Application requirements for stainless steel fasteners in pressure vessels
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There is a pressure vessel containing flammable and explosive materials, for which type A flat-welding equipment flanges are selected. Regarding the material for the flange fasteners, the supplier stated that stainless steel cannot be used. Upon investigation: 1. For pipe flanges, commercial-grade bolts are not permitted as fasteners; only specially designed bolts may be used. 2. The technical requirements for fasteners in the standard NB/T47027 for equipment flanges state: 4.1. The materials of fasteners shall comply with the provisions of GB150 and NB/T47020. GB150 does not specify any special requirements for high-alloy steel fastener materials ； There is no option for S30408 in the matching table in Table 2 of NB/T47020. 4.6. Fasteners made of alloy steel shall be blued. It has been found that blueing, also known as blackening treatment, only affects the outer surface. So when using stainless steel studs, appearance requirements become meaningless, as it is not in its original stainless steel color anyway. 3. In the verification of the flange in SW6-2011, S30408 is an optional material for the fasteners ； In summary: For customers who require all-stainless steel components and whose working medium is flammable or explosive, can stainless steel 304 studs be used? The contradiction is: 1. The grade of the stainless steel is A2-70, which is not a professional-grade material. It does not apply to HG20592; however, this is a specification for pipe flanges – can it be ignored? What should be done when stainless steel must be used in the pipes? 2. Are the bolt materials listed in Table 2 of NB/T47020 all the materials that can be used? 3. For customers who require a fully stainless steel appearance, is blueing still necessary? 4. Why is material 304 selected again in SW6-2011? 5. In practical applications, full stainless steel equipment is definitely something that has been seen and used; have you encountered similar problems when submitting inspections?
1. The equipment is made of stainless steel, but stainless steel studs are not necessarily required. The high linear expansion coefficient of stainless steel materials is actually detrimental to sealing. 2. Regarding the flammable and explosive media mentioned by the poster, commercial-grade studs and nuts are used. It is recommended to use long-neck butt-welded flanges, in combination with wound gaskets.
Stainless steel studs can certainly be used, but it is important to choose those that are \"strain-strengthened\". In pipelines where there is a high risk of toxicity or where liquefied petroleum gas is involved, it is definitely not appropriate to use commercial-grade stainless steel bolts such as A2-70; instead, special-grade bolts should be used. For explosive media, it depends on whether they are highly permeable; in principle, stainless steel can be used in such cases. However, commercial-grade products are not recommended. It is worth noting that the grades of austenitic stainless steel listed in SW6 are all based on the grades specified in GB150.2; the materials mentioned in GB150.2 belong to the category of special-grade studs/bolts.
For the flanges of stainless steel equipment in our factory, we use special fasteners along with stainless steel washers
Stainless steel studs can be used, and they shall meet the requirements regarding studs/bolts in material standards GB150.2.
The standards for pressure vessel flanges do not specify flanges made of pure stainless steel; only flanges lined with stainless steel are provided. There are no fasteners made of stainless steel either. The manufacturer can choose carbon steel fasteners; if the client insists on using stainless steel fasteners, HG/T20613 can be used, while nuts should be selected according to GB/T6175 or GB/T6176 (following the standards for pipe flanges).