Why flanges exist
Welding gives a permanent joint, but plants often need connections that can be opened. Pumps get removed, valves get replaced, and vessels need inspection. A flange provides a way to bolt two pieces together with a seal between them, so the connection can be taken apart and remade.
A flanged joint has four main parts: two mating flanges, a gasket between them, a set of bolts or studs with nuts, and the pipe or equipment each flange is attached to. The seal does not come from any one of these alone. It comes from the way they work together.
How the seal is made
When the bolts are tightened, they stretch slightly, like very stiff springs, and pull the flanges toward each other. That clamping force squeezes the gasket. The gasket is softer than the metal faces, so it flows a little into small imperfections on the surfaces and fills them. Once fluid pressure is applied, the joint also has to resist the force trying to push the flanges apart, which is why the bolts must hold a load above what pressure alone would demand.
Two things need to stay true for the life of the joint. The gasket must be compressed enough to seal at start-up, and the bolts must keep enough load on it as pressure, temperature and time act on the assembly. If the load drops too low, the joint leaks.
Flange faces and types
Flanges come in several styles. Weld neck flanges are welded to the pipe end and are common in demanding service. Slip-on flanges slide over the pipe and are welded in place. Threaded flanges screw onto pipe, which suits some small-bore or hard-to-weld situations. Blind flanges close the end of a line. Lap-joint flanges use a separate stub end and are helpful where alignment of bolt holes is awkward.
The sealing surface, or face, also varies. A flat face spreads load across a wide area and is often used with softer materials. A raised face concentrates load on a smaller ring, which helps the gasket seal. Ring-type joint faces use a metal ring in a groove and are used for severe, high-pressure service. The faces on mating flanges need to be compatible, and mixing types is a recognized cause of trouble.
Flanges are also grouped by pressure class under industry standards, so that a flange rated for a given service matches its partner and the pipe. Flange ratings generally fall as temperature rises, which is a reason to check them against the actual operating conditions rather than the label alone.
Gasket choices
The gasket must suit the fluid, temperature, pressure and the flange face. Common categories include:
- Compressed fiber or sheet gaskets, often used for water and general service at modest conditions.
- PTFE and other plastic gaskets, valued for chemical resistance but sometimes prone to creep, meaning they slowly flow under load and lose clamping force.
- Spiral wound gaskets, which combine metal windings and filler and are widespread in higher-pressure and temperature service.
- Metal and ring-joint gaskets, used where conditions are severe.
- Rubber gaskets, typical for water and some low-pressure lines.
Reusing a gasket is generally discouraged because it has already been compressed and may not seal again. A gasket that suits a harmless fluid may be unsuitable for a chemical, so materials should be checked against compatibility data, as with the broader question of pipe materials.
Why bolting practice matters
Many flange leaks trace back not to the gasket or flange but to how the bolts were tightened. Uneven or insufficient load is the main culprit. Some key habits are widely recommended:
- Clean and inspect the faces, studs and nuts. Damaged threads, rust or old gasket residue can prevent proper loading.
- Align the flanges before tightening. Forcing misaligned flanges together with bolts leaves residual stress and uneven gasket compression.
- Lubricate threads and nut faces with the product the procedure specifies, since friction strongly affects how much bolt load a given torque actually produces.
- Tighten in stages and in a cross pattern, often called star or criss-cross, rather than going around the circle. This brings the flanges together evenly.
- Make a final pass in a circular order so every bolt reaches the intended load.
- Use calibrated tools, and follow the written procedure, which supplies target values for the specific joint. The upkeep of tools falls under regular calibration.
Procedures for tightening, and the values used, come from the joint’s design and the gasket maker’s data. Guessing or relying on feel is a common source of leaks.
Common reasons joints leak
Beyond poor tightening, flange leaks often come from:
- Misalignment or pipe strain, where the line is pulling on the flange. Poor support is one cause, discussed in the article on pipe supports.
- Thermal cycling that loosens bolts over time, particularly when heating and cooling happen quickly. Movement in hot lines is covered under expansion joints.
- Vibration, which can work bolts loose.
- Damaged or dirty sealing faces.
- A wrong or worn gasket.
- Over-tightening, which can crush a gasket or yield a bolt.
Joints that leak steam, hot oil or hazardous fluids are a safety issue, not just a nuisance.
Safe practice around bolted joints
Opening a flange means opening a line, so the line must first be isolated, depressurized, drained, and where required purged and verified. Lockout tagout style procedures, permits and appropriate protective equipment apply. A bolt that is “just a little loose” on a live, pressurized joint should never be treated casually. Tightening live joints is restricted at many sites, and only trained personnel should do it under the site’s rules.
This is general information, not engineering advice. For any specific joint, follow the manufacturer’s instructions, the plant’s procedures and the applicable codes.