Double ferrule tube fittings
There are 3 main mechanical methods for connecting two lengths of tube together.
- Flared fittings – where the end of the tube is flared with a tool and then contained within a nut, sleeve and body.
- Single ferrule fittings – where the ends of the tube are placed into a fitting with a single compression ring, a body and nut.
- Double ferrule fittings – where the ends of the tube are placed into a fitting with two separate compression rings, body and nut. (Also known as the twin ferrule fitting, this is not strictly correct as the two rings are of different shape and length.)
For the purposes of this article the focus will be on the double ferrule fitting.
This style of fitting, like all the others, has one basic function. To securely join tubes together in a safe, seal tight assembly. Close tolerance machining on high quality CNC machines allows for consistent production of all components.
The double ferrule fitting has the ability to lock onto the tube with a ‘double bite’ feature. Each ring bites in to the tube giving two separate sealing areas. This style of fitting does so without transmitting torque or twisting the tube ensuring that the tube does not become ‘stressed’. Therefore, the mechanical properties of the tube are maintained. A further sealing point occurs at the bottom of the tube abutment. The abutment has an angle which the tube is forced into when the rings bite and drive the tube forward.
With stainless steel fittings the problem of galling is resolved by the use of silver plating. The internal parts 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 agent in the same way. However, Molybdenum Disulphide can be used as an alternative. These materials are selected in order to be compatible with the fluid media contained within them. For chemical stability the ideal 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 majority of sizes 1-1/4 rotations of the nut, from the finger tight position, will give the correct seal tight assembly. It is permissible to further ‘snug’ the fitting if a minor leak appears but this is normally unnecessary.
When access to any system is required 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 before the fitting shows any sign of a leak or movement. This is subject to certain constraints on the wall thickness of the tube. If the wall is too heavy the rings will not bite as well, as when the tube wall is too thin, the tube will collapse rather than allow the rings to bite fully. Reference to the manufacturers’ product information should be made in all instances. The tube should generally have a hardness of no more than 80 on the Rockwell ‘B’ scale.
There are a number of manufacturers of this product type around the world and they generally contain the term ‘LOK’ in their product name. The major manufacturers encourage users of this product type not to inter-change different products. It is deemed as sacrilege to mix parts from different producers. It must be realised that, with the possible exception of two manufacturers, all bar none are based on the same original design. (The same inventor then went on to design a further two versions of the original design, the two exceptions mentioned previously.) Interchangeability in this product is actually advertised as a positive by some companies. Those same companies quite often offer part number cross referencing in their product literature.
This product is used in many industries including the following:
- Oil & Gas
- Paper production
- High pressure systems.
- Food & Beverage
- Emission control
Fittings are produced in a vast range of styles with manufacturers offering tees, elbows, unions, male connectors, female connectors and many others. Reducers allow the installers to change from one tube size to another, even from metric to imperial. Unlike the flared fitting, there is no set standard to manufacture the double ferrule fittings to. However, certain unwritten rules apply and all producers use the same method of distinguishing between metric and imperial fittings. A step, or shoulder, is machined on to the metric fitting body below the hexagon portion, and on the front end of the nut. This is also clearly seen in all the manufacturers’ literature.
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