I can change a welding neck flange by a slip on flange

Yes.

Same size, pressure rating, flange face, type of gasket, material, ...

There are certain applications where they may not be interchangeable.

1. Weldnecks are required for severe cyclic conditions (ref B31.3 sect 308)
2. Slip-ons have a larger ID gasket contact area and may not work with certain size/pressure class spiral wound gaskets.
3. Slip-ons present a larger ID "crevice".
4. Methods of examination (NDT) are limited with slip-ons.

Regards,

donf

AND ensure that the slip-on weld is actually large enough. Very few welders understand that the minimum weld size has a thru-the-throat measurement equal to the wall thickness of the pipe. This gives a fillet weld size of 1.4 * pipe wall.

Jate....

It would be wise to question why the weld-neck was required at that location in the first place.

Many piping specifications I have dealt with require a (less expensive) slip-on flange as the first choice and a (more expensive) weld-neck as an acceptable substitution.

As "donf" pointed out, there are legitimate design and code reasons for going with weld-necks.

I would add to "donf's" list that slip-ons are preferred by the erectors because of ease of fit up (flnges can be rotated into position, then bolted up)

Please tell us more....

MJCronin
Sr. Process Engineer

Also, I believe weld necks are stiffer than slip on flanges. (from a stress analysis front)

slip-ons are ... (flanges can be rotated in position)

Click to expand...

You mean lap joint flanges?

@donf: you point out that weldnecks are required for severe cyclic service.
I once heard (or read) an engineer saying lap joint flanged joints are 'good' in thermal cycling conditions. Regardless of code requirements, which are of course a minimum, can you comment on that?

JATE,

What is the reason to change from weld neck to slip-on?

Curtis

RE: Lap-joint flanges; they are NOT good for thermal cycling. We have several that are giving us trouble, probably due to thermal cycling.

I've not looked specifically at benefits/problems with lap-joint flanges in thermal cycling applications, but...........

Because LJ flange hubs are not welded to the pipe, they cannot provide gasket compression loads as high as slip-ons or weldnecks. LJ flanges will start to rotate at lower bolting loads. Note the term "flange rotation" is confusing. It doesn't refer to axial rotation of the LJ flange around the stub end - it is used to describe the distortion or bending of the flanges as the fasteners are being tightened, which can affect the abililty to provide the desired seal.

Remember that flange type is only one of the elements involved. Look also at gasket, fasteners and assembly procedure.

donf