Safety in the Plant
Easy road to compliance with sour gas service
Roger Bours looks at how to protect high-investment, critical processing equipment from stress-induced cracking
Fig. 1. This graphic shows the internal workings of the safety relief valve.
P
roducing 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 pressurised 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 document has evolved through the decades, identifying which the limits for H_S partial pressure would be above which precautions against sulphide induced stress cracking (SSC) where to be considered. Guidance for selection of cracking-resistant materials as provided since the mid-1970s by the NACE (National Association of Corrosion Engineers) standard MR01- 75 which has been successfully applied for many decades resulting in satisfactory performance of process equipment under these harsh conditions. The general recognition of the NACE standard has been such that the NACE MR0175 was transformed into an international standard: the series ISO
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www.engineerlive.com 151546 consisting of three parts:
n NACE MR0175/ISO 151546-1: General principles for selection of cracking resistant materials.
n NACE MR0175/ISO 151546-2: Cracking-resistant carbon and low alloy steels.
n NACE MR0175/ISO 151546-3: Cracking-resistant Corrosion Resistant Alloys (CRA’s) and other alloys.
Where the original 1975 MR01 75 material selection standard was relatively transparent and considered as a workable document for equipment suppliers and users, the more recently introduced version has become dramatically more difficult to work with. As a simple indicator: the 1975 version of MR01 75 addressed the complete issue in about 30 pages, whereas the latest version consists of in excess of 120 pages. As an off-spin of the work done in the field of upstream
oil and gas production, reflected in MR0175, an additional standard was developed for selection of SSC-resistant materials used in corrosive petroleum refining environments (MR0103- 2003).
One of the most challenging impacts in the recent issues has been the identified need for the equipment user to determine
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