About Metal Seals
Function of a Resilient Metal Seal
The sealing performance of any elastic metal seal is based on the relative high specifi c 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-defi ned groove depth.
The graph on the previous page represents the compression and decompression characteristics of a standard spring energised C Ring. 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. In this example almost 80% of the max. 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 can lead to seal failure.
The total spring back or elastic recovery is situated from 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 will be clear that as soon as the flange separation equals the spring back, the seating load will fall back to zero. At that point the sealing performance will be highly questionable. Therefore it is strongly advised to design flange and bolts in such a manner that the flange rotation at the seal location is smaller than 20% of the total spring back. The latter is outlined with the useful spring back in the compression/decompression curve.
For safe operation the seal has to be maintained in the green section (line C-D) of the decompression curve. Depending on the number of variables it might be required to move point “D” upwards, i.e. reducing the useful spring back.
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 of the seal and the rate of seal compression. The seating stress will equal the linear load divided by the foot print width.
Linear loads vary from as low as 20 N/mm to more than 500 N/mm seal circumference. The seal width or footprint varies from less than 1 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.
Depending on the required tightness, the groove surface finish and the media to be sealed, a specifi c plating or coating shall be selected. For a softer plating or coating, the seating load of a low load C Ring may suffi ce to create the necessary stress to make the selected plating material fl ow. In case of higher temperature or when other service conditions dictate the use of harder plating, a spring energised seal may be the right choice.
At all times it is recommended to select the biggest possible cross section for a given diameter. By doing so, the useful spring back will be at its largest, enabling performance within the widest possible tolerance range for that given diameter (line C-D in the graph) and as such creating a more robust sealing solution. A higher spring back allows more flange rotation due to internal or external loads.
Not only the application but also the specifications determine the material to be used. In general however, high nickel alloys are most commonly used for C-Rings and spring energised C-Rings. High strength stainless steel and high nickel alloys are materials used for metal O-Rings.
Resilient metal seals often have to perform under extreme conditions. Standard solutions as given in this catalogue may not always suffi ce these requirements.
In case the application requires seal properties outside the scope of the standard designs, HTMS can develop a seal with the necessary physical properties.
Close cooperation with universities and material suppliers allows HTMS to optimise the seal characteristics.
Plating - Coating
With modern state of the art equipment, HTMS provides first class plating and coating services. Our in-house plating facility includes gold, silver, copper, nickel and tin plating. HTMS also runs a coating facility for applying PTFE as a soft layer on the mating surface of the seal.
The typical plating or coating thickness for seals is 50 microns. Adhering to the base material this layer will flow into the groove surface asperities under the seating stress. Softer materials such as tin and PTFE require a lower seating stress than for instance silver or gold. Nickel, being a relatively hard plating material, requires the highest seating stress.
Soft metal based plating can achieve a He-tightness of 10-10 Pa.m³/s. PTFE coating will have a limit of 10-6 Pa.m³/s because of the porosity of PTFE for Helium.