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Compression Fittings

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Single Ferrule Fitting	Double Ferrule Fitting

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

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 types of dynamic stresses.

Compression Fitting Specification and Identification

Fitting Manufacturer: State the manufacturer and the manufacturer’s reference code for the particular fitting that you require. Refer to the instrument piping specification.
Tube OD: CARE!! Metric and imperial compression fittings are incompatible. Refer to the instrument pipe specification to check which is used.
Tube Wall Thickness: Normally not required. If in doubt about what appears to be a very thick or thin wall, check with the manufacturer, as they normally set limitations on the tube wall thickness.
Pressure: Again, not normally required but quote if in doubt. Other types of compression fittings will be specified for high pressure applications.
Fitting Material: Must be quoted and will be as specified in the instrument piping specification.
Fitting Body Connection: Dependent on the application. A variety of body connections is available and must be quoted to complete the description of the compression fitting, e.g. male NPT straight, elbow or tee connectors, female connectors for parallel pipe thread, unions, reducing unions, etc.

Compression Fittings - Do.s and Don.ts

Do.s

Ensure that the compression fitting is free of dirt or any foreign material. The tube to be fitted must also be clean. The tube end must be cut square and any burrs should be removed.
Follow the manufacturer’s recommended procedure for tightening up the fitting when new. Swagelok compression fittings generally require 11/4 turns of the nut from fingertight to obtain an effective seal. (Small tubing - 2, 3 and 4 mm - only requires a 3/4 turn.)
Once tightened, check the gap between the nut and the body of the fitting with a Swagelok Inspection Gauge. If the gauge fits, then additional tightening of the nut is required.
When retightening a disconnected fitting, the nut will not require as much tightening as when making the connection from new. Swagelok fittings require about 1/4 of a turn from hand-tight to regain the seal. Use the inspection gauge to check.
Metric and Imperial compression fittings are incompatible, therefore check which is being used. Swagelok metric compression fittings have a stepped shoulder machined on the nut hex and the body hex. The letters MM will also be stamped on shaped body fittings.

Don'ts

Do not combine or mix parts from various compression fitting manufacturers. The components may have different dimensions and tolerances and a seal will be difficult to achieve.
Never turn the fitting body to make up the joint. The body should be held and the nut rotated.
Do not use stainless steel tubing with brass fittings. The metal tubing material should either be the same or softer than the fitting material.
Do not use tubing which is scratched, depressed or deformed. Similarly, do not force oval tube into a fitting. The tube should be cut back to sound material.
Do not use PTFE tape of any other thread sealing compound between the nut and the body of the compression fitting.
Never bleed down a system by loosening the compression fitting nut.