Compression fitting
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Compression fittings are used in plumbing and electrical conduit systems to join two tubes or thin-walled pipes together. In instances where two pipes made of dissimilar materials are to be joined (most commonly PVC and copper), the fittings will also be made of one or more compatible materials appropriate for the connection. Compression fittings for attaching tubing (piping) commonly have ferrules in them.
Compression fittings are also used extensively for hot and cold water faucets and toilet stop valves. These valves generally control water to one fixture, such as a toilet, to allow it to be serviced without turning off the water to the entire house. Compression fittings are well suited to this application, as these valves are usually located in confined spaces where copper pipe would be difficult to solder without creating a fire hazard.
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[edit] How they work
As the name implies, compression fittings form a tight seal by applying a compressive force to the pipes and pipe fitting. The fitting is compressed against the pipe with a force sufficient to eliminate all space remaining in the joint, thus preventing fluid from leaking.
The compression fitting is composed of an outer "compression nut" and an inner ring called a "ferrule" or "olive" in the UK. This is usually made of brass or copper. When the nut is tightened, it clamps-down on the ferrule, causing it to conform to the circumference of the pipe. Ferrules vary in shape and material according to the pipe material. To work properly, the ferrule must be oriented correctly. Typically the ferrule is fitted such that the longest sloping face of the ferrule faces away from the nut.
It is important to remember to not apply joint compound or teflon tape to a compression fitting's threads. The compression is the means of sealing the joint, not the sealing of the threads themselves. Pipe compound or teflon tape will frequently lead to a leak in the fitting by causing the fitting to loosen as a reaction to the compression. Joint compound is usually applied to the ferrule or Olive to seal imperfections in the fitting. In a standard threaded connection, pipe compound and teflon tape act to seal the threads from the water pressure.
In addition, it is critically important to the integrity of the fitting that excessive force is avoided in tightening the nut. If the fitting is overtightened, the ferrule will deform and cause leaks. Overtightening is the most common cause of leaks in compression fittings. As a general rule, a compression fitting should be "finger tight" and then tightened 1 turn with a wrench. The fitting should then be tested, and if slight weeping is observed, the fitting should be slowly tightened a bit more until the weeping stops.
[edit] Types of fittings
There are two common types of compression fitting, standard (British: Type-A/non-manipulative) and "flare" fittings (British: Type-B/manipulative). Standard fittings require no modifications to the tubing. Flare fittings require modification of the tubing with a special tool. Standard fittings are generally used for water and compressed air connections, while flare fittings are used for gas and high pressure lines.
A standard fitting can be installed using an ordinary wrench to tighten the surrounding nut. To remove it, a specialized pulling tool is often used to slide the nut and ferrule off the tube.
[edit] Advantages
Compression fittings are popular because they do not require soldering, so they are comparatively quick and easy to use. Compression fittings are especially useful in installations that may require occasional disassembly or partial removal for maintenance etc, since these joints can be broken and remade without affecting the integrity of the joint. They are also used in situations where a heat source, in particular a soldering torch, is prohibited; however, they are more expensive than other types of fittings and may require more space.
[edit] Disadvantages
Compression fittings are not as robust as soldered fittings. They should be used in applications where the fitting will not be disturbed and not subjected to flexing or bending. A soldered joint is highly tolerant of flexing and bending (such as when pipes knock or shake from sudden pressure changes). Compression fittings are much more sensitive to these type of dynamic stresses.
