Tri-Lateral Phased Array: A Novel Technique for Identifying Wet H2S Damage

Introduction

Tri-lateral phased array (TriLat) is a robotic phased array ultrasonic testing technique for the on-stream inspection of fixed equipment in wet hydrogen sulfide (H₂S) service and hydrofluoric (HF) acid service. TriLat identifies and quantifies wet H₂S damage, including hydrogen-induced cracking (HIC), stress-oriented HIC (SOHIC), and blistering in the base metal of carbon or low alloy steel equipment at relatively high productivity and resolution. It may also be used for crack detection in the parent metal of cladded or lined fixed equipment.

Within one unit, TriLat combines the power of two probes containing three angle beam sets to identify and quantify cracking at early stages. The result is inspection speeds up to ten times faster than traditional AUT systems, depending on probe size, and exceptional through-wall lateral resolution. Two axis focalization and high resolution allow for the identification of embedded cracks down to 0.039 inch (1 mm) in length, a data density 25 times greater than conventional tri-element or triplex automated ultrasonic testing methods.

The severity of the wet H₂S environment is determined by the content’s pH and level of H₂S (in ppm). In addition, the potential for equipment degradation is further influenced by temperature, presence of cyanides, exposure time, and nature of the base metal. In the refinery space, the equipment prone to wet H₂S damage is commonly found in, but not limited to, the following process units: sour water stripper, fluidized catalytic cracking unit (FCCU) light ends recovery, sulfur recovery, amine treating, FCCU and coker fractionation, crude, HF alkylation, and hydroprocessing.

Damage is regularly misclassified as minor cracks, laminations, and inclusions by other NDT techniques because of their lack of axial and lateral focalization through-wall and the data density required for proper identification and dimensioning for accurate fitness for service assessments. Conversely, TriLat’s multidirectional focalization customization effectively and efficiently confirms and quantifies wet H₂S damage mechanisms.

Wet H₂S Service and Damage Mechanisms

Equipment in wet hydrogen sulfide (H₂S) service, or sour service, environments are prone to a unique set of damage mechanisms that can cause catastrophic failure resulting in a loss of containment and/or personnel and impacting operational costs due to environmental remediation and reduced productivity.

Wet H₂S damage is seen in facilities that produce hydrocarbons including the oil and gas, chemical, and petrochemical industries. In the presence of moisture, hydrogen sulfide and iron react to form atomic hydrogen and iron sulfide. Atomic hydrogen, being the smallest possible impurity, diffuses into the material, filling flaws and non-metallic inclusions, and forming diatomic hydrogen gas (H₂) molecules that are too large to diffuse back out. Pressure from the expanding gas exacerbates the flaw creating blisters or cracks in the metal. Damage is more likely to occur in older or dirty steel and steel plates because of the prevalence of inclusions, laminations, and imperfections in these materials, thereby providing places for hydrogen to collect [1].

Non-metallic elongated inclusions are an ever-present issue that may or may not adversely affect mechanical properties and, therefore, overall material performance. Sulfur has dual effects, meaning that it can improve the machinability of material, but also has deleterious effects on other key service properties such as forgeability, ductility, toughness, weldability, and corrosion resistance.

Since the solubility of sulfur in iron and steel is very low (less than 0.01% at room temperature), it is usually present as a sulfide. The sulfide inclusions formed during the solidification of steel are predominantly manganese sulfide (MnS). MnS is a common inclusion that adversely influences the steel's mechanical properties, physical properties, and corrosion resistance. In the presence of cavities of nonmetallic inclusions, MnS and atomic hydrogen react to form H₂S gas (MnS+2H⁺↔H₂S+Mn²⁺). In this scenario, pockets of H₂S gas contribute to generating blisters on those cavities.

The most common wet H₂S damage mechanisms are:

Hydrogen-Induced Cracking (HIC): planar cracking or blistering that is caused by the pressurization of hydrogen gas in the mid-wall. Individual cracks are oriented parallel to the inner and outer surfaces (stationary cracking) of the metal, but over time, increased hydrogen concentration can cause the individual cracks to link together forming stepwise cracking.

Stress-Oriented HIC (SOHIC): staggered HIC cracking formed perpendicular to the principal residual or applied stress, resulting in a ladder-like crack array. It can occur in the heat-affected zones (HAZ) of welds and base metal in pressure vessels. It is not hardness dependent.

Sulfide Stress Cracking (SSC): cracking caused by corrosion and tensile stress in the presence of water and H₂S. It occurs on the surface of high-strength steel or in weld deposits and HAZs [2].

Understanding Stepwise Cracking