What types of pipes use necked slip-on flanges, and what types of pipes use necked butt-welding flanges?

Introduction to Alloy Steel SO Neck Slip-on Flange: As the name suggests, the Alloy Steel SO Neck Slip-on Flange is a slip-on flange or flange plate made of alloy steel. It belongs to the category of neck slip-on flanges. Alloy Steel SO Neck Slip-on Flanges are aesthetically pleasing, have a smooth and flat surface, are resistant to acids and alkalis, corrosion-resistant, and have strong performance characteristics. The English designation for neck slip-on flanges is SO.

Common specifications for alloy steel SO neck slip-on flanges are DN15-DN600 1/2"-24". This doesn't mean they're limited to these sizes; there's another standard from 24"-60", commonly used in machinery and chemical industries. Today, we'll continue discussing the pressure ratings of alloy steel SO neck slip-on flanges, which are divided into PN series (PN0.25MPa, PN0.6MPa, PN1.0MPa, PN1.6MPa, PN2.5MPa, PN4.0MPa, PN6.3MPa) and Class series (Class150, Class300, Class600, Class900, Class1500). Common materials for alloy steel SO neck slip-on flanges include ASTM A182 F1, F5a, F9, F11, F12, and F22. F91, A182F12, A182F11, 16MnR, Cr5Mo, 12Cr1MoV, 15CrMo, 12Cr2Mo1, A335P22, St45.8/Ⅲ; Alloy steel SO necked slip-on welding flanges have several standards, including the People's Republic of China National Standard: GB/T 9116-2010 Hubbbed slip-on-welding steel pipe flanges; the People's Republic of China Chemical Industry Standard: HG/T20592~20635-2009 PN series (European system); HG/T20592~20635-2009 Class series (American system).

Alloy steel SO neck slip-on flanges come in various sealing face types, including flat face (FF), raised face (RF), recessed face (MF), tongue and groove face (TF), groove face (GF), and ring face (RJ). Generally, raised face (MF), tongue and groove face (TG), and ring face (RJ) are used in pairs.

Alloy steel SO neck slip-on flanges are connected by double-sided welding and double-sided nuts. Double-sided welding involves welding the inner diameter first, then welding the outer steel pipe to the flange. This adds a second layer of protection to the weld joint. However, double-sided welding is more expensive than single-sided welding, but within a reasonable range; it's essentially just an extra weld. It's crucial to use a gasket when connecting flanges to prevent leakage.

The manufacturing process for alloy steel SO neck slip-on flanges includes forging and casting.

The sealing principle of alloy steel SO neck slip-on flanges: The two sealing faces of the bolts press against the flange gasket to form a seal, but this process can also lead to seal failure. To maintain a seal, a huge bolt force must be sustained, requiring larger bolts. Larger bolts necessitate larger nuts, meaning larger diameter bolts are needed to tighten them. However, the larger the bolt diameter, the more likely the flange will bend; the only solution is to increase the flange wall thickness. The entire assembly will require significant size and weight, a particular problem in offshore environments where weight is always a primary concern. Furthermore, fundamentally, a slip-on flange is an ineffective seal, requiring 50% of the bolt load to compress the gasket, leaving only 50% for maintaining pressure.

Advantages and disadvantages of alloy steel SO neck welding flanges:

Advantages: Neck welding flanges are still welding flanges, but the short neck increases flange strength and improves load-bearing capacity. Therefore, they can be used in higher-pressure pipelines.

Disadvantages: More expensive than plate slip-on flanges, and their shape makes them more susceptible to damage during transport.

There are a few details to keep in mind when using alloy steel SO weld neck flanges:

1. The material must be the same as the material of the pipe or fitting being welded; otherwise, welding will fail.

2. Before welding, always check the flange for cracks. Flanges with cracks should not be used.