SEAL SELECTION GUIDE

Load and Spring back

Metal C Ring Manufacturer The selection of the most suitable seal for extreme service conditions is often a delicate trade off between load and spring back of the seal. The compression load or seating load has a direct relation with the obtainable tightness where as the spring back of the seal determines how well this tightness is maintained with varying temperatures and pressures. For a given seal cross section and seal type it is generally true that with maximum load the spring back is lowest. And of course that seal with minimum load will generate the highest spring back. Looking at a Metal O Ring made from stainless steel, one with a thin wall and the other with a thick wall, the one with the thin wall will require lower load to be compressed to groove depth than the one with the thick wall, where the spring back of the latter will be lower than the one with the thin wall. Again looking at the same cross section Metal O Ring, a seal made from Inconel X750 ® or Inconel 718 ® will have a higher load and higher spring back than a similar wall thickness Metal O Ring made from stainless steel. Going over to Metal C Rings, there is a certain similarity with spring back and seating load. The big difference is that Metal C Rings compared with the same cross section Metal O Rings, typically generate a lower load, and thus a better spring back than Metal O Rings. To overcome the low load of Metal C Rings, the spring energised Metal C Ring was developed. This seal, generates a relative high and uniform seating load with a relative high spring back. The Oysterseal ® or Metal V Ring, by its shape and material selection generates a high spring back, but gives only a moderate seating load.

Compression or Seating Load

The compression or seating load is expressed in N/mm seal circumference. The higher the load, the better the sealing performance will be. Soft plating requires lower load then harder plating. Rough surface finish of the sealing faces require a higher load to obtain similar seal performance. Non plated seals typically require extreme high loads to obtain technical tightness. It will be difficult, regardless the applied load to obtain a high tightness with unplated seals.

Tightness, Plating

Depending on the required tightness, medium to be sealed and temperature, a different plating or coating may be selected. For tightness better than 10–9 mbar.l/s a soft plating is always required. Indium and Tin are soft platings. PTFE is a soft coating too, but the obtainable tightness is limited to 10–6 mbar.l/s., because of the own porosity of PTFE. Silver, Gold and Copper are medium soft and require a higher load seal to obtain tightness ranges of 10–9 mbar.l/s. Silver is the most common used plating. Nickel plating, being the hardest plating requires a high load seal selection.

Flange Rotation or Lift off

Big diameters and high temperatures play a decisive role in the selection of a metal seal. Flange rotation or lift off occurs under high pressure. The higher the pressure and the bigger the flange diameters, subjected to this pressure, the bolts will be subjected to a quite high stress and strain, resulting in flange rotation or lift off. For such service conditions the biggest possible cross section for the given diameter should be selected, in order to have the best possible springback characteristics.

Summarizing

The groove surface finish for any selected seal should be equal or better then Ra = 1,8. For tightness equal or better than 10–9 mbar.l/s, a surface finish better than Ra = 0,8 should be applied. Always select the biggest possible cross section for a given diameter. Select the softest plating allowed for the application. If bold load permits, choose the highest seal load available when medium soft plating such as silver, gold or copper is selected. For temperatures above 350°C it is advised to use Inconel as base materials. For pressures above 35 MPa it is advised to use C Ring type seals or vented O Rings, both with or without spring energising, depending on the required tightness. For high temperature applications and when an O Ring type seal is required a gas filled seal may be the better option.

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