How to find the right Compression Cap Fittings AI guide
Written by an AI model from its trained knowledge.
What the AI looks for in Compression Cap Fittings
- Tube outside diameter
- Match the cap to the tube's actual OD, and keep fractional and metric systems separate (1/4 in and 6 mm look alike but do not seal reliably together) A compression cap seals by crushing a ferrule onto the tube OD, so a near-miss size leaks or blows off under pressure.
- Fitting system
- Match the ferrule design to the job: brass sleeve or olive caps for low-pressure copper water or air lines, and two-ferrule instrumentation caps for gas, hydraulic or high-pressure lines Plumbing-grade compression caps are not built for the pressures and vibration that instrumentation fittings handle.
- Material vs. tube
- Use brass on copper or soft tubing and 316 stainless on stainless tubing; the ferrule must be harder than the tube A ferrule softer than the tube cannot bite or grip, and mixed metals can corrode galvanically in wet service.
- Plastic tubing support
- Fit a tube insert or stiffener whenever you cap PE, nylon or other plastic tubing Without an insert the plastic tube collapses under the ferrule and the cap creeps off over time.
- Make-up procedure
- For two-ferrule instrumentation caps, follow the maker's turns-from-finger-tight spec, which is commonly 1-1/4 turns for 1/4 in tube and larger and 3/4 turn for the smallest sizes, then check with the maker's gap gauge Under-tightening leaks and over-tightening deforms the ferrules, and the turn count is how these caps are qualified.
Brands that matter
- Swagelok: Reference two-ferrule instrumentation system; buy caps, nuts and ferrules all from the same system.
- Parker: CPI and A-LOK instrumentation compression lines, plus brass compression fittings for general use.
- Hy-Lok: Two-ferrule instrumentation fittings in stainless and brass, widely used in process plants.
- Ham-Let: Instrumentation compression fittings, common in stainless gas and process lines.
- Anderson Metals: Brass sleeve-type compression fittings for copper and plastic tubing in plumbing and low-pressure air.
- Brennan Industries: Broad catalog of hydraulic and instrumentation tube fittings, including compression caps.
What to avoid
- Mixing nuts, ferrules and caps from different makers on two-ferrule instrumentation fittings, which manufacturers do not support
- Putting PTFE tape or sealant on the nut threads, since the seal is made at the ferrule and not the threads
- Using plumbing brass sleeve caps on hydraulic or high-pressure gas lines
- Reusing a ferrule that was already swaged on a different tube end
Questions
Do I need a cap or a plug?
A cap fits over the end of a bare tube, while a plug goes into the open tube port of a fitting body; choose by whether a tube or a fitting port is left open.
Can I remove and re-fit a compression cap?
Usually yes on the same tube end, by snugging it back to the same position plus a slight extra turn, but the swaged ferrule stays on the tube, so a fresh tube end needs new ferrules.