Damage Control: Wet H2S Damage Mitigation

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

As outlined in the previous two issues of Damage Control, wet H₂S damage mechanisms, including hydrogen blistering, hydrogen induced cracking (HIC), stress-oriented hydrogen induced cracking (SOHIC), and sulfide stress cracking (SSC) can require significant engineering and inspection resources to effectively manage in many refineries, process units, and pipelines worldwide. The best way to minimize long-term inspection and maintenance costs related to wet H₂S damage is to prevent the damage from occurring in the first place (or at least delay damage initiation and slow down damage progression rates). This can often be accomplished through good engineering design, fabrication, inspection, and process operating practices. This version of Damage Control will offer practical steps to mitigate these different forms of wet H₂S damage.

Equipment Design, Fabrication, and Repair Considerations

SOHIC and SSC are often driven by tensile weld residual stresses near weld deposits. Furthermore, high-hardness welds and heat affected zones (HAZs) are known to increase the propensity for SSC. Given this, specifying post weld heat treatment (PWHT) during the equipment design/fabrication phase offers owner-users an actionable step to mitigate these forms of in-service wet H₂S damage [1]. Specifically, PWHT significantly relaxes weld residual stresses (see Figure 1) and offers beneficial tempering or softening of high hardness weld cover passes and attachment welds that routinely serve as initiation sites for SSC and other forms of cracking [2,3]. As Figure 1 suggests, the higher the PWHT hold temperature and the longer the hold time, the more the residual stress relaxation. In general, PWHT is less effective at preventing HIC and hydrogen blistering as these damage mechanisms often occur in the base metal away from welds, but overall, PWHT does notably reduce the risk for unstable brittle fracture [4].

For in-service repairs, local PWHT layouts should be carefully engineered, and spot/bullseye layouts should generally be avoided to minimize the likelihood of inducing detrimental residual stresses from severe metal temperature gradients [5]. Any local PWHT configuration should be evaluated by an experienced engineer and may require advanced analysis to legitimize. Furthermore, alternatives to PWHT such as controlled deposition welding (temper bead techniques) and weld preheat in lieu of PWHT can promote softening of high-harness welds and HAZs, but these approaches offer little relaxation of weld residual stresses. This assertion is demonstrated in Figure 2, where weld residual stress relaxation is compared for no weld preheat and a 300°F preheat. These results are based on computational weld simulations of a 4-pass nozzle-to-head junction weld [3]. Residual stresses through the thickness of the head (adjacent to the weld deposit) reveal a stress relaxation on the order of 10% for a 300°F preheat. In contrast, conventional PWHT generally relaxes nominal residual stresses by approximately 70% - 80%. Based on the lack of residual stress relaxation, alternative methods to PWHT should be carefully considered, and in many cases excluded, for any repair situation in wet H₂S or aggressive environmental service [3].

Weld and HAZ hardness checks should also be utilized following fabrication or weld repairs, with 200 BHN reflecting a common practical hardness limit utilized to mitigate SSC [1]. Studies have also shown that low heat input welds (e.g., single pass welds) may exhibit an increased propensity for SOHIC in the HAZ adjacent to the weld deposit [6]. It is noted that performing PWHT on equipment that exhibits damage may propagate existing cracks or exacerbate HIC/blistering. Additionally, in certain wet H₂S environments, a properly engineered hydrogen bake-out (i.e., raising the metal temperature to allow for gradual hydrogen outgassing) may be recommended prior to conducting any weld repairs to improve weldability and reduce the risk for hydrogen cracking. Low-hydrogen electrodes should also be used for all fabrication and repair welds.