environmental
and external loads.
- Plans, including sufficient detail and dimensions to evaluate
the design.
- Strength calculations for normal operating and emergency
conditions.
- Bill of Materials including material specifications as
necessary.
- Fabrication specifications including welding, heat treatment,
type and extent of NDE.
4.3 Equipment certification
4.4 Materials
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.
4.4.5 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 Design pressure and temperature
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 Design safety factors
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.
4.6.4 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 Construction and testing
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.
4.8 Hydrostatic test pressure
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 Protective and pressure relief
devices
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 Bulk storage vessels
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.
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