There are 3 primary mechanical procedures for connecting two lengths of tube together.
Flared fittings – in which the end of the tube is flared with a tool and then included within a nut, sleeve and body.
Single ferrule fittings – where the ends of the tube are put into a fitting with one compression ring, a nut and body.
Dual ferrule fittings – where the ends of the tube are placed into a fitting with two separate compression rings, body and nut. (Also called the twin ferrule fitting, this isn’t strictly correct as the two rings are of different shape and length.)
This type of fitting, such as all the others, has one basic purpose. To securely join tubes together in a safe, seal tight assembly. Close tolerance machining on top quality CNC machines allows for constant production of all components.
The double ferrule fitting is able to lock onto the tube with a ‘double bite’ feature. Each ring bites in to the tube giving two different sealing areas. This style of fitting does so without transmitting torque or twisting the tube ensuring that the tube doesn’t become ‘stressed’. Click at ECO Fitting,if you want to know what to do before buying stainless steel.
With stainless steel fittings the problem of galling is solved by the use of silver plating. The internal areas of the nut are the only areas that are plated.
Materials such as high Nickel alloys, high temperature alloys, duplex and super-duplex’s are coated with an anti-gall representative in precisely the same way. However, Molybdenum Disulphide can be utilized as an alternative. These materials are chosen in order to be compatible with the fluid media contained within them. For chemical stability the perfect situation is that both tube and fittings are manufactured from the same material type.
There are strict assembly instructions to follow with this product type. The rule states that for the vast majority of sizes 1-1/4 rotations of the nut, from the finger tight position, will give the correct seal tight assembly. It’s permissible to further ‘snug’ the fitting if a minor leak appears but this is normally unnecessary.
When access to any system is needed these fittings provide an excellent ‘break-in’ point. The joint can be made and re-made many times over without loss of sealing integrity, providing the correct installation instructions are followed.
The strength of the fitting is such that the tube contained will burst until the fitting displays any sign of a leak or motion. This is subject to certain constraints on the wall thickness of the tube. If the wall is too thick the rings won’t bite as well, as when the tube wall is too thin, the tube will fall instead of allow the rings to bite fully.
There are numerous manufacturers of this product kind around the world and they generally contain the term ‘LOK’ in their product name. The major manufacturers encourage users of the product type not to inter-change different products. It’s deemed as sacrilege to combine parts from different producers.