Compression fittings
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A compression fitting is a fitting used in
plumbing Plumbing is any system that conveys fluids for a wide range of applications. Plumbing uses pipes, valves, plumbing fixtures, tanks, and other apparatuses to convey fluids. Heating and cooling (HVAC), waste removal, and potable water deliv ...
and electrical conduit systems to join two
tubes Tube or tubes may refer to: * ''Tube'' (2003 film), a 2003 Korean film * ''The Tube'' (TV series), a music related TV series by Channel 4 in the United Kingdom * "Tubes" (Peter Dale), performer on the Soccer AM television show * Tube (band), a ...
or thin-walled
pipes Pipe(s), PIPE(S) or piping may refer to: Objects * Pipe (fluid conveyance), a hollow cylinder following certain dimension rules ** Piping, the use of pipes in industry * Smoking pipe ** Tobacco pipe * Half-pipe and quarter pipe, semi-circula ...
together. In instances where two pipes made of dissimilar materials are to be joined (most commonly PVC and
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
), the fittings will be made of one or more compatible materials appropriate for the connection. Compression fittings for attaching tubing (piping) commonly have
ferrule A ferrule (a corruption of Latin ' "small bracelet", under the influence of ' "iron") is any of a number of types of objects, generally used for fastening, joining, sealing, or reinforcement. They are often narrow circular rings made from me ...
s (or ''olives'' in the UK) in them, and are sometimes referred to as flareless fittings. There are also
flare fitting Flare fittings are a type of compression fitting used with metal tubing, usually soft steel, ductile (soft) copper and aluminum, though other materials are also used.  In a flare fitting the tube itself is "flared" i.e. expanded and deformed a ...
s that do not require ferrules/olives. Compression fittings are used extensively in hydraulic, gas, and water systems to enable the connection of tubing to threaded components like valves and tools. Compression fittings are suited to a variety of applications, such as plumbing systems in confined spaces where copper pipe would be difficult to solder without creating a fire hazard, and extensively in hydraulic industrial applications. A major benefit is that the fittings allow easy disconnection and reconnection.


How they work

In small sizes, the compression fitting is composed of an outer ''compression nut'' and an inner ''compression ring'' or
ferrule A ferrule (a corruption of Latin ' "small bracelet", under the influence of ' "iron") is any of a number of types of objects, generally used for fastening, joining, sealing, or reinforcement. They are often narrow circular rings made from me ...
(in the UK referred to as an "olive") that is typically made of brass or copper or steel. Ferrules vary in shape and material but are most commonly in the shape of a ring with beveled edges. To work properly, the ferrule must be oriented correctly, in the case of copper olives they are normally barrel shaped and this means they cannot be fitted incorrectly, but where this is not so particularly in Hydraulic and high pressure applications the ferrule is fitted such that the longest sloping face of the ferrule faces away from the nut. When the nut is tightened, the ferrule is compressed between the nut and the receiving fitting; causing both ends of barrel shaped copper olives to be clamped around the pipe when the middle of the ferrule bows away from the pipe, in the case of hydraulic style ferrules they currently have one end which is larger with a 45 degree chamfer which tapers away (from installation contact with the nut) and the small end generally has two internal biting edges, for applications demanding much higher pressure, that penetrate the outside diameter of the tube, the fittings must be tightened to guidelines as per DIN2353 as not to exceed the elastic limit of the steel ferrules, The result is that the ferrule seals the space between the pipe, nut, and receiving fitting, thereby forming a tight joint. The clamping support of the pipe by the force at the taper at both ends help prevent movement of the pipe in the fitting, but it is only the taper at the receiving fitting itself that needs to seal completely, since if it does seal (to both the pipe and the compression fitting) then no fluid can get to the nut threads or the taper at the nut end to result in any leaks. As a result some similar fittings can be made using an olive with only one taper (or a fixed cone sealed to the pipe) where the sealing at that taper prevents fluid from reaching the nut. Larger sizes of compression fitting do not have a single nut to compress the ferrule but a flange with a ring of bolts that performs this task. The bolts have to be tightened evenly. Thread sealants such as joint compound (
pipe dope Pipe dope is any thread lubricant, thread sealing compound, or anaerobic chemical sealant that is used to make a pipe thread joint leak proof and pressure tight. It is also referred to as "thread compound" or "pipe thread sealant." Although c ...
or
thread seal tape Thread seal tape (also known as PTFE tape, Teflon tape, or plumber's tape) is a polytetrafluoroethylene (PTFE) film tape commonly used in plumbing for sealing pipe threads. The tape is sold cut to specific widths and wound on a spool, making it ...
such as PTFE tape) are unnecessary on compression fitting threads, as it is not the thread that seals the joint but rather the compression of the ferrule between the nut and pipe. However, a small amount of plumber's grease or light oil applied to the threads will provide lubrication to help ensure a smooth, consistent tightening of the compression nut. It is critical to avoid over-tightening the nut or else the integrity of the compression fitting will be compromised by the excessive force. If the nut is overtightened the ferrule will deform improperly causing the joint to fail. Indeed, overtightening is the most common cause of leaks in compression fittings. A good rule of thumb is to tighten the nut first by hand until it is too difficult to continue and then tighten the nut one half-turn more with the aid of a wrench; the actual amount varies with the size of the fitting, as a larger one requires less tightening. The fitting is then tested: if slight weeping is observed, the fitting is gradually tightened until the weeping stops. The integrity of the compression fitting is determined by the ferrule, which is easily prone to damage. Thus care should be taken to when handling and tightening the fitting, although if the ferrule is damaged it is easily replaced.


