Failure analysis of cracks occurred on SS304L tubes of shell and tube heat exchanger during its first start up in phases 23 gas refinery of South Pars Gas Complex

摘要

Stress corrosion cracking in austenitic stainless steels is one the predominant failure root causes in oil and gas refineries. In this paper failure analysis of SS304L tubes in a shell and tube heat exchanger within one of the gas refineries in South Pars Gas Complex (SPGC) has been investigated. Tube side medium in this heat exchanger was sour gas coming from high pressure separators and shell side medium was low pressure steam in order to heat the gas during winter to prevent hydrate formation. To make sure of hydrate prevention, there is also a continuous injection of mono-ethylene glycol (MEG) into the gas stream before introducing the gas heater. This MEG may contain high levels of chloride and calcium ions which may cause problems in injected lines and proceeding equipments. This gas heater failed after a few days of startup. In visual inspection many cracks observed on the tubes and also deposits found inside the tubes. Samples taken from the deposits and XRD analysis were performed. Also, specimens were cut from the cracked area of tubes and analysed by scanning electron microscope. Diffraction pattern of XRD analysis indicated remarkable content of complex phases of chromium and calcium elements in the composition of the deposit. SEM images from cracked area demonstrated branched intergranular cracks. High chloride content of injected MEG and chromium depletion around the grain boundaries in the presence of tensile stress due to tubes internal pressure concluded as root causes of failure which can caused CLSCC.