PSI - Issue 60

Harikrishna S / Structural Integrity Procedia 00 (2019) 000–000

Harikrishna S et al. / Procedia Structural Integrity 60 (2024) 700–708

701

1.0 Introduction Reinforced cement concrete (RCC) structures have innumerable applications in all spheres of infrastructure. Concrete can be casted in a variety of shapes &sizes and can be designed to support any types of load conditions. RCC structures can be in service for decades without much maintenance, which qualify them as a desirable material for various construction jobs. But in industrial facilities, these structures are subject to a highly adverse conditions which is affecting their healthiness. Exposure to the corrosive environment, thermal changes, and acid attacks are not uncommon. Such RCC structures must be designed to resist all such exposures. An example of the same is hydrocarbon processing industry (HPI). Sulphur Recovery Unit (SRU) is a typical process plant in HPI. The crude oil, which is the ore for all petroleum products contains significant amount of sulphur. In oil refining industry, after distillation the fuels are hydrotreated for removal of sulphur to meet the applicable emission norms. The sulphur thus separated is generated as hydrogen

sulphur gas (H 2 S), absorbed in DEA (Di-Ethanol Amine) and sent to the Sulphur Recovery Unit (SRU) to recover sulphur. The generated molten sulphur is maintained in a liquid phase (at temperatures ranging from 130 °C to 160 °C) & stored in RCC sulphur pits, constructed below ground level to facilitate gravity flow. These sulphur pits are made with thick RCC walls & are covered with heavy pre-cast RCC slabs also. An acid resistant brick lining is also provided over walls.

Fig. 1. Typical layout of sulphur pit[1].

1.1 SIGNIFICANCE Over a period in service, minor pores may be developed in brick lining and intrusion of deleterious materials can happen. Thus, RCC walls are exposed to acidic gases, acidic vapor, sulfuric acid vapor fumes, moisture etc at high temperature. An acidic environment lowers the pH value of the concrete and leads to impacting the passive film protecting the reinforcing rebars. Also, the sulphur vapor can combine with moisture to create sulfuric acid (H ₂ SO ₄ )/ sulphurous acid (H ₂ SO ₃ ). This combination of high acidic environment & high temperature results in an extremely aggressive and corrosive environment. Sulphur can react with water at temperatures greater than 80°C. Sulphur can cause localized corrosion of mild steel when they are in direct contact. Iron sulphide films/scales also may form on the surface of the steel. These all result in delamination, spalling, cracks, and reinforcement corrosion instituting threat to structural integrity. Also, deterioration of roof slabs of pits also happens due to exposure to vapour forms resulting in corrosion of steel reinforcement, spalling of concrete, deposition of sulphur etc. Proper identification of such damages, their root causes and repair methodology shall aid HPI industry in effectively handling the risk and mitigating unplanned shutdown of facilities. 2.0 System Description As a case study approach, this paper deals with the concerns related to sulphur attack in RCC sulphur pit in Sulphur Recovery unit of Bharat Petroleum Corporation Ltd -Kochi Refinery (BPCL-KR). BPCL-KR is located at Ambalamugal in Ernakulam district of Kerala. It is a 15.5 MMTPA refinery fully owned by Bharat Petroleum Corporation Ltd, and its major products include LPG (Liquefied petroleum gas), Motor Spirit, Naphtha, Aviation Turbine Fuel, Kerosene etc. One of the process plants in KR is Sulphur Recovery Unit-II (SRU-II) which was commissioned in 2008. The sulphur pit there has two compartments, each having a dimension of around 14m(L) x 5m (B) x 4.2m(H). The pit is formed by RCC walls of 550mm thick all around and a middle wall separating the two compartments. The wall is coated with