Plastic tubing shows up all over a plant: instrument air to valve actuators, sample lines, lab panels, chemical injection. Techs often grab the same stainless compression fittings they use every day and push the plastic tube right in. It seals at first. A few weeks later the line pulls out, or the joint starts weeping after the tube creeps. In my 20 years in the instrumentation valve and fitting business, the missing piece is almost always the same: a tube insert.
Why Soft Tubing Needs Support
A two-ferrule compression fitting works by having the ferrules grip and seal on the outside of the tube. Stainless tubing is stiff enough to push back against that load. Soft tubing like nylon and polyethylene is not. Without something inside it, the tube wall collapses under the ferrules, the grip is weak, and the plastic keeps cold-flowing away from the ferrules over time. That's how you get a joint that passed the first check and leaks later.
A tube insert is a small stainless sleeve that slides into the bore of the tubing. It gives the ferrules something solid to compress the tube wall against, so the fitting can grip and seal the way it was designed to.
Match the Insert to the Tube
An insert is sized by both the tube OD and the tube ID, so wall thickness matters. A 1/4" OD nylon tube and a 1/4" OD polyethylene tube may not have the same wall. Check the actual tube ID before you order, and pick the insert made for that OD and wall. Too small and it won't support the wall. Too big and you'll fight to push it in or split the tube.
Also confirm the tubing itself is right for the service. Plastic tubing ratings drop as temperature rises, and different plastics handle chemicals differently. Check the tubing manufacturer's pressure and temperature data for your conditions. If the line runs hot or near a heat source, it may be a job for stainless. Our tubing page covers the stainless, Teflon and nylon tubing we stock.
How to Make Up the Connection
- Cut it square. Use a cutter made for plastic tubing, not a knife or side cutters. A crushed or angled end won't seat at the bottom of the fitting.
- Push the insert in fully. The insert's shoulder should sit against the tube end. Don't hammer it in; if it fights you, recheck the tube ID.
- Bottom the tube in the fitting. Push the tube with its insert all the way into the fitting body until it stops, then snug the nut finger-tight.
- Pull up per the catalog. Mark the nut and tighten the turns the fitting manufacturer specifies for that size. Hold the body with a backup wrench so you don't twist the line.
When you're making up small stainless fittings at the bench, the Fitting Tool holds stainless steel tube fittings from 1/16" to 1/2" steady so both hands are free for the wrench work.
Same Fittings, Your Choice of Brand
DK-Lok tube fittings are 100% interchangeable and intermixable with Swagelok and Parker A-Lok for like size and material, properly assembled per the DK-Lok catalog. In ASTM F1387 testing witnessed by TÜV Rheinland, 14 combinations of intermixed DK-Lok and Swagelok nuts, front ferrules, back ferrules and bodies passed every test, with "No failure observed." See also the Parker A-Lok interchangeability letter.
Leak Test What You Touched
Once the line is pressurized per your site procedure, apply Gator Leak Detector all the way around each joint you touched and watch for bubbles or foam. On male connectors and other NPT-to-tube fittings, check the pipe-thread joint as well as the tube nut, since the NPT side is often skipped. If something leaks, isolate and depressurize before putting a wrench on it. Never tighten a fitting under pressure.
We're Here to Help
Not sure which insert fits your tubing? A-Best Industrial Inc. Fittings, Valves, and Tubing stocks DK-Lok compression tube fittings and inserts and can help you match them to your line. Call 310-833-3353 or email sales@abesttube.com. We're open Monday–Friday, 8:00 AM–5:00 PM Pacific, with after-hours emergency service.