As a by-product of the hydrocarbon refining process, molten sulfurous compounds are generated, one of which is elemental sulfur. When conveyed and contained in a refining or rehandling facility, molten sulfur is generally stored in subsurface tanks of structural concrete construction. Recognizing molten sulfur exists at 300oF, these tanks, or Sulfur Pits, are subject to an extremely harsh environment daily. Typically constructed of reinforced concrete, Sulfur Pits can suffer severe deterioration from construction "defects." These defects can be associated with substandard original concrete mixture ingredients, such as having an improper portland cement type, reactive coarse aggregates or the composite mixture having an elevated Water-to-Cement Ratio. Additionally, should routine Sulfur Pit mechanical and physical maintenance not be performed regularly, aggressive deterioration mechanisms can initiate and significantly affect the integrity of the "box-in-the-ground."
A major refining company in Texas began to experience degradation to their below grade reinforced concrete Sulfur Pit when water began to enter the pit through cracks in the concrete. The ingress of water into Sulfur Pits is of serious concern as it mixes with the sulfur, forming sulfurous acidic compounds - extremely damaging to concrete.
Having extensive experience, developed by performing numerous Sulfur Recovery Unit (SRU) repair projects, Structural Preservation Systems (SPS) educated the Owner about the unique aspects associated with Sulfur Pit repair. Enlightened, the Owner realized the benefit of having SPS' Sulfur Pit expertise on-board and awarded them the project. SPS began by performing a Condition Survey and Evaluation (CS&E) to assess the condition of the existing structure. This process identifies the root cause of located concrete deterioration, qualifies and then quantifies the required repairs at a structure. The CS&E revealed extensive and severe deterioration affecting the structural integrity of the concrete walls. As such, a comprehensive restoration would need to be implemented or the Owner risked complete structural failure (i.e., wall implosion) of the Sulfur Pit.
The Sulfur Pit metrics were 15 feet in width by 30 feet in length with an average depth of 5 feet. The pit, considered to be relatively small as far as Sulfur Pits go, had a sloped floor on which the heating steam coils rested. Time, as with most SRU projects, was crucial as it had to be completed during a planned shutdown of the facility in less than two weeks. Short-duration outages generally require extensive preplanning and logistical support. For this project, two crews of SPS personnel were required to work around-the-clock on the pit repair.
Beginning the repair process, SPS removed all loose, delaminated concrete from the existing pit walls, which equated to the removal of partially detached concrete nearly 4 to 6 inches deep. Furthermore, sulfur impurities called carsul, or "sulfurcrete," had settled to the bottom of the pit, overlaying the pit floor and needed to be excavated using pneumatic chipping hammers. All leaking cracks along wall surfaces were sawcut, impacted with lead wool and covered with rapid setting hydraulic cement, employing an age-old process called "chinking" that stopped water ingress. To help prolong the life of the Sulfur Pit, SPS recommended and installed high-performance concrete materials incorporating dense, shrinkage compensating, sulfate-resistant cements and non-reactive coarse aggregates. An engineered structural liner was designed, incorporating extremely durable materials of construction including reinforcing, anchorage and waterstops that were cast-in-place within the high-performance concrete matrix.
One unique element to this project was the fabrication and installation of new Sulfur Pit modular roof panels. The previous roof panels had also experienced extensive deterioration, much like the pit, and needed to be removed and replaced. SPS designed, fabricated and installed the new roof panels. The reinforced concrete modular roof panels were pre-cast off site in a controlled environment incorporating the same high-performance concrete as that used in the Sulfur Pit wall restoration. The roof panels, once cured, were then transported to the job site and installed in a sequential manner by SPS to ensure a water-tight roof structure for the pit.
This challenging project was completed safely, on schedule and within budget - allowing the facility to maintain its scheduled start-up date. The Owner was extremely pleased with the innovative approach to the project as well as the end result of SPS's efforts. Clearly, the Owner found the CS&E process to be a valuable tool and one of the main factors for the repair program's success. By conducting the assessment at the onset of the project, SPS was able to provide the Owner with accurate numbers for the cost of the repair and a realistic construction schedule duration.