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Robert Jenkins pressure vessel

The vessel was 34m long in two sections which were joined by a flanged and bolted connection. One section was ~4.5m in diameter with a wall thickness of 41mm. The second section, in which the failure occurred, was 2.7m in diameter with a wall thickness of 28mm. The vessel was constructed according to ASME Section VIII. The larger diameter section was post-weld heat treated, while the failed section was in the as-welded condition.

The failure initiated in the vicinity of the fillet weld of a manhole compensating plate on the main shell. The fracture, which was brittle, extended in two directions through the strake containing the manhole into the adjacent strake on one side and the dished end on the other before arresting. The overall length of the fracture was 4.3m with a subsidiary crack of 1.5m long present on the other side of the manhole.

The vessel material was a C-Mn ASTM 515 Grade 70 (1967) steel for intermediate and high temperature use produced in the UK to coarse grained practice and supplied in the as-rolled condition. The yield strength of the material was in the region of 300N/mm2 and the Charpy properties of the strake in which the fracture initiated were poor: 8-20J longitudinal and 11-14J transverse at 10°C with fracture appearances of at least 90% crystallinity.

 

Examination of the fracture faces found evidence of an initial crack 115mm long at the fillet weld toe of the compensating plate on the external surface of the shell. This defect extended approximately halfway through the plate thickness. Fracture mechanics analyses carried out as part of the failure investigation showed that the applied stress intensity factor at the defect was close to the measured values of fracture toughness obtained at 12°C.

The origin of the initiating defect was thought to be hydrogen cracking in the HAZ of the fillet weld. Regions of martensite with hardnesses in the range 500-515 VPN were found near the toe of the fillet weld. A hardness survey of the area of the toe of the weld measured hardnesses from 393 to 496 VPN. (In comparison, the hardness of the parent plate was 190 VPN). With a carbon content of 0.33%, the plate was considered to be liable to hydrogen cracking in the high hardness regions. Although the manufacturer's recommendations for drying the electrodes were followed (150°C for 1 hour), it was suggested by the investigators that this was insufficient given the hardness levels present in the HAZ.

Tests on the failed plate showed the chemical composition in the initiation region to be susceptible to liquation cracking. It was postulated that the initiating defect may have formed as a liquation crack which extended by hydrogen cracking under the conditions of high restraint and residual stresses present at the fillet weld toe.

An investigation was made into the effects of post-weld heat treatment on the welds in the as-welded failed section of the pressure vessel. It was found that little improvement in toughness properties was obtained in the range of temperatures associated with hydrotesting although the peak welding residual stresses were reduced which would be beneficial.

This failure arose from a material problem: the toughness of the parent plate was low and it was susceptible to liquation cracking and formation of high hardness zones in the HAZ, leading to probable hydrogen cracking. The selection of a fine grained steel with superior toughness properties meeting specified levels would give greater protection against this type of failure during hydrotest or during service should the temperature be reduced below normal operating temperature.

The effect of temperature on material toughness is critical. Material selection should take into account all possible service temperatures, including pressure testing temperatures. The possibility of hydrogen cracking needs to be considered when establishing welding procedures.


This is a case history taken from Report 632/1998 . For further case histories, please contact us

Professional & WJS members and non-members of TWI can obtain further case histories by reading the following article:-

Hayes B 
Six case histories of pressure vessel failures 
Engineering Failure Analysis, vol 3, no 3. 1996. pp.157-170.

For more information, please email contactus@twi.co.uk

Robert Jenkins Pressure vessel failure
Robert Jenkins Pressure vessel failure
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