Compression fittings look interchangeable on a catalogue page. In service, on a vibrating pump skid or a hot steam line, small spec mistakes turn into weeping joints, frozen nuts, and unplanned shutdowns. Here is the checklist we hand buyers who ask us what actually goes wrong – drawn from the failure modes we see every week as a manufacturer.
Check 1: Match the ferrule system, and never mix brands
Single-ferrule (37-degree flare) and double-ferrule (bite-type, DIN 2353) fittings seal in completely different ways. The front ferrule of a bite-type fitting cuts into the tube to form the grip; the flare system instead seals on a formed lip. Mixing a tube and fitting from different systems – or even mixing brands within the twin-ferrule family – is the single most common cause of leaks under vibration. Specify one system across the whole line and keep spare parts to that standard. If you are unsure which family a legacy installation uses, our field guide to compression fitting leaks walks through the visual differences before you disassemble anything: https://ferrulex.com/blog/compression-fitting-leaks-causes-fixes/
Checklist questions:
– Is the installed base single- or double-ferrule?
– Are replacement fittings the same brand and series as the original?
Check 2: Ask for material traceability, not just “stainless”
“Stainless” covers 304, 316, 316L, and sometimes re-stamped 304 sold as 316. In chloride-rich or coastal environments, the difference is corrosion, not price. Require mill test reports (MTRs) with heat numbers so the certificate matches the shipment, not a generic grade claim. For steam or high-temperature service, ask specifically about carbon content and stabilization.
Checklist questions:
– Will the supplier provide MTRs with heat numbers per lot?
– Is the grade stamped on the body, and does it match the paperwork?
Check 3: Verify tube OD and wall before blaming the fitting
The number-one cause of weeping joints is not the fitting — it is tubing outside spec. Undersized OD means the ferrule cannot bite; oversized OD means the nut will never pull up. Wall thickness matters just as much: too thin and the tube collapses during tightening; too thick and the ferrule cannot form properly. Order tubing to the fitting manufacturer’s tolerance table, and inspect a sample cut rather than trusting the box label. When reading line drawings, the dash size chart is the quick reference that maps dash numbers to actual OD in inches: https://ferrulex.com/blog/dash-size-chart/
Checklist questions:
– Does the tube certificate list OD tolerance and wall, not just alloy?
– Has anyone measured a cut sample from this batch?
Check 4: Derate for temperature and pressure together
A fitting rated 5000 psi at 20 degrees C is not a 5000 psi fitting on a steam line. Pressure ratings fall as temperature climbs, and the curve differs by material and ferrule design. Get the derating table for the exact series you are buying, size for the worst-case process condition, and add vibration allowance where reciprocating equipment is nearby.
Checklist questions:
– What is the rating at the highest process temperature, not ambient?
– Is there a pulsation or vibration allowance in the spec?
Check 5: Decide stock vs custom with total cost, not unit price
Standard fittings ship in days, and replacements are on the shelf. Custom machining makes sense when the geometry is unusual, volumes justify the tooling, or a standard part would force redesign of the whole assembly. Get quotes both ways before defaulting to either – the cheap path is often the one that keeps spares on the shelf.
Checklist questions:
– Has the custom quote been compared against the nearest standard part?
– Who holds the drawing and can remake the part in three years?
Final word
Most compression fitting failures trace back to one of these five checks skipped during procurement. Run the list before the PO goes out – it is cheaper than the shutdown after it.