SA-193 Gr.B6 in sour service

[mitulmech]

Dear Group Members,

I am working on electric heater material of construction is 13%Cr.
Service is high pressure CO2 Service
Design Pressure = 320 Barg
Design Temp = 85 / -20 °C

Equipment is under sour service and NACE MR0175 is applicable.

Can I use SA-193 Gr.B6 for the flange bolting (Non exposed bolting).....I could only find in part II of MR0175 i.e. permitted bolting are SA-193 B7M and SA-193 L7M in Part-II??

Please advice is it allowed or not.

If not what alternative material we can use.

 

[CoryPad]

Yes, you can use parts that meet Grade B6 according to ASTM A193. They need to be austenitized, quenched and double tempered (621 [°]C minimum) with 22 HRC maximum hardness. The hydrogen sulfide partial pressure shall be 10 kPa maximum, and the pH shall be ≥ 3.5. You can review this information in ISO 15156-3, the successor to NACE MR0175.

 

[OGMetEngr]

If the bolting truly is "non-exposed", sour service resistance is not applicable. If the bolting has the potential to be "exposed" to H2S containing fluids in the event of a leak, then the bolting should be sour service resistant in accordance with NACE MR0175 / ISO15156.

 

[mitulmech]

Thanks for reply.

Why Part - 3 of ISO 15156-3 does not contain any information on acceptable bolting material like Part - 2 specifies B7M and L7M?Is there any reason behind it?

CoryPad, I understand your explanation is based on the Table No.A.23 of the ISO 15156-3. I found no hardness value has been specified for B6 and B6X material in ASME Sec.II Part A. Do not know the reason.

OGMetEngr, I checked again insulation specification and flanges are insulated hence we need to consider this as exposed bolting. Still can we use B6 bolting with heat treatment and hardness limitations as specified in Table no.A.23.


Additionally generally client specifications provide bolting material selection based on temperature and service. For sour service B7M upper temp limit is 427 °C. What if temperature is higher than this say 550 °C with also sour service is present during certain duration like start up or shutdown. What material we need to specify client table specifies SA-193 B16 for high temp, non-sour service. Please advice.

 

[OGMetEngr]

The inclusion of specific acceptable carbon and low-alloy steel bolting in Part 2 is most likely a carryover from previous versions of the standard. I'm not sure how far back it was included, but I know it was in the 1997 version. There are lots more instances of this in the standard as well.

As for Part 3 not including a similar section, I would imagine because the CRA acceptability is much more environment specific (pH, H2S partial pressure, chloride concentration, temperature). Also, CRAs are oftentimes used in more severe environments, so an "acceptable" list would be difficult to generate.

ASME doesn't have the hardness limitation for B6 as all B6 material isn't necessarily used in sour service. If the B6 isn't used in sour service, a hardness limitation may not add any value to the material. The 2011 version of Section II does have a hardness limit for B6X (26HRC max), but this is not in accordance with NACE MR0175/ISO15156 which limits it to 22HRC.

Table A.23 is intended for less stressed components (i.e. - it excludes valve stems, casing and tubing hangers). Table A.18 would be more suitable for bolting. As for using B6 for bolting, it's hard to make that decision without all variables. At 427C high temperature attack would be more of a concern than sulfide SCC, and NACE MR0175/ISO15156 would no longer apply.

 

[CoryPad]

I agree with OGMetEngr, Table A.18 is the correct choice and the one I used to make a recommendation. I also agree that it would be difficult to add a table for suitable bolting in part 3.

Hardness limits are more important for sour service/hydrogen embrittlement (ISO 15156) than for high temperature service/creep (ASME B&PVC).

If the temperature requirements are above low carbon steel (e.g. B7M) limits, then another alloy (e.g. austenitic steel) is needed. Use a combination of the codes/standards to select a material that meets the requirements.