The partial pressure of H₂S (ppH₂S ) is used throughout ANSI/NACE MR0175/ISO 15156 to set the production environment limits for carbon steels, low alloy steels and corrosion resistant alloys. However, NACE MR0175 does not specify if the user should consider operation pressure, design pressure or an intermediate value. So, what is NACE MR0175 intent when it comes to H₂S partial pressure? operating or design pressure to be used in the calculations? Here, Oil & Gas Corrosion addresses these questions.

First, we have to define the partial pressure of H2S (ppH2S ) which is calculated as:

                 ppH2S = system total absolute pressure x mol fraction of H2S present

Is the system total absolute pressure the design or the operating pressure? This is one of the most  frequently asked questions and this video clarifies if design or operating pressure should be used.

partial pressure of H2S, design or operating conditions? from Ivan Gutierrez on Vimeo.

At Oil & Gas Corrosion we help clients to determine the partial pressure of H₂S. We have the expertise to help you decide if you should use the operating or design pressure. We do this by evaluating the production conditions (pressure, temperature, produced water etc.), the design and evaluating the risks and consequences. We are here to help you with sour service materials selection and partial pressure calculations.

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This video makes reference to an inquiry sent to the NACE MR0175 maintenance panel. The inquiry is below:

QUESTION: What is NACE’s intent when it comes to H2S partial pressure? i.e., when calculating the H2S partial pressure per Annex C.1 and C.2 in NACE MR0175/ISO 15156-2, should the operating or design pressure be used in the calculations? I believe operating pressure should be used. Could you please confirm?

ANSWER:

15156-1 Para 6.1 states that the user shall define the service conditions including unintended exposures (e.g., resulting from failure of primary containment). These service conditions become the basis for calculating H2S partial pressure. It is up to the user to decide whether to use operating or design pressure for partial pressure calculations.

Reference: ISO 15156 MP Inquiry #2009-21 (NACE MR0175 Part 2 Annex C)