Pressure Vessel and Heat Exchanger External Bolting Material Selection

[mitulmech]

Dear group members,

I am little confused about following question on external bolting selection for girth flange (w.r.t to flange material / sour service / temperature / Allowable stress and Flange design)

1.Currently we are using SA 193 B6 bolt (13%Cr S41000) for SA 182 Gr.F6a (13%Cr S41000) Flanges to match bolt material with flange material for high pressure (320 bar), Carbon Dioxide (CO2) service electric heaters.

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Due to high pressure bolt size is coming very high (3" to 4") and equipment is in sour service so hardness can be the issue to comply NACE MR0175 hardness requirement.

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Please highlight any issues face in past with B6 material and what alternative bolt material can be use.
2.General practise is to use LAS bolting with SS flange however in which case we need to go for SS bolting for SS flanges??
3.In case of using alloy steel bolting with Stainless Steel Flanges, What are the hidden problems like galvanic corrosion and differential thermal expansion??
4.What are other problems of going with SS bolting??
5.For stainless steel bolting, Which are acceptable bolting material grade grades as per NACE MR0175 as NACE does not specify the acceptable grades for SS bolting like B7M and L7M is specified in part-2.

Please share your views.

 

[metengr]

For process piping design, the evaluation of bolting materials depends on the following;

1. Selection of the applicable piping code.

2. Piping code bolting material. The reason for using bolting material listed in the piping code is to provide quantifiable allowable stress values and maximum and/or minimum allowable use temperatures in design.

3. Narrowing down the bolting material options in the piping code requires detailed engineering evaluation of the specific service environment; external exposure and internal process fluid, range of service temperature, corrosion exposure(general, galvanic and/or crevice), thermal expansion exposure and affect on bolt preload, NACE compliance, if applicable, material cost and availability.