A full failure investigation of the cracks that caused a leak in 24” syntheses gas pipeline leading to the plant shutdown, is addressed in this paper. The through-wall cracks were initiated from the thermowell hole and propagated longitudinally. This investigation confirms that the failure mechanism is of a High Temperature Hydrogen Attack (HTHA) initiated from stress risers, introduced on account of improper field fabrication.
Key words: Field fabrication, High temperature hydrogen attack, API 941, 2.25Cr 1Mo Steel.
The scope of work for this paper is to identify the cause(s) of this failure.
A leak has been discovered at the thermowell weld joint and this leak was increasing with time and was controlled by supplying steam and nitrogen to maintain the proper temperature. A plant shutdown was required to repair the leakage. The main pipe diameter size is 24" and 67 mm thick and the pipe material of construction is A 369/A 369M-FP22 (2.25Cr, 1Mo). The thermowell pipe material is Stainless steel 316. The operating Pressure is around 150 kg/cm2g and Temperature is around 450°C. The process fluid is Synthesis gas which consists of Hydrogen, Nitrogen, and Argon.
The pipe being operated in high temperature hydrogen service, metallographic examination was thoroughly done from all aspects, especially for the occurrence of HTHA (High Temperature Hydrogen Attack).
RESULTS AND DISCUSSION
Martensite and ferrite are expected to have similar hydrogen transport properties since both are open structure. The open-packed structures like a body centered tetragonal (BCT) i.e. martensite (α’) & body centered cubic (BCC) i.e. ferrite (α), provide greater hydrogen diffusivities than close-packed structures like face centered cubic (FCC) austenite (). The transport aspect is important because of the lower hydrogen solubility but higher hydrogen diffusion in ferrite (BCC) and martensite (BCT) which is opposite from austenite (FCC). The solubility of hydrogen in austenite is three orders of magnitude higher than the solubility in ferrite. The reported values for hydrogen diffusivities in ferrite at room temperature range from 10-4 to 10-6 cm2/s, whereas the common reported value for austenite is 10-12 cm2/s. so, the steel microstructure plays a big role in the susceptibility to hydrogen damage.
The thermowell hole was made by the vendor to relocate one thermowell from a flanged nozzle to the main pipe. Unfortunately, no documents were available describing the technique that the vendor followed to make this hole. However, these irregular edges obviously acted as stress risers to initiate crack in a pressure pipe and in addition they resulted in high residual stresses (strains) due to plasma cutting, which will make this specific area susceptible for damage to high pressure, high temperature and high percentage of hydrogen. That was one major contributor to this failure. The second one was the inadequacy of weld size & external reinforcement which could have resulted in the formation of excessive strained zone in the thermowell hole vicinity which is highly susceptible for metal degradation like HTHA at such operating conditions.
It is possible for the hydrogen molecular to dissociate into atomic form at temperature 450oC. The hydrogen will react with the carbon in the steel matrix to form methane (CH4). The fissures were formed by the methane pressure build up at the grain boundaries in the material.
•Improper workmanship without following Standard Engineering Practice had resulted in sharp edges & stress risers which significantly rendered the metal susceptible to the HTHA attack.
•Severe undulations, high residual stresses and undesirable microstructural changes in the vicinity of thermowell hole promoted the cracks initiation and propagation in longitudinal direction.
•High Temperature Hydrogen Attack HTHA in the vicinity is attributed to poor PWHT and improper hole made in a pressurized piping.