Slip On Flanges

Slip On (SO) flanges are popular in piping systems primarily because of their ease of installation and cost effectiveness compared to other flange types, like Weld Neck flanges. They are designed to simply slip over the end of the pipe, which means they require less precision when cutting the pipe to length. This loose fit also significantly simplifies the task of aligning the bolt holes with the mating flange or equipment nozzle, reducing labor time and overall installation costs. Since they require less material than Weld Neck flanges, their initial purchase price is also generally lower.

Installing a Slip On Flange is a straightforward process that involves two key welds. First, the flange is slid over the pipe end and is positioned so that the pipe's end is slightly set back from the face of the flange, typically leaving a 1/8 inch (3 mm) gap. This small gap prevents the welding arc from damaging the flange face and helps minimize stress concentration. Once the flange is correctly positioned and its bolt holes are aligned with the adjacent connection point, it is secured by performing two fillet welds. The first weld is applied on the outside of the flange hub where it meets the pipe, and the second is applied on the inside of the flange bore where the pipe end sits. This double welding ensures a strong, sealed connection.

To accurately measure a Slip On Flange for identification or replacement, you need to determine four critical dimensions. You must measure the Outside Diameter (OD) of the flange (the total width across the flange edge to edge) and the Inner Diameter (ID) or Bore, which is the diameter of the center hole that slips over the pipe. For flange mating, the most vital measurement is the Bolt Circle Diameter (BCD), also called the Pitch Circle Diameter (PCD). This is the diameter of the circle that passes through the exact center of all bolt holes, and it must match the BCD of the mating flange. Finally, you should measure the overall Flange Thickness (T) and note the number and size of the bolt holes, as these measurements correlate directly to the flange's Pressure Class (e.g., 150#, 300#) as defined by industry standards like ASME B16.5.