Why we use metal seals? Metal seals are mainly used in extreme conditions where you cannot use regular elastomeric seals. Specially in the extremes of Temperature – from cryogenic to above 300 deg C, Pressure – above 100 bar, ultra high vacuum – 1 x 10-9 mbar.lit/sec and above, Radioactive environment and so on. One should go for the metal seals only in the case when regular elastomeric seals do not do the job. Commonly used material for these seals are, SS321, Alloy 600, Alloy 718, Alloy 750.
How does a metal seal work? A metal seal is placed in the groove of a flange. The other mating flange compresses the seal by 20% (15 – 25% is the range of compression) in to the groove. Once the seal is compressed a seating load is created which is a direct function of seal material, size and its wall thickness. The flange waviness creates an impression on the seal surface. This impression/ foot print is the leak prevention zone. The wider the foot prints better the sealing. To achieve higher leak tightness the seals are altered by adding springs,surface coating/plating. Higher the seating load accompanied with surface coatings/plating will give better leak tightness.
Metal seals need to avoid permanent deformation. To avoid this deformation seals are placed in the grooves of the flange. These grooves are made 80% deep of seal height. On complete compression of these seals they can get maximum 20% compressed, which is optimum to achieve max leak tightness. If grooves cannot be made then retainers are required to prevent over compression. The flange Ra value i.e. the roughness plays a very important role in achieving the desired leak tightness. Enough bolt load is required to compress the seal for creating sufficient seating load. The number of bolts used for clamping plays an important load in applying load on the circumference of the seal.
Selecting the optimum seal for any application is often a tradeoff between available load, required tightness, surface finish and cost. Temperature, Pressure and type of medium are important criteria’s in seal selection.
Select the biggest possible cross-section for a given diameter. A bigger cross section always gives higher spring back. The impact of the weld region is smaller. The variations in operating temperatures are better compensated.
The material and plating/coating of seal also plays an important role in the performance of the seal. It is important for the designer to understand what material is suitable for the application along with the plating type. In case any help is required Kwality Products can provide guidance based on our experience.
Available load is very important in deciding the seal sealing criteria. If load available is high enough one should go for seal with bigger cross section. Higher the cross sections better the spring back range.
Plating/Coating is done on the seal surface. The purpose of plating/coating is to provide high leak tightness. The soft layer of plating is created so that it flows into the waviness of the flanges. Due to this the unevenness of flanges is filled and high leak tightness is achieved. The type of plating is decided based on application. Following are the types of plating/coating done on the seal.