Neutralization Pits – Measures to Extend Service Life

Abstract

Some processes in the oil/gas plant generate wastewater that contain acids/alkalis with pH varying from 1 to 14. Stringent environmental norms require wastewater to be neutralized prior to disposal, which is normally carried out inside the concrete neutralization pits. These concrete pits tend to deteriorate in long-term as concrete and steel components have prolonged exposure to aggressive environment. This paper presents investigations undertaken to ascertain the main causes of concrete deterioration and efficient repair method adopted by GASCO for achieving improved service life and optimizing maintenance cost. Though pits are designed for a service life of 30years, defects such as cracking and spalling of concrete, peeling of coating, corrosion of reinforcement and embedded steel are commonly observed within the first 5 years of construction. Study of existing design and construction specifications were carried out followed by visual inspection and investigative tests such as chemical analysis, NDT tests and petrographic analysis of concrete to identify the sources of deterioration. After identification of the root cause, repair options were evaluated and remedial measures were recommended. These tests revealed significant concentration of H2SO4 & NaOH in the wastewater and sulphate content in concrete. Improper selection of coating material led to peeling/delamination, resulting in acid attack on concrete and subsequent corrosion of reinforcement. In general, even concrete comprising of sulphate resistant cement cannot provide complete resistance to sulphuric acid, without additional acid resistant coating. Sulphuric acid is highly corrosive and reacts with calcium compounds in concrete, to produce soluble calcium sulphate (CaSO4) and water (H2O) resulting in concrete disintegration. The corrosion of reinforcement leading to an increase in volume of reinforcement and causing cracking/spalling of concrete is the consequence that can jeopardize the structure integrity. Remedial measures involve the use of micro concrete with silica to repair the damaged surfaces and a protective coating of dense epoxies with silica fill. Use of silica in concrete fills the pores and decreases permeability. Epoxy coating has excellent bond, low shrinkage and moisture resistant properties and it prevents buildup of stresses that can cause delamination. Recommendations for improved service life include proper selection and optimizing embedded items in aggressive environment, proper selection of cement, water cement ratio, curing, and suitable coating. In addition, it is also recommended to use mechanical mixers to achieve a homogeneous and neutral liquid within the pit. Chemical attack on concrete structures is an important issue in wastewater treatment units. Early detection of deterioration with the application of sound remedial techniques and materials based on comprehensive investigations ensures integrity of the structure besides reducing repair costs and plant outages. This issue is common for process plants and the explored measures can be applied across the industry.