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Flange Basics: ANSI Classes, Face Types & How to Spec the Right Flange

By MKS Pipe & Valve Technical Team | May 28, 2026

A practical guide to pipe flanges covering ANSI pressure classes, the main flange types, raised and flat and ring type joint faces, and how to spec a flange that mates correctly.

A flange is one of the simplest looking parts in a piping system and one of the easiest to get wrong. Spec the class, type, or face incorrectly and the joint will not seal, no matter how carefully it is bolted. Here is what a trade reader needs to know to get it right.

What a Flange Does

A flange creates a bolted, serviceable connection between two pieces of pipe, or between pipe and a valve, pump, or vessel. Unlike a welded joint, a flanged joint can be unbolted, opened, and reassembled, which makes it the connection of choice anywhere you need to break the line for maintenance, inspection, or component swaps. The seal is made by a gasket compressed between two flange faces and held by a ring of bolts.

Pressure Classes

Flanges are rated by pressure class, commonly 150, 300, 600, 900, 1500, and 2500. It is important to understand that a class is not a single pressure number. It is a pressure-temperature rating. The pressure a given class can actually hold depends on the flange material and the operating temperature, and the allowable pressure drops as temperature climbs.

That means a Class 150 flange in one material at ambient temperature carries a very different rating than the same class in another material running hot. Higher class numbers provide more capacity for a given material, which is why both flanges in a joint, along with the bolting, must share the same class. Mixing classes is a recipe for a leak or a failure.

The Main Flange Types

Several flange types exist because piping systems face very different demands.

Weld neck: A tapered hub welded to the pipe with a single butt weld. Strong and well suited to high-pressure, high-temperature, and cyclic service.
Slip-on: Slides over the pipe end and is fillet welded inside and out. Easier to align and lower cost, common on moderate-pressure lines.
Socket weld: The pipe seats into a socket and is fillet welded. Used on smaller, higher-pressure lines where a clean internal bore matters.
Threaded: Screws onto threaded pipe with no welding. Useful where hot work is not allowed or not practical.
Lap joint: Pairs with a stub end and lets the flange rotate freely for bolt-hole alignment. Handy when frequent disassembly or alignment freedom is needed, and it lets a less expensive flange material back a costlier stub end.
Blind: A solid flange with no bore, used to close off the end of a run, a vessel nozzle, or a future tie-in point.

Face Types

The flange face is the surface that contacts the gasket, and it has to match the mating flange.

Raised face (RF): A small raised area around the bore concentrates the bolt load onto a smaller gasket area for a better seal. This is the most common facing in industrial service.
Flat face (FF): A full flat mating surface, often specified when bolting to cast iron equipment or flat-faced equipment flanges, where a raised face could crack the brittle casting.
Ring type joint (RTJ): A machined groove holds a metal ring gasket that seals metal to metal, used in high-pressure and high-temperature applications.

The rule is simple. The facing must match on both flanges, and the gasket must suit that facing. An RF flange does not mate properly to an FF flange, and an RTJ groove needs the correct ring, not a flat gasket.

How to Spec a Flange

When you order a flange, confirm every one of these so both halves of the joint agree.

Nominal pipe size
Pressure class
Flange type (weld neck, slip-on, and so on)
Facing (RF, FF, or RTJ)
Bolt pattern, meaning the bolt circle and hole count, which follows from the size and class
Material, matched to the medium and temperature

Get the bolt pattern, class, and facing aligned on both flanges and the joint comes together cleanly. Miss any one and it will not. If you are also weighing flange material against the rest of the line, our carbon steel versus stainless steel pipe guide is a useful companion.

Spec It Once, Order It Right

MKS Pipe & Valve has stocked flanges and supported Midwest contractors and plants since 1946, and we can pair the right flange with a matching gasket and bolting so the whole joint is rated as a set. When a standard part will not do, our in-house machine shop cuts custom gaskets and fabricates specialty assemblies to spec.

Call (888) 665-2696, email info@mkspvf.com, or contact our team with your specs. You can also browse flanges and fittings at shop.mkspvf.com.

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Frequently Asked Questions

What do ANSI flange classes like 150 and 300 mean?

A flange class such as 150 or 300 is a pressure-temperature rating designation, not a single pressure number. The actual pressure a class can hold depends on the flange material and the operating temperature, with capacity dropping as temperature rises. Higher class numbers carry more pressure for a given material, which is why both halves of a joint must share the same class.

What are the main types of pipe flanges?

The common types are weld neck, slip-on, socket weld, threaded, lap joint, and blind. Weld neck flanges suit high-pressure and cyclic service, slip-on and socket weld are easier to fit on smaller lines, threaded flanges avoid welding, lap joints allow alignment freedom, and blind flanges close off the end of a run. The right choice depends on pressure, pipe size, and how the joint will be assembled.

What is the difference between raised face and flat face flanges?

A raised face (RF) flange has a small raised area around the bore that concentrates gasket load, while a flat face (FF) flange has a full flat mating surface often used with cast iron equipment. Ring type joint (RTJ) flanges use a metal ring in a machined groove for high-pressure and high-temperature sealing. The facing must match on both flanges, and the gasket has to suit that facing.

How do I spec a flange correctly?

Match the nominal size, pressure class, facing, bolt pattern, and material on both halves of the joint, then select a gasket and bolting rated for the same conditions. A mismatch in any of these leaves the joint unable to seal or hold pressure. MKS can pair the correct flange with the right gasket and bolting and fabricate specialty assemblies in-house.