Design Factor (f) 2
Design Factor (f) 2 : as with most engineering systems a factor of safety is applied to pipelines: this is known as the design factor: and may have a significant contribution when deciding on the route of a pipeline. The design factor is the relationship between the maximum stress allowable at a particular location and the specified minimum yield strength of the pipe material. [The specified minimum yield strength of a pipe is not the point at which a pipe section would fail catastrophically, ie the ultimate tensile strength, but that stress which would cause a section of pipe material to elongate by 0.5% of its original length. For example, for most steels used in gas transmission pipelines the ultimate tensile strength is between 15%-70% greater than the specified minimum yield strength.] For most pipelines on land the calculation of stress levels in the pipe wall is generally confined to those stresses resulting from the internal pressure of the fluid. Currently, the design factor for pipelines in open country (or 'rural') is limited to a maximum of 0.72. Where pipelines approach built-up areas this generally reduces to 0.3 for gas pipelines but higher values up to 0.5 may be allowed for gas pipelines designed to IGE/TD/18 if the nominal wall thickness is not less than 19.1 mm or if additional external protection is afforded or if a safety evaluation is carried out. Pipelines in heavily built-up areas (ie town and city centres) are limited to 7 barg operating pressure and the design factor becomes almost irrelevant. The 0.3 design factor was selected by analysis of experimental data obtained from pipeline failure tests. It was found that pipelines with a design factor of 0.3 or less would generally not fail by long-running fractures: they would tend to leak rather than rupture (the 'leak before break' principle)
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