Case Study - Failure Caused by Mercury Embrittlement in MCHE Main Cryogenic Heat Exchanger in a LNG Plant

Abstract

This is a case study explaining the failure of an inlet nozzle to a Main Cryogenic Heat Exchanger (MCHE) used in a LNG plant, causing the refrigeration compressors to be in shutdown for 6 weeks. This resulted in loss of LNG production. The failure analysis of the nozzle indicated the cause of failure was Mercury embrittlement that is made of Aluminum, which caused flange leakage. Main Cryogenic Heat Exchanger (MCHE) is made of aluminum and part of refrigeration process in a LNG plant MR (mixed refrigerant) loop. The Mercury embrittlement is a form of liquid metal embrittlement (LME) and a complex metal fracture mechanism that occurs without any warning indications. Mercury embrittlement, being a significant problem in LNG Plants using aluminum MCHE, have led to number of major plant incidents worldwide. Mercury is always present in natural gas feed stocks, sometimes in quantities sufficient to cause severe attack and failure of Aluminum heat exchanger. To avoid Mercury embrittlement failure in aluminum heat exchanger tight limits have been placed on allowable mercury levels in natural gas passing through Aluminum heat exchanger. The natural gas is pretreated with mercury removal units before entering the refrigeration units in a typical LNG plant. Root cause of this failure is identified as the failure of mercury removal units to remove the mercury efficiently. In this case study, Different type of mechanisms has been highlighted by which the mercury degrades the aluminum heat exchanger that includes LME, Amalgamation and Amalgam Corrosion. The importance of mercury removal units in LNG plants is emphasized. Different types of mercury removal technology are explored and discussed. This case study Introduce the newly developed high activity mercury removal absorbents that allows greater flexibility in the design of LNG plants. These absorbents allow for smaller beds, which coupled with new reactor designs improve savings in compression costs. It is now possible to locate the mercury removal units upstream of the main gas processing plant and thus avoid mercury emissions and contamination of any co-produced Natural Gas Liquids (NGL) in a LNG plant.