What are tubes? why they differ from pipes?
Tubes are a thin hollow structure designed to provide an enclosed pathway for fluids to flow. In the case of tubing, it is usually manufactured from rolled or extruded metal.
The main difference between the tube and pipe is that pipe is threaded at the end to form a connection. Tubes are never threaded and a device called a tube fitting must be used to couple a section of tube to another tube. The wall thickness of a tube is simply too thin to support threads.
Compression type tube fitting:
The most common type of tube fitting, which uses a compressible ferrule to perform the task of sealing fluid pressure. The essential components of a compression tube fitting are the body, the ferrule, and the nut. You must not thread instrument tubing and use a nut to make a connection.
When the nut is tightened the twin ferrules are forced into the tube and against the sides of the connector. They make a metal to metal seal. This seal is very effective if the nut is tightened correctly. It is good for pressures to at least 10,000 psi.
When initially assembling compression-style tube fittings, you should always precisely follow the manufacturer’s instructions to ensure correct compression.
Making a Compression Fitting:
Simply insert the tubing into the tube fitting. Make sure that the tubing rests firmly on the shoulder of the fitting and that the nut is finger tight
Hold the connector tightly with a backup wrench. Turn the nut about 1 1/2 turns (from finger tight). This is enough to seal the connection properly.
Over-tightening of a compression fitting is just as bad as under-tightening, as the fitting cannot form a robust seal once the ferrule and tube have been deformed.
Fitting connectors Types:
Tube fittings designed to connect a tube to pipe threads are called connectors. Manufacturers make connectors to fit all systems. They also. make connectors to connect two different systems. Tube fittings designed to connect one tube to another are called unions
A variation on the theme of tube connectors and unions is the bulkhead fitting. Bulkhead fittings are designed to fit through holes drilled in panels or enclosures to provide a way for a fluid line to pass through the wall of the panel or enclosure.
Tubing elbows are tube connectors with a bend. These are useful for making turns in tube runs without having to bend the tubing itself. Like standard connectors, they may terminate in male pipe thread, female pipe threads, or in another tube end.
Tee fittings join three fluid lines together. Tees may have one pipe end and two tube ends (branch tees and run tees), or three tube ends (union tees).
A variation of the theme of union tees is the cross, joining four tubes together: