In 1997, Altona Refinery replaced reformer feed/effluent heat exchanger shell/channel joint studs in UNS N07750 in order to improve the stress relaxation properties of the studs, and therefore the reliability of these joints. During the subsequent 6 years of service, some minor joint leakage occurred, which resulted in steam bands around the joint being recommissioned and operated continuously.
During a 2003 turnaround, the exchangers were disassembled for inspection, and a number of studs were found to be cracked and/or fractured. A failure analysis identified the damage mechanism to be consistent with a form of hydrogen-driven intergranular stress corrosion cracking; UNS NO7750; steam; bolt; in a high temperature steam/water environment. A literature review identified the optimal UNS N07750 microstructure for resistance to this damage mechanism, and further work was conducted to characterise the properties of the as-received alloy in relation to this optimal condition.
This paper presents the results of all work conducted, and provides an in-depth examination of this rather complex alloy system in terms of some counter-intuitive results concerning grain boundary carbide precipitation and resistance to SCC. This reiterates the complex nature of materials and damage mechanisms, identifying yet another "exception to the rule". Finally, this paper discusses the necessary compromise between fracture toughness and resistance to the intergranular stress corrosion cracking mechanism, and suggests further work which may be useful in further characterising the damage mechanism.
Background
Altona Refinery is a 100 tbd nameplate refinery, situated in Melbourne, on the South-East coast of Australia. The refinery produces a full range of petroleum fuels, i.e. gasoline, jet fuel and diesel, as well as liquefied petroleum gas. The refinery produces gasoline blendstock from pre-treated naphtha in two reformer units. These units utilise several spherical reactor vessels in series with preheat fired furnaces. A mixture of heated naphtha and hydrogen passes through a fixed bed of platinum/rhenium catalyst in each reactor, which converts olefins to paraffinic material. The reaction is endothermic, and so preheat furnaces are required to provide heat to each reactor feed stream. The initial reactor feed is also preheated prior to the first furnace in a series of shell and tube heat exchangers, which exchange heat with the reactor effluent material from the last reactor. Figure 1 provides a process flow diagram for a reformer unit.
Typical operating conditions for the reactors are 1550 KPa and 480-520°C. The hotter feed/effluent exchangers operate at feed outlet temperatures (shell) of 430oC, and effluent inlet conditions (channel) of 510oC.
The refinery has had a history of emissions (low-level leakage) from the shell-to-channel joint of these exchangers. The cause of this was thought to be a combination of stress relaxation in the bolts used to close the joints and the inherent design of the exchanger nozzle joints, which exacerbated the tensile loading on the shellto- channel joint. The bolt material used (ASTM A193 Grade B16) is a Fe-Cr-Mo-V ferritic alloy of typical composition 1Cr, 0.6Mo, 0.3V. It has good tensile and stress relaxation (creep) properties, and is normally the bolt material of choice for the operating temperature range. It should be noted that, while the exchanger shell and channel are insulated using mineral wool and steel cladding, the joint is not, and hence the average bolt temperature is somewhat lower than the exchanger surface temperature.
