Slip On Flange

Slip-on flanges, which are also called "hubbed flanges," have a very low hub. Most of the time, one or two fillet welds (one outside the flange and one inside the flange) are used to connect this type of flange to a pipe. However, it is possible to use only one fillet weld. Slip-on flanges come in a wide range of sizes and are most often used in applications with low pressure (ASME class 600).

The bore size (internal diameter) of a slip-on flange is bigger than that of the connecting pipe. This lets it slide onto the pipe (slip-onto the pipe). There isn't a full penetration weld between the pipe and the flange, so you can't use it as much because the weld isn't as strong.

But if there isn't enough space for a welding neck flange, a slip-on flange might be a good alternative.

Slip-on flanges have benefits like lower costs to buy, less skill needed to make the weld, less accuracy needed to cut the pipe, and no need to prepare the weld for a pipe end weld.

Slip-on flanges have some restrictions when it comes to:

When two welds are used instead of one (usually).

Welding neck flanges are 30% stronger, but they have less mechanical strength.

Not good enough for cyclic loading.

Not being able to test welds in some non-destructive ways.

ASME class 1500 sizes 2.5 inches and larger are not available.

ASME class 2500 doesn't have all sizes.

Above ASME class 600, steel is rarely used.

Even though it's cheaper than a welding neck flange, there's not much of a price difference if you have to do two welds and check both of them with non-destructive testing.

Non-Destructive Testing (NDT) Techniques

Slip-on flanges are not used with any particular NDT techniques. A dye penetrant test can be done on the weld if the slip-on flange is fillet welded. It is possible to look at the mating faces and pipe.

ASME Standards

The standards ASME B16.5 and ASME B31.3 spell out the rules for installing, maintaining, and testing slip-on flanges.