Lens ring gaskets are characterized by a spherical sealing surface that touch the conical surface of the flange. These gaskets are used in special application with specific values of pression, temperature and changes during the time.
Usually you can find these gaskets on flange connections of high-pressure reactors where there are heavy working conditions and a perfect sealing is needed. In this case a loss of material could comport high risks both for the workers and the environment.
A weld ring gasket is a sealing system thought for critical applications where a weathertight joint is needed (heat exchangers, thermic reactors, chemical plants, etc.).
The main reasons to install a weld ring are dangerous fluids containment and difficult work conditions (thermic shocks, vibrations, axial or lateral movements of the flange, etc.)
The single gasket is made of a couple of rings with a mirrored cross section. Every ring is welded singularly to his flange and then welded together on their external diameter; in that way you can have a completely welded joint granting the weathertight sealing.
To avoid welding problems like galvanic corrosion or different heat expansion the material of the gasket must be the same or as similar as possible to the flange material.