The Pressure Equipment Regulations 1999

2.2.    DESIGN

Design for adequate strength

2.2.1 The pressure equipment must be designed for loadings appropriate to its intended use and other reasonably foreseeable operating conditions. In particular, the following factors must be taken into account:

• internal/external pressure,

• ambient and operational temperatures,

• static pressure and mass of contents in operating and test conditions,

• traffic, wind, earthquake loading,

• reaction forces and moments which result from the supports, attachments, piping, etc.,

• corrosion and erosion, fatigue, etc.,

• decomposition of unstable fluids.

Various loadings which can occur at the same time must be considered, taking into account the probability of their simultaneous occurrence.

2.2.2 Design for adequate strength must be based on:

• as a general rule, a calculation method, as described in 2.2.3, and supplemented if necessary by an experimental design method as described in 2.2.4, or

• an experimental design method without calculation, as described in 2.2.4, when the product of the maximum allowable pressure PS and the volume V is less than 6 000 bar-L or the product PS-DN less than 3 000 bar.

2.2.3 Calculation method

(a) Pressure containment and other loading aspects

• The allowable stresses for pressure equipment must be limited having regard to reasonably foreseeable failure modes under operating conditions. To this end, safety factors must be applied to eliminate fully any uncertainty arising out of manufacture, actual operational conditions, stresses, calculation models and properties and behaviour of the material.

• These calculation methods must provide sufficient safety margins consistent, where applicable, with the requirements of section 7.

• The requirements set out above may be met by applying one of the following methods, as appropriate, if necessary as a supplement to or in combination with another method:

  • design by formula,

  • design by analysis,

  • design by fracture mechanics;

(b) Resistance

• Appropriate design calculations must be used to establish the resistance of the pressure equipment concerned.

• In particular:

  • the calculation pressures must not be less than the maximum allowable pressures and take into account static head and dynamic fluid pressures and the decomposition of unstable fluids. Where a vessel is separated into individual pressure-containing chambers, the partition wall must be designed on the basis of the highest possible chamber pressure relative to the lowest pressure possible in the adjoining chamber,

  • the calculation temperatures must allow for appropriate safety margins,

  • the design must take appropriate account of all possible combinations of temperature and pressure which might arise under reasonably foreseeable operating conditions for the equipment,

  • the maximum stresses and peak stress concentrations must be kept within safe limits,

  • the calculation for pressure containment must utilise the values appropriate to the properties of the material, based on documented data, having regard to the provisions set out in section 4 together with appropriate safety factors. Material characteristics to be considered, where applicable, include:

    • yield strength, 0.2% or 1.0% proof strength as appropriate at calculation temperature,

    • tensile strength,

    • time-dependent strength, i.e. creep strength,

    • fatigue data,

    • Young’s modulus (modulus of elasticity),

    • appropriate amount of plastic strain,

    • impact strength,

    • fracture toughness,

    • appropriate joint factors must be applied to the material properties depending, for example, on the type of non-destructive testing, the materials joined and the operating conditions envisaged,

    • the design must take appropriate account of all reasonably foreseeable degradation mechanisms (e.g. corrosion, creep, fatigue) commensurate with the intended use of the equipment. Attention must be drawn, in the instructions referred to in section 3.4, to particular features of the design which are relevant to the life of the equipment, for example:

    • for creep: design hours of operation at specified temperatures,

    • for fatigue: design under number of cycles at specified stress levels,

    • for corrosion: design corrosion allowance;

(c) Stability aspects

• Where the calculated thickness does not allow for adequate structural stability, the necessary measures must be taken to remedy the situation taking into account the risks from transport and handling.

2.2.4 Experimental design method

• The design of the equipment may be validated, in all or in part, by an appropriate test programme carried out on a sample representative of the equipment or the category of equipment.

• The test programme must be clearly defined prior to testing and accepted by the notified body responsible for the design conformity assessment module, where it exists.

• This programme must define test conditions and criteria for acceptance or refusal. The actual values of the essential dimensions and characteristics of the materials which constitute the equipment tested shall be measured before the test.

• Where appropriate, during tests, it must be possible to observe the critical zones of the pressure equipment with adequate instrumentation capable of registering strains and stresses with sufficient precision.

• The test programme must include:

  (a)

  A pressure strength test, the purpose of which is to check that, at a pressure with a defined safety margin in relation to the maximum allowable pressure, the equipment does not exhibit significant leaks or deformation exceeding a determined threshold.

  The test pressure must be determined on the basis of the differences between the values of the geometrical and material characteristics measures under test conditions and the values used for design purposes; it must take into account the differences between the test and design temperatures;

  (b)

  where the risk of creep or fatigue exists, appropriate tests determined on the basis of the service conditions laid down for the equipment, for instance hold time at specified temperatures, number of cycles at specified stress-levels, etc;

  (c)

  where necessary, additional tests concerning other factors referred to in 2.2.1 such as corrosion, external damage, etc.