Chloride stress corrosion cracking of precipitation hardening S.S. impellers in centrifugal compressor. Laboratory investigations and corrective actions
Introduction
The centrifugal compressor which is the centre of this case of failure is installed at a natural gas compression station (gas lifting service).
The rotor is composed of six impellers, three for the first and three for the second phase. The 1st phase impellers, of precipitation hardening stainless steel 17-4 PH (ASTM A 705 Gr.630 mod.) [1], have been constructed by welding the blades to the counter-disc.
Even though no hydrogen sulphide was present in the process gas, according to the design data, the impellers have been heat treated to keep their hardness below 33 HRC (311 HB), in compliance with the NACE MR0175 standard [2], as requested by the customer in anticipation of possible variations in the fluid composition.
Approximately one year after start up, and after a period of even operation according to the plant requirements, the compressor failed. An investigation was therefore carried out to establish the cause and identify the corrective actions. Meanwhile the compressor was put again into operation with a new rotor.
Section snippets
Visual inspection
After the compressor disassembling and the rotor removal it was evident that the 2nd stage impeller was catastrophically broken, completely “exploded” with detachment of the disc from the counter-disc. A lot of pits were also visible on the 1st stage impeller, as well as mechanical damaging marks on the 3rd stage impeller. An accumulation of brown-ochre solid deposits was finally noticed, probably carried over by the process gas, as shown in Fig. 1.
A more careful visual inspection of the 1st
Discussion
In order to establish the most probable cause of the centrifugal compressor rotor failure all the results from the experimental study have been carefully examined. Taking into consideration, first of all, the intrinsic properties of the impeller material, no abnormal parameter was found, regarding both the mechanical characteristics and the chemical analysis.
A particular care must be devoted to the serious localised corrosion phenomenon (pitting) noticed on the damaged impellers. It is well
The remedial actions
After identifying the type of phenomenon (stress corrosion due to chloride) and the cause of damage (unsuitable gas treatment) a research was begun to establish the best technological solution for the problem.
First examined was the possibility of changing the environmental conditions and prevent the stress corrosion by removing from the handled fluid the chemical species which had caused that phenomenon, that is, chloride. In short it consists in separating, in a more effective way, the water
Conclusions
Considering the impossibility of changing the environment, the search for possible remedial actions of the chloride stress corrosion cracking failure of the centrifugal compressor impeller led to consideration of impeller construction.
In order to find alternative materials, tests were used which must be considered very severe for the high chloride concentration, the temperature and the stress to which the samples have been submitted (in excess of yield). Without disregarding some “deviations”
References (7)
- ASTM A 705. Standard specification for age-hardening stainless steel forgings. Annual Book of ASTM Standards...
- ANSI/NACE Standard MR0175-98 (Item No.21302). Standard material requirements — sulphide stress cracking resistant...
- Metals handbook. 9th ed. Failure analysis and prevention, vol. 11, 1986, p....
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