How do customers allow for surge and fatigue loading in the design?
Surge and fatigue occur in pipelines due to the normal operations of, for example, pumps shutting down or valves being operated quickly. Due to the incompressible nature of liquids the phenomenon is usually associated with water distribution mains and pumped sewer mains.
Surge can be described as short term pressure rises above the static operating pressure. This is generally as a result of water hammer where the sudden changes in fluid velocity within the pipeline, as pumps and valves are operated, are converted to increases in fluid pressure. As the velocity stabilises the fluid pressure reverts to its static operating pressure. Extensive testing has confirmed that PE100 pipe can be used in the following surge conditions;
|Overall service coefficient||Typical application||Surge pressure above Maximum Operating Pressure (MOP)|
Fatigue is associated with the repeated operation of the pumps and valves over a long period causing cyclic pressure variation. It is the frequency of these events as well as the amplitude of them that is critical. Under these conditions the theory of linear fatigue damage accumulation applies (Miners rule; ISO 13760).
With the introduction of high toughness PE100 pipe there have been few, if any, problems reported in these pipes operating under surge and fatigue conditions. The empirical evidence has been backed up by extensive laboratory testing. Fatigue is not a concern with high toughness PE100, and surge pressure well in excess of MOP can be sustained without damage.
The reference document in the UK is the ‘Water Industry Guidance Note' (IGN 4-37-02 ) which requires no downgrading of PE100 pipes operating under fatigue loading. Moreover PE100 materials can withstand surges of up to twice the MOP avoiding any need for derating.
It is recommended that, for pipelines where surge and fatigue conditions will apply, a detailed surge analysis is carried out to identify the peak surge pressures. The simple calculations can then be carried out with reference to the design curves in IGN 4-37-02.
UK Water Industry Information & Guidance Note IGN 4-37-02, March 1999, Issue WRC plc.
Marshall G.P., Brogden S: Pipeline Dev Ltd, Shepherd M.A.: Thames Water Utility Ltd. Evaluation of Surge and Fatigue Resistance of PVC & PE Pipelines Material for use in the UK Water Industry