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Stud Bolt Selection. 1

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irfan202

irfan202

Mechanical
Jun 30, 2012
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Dear All

I have two question regarding material selection for Stud Bolt.

1. Can i select B7 alloy steel stud bolts for SS 316 or SS304 flanges , if it is acceptable how can i justify galvanic corrosion issue due to dissimilar metal contact issue. Any standard reference is highly appreciated. Line class 150, Design Pressure 10 Bar. Design temp 122 F, Service water.

2. Is there any reference or standard to stud selection as per environmental condition, means stud or bolt shall be galvanized or non galvanized or any type of coating is applicable.

3. if all stud are non coated and later on we observed corrosion on studs can we apply the paints or cold galvanizing on all stud without removing from the flange joints.

Please send me any standard reference.

Regards
 
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1. You will not get corrosion unless there is an electrolyte. Hence if the flange is going to be dry as inside a plant corrosion will not occur. If outside and the environment is such that rain will be brief and heavy followed by a drying atmosphere then corrosion will not be a concern. If the environment is damp, wet, near sea then corrosion will occur. you could isolate the CS studs from the ss flanges with isolating washers and thus avoid the dissimilar metals issue. It is quite common for ss stub flanges to be used with galvanised CS backing flanges and galvanised stud bolts.

Galvanising is a sacrificial protection and will not last in a corrosive environment. Some comapies use zinc plated bolts but this lasts but a short time as the amount of zinc is no where near that of HDG.

2. This is more a corrosion philosophy issue and standards are not neant to be design guides. i dont know of such a standard. Generally companies have standards based upon experience in their industry. The durability requirements based on risk, design life etc are used to determine materials of construction.

3. Many stud bolts are protected from corrosion by coatings in grease, mechanical or hot dip galvanising, PEEK, ECTFE or other material. If corrosion is observed it comes down to has it reduced the effectiveness of the stud bolt? Some oil industry companies change their stud bolts routinely. You could installed cathodic protective zinc cups on the ends of the stud bolts and this will provide the same sacrificial protection.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
 
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Just curious why you are using B7 for SS flange ? B8 is commonly used. Are you trying to save cost ? Good engineering judgemnet takes precedence of anything else.
 
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B7 also overcomes the difficulty of galling. Substantial savings can be made by use of ss stub flanges with cs backing flanges. Also a stub end butt weld can be fully examined radiographically whereas a SORF flange cannot be. A stub end and loose backing flange reduces the fitting time compared to a WNRF or SORF flange.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
 
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B8 or B8M class 1 bolts/studs should be avoided if at all possible. They are low strength and their use in piping connections will seriously limit your available gasket choices. They simply cannot develop the compressive load needed to seat many gasket materials; especially in class 150 flanges. If you do opt for stainless hardware specify and make sure that your supplier supplies strain hardened B8 class 2.
 
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Thanks for all of you to reply my query. PTFE or durabolt (epxy coated) is one the good option to avoid galling , corrosion every thing . My contractor already installed all non coated B7 stud bolt and now suggesting cold galvanizing / painting and sealing between flange to avoid corrosion. ASME B 31.1 & ASME B 16.5 is our code. Can we apply ?
 
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B31.1 dictates specific bolting materials and B7 is one of them, so your code requirements are met. I don't remember seeing (not saying it isn't there) some of the other materials suggested in posts above in B31.1. B31.1 doesn't care whether the hardware is coated or not.

Now is the point at which the question; "what does the contract call for" is pertinent. If it didn't specify bolting hardware coating, you are stuck and if you want corrosion protection, you will have to pay to have these removed and coated hardware installed. If it calls for coated hardware, then the contractor will have to remove what is there and install what was specified at his expense.

I agree with the recommendation of not using B8 due to the tendency to gall, not that I don't use them on a regular basis with lots of anti-seize and crossed fingers. (As long as there isn't the persistent presence of an electrolye to consider as part of the mix.)

rmw
 
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Coated (Xylan) bolting will provide improved atmospheric corrosion resistance and consistent effective thread lubricant for assembly, however this coating should not be relied upon to provide chemical resistance. It won't.
 
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