Abstract

In our plant, sulphur is recovered from gas in Sulphur Recovery Units and stored in Liquid Sulphur tanks, before granulation and pelletizing for shipment. Underground concrete storage pits form an important element of the Sulphur Recovery, Treatment / export cycle, providing an intermediate containment to ensure that molten sulphur temperature is maintained within the operating range. These concrete pits tend to deteriorate rapidly due to high temperatures and contact with concentrated sulphur. This paper discusses the pit design and construction practices, and efficient repair/maintenance techniques of ageing containments to ensure improved service life while optimizing maintenance costs.

The structure consists of reinforced concrete walls, base and top slab, which supports the Sulphur transfer pumps and associated piping / instrumentation. At bottom, steam coil system is provided where Low Pressure steam is used as heating medium to maintain molten Sulphur at high temperatures.

Primarily two concepts are followed for protecting Sulphur pits: (a) Internal surface lining with acid resistant / refractory bricks and insulation, and external waterproofing, or (b) Externally applied waterproofing and thermal insulation, with no internal coating or lining.

Since exposure to high Sulphur concentration and high temperature renders the concrete susceptible to sulphate attack and deterioration, it is essential to carry out periodic inspection and maintenance works in these pits. The top slab is susceptible to higher corrosion due to contact with vapours.

Evaluation of existing pits with above designs revealed contrasting conditions. Refractory / brick lined pits indicated severe deterioration of lining, with traces of corrosion in concrete surfaces. Underside of slabs also showed deterioration of tiles, however, slab concrete was relatively in better condition. Unlined pits revealed the concrete surfaces of walls and base slab to be in reasonably good condition. However, underside of top slab was found to have severe deterioration, beyond repair mandating replacement. Physical and chemical tests indicated reasonably good condition of reinforcement steel, retention of concrete strength, and no loss of concrete cover. However, sulphate content near the surface was found to exceed code permissible limits.

Repair procedures for lined and unlined pits vary, with primary focus in lined pits being selection of appropriate lining for Sulphur resistance and thermal insulation, while for unlined pits, the main concern is selection of suitable concrete mix design and ensuring proper compaction and curing.

Deterioration of concrete in process critical structures like liquid sulphur pits is an important issue, which could potentially impact the Sulphur recovery and export process. Both lined and unlined pits are found to be suitable for the purpose, however, each has its own pros and cons. Maintenance and repair procedures need to be in place specific to the type of design and construction of these pits, to improve service life.

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