Producing or processing crude oil, especially where (even small amounts in the range of ppm) hydrogen sulphide (H2S) is present, has been known to cause considerable difficulties and damages to related process equipment. The exposure of certain pressurized metallic parts/components to this so-called “sour” service has been known to initiate stress-induced cracking with often catastrophic failures of high-investment, critical processing equipment such as safety valves.
The consequences of sudden failure of components used in the oil and gas industries have led to the creation of relevant engineering guidance documents. The NACE (National Association of Corrosion Engineers) document identifies which the limits for H2S partial pressure would be above which precautions against sulphide induced stress cracking (SSC) where to be considered. The general recognition of the NACE standard has been such that the NACE MR0175 was transformed into an international standard: the series ISO 151546 consisting of 3 parts:
•NACE MR0175/ISO 151546-1: General principles for selection of cracking resistant materials
•NACE MR0175/ISO 151546-2: Cracking-resistant carbon and low alloy steels
•NACE MR0175/ISO 151546-3: Cracking-resistant Corrosion Resistant Alloys (CRA’s) and other alloys
One of the most challenging impacts in the recent issues has been the identified need for the equipment user to determine the specific composition of the sour gas in the specific service or installation. Based on the available input the most suitable corrosion-resistant material for constructing the equipment can be determined in close liaison with the equipment supplier. Offering in theory a good approach, this procedure becomes quite cumbersome when the relevant equipment involves castings – often produced in minimal quantities for economical reasons – or specially engineered machined-from-forging based designs.
