Slip-on Flanges Explained

What is a slip-on flange?

Slip-on flanges, also known as ‘hubbed flanges’, have a hub with a very low profile. This type of flange is usually connected to a pipe by one or two fillet welds (one outside the flange and one inside the flange), it is however possible to use only a single weld. Slip-on flanges are produced in many sizes and favoured for lower pressure applications (ASME class ≤ 600).

Slip-on Flange Cross Section

Slip-on Flange Cross Section

A slip-on flange’s bore size (internal diameter) is larger than that of the connecting pipe, which allows it to slide/slip onto the pipe (slip-onto the pipe). There is no full penetration weld between the pipe and the flange, thus there are limitations for its usage due to lower weld integrity. 

However, if a welding neck flange cannot be used due to space limitations, a slip-on flange may be a suitable alternative.

Slip-on Flange

Slip-on Flange

Advantages of slip-on flanges relate to lower procurement costs, less required skill for making the weld, lower accuracy requirement for pipe cutting, and no weld preparation requirements for a pipe end weld. 

Limitations for slip-on flanges relate to:

  • The use of two welds instead of one (usually).
  • Reduced mechanical strength (welding neck flanges are 30% stronger
  • Unsuitability for cyclic loading.
  • Inability to perform some non-destructive tests on welds.
  • Unavailable in 2.5-inch size and over in ASME class 1500.
  • Unavailable in all sizes in ASME class 2500.
  • Generally not used above ASME class 600.

Although cheaper than a welding neck flange, there is little price difference if two welds are performed and both must be checked using non-destructive testing techniques.


Non-Destructive Testing (NDT) Techniques

There are no specific NDT techniques used in conjunction with slip-on flanges. If the slip-on flange is fillet welded, then a dye penetrant test may be performed on the weld. A visual inspection of the mating faces and pipe is possible.


ASME Standards

The standards ASME B16.5 and ASME B31.3 specify the requirements for slip-on flange installation, maintenance, and testing.

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