Compression fitting

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A compression fitting 15 mm isolating valve

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 (also known as olives) in them.

Compression fittings are also used extensively for hot and cold water faucets (taps) 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.

Compression fittings are the industry standard for chemical, oil and gas, R&D, Bio-tech, and the Semiconductor industry.^{[citation needed]} They are used due to their ability to provide leak tight seals. These fittings can be remade.

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 "compression ring". In the UK, this brass ring is referred to as a "ferrule" or "olive". 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 but are most commonly in the shape of a beveled ring and are most commonly made out of brass or 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.

When the fitting is tightened, the olive, or compression ring, which starts off, in cross-section, as something similar to a curved bracket - ) - with a thickened middle, like a saucer, is compressed across the width of the ring (from top to bottom of the "bracket", as shown above), causing it to curve across a shorter distance and increasing the concavity of the ring in the same manner that the middle of a piece of paper, lying on a table, will rise upwards as its ends are brought together. The compression ring therefore effectively becomes thicker as the fitting is tightened (albeit with a hollow under the ring between where the two edges of the ring touch the pipe), and this arch between the outside of the pipe and the inside of the fitting is what seals the joint.

Joint compound (a.k.a. pipe "dope") or Thread seal tape (a.k.a. PTFE tape or Teflon tape) is not applied to a compression fitting's threads. Joint compound and teflon tape act to seal the threads from the water pressure in normal threaded connections, but serve little purpose in compression seals and may actually serve to weaken them. The compression is the means of sealing the joint, not the sealing of the threads themselves. Pipe compound or PTFE tape frequently leads to leaks in the fitting; in the case of teflon tape, it allows a gap to form between the compression ring and nut. Joint compound is sometimes applied to the ferrule or olive to seal imperfections in the fitting, but really serves no purpose as the sealing is enacted through the compression ring itself. Rarely does joint compound or teflon tape compensate for the loss of an air/watertight seal if the compression ring becomes "ovalled" (due to over-tightening while in a misoriented position, in relation to the compression nut), or if there is some factory defect in the product. In these instances, the compression fittings are simply replaced.

It is important to the integrity of a compression fitting that excessive force is avoided in tightening the nut. If the fitting is overtightened, the ferrule frequently deforms and the compression fitting fails. Overtightening is the most common cause of leaks in compression fittings. Generally, compression fittings are tightened to "finger tight" and then tightened a half-turn more with a wrench. The actual amount varies with the size of the fitting; a larger one requiring less. The fitting is then tested. If slight weeping is observed, the fitting is slowly tightened until the weeping stops.

The most important component in a compression seal is the compression ring itself. The ring is a thin piece of brass and is extremely prone to damage. Care should be used when installing them (although if they become defective, it is easy for a plumber to install a replacement). Masterful plumbers have been known in some instances to replicate the sealing action of the compression ring by using an artfully "woven" piece of teflon tape. This is usually attempted as a temporary solution to maintain a seal and prevent a leak until a new fitting/ring can be procured.

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 puller is often used to slide the nut and ferrule off the tube.

Advantages

Compression fittings are popular because they do not require soldering, so they are comparatively quick and easy to use. They require no special tools or skills to operate. They work at higher pressures and with toxic gases. 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.

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. They are also bulkier, and may be considered less aesthetically pleasing than a neatly soldered joint.

See also

• AN thread
• Flare fitting
• JIC fitting

Notes and references

External links

• Diagrams and explanations at diymate.co.uk
• Jointing Copper Tubes: Compression Joints, UK Copper Board