Pipe Pressure Considerations And Definitions
Surge is defined as a rapid, very short-term pressure variation caused by an accidental, unplanned event such as an emergency shutdown resulting from a power failure. Surge events are characterised by high pressure rates with no time spent at the peak pressure.
In contrast, fatigue is associated with a large number of repetitive events. Many materials will fail at a lower stress when subjected to cyclic of repetitive loads than when under static loads. This type of failure is known as (cyclic) fatigue. For thermoplastic pipe materials, fatigue is only relevant where a large number of cycles are anticipated. The important factors to consider are the magnitude of the stress fluctuation, the loading frequency and the intended service life. Where large pressure fluctuations are predicted, fatigue design might be required if the total number of cycles over the intended lifetime of the pipeline exceeds 25,000. For smaller pressure cycles, a larger number of cycles can be tolerated.
Pressure range is defined as the maximum pressure minus the minimum pressure, including all transients, experienced by the system during normal operations.
Diurnal pressure changes
Diurnal pressure changes are gradual pressure changes which occur in most distributor pipelines as a result of demand of variation. It is generally accepted that diurnal pressure changes will not cause fatigue. The only design consideration required for this type of pressure fluctuation is that the maximum pressure should not exceed the pressure rating of the pipe.