Slip-on flanges are preferred to weld-neck flanges by many users because of their initial low cost and ease of installation. Their calculated strength under internal pressure is about two-thirds of that of weld-neck flanges. They are typically used in low-pressure, low-hazard services such as fire water, cooling water, and other services.

The pipe is “double-welded” to both the hub and the bore of the flange, but, again, radiography is not practical. MP, PT, or visual examination is used to check the integrity of the weld. When specified, the slip-on flanges are used on pipe sizes greater than NPS 2¹⁄₂ (DN 65).

Several different flange types are permitted by the standards, as principally covered by ASME B16.5. These include different types of attachment to the pipe including threaded, lapped, and welded as covered in more detail. Of the welded type, most flanges are the butt-welded, slip-on, or socket-welded types.

The majority of flanges are butt-welded, which are more commonly referred to as the welding neck flange. Socket-welded flanges are typically limited to small-diameter connections less than NPS 2 (DN 50). Slip-on flanges fit over the outside diameter of the pipe and are attached with fillet welds at both the pipe end and off the hub end of the flange.

Typical restrictions on the use of slip-on flanges include:

#1 While available in most pressure classes, slip-on flanges are more typically limited to Class 300 or lower pressure rating in process plant piping. The available raised face gasket seating area can preclude commonly used gaskets in the case of slip-on flanges for higher class piping.

#2 They are limited to services with design temperatures below 750°F (400°C).

#3 They should not be used where the specified corrosion allowance exceeds 0.125 in (3 mm).

#4 Many pipe designers are reluctant to use slip-on flanges for higher pressures, since (1) the joint between the flange and pipe is not as strong as in the welding neck type; and (2) the junction of the flange and pipe is more susceptible to corrosion.