What is the minimum partial pressure of H2S that requires compliance with NACE MR0175? What is the maximum ppm of H2S allowed without NACE MR0175?
ANSI/NACE MR0175/ISO 15156 indicates that in production conditions with partial pressure of H2S (ppH2S) below 0.3kPa (0.05psi) no special precautions are required for carbon and low alloy steels. However highly susceptible carbon and low alloy steels (high strength, heavily cold worked etc) can still be susceptible to H2S related damage mechanisms even at ppH2S below these limits. Furthermore, the risk of Hydrogen Induced Cracking is present even at partial pressure of H2S (ppH2S) below 0.3kPa (0.05psi).
For corrosion resistant alloys (CRAs) there is now ppH2S low limit, which means precautions need to be taken whenever the possibility of H2S is present.
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NACE MR0175 / ISO 15156-2 Para 7.1.2 says “If the partial pressure of H2S in the gas is equal to or greater than 0.3kPa (0.05psi), SSC-resistant steels shall be selected using A.2” Could I please get clarification regarding the sour service H2S partial pressure cut-off value as NACE MR0175 / ISO 15156 use both imperial and metric units. The conversion of 0.05 psi is 0.345 kPa. Please clarify if the H2S pp cut-off for sour service is 0.3 kPa or 0.345 kPa (conversion from 0.05 psi).
The value was rounded off when converting from 0.05 psi. This limit is actually not a very sharp and accurate value and this is why 0,3 kPa value was taken as a practical engineering value. Note, however, that even below this H2S partial pressure, some materials can be susceptible to SSC as stated in 7.1.1. This means that in principle there is no cut-off value for sour service. It is just that below this value, only particularly sensitive steels can be susceptible to SSC. It is up to the user to make sure that the steel used is resistant below 0.05 psi (0.3 kPa).
This question is in relation to NACE MR0175/ISO 15156-2 Clause 7.1.2
Reference: ISO 15156 Maintenance Panel Inquiry #2009-18