Abstract
During a pre-weld bakeout, hydrogen efflux was monitored in real time using the hydrogen collection method at high temperatures. A circular section of a 50 mm thickness alkylation unit acid storage vessel dome which had developed severe hydrogen blistering was removed, and pre-weld bake-out applied using heat pads placed within twelve inches of the cut-away section perimeter, on the dome’s exterior face. A ring of eight hydrogen collection probes were placed under the heat pads, about six inches from the cut-away perimeter. One probe became blocked and failed to deliver measurement. The remaining seven delivered flux values indicating effective bake-out at five sites, and the need for extended bake-out at two neighbouring sites. The efflux trend at these sites strongly indicated initially high concentrations of hydrogen liberated by the bake-out. Corresponding efflux measurements at corresponding positions on the dome’s interior face were much lower. A model of hydrogen diffusion for the bake-out, incorporating a high density of trapping sites located at depths of 5-15 mm beneath the monitored exterior face provided model flux transients consistent with field flux transients and spot measurements. The work demonstrated that pre-weld heat treatment can be assessed, extended or reduced in real time, on evidence provided by spot high temperature hydrogen efflux measurements at a small number of sites. An additional bake-out is presented in which near zero hydrogen effluxed.
