Flanged Joints - Joining Weld Neck and Slip on Flanges 1

[beh188]

Are there any design issues that need to be considered when you have a joint containing flanges of different types? For example, say you have a flanged joint where one side is a raised face weld neck flange and the other side is a raised face slip on flange. Assuming the slip on flange is appropriate for the application, and the flanges are of the same pipe size and class, are there any joint integrity issues?

I would assume you need to pick a gasket that is suitable to be used with the slip on flange, but assuming you do that, is there anything to be concerned about regarding joint integrity?


Thanks.

 

[DSB123]

The obvious is that the Weld neck would be stronger so you need to ensure that bolt loads etc are calculated for the slip-on.

But the first question is why on earth you would wanrt to do this in the first place unless you are mating with an existing flange.

 

[Duwe6]

Under ASME B31.3, either flange is 'fully rated' and does not require any special considerations other than insuring that the Slip-On flange has a full-sized weld to the pipe.

I have seen [& rejected] many, many undersize S-O flange welds. Unless engineered [with calc's] otherwise, it requires a fillet weld 1.4 times the pipe wall thickness 'T'. This gives a through-thickness in the fillet of 'T' [the wall thickness of the pipe].

 

[moltenmetal]

The only issue is that a special gasket required any time a slip-on is used, with a bore matching the OD of the pipe rather than its ID. The issue is the unsupported edge of the gasket if you use one with a bore smaller than the pipe OD. No problem with composition-type gaskets, but if you use a spiral wound type, can cause the gasket to come apart and spool into the bore. Note that the gasket suppliers are often unaware of this problem, specifying the same gasket whether the flanges are RFSO, RFSW or RFWN.