NACE compliance for non-pressure retaining parts of a vessel

[Nandhu28]

Hi all.....

Do we really need to check the NACE compliance(MR 0175/ ISO 15156) for non-pressure retaining parts of a vessel operating in sour service?
Because for the non-pressure retaining parts, we will not get partial pressure of H2S. So only having the temperature and chloride concentration, how can we check the compliance....

thanks in advance

regards,
Nandhini

 

[OGMetEngr]

MR0175 applies for materials in contact with H2S containing fluids, or bolting that would be "exposed" to H2S in the event of a leak, or base material that would be exposed in the event of a breach of cladding. The standard provides methods for mitigation of cracking due to these H2S containing fluids.

You state that the pressure vessel is operating in sour service. This implies that the fluid contains H2S. Is this a heat exchanger or something similar, with sour fluid on one side and chloride-containing water on the other?

 

[Nandhu28]

Thank you for your prompt reply.

It is a Separator vessel and made up of Carbon steel with 6mm corrosion allowance with an internal epoxy novalac ceramic lining.

Process conditions are
Operating temp of vessel = 90 deg.C
Operating Pressure of vessel = 174 psi
Chloride content = 149549 ppm
H2S mol% = 0.31

Removal internals (weir plate, demister, etc.) of this vessel are in 316L Stainless steel.As per NACE MR0175- Table A.2, it is not acceptable at this process conditions.

My query here is, for removable internals i.e. for non- pressure retaining parts which is in equilibrium pressure, can we check NACE compliance ......? Because the standard is silent regarding this.

 

[OGMetEngr]

The standard gives conditions and material properties that minimize the risk of cracking due to H2S. It's mainly used for parts that contain or control pressure, as a release of H2S containing fluids is undesirable for a number of reasons. The internals you specify sound as if they will be subjected to the same H2S concentration as the inside surface of the vessel. Even in the absence of H2S, I'd be concerned about pitting/crevice corrosion and chloride-induced stress corrosion cracking with 316L at these temperatures and chloride concentrations. Whether it matters or not if these corrosion mechanisms affect the function of these parts is for you to decide. I'd also recommend considering the condition of the 316L (welded, cold-worked, sensitized, etc.) as well as the chemistry of the fluid (oxygen or other reducing species). If you have no idea what I'm referring to then I'd also suggest consulting with a corrosion/metallurgy expert.