Pipe screw connections

Elements used to create removable connections for pipelines and devices. They are manufactured for three pressure stages (LL - very light, L - light, S - heavy). A distinction is made between three different types of design.

  • Screw-in fittings

These include a threaded pin to screw in to device bore on one end, with the pipe pushed into the other end. Screw-in thread in accordance with DIN 3852.

  • Fittings:

These include connection elements on both sides to connect the pipes to one another.

  • Swivel fittings

These are elbow fittings, in which you can select the outlet direction before the fitting is tightened (Figure R 29).

In terms of construction, a distinction is made between (Figure R 30):

Cutting ring fittings (a) (DIN 2353, DIN 3861):

The pipe is held by a cutting ring that embeds itself. It is the most common type of fitting, but can become problematic in the case of strong impacts or oscillations.

Cutting ring fittings are preferred for metal pipes. However, they can

also be used for plastic pipes, but then need a reinforcing sleeve. However, this will make the cross section of the pipe smaller.

Cutting ring fitting with soft-sealing elements (b):

The sealing and holding functions are performed by two construction elements, a sealing ring and a retaining ring.

The elastomer sealing lip is vulcanized on the metallic supporting body and is located inside the fitting taper after it is fitted. The retaining ring performs the holding function according to the cutting ring principle in connection with the union nut.

Fitting with clamping ring (c):

The connection is made by a fine-ribbed clamping ring, which only leaves minimal impressions inside the pipe and therefore reduces the notch effect.

Flare tube fitting (d):

The end of the pipe is flared and held by the supporting ring and union nut. An intermediate piece that is conical on both sides performs the sealing function together with the elastic sealing rings and the frictional connection between the fitting and the pipe. Flare tube fittings with a metallic seal, connectors with a UNF thread, in accordance with SAE 7514/ISO 8434 are widely used, particularly in the USA.

Welded nipple connection (e) (DIN 3865):

The pipe is welded using a nipple, which is used in the normal fitting. The sealing function on the welded nipple is often performed by an elastic sealing ring.

Depending on the design, a distinction is made between straight, angular, T-shaped fittings (Figure R 31) and L-shaped and cross-type fittings.


Figure R 29: Swivel screw fitting


Figure R 30: Pipe screw fittings


Figure R 31: Different fitting shapes