An ASME B16.5 Class 1500 Slip-On (SO) Flange is designed to slide over the outside diameter (OD) of a pipe. It is typically welded both internally and externally using fillet welds to seal the connection and provide structural strength. HT PIPE supply all different types and scales of flanges. Please feel free to contact us for further product information and pricing.
The Class 1500 designation represents a severe-service pressure rating. At room temperature, a Class 1500 flange made from standard carbon steel (like ASTM A105) can handle working pressures up to 3,705 psi (255.4 bar). As the operating temperature increases, this allowable pressure drops significantly.
Critical Application Constraints
While highly convenient because they do not require precise pipe-cut lengths, slip-on flanges are subject to strict limitations in high-pressure classes:
Size Restrictions: While the ASME B16.5 standard officially defines Class 1500 dimensions up to Nominal Pipe Size (NPS) 24, many engineering specifications restrict the use of slip-ons to smaller diameters (typically NPS 2-1/2 or smaller), preferring weld-neck flanges for larger high-pressure lines.
Cyclic Loading: Because they rely on fillet welds rather than a full-penetration butt weld, slip-on flanges exhibit lower fatigue strength. They should be avoided in systems subject to severe thermal cycling or heavy mechanical vibration.
Key Installation & Welder Tolerances
When fitting a Class 1500 Slip-On flange in the field, layout crews and quality inspectors focus on several strict ASME guidelines to ensure a high-integrity seal:
The Retraction Gap (Setback): The pipe must not be bottomed out completely flush with the flange face. Welders must pull the pipe back from the flange face by a distance equal to the nominal pipe wall thickness plus roughly 1/16 in (1.6 mm). This ensures that the internal fillet weld does not damage or distort the gasket seating surface.
Dual Fillet Welding: Slip-on flanges require two fillet welds: one on the outside hub of the flange, and one inside the bore near the flange face.
Bore Tolerances: ASME B16.5 limits the tolerance on the internal bore of slip-ons. For sizes up to NPS 10, the bore must be within +0.03 in (+0.8 mm) / -0.0 in of the nominal dimension to prevent excessive gaps that would compromise the strength of the fillet welds.
ASME B16.5 Class 1500 Slip-On (SO) Flange Dimension
