Flanged joint on a vertical hot-water riser, raised faces and bolting exposed

Flanged joints — Inzonex Modular Insulation, hot-water skid

Flanged joints / Types

Flange types

Five types cover almost everything in a plant room, and they are not interchangeable.

The type is decided by how the flange attaches to the pipe, not by the pipe. That choice sets what the joint can carry, how long it takes to fit, and whether it can be taken apart twice. On a hot-water circuit it also sets how much metal is standing in the air. A weld-neck hub is a longer thermal path than a slip-on collar, and both are heavier than the pipe they terminate.

The five

What each one is

Weld-neck

A tapered hub butt-welded to the pipe, so the bore is continuous and the load runs into the pipe wall rather than into a fillet. It is what a header or a pump connection should be. It is also the most expensive and the slowest to fit, because the weld is a full penetration butt.

Slip-on

The pipe slides through and is fillet-welded inside and out. Quicker, cheaper, tolerant of a length cut slightly wrong. Weaker in fatigue by a wide margin. It belongs on branch work rather than on the discharge of a pump that starts and stops all day.

Blind

A blind flange has no bore. It closes a branch kept for a future connection, or blanks a vessel nozzle. On a live header it is a solid disc of hot metal, with no insulation, no flow through it and nothing behind it. It is routinely the hottest surface in a plant room.

Threaded

Screwed onto a threaded pipe end with no welding at all. It appears where hot work is not allowed. It cannot be relied on where the joint sees vibration or repeated heating and cooling.

Lap-joint

A stub end welded to the pipe with a loose backing flange behind it. The backing flange spins, so the bolt holes can be aligned after the pipe is up. Used on lined and on stainless systems where the expensive material stops at the stub.

Faces

Raised, flat and ring-joint

Raised face

A narrow band around the bore standing proud of the rest of the flange. Concentrating the bolt load onto a small area is what seats the gasket, and it is the default on steel pipework.

Flat face

The whole face is one plane. It exists for cast iron and for GRP: a raised face bolted against a cast body levers the rim and cracks it. If one side of a joint is cast, both sides are flat-faced and the gasket is full-face.

Ring-type joint

A machined groove taking a metal ring, sealing by controlled deformation rather than by squeezing a soft sheet. It belongs to high pressure and high temperature work and is not something a hot-water circuit needs.

On the skid

What this changes for insulation

The hub is not the pipe

A weld-neck joint is two hubs, two rims and the bolting. It stands in still air with a clear path to it on every side. It is counted as its own item rather than folded into a metre of pipe.

It has to come apart

A joint exists so that something can be removed. Anything glued, wired or foamed over the bolt heads gets cut off the first time the pump is lifted. It does not go back on. The module here is cut to the joint and closes onto the same geometry.

Reference

Type against duty

Type	How it joins	Where it belongs	What limits it
Weld-neck	Butt weld to the pipe	Headers, pumps, exchanger ports	Cost and welding time
Slip-on	Fillet weld both sides	Branches, low-cycle runs	Fatigue life
Blind	Bolted, no bore	Spare branches, vessel nozzles	Uninsulated hot plate
Threaded	Screwed, no hot work	Where welding is prohibited	Vibration and thermal cycling
Lap-joint	Stub end and loose backing	Lined and stainless systems	More parts, more cost

Type · How it joins · Where it belongs · What limits it

Asked about flange types

Questions.

Answers distinguish modelled heat-loss estimates from equipment selection and service guidance.

What is the difference between a weld-neck and a slip-on flange?

A weld-neck has a tapered hub butt-welded to the pipe, so the bore is continuous and bending load passes into the pipe wall. A slip-on sits over the pipe end and is fillet-welded, which is faster and cheaper but far weaker in fatigue. Weld-neck on a pump connection, slip-on on a branch.

What is a blind flange used for?

It closes a flanged branch that has no pipe on it. That is a spare connection kept for future plant, or a vessel nozzle not in use. On a live header it is a solid uninsulated disc at line temperature. It is very often the hottest thing anyone can touch in the room.

Can you bolt a raised-face flange to a flat-face flange?

No, not onto cast iron or GRP. The raised face concentrates the bolt load on a narrow band, and it levers the rim of the softer body until it cracks. Where one side is cast, both sides are flat-faced and the gasket is full-face, out to the bolt holes.

Which flange type is best for a hot-water plant room?

Weld-neck on anything carrying load or cycling - pump suction and discharge, exchanger ports, the main header. Slip-on is acceptable on branch work. What matters more than the type is that the joint can still be undone after it has been insulated.