Abstract
Refining industry cooling systems are prone to many different corrosion deterioration mechanisms, one of which is Microbiologically Influenced Corrosion (MIC). Bacteria are ubiquitous, they have been implicated in a number of major failures and their control measures have a high economic impact on the refining industry. This work investigates the unexpected failures and leaks observed in several seawater heat exchanger tubes, which resulted in plant shutdowns and significant production losses. Both metallurgical and molecular microbiological methods were deployed to comprehend the root cause of these failures. Metallurgical analysis in all examined tubes (70-30 Cu-Ni and Al-Brass) revealed localized pitting with circular and cup-like morphology typical of those formed by MIC attack. The DNA subsequent sequential analysis revealed the co-existence of various strains of Acid Producing Bacteria (APB) and Iron Oxidizing Bacteria (IOB) in the collected corrosion products with a noticeable absence of Sulfate Reducing Bacteria (SRB). Lack of adequate chlorination and presence of hydrocarbons and other organic matters that naturally end up in the open transport channel as a nutritional source led to flourishing microbial population including corrosion-causing species and subsequent rapid MIC attacks. Cost effective short and long term action plans including criticality assessment, timed replacements, selective cleaning, chlorination dosage adjustments and carbon source identification monitoring were considered. The study resulted in the development of a comprehensive control and monitoring plan to safeguard the integrity of the system.
Introduction
Cooling water systems are designed to control the process temperatures and transfer waste-heat from the process. There are a number of water sources that can be used as cooling media such as potable water, river water, ground water, sea water and recycled water (tertiary treated sewage effluent). In the oil and gas industry close to the sea, the favorable medium for cooling is seawater because it is readily available, inexpensive, has an ability to absorb large amounts of heat per square area, and can be safely discharged.