Tubing Variables

  • Wall Thickness
  • Outside and Inside Diameter
  • Type of Material
  • Seamless or Welded
  • Hardness
  • Concentricity and Ovality
  • Surface Finish

Wall Thickness

The wall thickness is directly related to the pressure rating of the system. If the wall thickness is too thick, it will not allow the ferrules to deform the tube. If the wall thickness is too thin, it can result in a blowout.

Outside & Inside Diameter

When designing a fluid system the Outside Diameter and Inside Diameter need to be carefully selected to ensure sufficient flow and pressure rating. SS tubing (Type 304, 304L, 316, 316L) by EN ISO 1127 (D4, T4 tolerance for 3 to 12mm; D4, T3 tolerance for 14 to 50 mm) or equivalent.

Type of Material

Seamless or Welded

  • Seamless tube gives a 20% higher pressure rating
  • Welded seams can reduce the corrosion resistance of the tube


Tubing must be softer than the fitting material. Hardness of SS Tubing used with twin ferrule tube fittings should be 90 HRB (200 HV) or less.

AlloyEN NumberHardness (HRB)
3161.440490 max
316L1.443590 max
Duplex1.446295 max
S Duplex1.4410100 max
6Mo1.454790 max
904L1.453990 max
Monel 4002.436075 max
8252.485890 max
6252.485693 max
C-2762.481993 max

Concentricity and Ovality

  • Non-concentric tubing has a lower pressure rating
  • Oval tubing may not fit through the ferrules

Surface Finish

Pipes: Acid Pickling (AP)

Tubes: Acid Pickling (AP), Mechanical Polishing (MP), Bright Annealing (BA) and Electro Polishing (EP).

Surface Finish is very important for proper sealing. Tubing with any kind of depression, scratch, raised portion, or other surface defect will be difficult to seal, particularly in gas service.

Surface ConditionSurface Roughness (μm)Surface Roughness (μinch)

For more information contact our sales team at sales@nvfcl.com or call 01942601209