Connect, no.156, September/October 2008, p.6

Safety and reliability concerns with P91 pressure vessels and piping

There are currently concerns within industry regarding the integrity of Grade 91 steel piping and boiler headers in elevated temperature service.

Grade 91 steel was introduced in the 1980s as an advanced ferritic steel and has been used extensively since then as P91 in high temperature headers and steam piping in new plant as well as in retrofit refurbishment applications.

Evidence from recent premature weld failures in P91 steel together with long term data on cross-weld samples now suggests that design may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000hrs leading to safety and reliability concerns worldwide.

As a result, ASME is considering a reduction in the allowable stress for P91 welded pipe. Consequently, the stress imposed on a welded pipe currently being commissioned or in service is greater that which is now allowable. In turn, the expected lifetime may be reduced considerably. The performance of P91 components is also highly sensitive to fabrication heat treatment and there have been instances where improper control has led to worries regarding premature failure. TWI is amassing an industry experience database in this area with the aim of assisting plant operators to assess the risk of premature failure. To find out more please contact power@twi.co.uk