Stress Critical lines

Flexibility as defined in the design codes is a measure of the ability of a piping system to absorb its own expansion and that of connected equipment to ensure that the loadings do not cause :

• Fatigue failure of the piping system due to repeated overstress.
• Failure of the piping system due to excessive plastic strains.
• Leakage at any flanged joints.
• Overloading of valves and other components in the piping system which would impair their operation.
• Detrimental distortion of connected equipment from excessive thrusts and movements and machinery misalignment.

A flexibility analysis is required if the Stress Engineer is in any doubt as to the ability of the system to satisfy the above requirements. Typically a comprehensive analysis is not required if :

• They are duplication of successfully operating installations.
• They may readily be judged adequate by comparision with previously analyzed systems.
• They are of uniform size, have no more than two points of fixation, no intermediate restraints and fall within the limitations of the empirical equation from ASME B31.3.

The following guidelines may be used for defining stress critical lines :

• Process lines to and from strain sensitive eqipments such as pumps, compressors and blowers.
• Lines to and from steam turbines and reactors.
• Lines with operating temperature differences from base temperatures >= 180°C for pipe sizes 6" and larger and >= 330°C for pipe sizes 4" and smaller.
• Process lines in toxic services for verification of flange leak tightness.
• Two phase flow piping such as blowdown, and flare piping.
• Non-metallic piping with design temperature above 50°C.
• All category "M" piping as defined in ASME B31.3.
• Lines connected to wellheads (applies to onshore and offshore oil and gas installations)
• Lines subject to excessive deflection resulting from differential settlement (e.g. lines connected to storage tanks), structural and/or equipment displacement.

Typically, engineering analysis, by visual inspection and quick manual calculations, are performed on the following systems:

• Lines >= 4" to air coolers.
• Lines >= 16".
• Lines with operating temperature differences from base temperatures >=80°C for piping sizes 6" and larger.
• Lines subjected to vacuum or external pressure such as jacketed lines.
• Relief systems - closed or relieving to atmosphere