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Fitness for purpose of flanges built up by welding

At a late stage in the construction of a gas processing facility it was found that a number of flanges for the high pressure piping had been purchased to a lower strength specification than the pipe itself. A subsequent analysis suggested that some flanges might be over-stressed in service. The construction contractor decided to rectify this situation by building up the flange hubs by welding. Although finite element analysis indicated that the strength requirements for the flange hubs would be met by this practice, insufficient attention had been given to the effects of welding on the Charpy toughness of flange substrate or the toughness of the weld metal overlay itself. The operating oil company was therefore concerned over the risk of fracture initiating from the weld overlay or its heat affected zone, particularly at the minimum design temperature.

TWI was asked to carry out a fitness for purpose analysis. This dealt with a variety of combinations of pressure and temperature. The original design specified a minimum operating temperature of -46°C. Whilst low temperatures can arise in the pipework because of rapid depressurisation, by definition, the pressure during these excursions will be lower than the normal operating condition. Following extensive process modelling (elsewhere), the operator decided that more realistic estimates of the extremes of pressure and temperature were 430bar at -29°C under normal operation to 18bar at -42°C during repressurisation following a rapid blow-down. These data was supplied to TWI who then carried out an engineering critical assessment (ECA) according to the procedures of PD 6493 (now BS 7910) using the revised process conditions.

It was found that the flanges were fit for service providing that no surface breaking flaws greater than 2mm deep by 8mm long were present. It was recommended that in order to avoid the occurrence of such flaws, or of the possibility of them remaining undetected, the toes of all weld overlays should be inspected with particular care using a combination of ultrasonic and magnetic particle inspection. It was further recommended that any indications should be ground back to sound material.

On this basis, the operator decided to accept the built up flanges. This decision was of great commercial value since, to obtain replacement flanges, would have led to an extensive delay in bringing the plant on stream.

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