What Is the Meaning of SO Flange?

If you’re specifying flanges for pipe systems and see “SO flange,” you’re looking at a Slip-On flange. Unlike some high-cost options, SO flanges slide over the pipe and are joined with fillet welds—inside and out—making them easy to align and install in low- to medium-pressure systems.

How SO Flanges Work

Slip-On flanges have a slightly larger bore than the pipe, allowing them to slide over it before being welded on both sides. After welding, you add bolts and a gasket to complete the seal.

Easy installation: The flange simply slides on before welding, making alignment easier compared to flanges requiring more precise fit-up.
Good leak resistance: Once properly welded inside and out, slip-on flanges offer solid sealing, especially in smaller-diameter piping.

What Makes SO Flanges Popular

Slip-On flanges have several advantages that make them stand out:

Lower cost: Their flat design and simple manufacturing process reduce material and machining expenses.
Easy alignment: They don’t require the same tight fit as weld-neck types, speeding up installation and reducing labor.
Reliable for standard service: When used within recommended pressure and temperature ranges, the inner and outer welds provide a dependable seal, especially in smaller-diameter pipes.

These features make SO flanges a practical choice for a wide range of industrial systems.

Choosing the Right SO Flange for Your System

Before specifying a slip-on flange, make sure each detail matches your project requirements:

Match pipe dimensions: The flange should correspond to your pipe’s nominal diameter and wall thickness suited for your system’s conditions.
Gasket and face compatibility: Slip-on flanges can use raised face or flat face gaskets based on your pressure and sealing needs.
Bolt alignment: Most Slip-On flanges follow ASME (B16.5, B16.47) or DIN standards, so check that the bolt-hole spacing matches the connected components.
Material selection: Choose the right material for your fluid and operating conditions—stainless steel flanges resist corrosion, while carbon steel options are more cost-effective. Check out Longan’s stainless steel flanges or ASME-grade options if you need compliance with U.S. standards.

When Slip-On Flanges Aren’t the Best Fit

While SO flanges cover many use cases, they’re not suitable for everything. Avoid them in applications that require:

High-pressure systems where stronger weld-neck flanges offer better structural support.
Frequent temperature or pressure cycling, which can weaken welds over time.
Large-diameter piping, where the welded structure of other flange types performs better under stress.

In such cases, options like weld-neck or socket weld flanges may be more appropriate.