Reducing Flanges: When Do You Need One?

When two different pipe sizes have to meet at a flanged joint there are two routes: weld in a reducer and use a normal flange, or use a single piece reducing flange. Which is right depends on the location and the service.

What a reducing flange is

Its outside dimensions and bolt circle match the flange of the larger size, while the hub and bore are machined for the smaller pipe. A 4" pipe can therefore connect directly into a 6" flanged joint. Weld neck, slip-on and threaded versions exist.

When it is preferred

• Where space is tight. A reducer plus flange needs at least two welds and extra length; a reducing flange is one piece.
• Where weight matters. Particularly on platforms and skids.
• At equipment connections. Where a pump or valve size differs from the line size.

When it is not

If flow behaviour matters, the sudden change in bore creates turbulence and pressure loss. Near measurement points, at pump suction and where cavitation is a risk, a concentric or eccentric reducer giving a gradual transition is the better answer. On horizontal lines, eccentric reducers prevent liquid pooling.

When ordering

Give both sizes together (for example 6" × 4") plus pressure class, face type, material grade, standard and, for weld neck versions, the schedule of the smaller pipe. Without the schedule the hub bore is machined incorrectly.

See our butt weld fittings page for the reducer alternative, and send your list through the enquiry form.

FAQ

Is a reducing flange available in every size combination?
Standard tables define the common combinations; unusual pairs are made to order and take longer.

Can a reducing flange be used on a measurement line?
It is not recommended. The sudden change of bore distorts the flow profile and reduces measurement accuracy.

Which size is written on the order?
Both: the larger flange size and the pipe size to be connected, together with the pipe schedule.