How to find the right Compression Union Fittings AI guide
Written by an AI model from its trained knowledge.
What the AI looks for in Compression Union Fittings
- Tube OD match
- Size the fitting to the tube's outside diameter, not a nominal pipe size; each end must match its tube OD exactly. Compression fittings seal on the tube's outer surface, so a near-miss size will leak or blow off.
- Imperial vs metric
- Do not interchange close sizes such as 6 mm and 1/4 in, or 10 mm and 3/8 in. The diameters are close enough to assemble but not to seal reliably under pressure.
- Ferrule system and brand
- For double-ferrule instrumentation fittings, keep the body, nut and ferrules from one manufacturer, and follow that maker's turns-past-finger-tight instructions, checked with its gap gauge. Ferrule geometries differ between makers, and mixed components are not rated to seal.
- Material vs tube and media
- Use brass with copper or plastic tubing for water and air, and stainless steel (typically 316) with stainless tubing for corrosive, high-pressure or instrumentation service. Mismatched hardness or metals causes poor ferrule bite or galvanic corrosion.
- Plastic tubing support
- Fit a tube insert or stiffener in polyethylene, nylon or PEX-type tubing used with metal-ferrule compression fittings. Soft tubing collapses under the ferrule without support and pulls out or leaks.
- Potable water compliance
- For drinking water, require lead-free brass certified to NSF/ANSI 61 and NSF/ANSI 372. Standard brass can contain lead that is not permitted in potable water systems.
Brands that matter
- Swagelok: Reference standard for stainless two-ferrule instrumentation unions, including reducing versions.
- Parker: Offers CPI and A-Lok instrumentation fittings, plus brass compression fittings for general service.
- Hy-Lok: Two-ferrule instrumentation fittings in a Swagelok-style design.
- Ham-Let: Instrumentation compression fittings in stainless and other alloys.
- Brennan Industries: Broad catalog of industrial tube fittings, including compression unions.
- Anderson Metals: Brass compression fittings for copper and plastic tubing in light-duty water and air lines.
What to avoid
- Mixing nuts or ferrules from different brands on instrumentation compression fittings.
- Using a 6 mm fitting on 1/4 in tube, or any similar metric/imperial near-match.
- Clamping metal ferrules on soft plastic tubing without an insert.
- Overtightening brass compression nuts, which deforms the sleeve and the tube.
- Non-lead-free brass unions on drinking water lines.
Questions
What is the difference between a compression union and a compression coupling?
In tube fittings, a compression union is a straight connector that joins two tube ends with a compression nut and ferrule on each side. Many catalogs use 'coupling' for the same part. A reducing union joins two different tube ODs.
Can I reuse the ferrules when I take a union apart?
You can usually reassemble the union with the same ferrules still swaged onto the tube, using the maker's remake procedure. A new joint needs new ferrules.
Compression Union Reducers
A reducing union joins two different tube ODs, so match each end exactly to its own tube size and material. On instrumentation fittings, keep both ends from the same manufacturer's ferrule system.
Compression Union Straights
A straight union joins two tubes of the same OD. Choose by tube OD (imperial or metric), material and ferrule system as described for compression unions, and add inserts if the tubing is plastic.