IFMASS10

166

Leak testing - LT

Magnetic particle testing - MT

Penetrating fluid test - PT

Visual testing - VT

Standards for NDT of welded joints include VT, ET, MT, PT, RT and UT.

3. DESIGN OF PRESSURE VESSELS

Pressure equipment must be designed for prescribed loading according to its purpose

and other predictable conditions. In order to establish corresponding security limits for all

possible errors general available calculation method and suitable safety coefficients

should be used in design. Allowed stresses for pressure equipment have to be limited by

operating conditions.

Given design requirements could be fulfilled by one of design methods or by their

combination, corresponding to considered case, applying experimental method with

empirical equations, analytical methods or using fracture mechanics approach.

3.1. Design using experimental method

Pressure equipment design evaluation, entirely or in parts, could be performed by

using of adequate testing program on the representative equipment sample or equipment

category. Program must define testing conditions and acceptance criteria for the design.

Real values of basic dimensions and material properties must be established before tes-

ting. When risk of deformation or fatigue is present, adequate testing is defined based on

equipment operating conditions, for example time of exposure to given temperature,

number of cycles for determined load level. If necessary, additional testing concerning

other factors such as corrosion or external damages could be required.

3.2. Design using analytical procedures

Design of pressure vessels by analytical methods implies for vessel and components

shapes for which standard calculation methods and regulations are available.

Regulations in general comprise clauses which are obligatory for designer. Different

influences which may appear in loadings and stresses of vessels are quoted, but designer

is free to evaluate and consider all factors such as category, class, purpose, operating

conditions. Procedure of analytical calculation of strength and stability of pressure vessels

is defined by particular standards.

3.3. Design using of fracture mechanics

Design of pressure vessel by fracture mechanics approach includes designing for

determined operating life, considering following operating life phases:

–

Phase without fractures (initial fractures are not detected on the product)

–

Fracture appearance in service (the appearance of the initial fracture)

–

Fracture propagation (crack extension)

–

Phase of operating life before fracture (fracture process phase)

The effect of individual phase till final fracture of pressure equipment is depended on

service stress intensity, base material properties and structure design.

Design using the rate of crack growth is effective in the case of negligible effect of

initial crack, high toughness of material, or high mean stress and low stress amplitude.