Types of fittings

There are two 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 fitting Flare fittings are a type of compression fitting used with metal tubing, usually soft steel, ductile (soft) copper and aluminum, though other materials are also used.  In a flare fitting the tube itself is "flared" i.e. expanded and deformed a ...
s require modification of the tubing with a special tool. Standard fittings are typically used for water, hydraulic and compressed air connections, whereas 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. If the ferrule is difficult to remove it can be weakened with a cut, care being taken to not nick the pipe while cutting.


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 connections 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, or where it is difficult to remove remains of water from inside the pipe which prevent the pipe heating up to allow soldering.


Disadvantages

Compression fittings are not as robust as soldered fittings. They are typically 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 e.g. caused by
water hammer Hydraulic shock (colloquial: water hammer; fluid hammer) is a pressure surge or wave caused when a fluid in motion, usually a liquid but sometimes also a gas is forced to stop or change direction suddenly; a momentum change. This phenomenon com ...
). 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. Compression fittings work best when tightened once and not disturbed. Some compression connectors may never be reused, such as a ferrule ring type. It can never be reused once they have been compressed. This connector is directly placed over the pipe and the nut is tightened compressing the ferrule between the pipe and the body of the fitting. Compression of this ferrule also results in deformation of the copper tubing. If a compression type connection needs to be redone, more often than not the compressed copper/ferrule would need to be cut off and a new ferrule is to be used on a clean non-compressed piece of pipe end. This is to assure a leak proof sound connection.


See also

*
AN thread The AN thread (also A-N) is a particular type of fitting used to connect flexible hoses and rigid metal tubing that carry fluid. It is a US military-derived specification that dates back to World War II and stems from a joint standard agreed upon ...
*
Cutting ring fitting Cutting ring fittings are flareless fittings used for connections in fluid applications. They are widely used worldwide in hydraulic systems. The cutting ring fitting consists of the body, the cutting ring and the nut. On assembly, the two cuttin ...
*
Flare fitting Flare fittings are a type of compression fitting used with metal tubing, usually soft steel, ductile (soft) copper and aluminum, though other materials are also used.  In a flare fitting the tube itself is "flared" i.e. expanded and deformed a ...
*
JIC fitting SAE J514 37° JIC Flare
SHF, Inc. JIC fittings, defined by the
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