ABSTRACT

A method was developed to screen refining equipment, includingtrayed towers, for wet hydrogen sulfide (H2S) environments. After applying process modeling to simulate exposure environments, results were used to predict whether an aqueous phase was likely to occur and to predict concentrations of the dissolved sulfides and other corrosive species in that aqueous phase. Environments conducive to wet H2S cracking were assessed. Examples were given to demonstrate the calculation method. For trayed towers, operating temperature was shown to be a key parameter in determining if an ordinarily assumed dry tower will be prone to aqueous condensation. Tower trays at lower temperatures were more likely to be exposed to a free water phase. Because some levels of H2S are inevitable in refinery streams, as long as operating conditions allow an aqueous phase, the resultant wet H2S concentration should not be overlooked. Dissolved sulfide levels on different trays can be predicted using the method. For other types of equipment, water content and temperature can be used to determine if wet H2S damage is likely.

INTRODUCTION

Sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) in wet hydrogen sulfide (H2S) environments are common and costly failure modes for carbon steel equipment in the petroleum refining industry.l-3 Common mitigation methods include metallurgy upgrades, stress relief, inhibitor injection, and/or control of corrosives. Since process environments dictate whether a piece of equipment is susceptible to wet H2S cracking, a cost-effective approach to damage control is to identify the potential severity of cracking by screening the process environments before applying mitigation methods. The concept of environment screening is not new. The same approach has been used, especially by plant personnel, to set priorities in inspection and maintenance for other types of aggressive service, usually involving condensing waters and corrosive chemicals. A very rudimentary and conservative estimate comes from the use of steam tables, or other simple algorithms based on steam tables, to estimate whether a piece of equipment is likely to be exposed to an aqueous phase. This inaccurate and overly conservative method often results in unnecessary inspection, corrosion control, and maintenance costs. Even if the existence of an aqueous phase can be determined, it is not easy to estimate the amount of dissolved sulfide and other relevant corrosives, which will impact the extent and severity of cracking.4 A calculation method based on an accurate stream property determination was developed to establish whether a piece of equipment is prone to wet H2S cracking based upon well-known criterion.4 This method can be applied to piping, exchangers, vessels, and trayed towers.

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