Consider the degradation mechanisms: CO2/H2S weight loss corrosion and H2S cracking, principally SSC. The former is a longer term phenomenon, whilst the latter could occur much more rapidly. Thus, with CO2/H2S weight loss corrosion, the tendency is to work either with normal operating conditions, or if the end user's risk tolerance is low, maximum operating conditions, i.e. normal operating pressure, or maximum operating pressure. Since the SSC mechanism could occur much more rapidly, and possibly have more catastrophic results, the tendency is to calculate the H2S partial pressure for use with ISO 15156 using the maximum possible system pressure under normal conditions, where a relief scenario is not considered normal. By definition, that tendency would necessitate use of the design pressure.
Being familiar with the thrust of the question, the problem centres on the adoption of the maximum flange pressure-temperature rating of the piping as the design pressure (even for pressure vessels) which, generally, bears no relation to the actual process design conditions that could lead to cracking. This results in higher partial pressures of H2S for the same mole fraction in the gas, which in many cases forces the selection of costlier CRA materials since the ISO 15156 H2S partial pressure thresholds of the lower alloys are surpassed. Using the relief valve set pressure as the basis for determining the H2S partial pressure for use with ISO 15156 is fraught with problems, such as:
1. Accumulation, overpressure and relieving pressure
2. Location of the pressure relief valve in the pressure system
The end user that you are working with has reasonably clear rules on the determination of design pressure and they should be held to applying them. It is understood that the end user has great difficulty in getting to grips with setting design pressures at the phase of projects where materials selection decisions have key importance, but it is not recommended to let them go down either their current route of simply setting design pressures at the maximum flange rating, or conversely, using relief valve set pressure to ascertain H2S partial pressure. They should be forced to apply their rules and set a sensible, realistic design pressure for you to undertake materials selection studies.
Steve Jones
Corrosion Management Consultant
All answers are personal opinions only and are in no way connected with any employer.
