Connecting Different Types of Flanges

I don't know who told you this or suggested that you can connect two different classes together. You cannot! I repeat you cannot connect two different flange classes together. Period.

They have too much that is not in common.

Such as:

• Outside diameter of flange
• Diameter of the bolt circle
• (sometimes) the number and size of bolts
• the outside diameter of the gasket face
• (Sometimes) the Inside diameter of the flange bore

With that settled, maybe you heard someone say that you can connect two different kinds of flanges (with-in the same flange class). By this I mean you can connect a Weld neck flange to a Slip-on flange of the same class. Kind of flange is not the same as class of flange.

Go back to the person who told you that and ask them to explain what they mean. Have them show you an example.

Have a look to standards DIN, ASME, GOST, etc. for flanges dimensions and you will find to many differences. It is impossible to connect two different types of flanges!

does two different flange class can be connected to each other?

You said different flange class, how do you connect two different flange classes?

But, sometimes-and due to short of flanges and similarity of dimensions- the pipe fitter/erector can assemble two flanges with different sizes and different classes together, but this is very dangerous due to the limited gasket width and the abrupt change in inner diameters.

For example, for ASME Flanges B16.5, the Bolt Circle Diameter for both NPS 1.1/4" 150# and NPS 1" 300# are the same (88.9 mm) and the Number of Holes are the same (4 holes) with different hole diameter (15.8 mm for 1 1/4" 150# & 19 mm for 1" 300#). In this case the net gasket width in contact reaches 3.8 mm only which is very limited.

All of us don't recommend that bad trial and we ask to assemble the two flanges with the same size and same class. And I agree PennPiper and AUREL MARES.

Those are all very good answers. The whole reason we have different classes of flanges is to prevent one from trying to do something dangerous while at the same time not over doing things and spending extra money. Most flanges have allot of extra strength designed into them to cover for people who try and do these silly things but I would not push it. I would not suggest anyone to "experiment" by doing anything other than using the proper matching class of flanges. I don't know of a standard that would allow this but if someone did try this I would personally call it being stupid or ignorant according to how much experience he has.

pipewelder

Respectfully, this situation is covered under ASME B31.3 Paragraph 312.1 (the main point being the joint rating is to be taken as the lower of the classes); small bore flanges <3.5" do mate in differing classes. The only outstanding issue is why would you need/want to do this? Unless you are in a pinch and out of stock or something.