Flange types: weld neck, slip-on and blind

What a flange does

A flanged joint consists of two flanges, a gasket between them and a bolt set that presses the flanges together. Unlike a welded joint, it can be opened again. Flanges are therefore used where a line must be taken apart: at valves, pumps, vessels and instruments, at spool ends that are joined on site, and where a line must be cleaned, inspected or extended later.

The type of flange determines how it is attached to the pipe, how strong the joint is and how much work it takes in the workshop. The choice is not free: the piping specification states per line which flange type, pressure rating and facing apply.

Standards

Two standards cover most industrial flanges:

• EN 1092-1 for steel flanges in PN ratings (PN 6, PN 10, PN 16, PN 40 and higher). Each flange type has a type number.
• ASME B16.5 for steel flanges in Class ratings (Class 150, 300, 600 and higher), in sizes up to NPS 24.

The standards fix the dimensions: outside diameter, bolt circle, number and size of bolts, thickness and, for hubbed flanges, the length of the hub. Two flanges of the same standard, size and rating fit together, whatever the manufacturer.

Weld neck flange

A weld neck flange has a long, tapered hub that ends in a bevel. The pipe is joined to the hub with a butt weld. In EN 1092-1 this is type 11; in ASME B16.5 it is the welding neck flange (WN).

Properties:

• The tapered hub spreads the stress from the flange into the pipe. The joint performs well at high pressure, high or fluctuating temperature and under cyclic loading.
• The bore of the flange matches the inside diameter of the pipe, so there is no step in the flow. The bore is therefore ordered with the same wall thickness or schedule as the pipe.
• The butt weld can be inspected by radiography or ultrasonic testing, like any other butt weld in the line.
• The flange is heavier and more expensive than a slip-on flange, and the pipe must be cut to an exact length, because the flange adds its full length to the line.

Weld neck flanges are the usual choice in process plants, steam lines and lines with a high pressure rating.

Slip-on flange

A slip-on flange has a bore slightly larger than the outside diameter of the pipe. The pipe slides into the flange and is fixed with two fillet welds: one on the outside at the back of the flange (at the hub, if there is one) and one on the inside, near the face. In EN 1092-1 the plate flange for welding (type 01) and the hubbed slip-on flange (type 12) work this way; in ASME B16.5 it is the slip-on flange (SO).

Properties:

• The pipe can be positioned in the flange before welding, so length and orientation are easier to adjust.
• The flange is lighter and cheaper than a weld neck flange.
• Two fillet welds take more welding time than one butt weld, and fillet welds are not inspected by radiography in the same way.
• The joint is less strong and less resistant to fatigue than a weld neck joint.

Slip-on flanges are common in utility lines, cooling water, low-pressure air and building services, where pressure and temperature are moderate and loads are mostly static.

Blind flange

A blind flange is a solid disc with the bolt holes of a flange but without a bore. It closes the end of a line, a nozzle or an opening in a vessel. In EN 1092-1 this is type 05; in ASME B16.5 it is the blind flange (BL).

Uses:

• closing a branch or nozzle that will be connected later;
• closing a line for a pressure test;
• providing an access point for cleaning or inspection.

A blind flange carries the full pressure of the line on its face. It is not welded, only bolted.

Comparison

Other types

Besides these three, a piping specification may include:

• socket weld flanges, in small sizes, where the pipe is inserted into a socket and fillet welded;
• threaded flanges, without welding, for small sizes and low pressure;
• lap joint or loose flanges with a stub end or collar, where the flange itself can rotate; useful for bolt hole alignment and for stainless steel lines with a carbon steel flange.

Facing

The face of the flange determines which gasket is used. The most common facings are:

• raised face (EN 1092-1 type B, ASME RF): the gasket sits on a raised ring inside the bolt circle;
• flat face (EN 1092-1 type A, ASME FF): the whole face is flat; used with cast iron equipment and some plastic or lined components;
• ring type joint (ASME RTJ): a groove in the face holds a metal ring gasket; used at high pressure.

Two flanges in one joint must have matching facings. A raised face flange against a flat face flange on cast iron equipment can crack the equipment flange when the bolts are tightened.

Points to check

When a flange is selected or ordered, the following must match:

1. the standard (EN 1092-1 or ASME B16.5) and the type;
2. the nominal size (DN or NPS) and the pressure rating (PN or Class);
3. the material, compatible with the pipe and the medium;
4. for a weld neck flange: the bore, matching the wall thickness of the pipe;
5. the facing, matching the opposite flange and the gasket.

All of this comes from the piping specification. On the isometric drawing and in the bill of materials, each flange appears with its full description, together with the gasket and bolt set of the joint.

In FlowCAD3D

The component library contains flanges from EN 1092-1 and ASME B16.5, among about 650 parts from standard datasets. Components are placed with their actual installation lengths, and gaskets and bolt sets are added automatically at each flanged joint. Specifications (piping classes) warn or block when a flange does not match the class of the line. The bill of materials lists flanges, gaskets and bolt sets, and can be exported to XLSX and CSV. Bolt lengths are indicative.