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Warm pre-stressing of pressure vessels

Connect, no.66, April 1995, p.1

High pressure testing trials highlight enhanced structural integrity and cost benefits

WPS brings many operational advantages to vessels such as these. Courtesy ICI
WPS brings many operational advantages to vessels such as these. Courtesy ICI

Proof testing of components and structures is often required for fabrication codes and revalidation purposes. The Structural Integrity Department at TWI has been investigating warm prestressing effects from pressure tests since the early 1980s. Results of investigations have already allowed some major chemical and oil and gas companies to make substantial savings.

Warm pre-stressing (WPS) increases fracture toughness (K c), enhancing the structural integrity of a component; reduces the need for post-weld heat treatment (PWHT) and justifies the avoidance of in-service repairs of defects.

In pressure tests a vessel is taken up to 1.5 times design pressure, above design temperature. As the temperature is raised, material toughness increases. Deformation will occur more readily around any defect present.

Engineers at TWI have shown that this deformation in turn increases low temperature fracture resistance in subsequent service - which gives enhanced structural integrity.

When a pressure vessel is put under the greater pressures involved in WPS, then the only further in service tests needed are regular NDT (non-destructive testing) checks for any propagating planar defects. (Ultrasonics will detect embedded flaws and magnetic particle inspection tests for surface flaws).

This work is now an ongoing part of TWI's core research programme, with further related projects scheduled for sponsoring companies.

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