Threaded Flange (THD)

What thread form does the Threaded Flange (THD) use?

A threaded flange (THD) is machined with an internal tapered thread — NPT or BSP — and is screwed directly onto threaded pipe. In the American system the taper thread is dimensioned to ASME B1.20.1 (NPT).

The flange is offered with two sealing faces:

• RF — Raised Face
• FF — Flat Face

Why can a threaded flange be installed without welding?

Because the connection is made by screwing the internal tapered thread onto the mating threaded pipe, no welding is required for the joint. This makes the THD flange suitable for non-weldable sites and for fire hazard areas where welding is forbidden.

What is the risk of thermal expansion on the threaded joint?

The threaded connection is mechanical rather than fused, so thermal expansion may loosen the thread joint. This behaviour is the reason the type is restricted to lower-risk services.

In which working conditions must a threaded flange not be used?

THD flanges are intended for low pressure service, non-weldable sites, fire hazard areas where welding is forbidden, and small bore gas & water piping. They are not for high temperature, cyclic heat, or toxic flammable fluid.

Which standards apply to threaded flanges?

The threaded flange is covered by the following standards across the major systems: