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In NACE MR0175, what does the uncertainty below 0.3 kPa mean?

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Question: There is the sentence in the note 1 of Figure 1 in ISO 15156-2: “The discontinuities in the figure below 0.3 kPa (0.05 psi) and above 1 MPa (150 psi) partial pressure H2S reflect uncertainty with respect to the measurement of H2S partial pressure (low H2S) and steels performance outside these limits (both lower and higher H2S).” I understand the above sentence, and if I will use the carbon steel and low-alloy steel in the sour service above 1 MPa (150 psi) of partial pressure of H2S, what can I do? Should I require a special laboratory test imitating the H2S partial pressure and pH in the service for SSC of the carbon steel and low-alloy steel? Which solution can I use in the special laboratory test? NACE TM0177 A solution or the imitating solution in the service?

Answer:

The following response must be seen in the context of NACE MR0175/ISO 15156-2, Clause 7.

1. NACE MR0175/ISO 15156-2, Fig. 1 is a schematic definition of Regions of environmental severity with respect to SSC of carbon and low alloy steels. As mentioned in Paragraph 7.2.1.4, qualification for the use of a material not listed in Annex A for use in one or more of the Regions of Fig. 1 is always dependent on reported field experience or laboratory testing.
There is little documented evidence that describes the SSC resistance of carbon and low alloy steels in H2S-containing environments outside the H2S limits of Fig. 1. The Note quoted reflects this.

2. The equipment user must decide whether the listing of a steel in Annex A serves as an adequate guide for its behavior in H2S-containing field environments that might be more severe with respect to SSC than those represented by the SSC testing methods normally used; see Annex B.1a).
For qualification for a specific application all the test conditions must be at least as severe, with respect to the potential mode of failure, as those expected to occur in field service.

This question is in relation to NACE MR0175/ISO 15156-2 Clause 7.2.1.2, Fig 1

Reference: ISO 15156 Maintenance Panel Inquiry #2005-17

 

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