Condition assessment and finite element modelling of a sulfur pit structure in the sulfur recovery unit of a gas plant

Sour crudes and gas with sulfur content more than 1.5% are encountered predominantly in the Saudi Arabian Oil/Gas fields. The molten sulfur compounds at temperatures ranging from 130 °C to 160 °C, extracted from the hydrocarbons are stored in large subterranean reinforced concrete tank commonly referred to as sulfur pits. These sulfur pits have steam heaters at the base of the pits to maintain sulfur in liquid phase. The reinforced concrete sulfur pits exposed to a hostile, corrosive and hazardous environment suffer from extensive deterioration in a short span of time (less than 10 years). Corrosion of reinforcing steel, delamination and spalling of concrete cover occurs in the walls and the roof of the sulfur pit. Heavy deterioration is generally observed in the soffit of the roof slab with incidents of the roof collapsing. The sulfurous acid fumes, which are formed when the molten sulfur is exposed to water or steam, attack the reinforcing steel, causing corrosion. This paper presents a case study of the assessment of deteriorated sulfur pit based on non-destructive and partial destructive investigations and development of repair strategy to extend its service life.