ABSTRACT

Cooling systems are prone to many corrosion deterioration mechanisms, one of which is Microbiologically Influenced Corrosion (MIC). Microbes are ubiquitous, they have been implicated in many major failures, and their control measures have a high economic impact on the oil and gas industry. This work investigates unexpected leaks observed on several cracked admiralty brass tubes that failed after 7 years in operation. Both metallurgical and molecular microbiological methods were used to understand the root cause of these failures. Scanning Electron Microscopy (SEM) analysis in all examined tubes (Cu-Zn alloy) showed localized pitting associated with cracks. X-Ray Diffraction (XRD) analysis carried out on the corrosion products confirmed presence of ammonic compounds (copper nitrate hydroxide-Cu2(NO)3(OH)3). The quantitative polymerase chain reaction (qPCR) technique used for microbiological DNA analysis revealed the coexistence of various strains of Sulfate Reducing Archaea (SRA), Acid Producing Bacteria (APB), Iron Reducing Bacteria (IRB) and Denitrifying Bacteria (DNB). It was postulated the lube oil cooler failed due to microbial assisted cracking driven by a collaborative metabolic reaction by the presence of microbial community. The study resulted in the development of a comprehensive control and monitoring plan to safeguard the integrity of the system and prevent damage recurrence in similar systems.

INTRODUCTION

In a hydrocarbon processing plant, a lube oil cooler of a closed cooling system leaked during operation. A total of 24 tubes out of 116 were identified leaking through hydrotest. This cooler is a part of a second pass reverse osmosis (RO) system that was in service for seven years. The RO system was dosed with a blend of chemicals to control corrosion including sodium nitrite and sodium molybdate. Design specifications and operating conditions of the unit is provided in Table 1. The system water composition is drinking water quality as prior to chemical dosing provided in Table 2. A tube sample was randomly selected out of the 24 failed tubes for metallurgical examination to identify the damage mechanism.

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