Detection of Flange Ring Groove Cracking in High Pressure Hydrogen Service

Inspect flange ring grooves without disassembly or shutdown!

The Problem: Gaining access to flange face grooves for inspection is time consuming and, if cracks are detected, can result in delays in returning the unit to service

The Solution: A unique NDT inspection technique that does not require direct access to flange face surfaces

Typical Cracked Ring Groove Flange Cross-Section

Ring joint flanges in high-pressure hydrogen service lines historically experience cracking in flange face grooves. Root cause for this cracking is related to stress corrosion cracking (SCC) mechanisms initiated by high stress levels in the groove area, and either chloride or caustic environmental conditions. This determination was made by metallurgical studies, which indicated the presence of transgranular multi-branched cracking.

Initially, flange ring gasket and groove geometry was assumed to be the principal contributor to the high stress levels found at the groove bottom. However, extensive crack propagation has been detected in flanges subsequent to re-engineering and machining of these surfaces. While the stress levels associated with the flange groove and ring gasket design must be considered, it is secondary to the applied stress level resultant from improper tensioning and alignment.

Sources for the corrosive agents have also been debated. One possible source is the neutralizing wash (soda ash) used during turn-around periods. Concentrations of the neutralizing wash, with low levels of chlorides, along the flange face ring groove have led to the belief that flange groove contamination may be occurring when the flange is returned to normal operating condition. This is particularly suspect at "dead legs" or "dead ends" in piping systems with blind flanges installed during turn-around. A second potential source, although not proven, is residual quantities of neutralizing wash collected along the flange groove that is hydrolyzed to caustic while at operating temperatures. This leads to the initiation of caustic stress corrosion cracking.

Typical C-Scan image of Cracked Ring Groove Flange

Harold Queen, of SI’s Sunrise, FL office, with support from a major petroleum refiner, has developed and demonstrated an off-stream NDT inspection method that can be done without flange joint disassembly and requires only a minimal amount of operator interpretation. Off-stream inspections have been successfully implemented during scheduled plant turn-around, with sufficient time available for flaw disposition activities.

Further work has been done to (1) fully determine the root cause of the cracking; (2) establish flaw acceptance criteria which could be used to support run / repair decisions for defects found during future inspections, and (3) develop on-stream inspection and monitoring systems. This additional work includes finite element stress analysis, material fracture toughness testing, fracture mechanics calculations, metallurgical and failure analysis and research, development and validation of an on-stream NDT inspection system.

For more information on SI's Ring Groove Inspection Services, please contact SI.