Wednesday, May 02, 2012

Oil Leaks Across All-Metal Joints

"A safe and reliable seal against liquids and gases under pressure cannot be achieved with compressive forces that produce elastic deformations at the interface areas only, regardless of the surface finish."1.

AN Flare Tube Connection

Translation:  To prevent leaks under pressure, tighten to achieve a permanent (plastic) deformation of the sealing surfaces. Sealing under pressure cannot be achieved with a non-permanent (elastic) force.

Maintenance Impact:
  1. Close torque tolerance requirement. Torque must be great enough to create localized plastic deformation but not too great that it cracks or excessively deforms the sealing surface. Torque must be accurate! Minimum torque to achieve permanent deformation on AN fittings might cause unacceptable flare damage when used on soft grades of aluminum tubing such as 3003-0 or 6061-0 temper. Same problem when using single-flares rather than stronger double flares on aluminum tubing. 
  2. If you replace one side of the connection then you need to replace the other side of the connection if it is the same hardness or less. Example, attaching a hard stainless steel hose fitting to a softer aluminum male hose nipple fitting. The softer male fitting is distorted and needs to be replaced. A new hose may not be capable of complying to the distorted surface created by the old hose.
  3. Two hard surfaces (such as stainless on stainless) are harder to deform and seal than a soft/soft (aluminum on aluminum) or a soft/hard (aluminum on stainless). Requires greater tightening. Use of conical seals can alleviate this problem.

Close-up of AN Flared Tube Connection

Conical Seal - soft pliable insert provides the plastic surface thereby protecting the fittings and reducing torque requirement to acceptable levels.

Gross Fitting Distortion from too much torque
4. All metal gaskets should not be re-used since they will have been deformed.
Deformed spark plug gasket
The practice of annealing spark plug gaskets and reusing them is faulted since annealing only restores the hardness and not the shape.

1. Butcher H. 1973, "Fundamental principles for static sealing with metal in high pressure field", ASLE Transactions Vol. 16, p.304-309.


  1. There are two main types of bolted Metal joints designs. In one method the bolt is tightened to a calculated clamp load, usually by applying a measured torque load. The joint will be designed such that the clamp load is never overcome by the forces acting on the joint (and therefore the joined parts see no relative motion).

  2. Single-plane three-hinged Metal joints make do with one-sided angularly flexible expansion joints, while multi-plane three-hinged systems for absorbing thermal expansion in three axial directions require at least two gimbal expansion joints that are angularly flexible on all sides. The following basic rules apply to angular compensation