Function of a Resilient Metal Seal

HTMS - High Tech Metal Seals

Seal Dynamics

The sealing performance of any elastic metal seal is based on the relative high specific contact load between the seal and the mating surface. This linear load or seating load is generated by the reaction of the seal (with or without spring) against its deformation by compressing the seal to a well-defined groove depth.

The graph above represents the compression and decompression characteristics of a standard metallic seal. The curve “A-B-C” gives the increasing linear load by compression rate, whereas the curve “C-D-E” represents the reduction of linear load when the seal flanges separate and compression is reduced.

The curve shows a plastic deformation of the metal seal. Point “B” on the compression curve is the transition point between elastic and plastic deformation. As a rule of thumb, in this point, almost 80% of the maximum linear load is achieved. Point “C” indicates the point of maximum compression (min. groove depth). Metal seals should be compressed approximately 20%, as higher compression rates can lead to seal failure due to the heavy plastic deformation.

The total spring back or elastic recovery is represented between point “C” to “E”. As a rule of thumb, the spring back varies between 4 and 6% of the original cross section of the seal. It must be clear that as soon as the flange separation equals the total spring back, the seal’s performance is over. Therefore it is strongly advised to design flange and bolts in such a manner that the flange movement / rotation at the seal location is smaller than 30 % of the total spring back. The latter is outlined with the useful spring back in the compression / decompression curve and is strongly dependent on several parameters:

  • Acceptable leak rate
  • Seal design
  • Hardware

Seating Stress

The initial line contact between the seal and the mating surface will gradually increase with the rate of compression to form a footprint. The width of the footprint depends on the seal type, the cross section and the compression rate. The seating stress (MPa) will approximately equal the linear load (N/mm) divided by the foot print width. (MPa =N/mm²)

Linear loads vary from as low as 20 N/mm to more than 500 N/mm seal circumference.

The seal width or footprint usually varies from less than 0.3 mm to about 3 mm for the bigger cross section seals.

Based on this, the seating stress varies from a minimum of 30 MPa to over 150 MPa. With a heavy duty spring, the seating stress can be increased to above 300 MPa.

The high seating stress is required to make the selected plating or coating flow into the irregularities of the flanges, thereby sealing off all leak paths.

HTMS - High Tech Metal Seals