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 151546 consisting of three 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.
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 the specific composition of the sour gas in the specific service or installation. The results of this assessment will form the basis for determining the specific material requirements in accordance with MR0175/ISO 151546. These inputs from the user will be the basis for working with suppliers to select the appropriate materials of construction of the subject equipment. 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. This is specifically valid for example for safety relief valves. Even when the user may be able to determine best suitable material grade for the specific application, the available executions of safety valves or pressure relief valves are limited. Special designs may be considered, taking into account the cost-related issues, type testing requirements and extended lead-times.
A pragmatic approach, offering economical advantages of considerable scale, is to combine the use of 'standard' NACE-compliant material safety or pressure relief valve with a bursting disc devices installed upstream. The availability of bursting or rupture discs and corresponding holder in suitable execution and materials is often nor problem whereas the positive sealing provided by this device will avoid the contact of the safety valve critical components with the sour gas environment. Only in case where an unacceptable overpressure situation exists the bursting disc will relief the pressure into the safety valve inlet which will handle the pressure relief as planned. After completing the discharge situation, measures can be taken to safely close-down the process or divert to a parallel mounted system, allowing cleaning and repair of the initial pressure safety combination. In this way the use of standard execution equipment will offer an effective protection with important economical advantages, without discriminating the long-term safety aspect of the relevant installation.
The consideration of additional equipment (the bursting disc upstream of the safety valve) will often result in major savings in both the initial investment (use of standard material grade, shorter lead times of initial parts and spares) as well as in reducing the maintenance costs of the installation (lower need for cleaning, possibility for in-situ testing of safety valve setting, shorter downtime).
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