Torque + Angle tightening strategy for pipes assembly

The use of natural gas as fuel in transport is becoming increasingly popular, which is due to a saving in fuel costs of at least 30% compared to traditional types. One of the requirements of UNECE Regulation No. 110 for the assembly of such vehicles is the use of mechanical pipe connections on compressed (CNG) or liquefied (LNG) natural gas supply lines.

The fuel line of the bus is connected to other elements of the system not by welding, but by means of two-ferrule mechanical fittings (see photo below). The fitting nuts are tightened using the "Torque + Angle" strategy. The tube is inserted into the fitting until it stops, the nut is tightened "by hand" (1 ... 3 Nm). Then the operator turns the nut 450 ° (one full turn with a quarter) using the most common wrench. Correctness of the tightening is checked by means of a straight gauge, which is inserted into the gap between the nut and the fitting body (the gap is shown by the red arrow). In this case, repeatability is achieved due to the high quality of manufacturing of all components of the fitting (body, nut, front and rear ferrules). It is also important to note that the purpose of applying the "Torque + Angle" strategy is not to achieve maximum clamping force in the threaded connection, but to correctly position the ferrules: the front ferrule is responsible for tightness, the rear one is for holding the fitting on the tube.

Fuel pipes assembly with two-ferrule fittings

Two-ferrule fitting: how it works