DIN Flange Types

DIN flanges come in several distinct configurations, each designed for specific piping requirements. The choice of flange type depends on factors including the operating pressure, temperature range, whether the connection needs to be disassembled for maintenance, and the pipe material being joined.

Weld Neck Flanges

Weld neck flanges are identifiable by their long tapered hub that transitions gradually from the flange face to the pipe wall thickness. This taper serves a structural purpose: it distributes mechanical stress across the hub rather than concentrating it at the base of the flange, which makes weld neck flanges the preferred choice for high-pressure, high-temperature, and cyclic-loading applications.

The butt-weld connection between the hub and the pipe creates a joint that can be fully radiographed for quality inspection, which is often a requirement in critical process piping.

Slip-On Flanges (Flat Flanges for Welding)

Slip-on flanges have a straight bore slightly larger than the outside diameter of the matching pipe. The pipe slides into the flange, and two fillet welds (one inside, one outside) secure the connection. They cost less than weld neck flanges and are easier to align during installation, which makes them common in lower-pressure systems where extreme structural integrity is not the primary concern.

The trade-off is that slip-on connections cannot be radiographed as effectively as butt-weld joints, and the fatigue life under cyclic loading is roughly one-third that of a weld neck connection.

Blind Flanges

Blind flanges are manufactured without a bore. Their purpose is to seal the end of a piping system, a pressure vessel nozzle, or a valve opening. They must withstand the full system pressure across their entire face, which means blind flanges experience significant bending stress, especially at larger diameters.

Blind flanges are commonly used during hydrostatic testing, when a section of piping needs to be isolated for maintenance, or as permanent closures on branch connections reserved for future expansion.

Threaded Flanges (Screwed Flanges with Neck)

Threaded flanges have an internal thread that matches the external thread on the pipe end. The primary advantage is that no welding is required for the pipe-to-flange connection, which makes them useful in situations where welding is impractical or prohibited, such as in explosive atmospheres or when connecting to equipment that cannot tolerate welding heat.

The limitation is that threaded connections are not suitable for high-temperature service because thermal cycling can cause the threads to loosen over time. They are also not recommended for applications involving significant bending loads.

Lap Joint Flanges

A lap joint assembly consists of two components: a stub end that is butt-welded to the pipe, and a loose backing flange that slides over the pipe before the stub end is welded in place. The backing flange is free to rotate, which greatly simplifies bolt-hole alignment during assembly.

This configuration is particularly useful in systems that require frequent disassembly for cleaning or inspection, and in situations where the piping system uses expensive corrosion-resistant alloys. The stub end can be made from the exotic alloy while the backing flange uses less expensive carbon steel, reducing overall material cost.

Loose Flanges

Loose flanges operate on a similar principle to lap joints but use a welded collar or ring rather than a forged stub end. The loose flange slips over the collar and can rotate freely for alignment. This design is common in large-diameter, low-pressure applications where the ability to easily align bolt holes across large flanged connections is a practical necessity.

Selecting the Right Type

The choice of flange type is driven by the specific demands of the application. For critical, high-pressure process piping, weld neck flanges are almost always specified. For general utility service at moderate pressures, slip-on flanges offer a cost-effective solution. Where welding is not possible, threaded flanges fill the gap. And where frequent disassembly or material cost savings are priorities, lap joint configurations provide the needed flexibility.