You've run the tubing, made up the fittings and the system looks clean. Before it goes into service, it needs a leak test. Skipping that step, or rushing it, is how a small weep at a fitting turns into a call-out at 2 AM. Here's the process I've used for years on instrument and process tubing.
Check Every Fitting Before You Add Pressure
The cheapest leak to fix is the one you catch with zero pressure in the line. Walk every connection first. Make sure each tube is fully bottomed in the fitting body and every nut has been pulled up the full amount the manufacturer calls for.
On new compression fitting installs, run a gap inspection gauge on each nut. If the gauge slips between the nut and the body, that fitting isn't tight enough. Find it now, not on the test.
If you have to tighten or remake a fitting, the Fitting Tool holds stainless steel tube fittings from 1/16" to 1/2" steady while you work, so you aren't fighting the part with a second wrench. Both tools are on our tools page.
Know Your Test Pressure and Test Medium
Follow your site procedure and the code that governs the system. Don't guess at the number. A few rules hold almost everywhere:
- Never test above the rating of the lowest-rated component in the system. That includes valves, gauges and hose, not just tubing.
- Isolate or remove anything that could be damaged or lift during the test, like transmitters, low-range gauges and relief valves.
- Hydrostatic testing is safer than pneumatic. Compressed gas stores a lot of energy, and if something lets go, it lets go hard. If you must test with gas, keep people back and follow your plant's pneumatic test rules.
Use a calibrated test gauge with a range that puts your test pressure in the middle of the dial, so you can actually see a small drop.
Bring Pressure Up in Stages
Don't go straight to full test pressure. Bring it up in steps, hold at each one and look the system over. Watch the gauge during the final hold for any drop.
On air or gas, check each fitting with Gator Leak Detector. Put it on straight from the squeeze bottle so it covers the whole connection, then watch for 5 to 30 seconds. Large leaks form bubbles, and fine leaks build a white foam. It's safe on oxygen and compressed gas lines and dries clean without leaving any residue on your fittings. Remember that temperature changes during a long hold can move the gauge a little, so don't chase a needle without confirming with leak fluid.
One rule I won't bend on: never tighten a fitting while the system is under pressure. Depressurize, lock it out, then fix it.
Fixing a Leak at a Tube Fitting
Most leaks on new installs come down to tube prep or make-up: a tube not bottomed, a burr, a scratched tube or a nut that wasn't pulled up far enough. Remake the fitting by the manufacturer's instructions, re-check it with the gap gauge, and retest.
On mixed jobs, techs often ask whether they can match a DK-Lok part to an existing Swagelok fitting. DK-Lok's ASTM F1387 test report, witnessed by TÜV Rheinland, tested 14 combinations of intermixed DK-Lok and Swagelok nuts, ferrules and bodies in 1/4" through 1" OD. Tests included hydrostatic burst, impulse, vibration, thermal cycling and repeated assembly, with "no failure observed." DK-Lok backs intermixing with same-size, same-material Swagelok components assembled per its catalog. DK-Lok also has a written statement that its two-ferrule fittings are intermixable and interchangeable with Parker A-Lok of like size and material, properly assembled per the DK-Lok catalog. Learn more on our DK-Lok page.
Write It Down
Record the test pressure, test medium, hold time, gauge used, result and who witnessed it. When someone asks a year from now whether that line was tested, you'll have the answer.
Want your crew sharper on installs and testing? A-Best Industrial Inc. Fittings, Valves, and Tubing offers a free Safety training course for new and existing customers.
We're Here to Help
Need fittings, tubing, gauges or tools for your next test, or a second set of eyes on a tricky system? Call A-Best Industrial Inc. Fittings, Valves, and Tubing at 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.