DIN 2353 Compression Fittings

DIN 2353 compression fittings are used to connect metric OD tube into a line without welding. Commonly called "Ermeto-type fittings" in the field in Turkey (the name comes from one manufacturer's registered trademark and is used here only to describe the connection type), these connections are 24° cone compression fittings with nut and ferrule, covered by DIN 2353 and ISO 8434-1. As the nut is tightened, the cutting edge of the ferrule bites into the tube in a controlled way; sealing is achieved by metal-to-metal contact in the 24° cone seat or by an O-ring sealed cone end. Because the ferrule bites into the tube, they are also known as bite-type fittings.

Types of stainless 24° cone compression fittings

• Tube-to-tube connections: straight couplings, 90° equal elbows and equal tees, with two or three compression ends.
• Port connections: male stud connectors and male stud elbows with one compression end and a BSPP, BSPT or NPT male thread on the other; male branch tees.
• Bulkhead coupling: a removable pass-through secured to a panel, tank wall or ship bulkhead with a locknut.

L and S series selection (DIN 2353 / ISO 8434-1)

ISO 8434-1 defines different pressure series for the same tube OD. Because the series changes the nut thread and body dimensions, parts from different series cannot be fitted together.

For example, for 12 mm tube the nut thread is M18x1.5 in the L series and M20x1.5 in the S series. The standard also defines an extra-light LL series for small-diameter pilot lines. Working pressure varies with size, series and material; the value is checked in the table for the selected part. Stock and production sizes are confirmed at the quotation stage.

Tube selection

These fittings are used with seamless hydraulic tube whose OD tolerance and hardness suit the ferrule. If the tube is too hard, the ferrule cannot bite in sufficiently; if it is too soft, it collapses inward under pressure. Wall thickness is selected according to line pressure.

Compression fitting assembly and common mistakes

1. Cut the tube square and clean, and deburr the inside and outside.
2. Slide the nut and ferrule onto the tube in the correct orientation; the cutting edge of the ferrule must face the fitting body.
3. Push the tube all the way to the tube stop inside the body; if the tube does not bottom out, the ferrule will not bite correctly.
4. Tighten the nut by the number of turns specified by the manufacturer. With stainless steel, pre-assembly using a separate pre-assembly body or machine makes it easier for the ferrule to bite in evenly.
5. When reassembling a joint that has been disconnected, bring the nut back to its previous position and tighten it slightly.

Overtightening damages the ferrule and the cone; undertightening can allow the tube to slip under pressure.

Which industries use them?

Ship engine room and deck hydraulics, shipyard maintenance and repair work, offshore power units, rolling mill and board mill power lines, fire pump skids, and the hydraulic circuits of chemical and food plants. Where a removable point is needed on a welded line, weld nipples and weld fittings are used; where a hose transition is needed, metric swivel-nut hose fittings are used.