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Lens ring flange joints and how they are machined


The Lens ring type of flange joint is used to create a seal in high temperature and high pressure piping systems. This joint differs from the standard raised face flange joint (which has a ‘flat’ sealing face), by having a sealing face angled at 20° down towards the centre of the bore. This type of joint is not used on very large flanges, with typical outside diameters of the lens ring gaskets ranging from 20mm up to 385mm. 

How does a lens ring joint create a seal?

The lens ring gasket is a made using a metal softer than the flange so that the gasket plastically deforms on compressive load, rather than the flange. The lens cross section includes a spherical sealing surface and requires special angles machined on the 2 connecting flange surfaces. In accordance with DIN 2696 and to meet ASME B16.5 standards, only certain specified grades of Carbon Steel, Stainless Steel can be used for lens ring gaskets.


Lens ring gaskets can withstand high temperatures and high pressures at the same time. This makes them suitable for the oil, gas, petrochemical and offshore industries. Typical installations include flanged joint connections on valves, pipe-work assemblies and vessel joints.


  • Compared with other types, lens ring gaskets will seal with a smaller bolt load, because the contact area is very small).
  • The contact surface between the spherical flange and the spherical lens ring gasket increases when strong pressure is applied.
  • The gasket’s design and elastic deformation properties provide an additional advantage of being able to reuse it several times.
  • Lens ring gaskets are less sensitive to over-stressing, due to distribution of compressible loads spreading over the gasket facings.

What does a lens ring flange joint look like?

The image below shows a section view of a typical lens ring flange joint.

 lens ring joint cross section.jpg

How can a damaged lens type of flange surface be machined?

This is achieved using a flange facing machine used with a lens ring type of tool post attachment. The video above shows this type of arrangement with the Mirage MM300e external mount flange facing machine. On the lens ring tool post the angle is pre-set at the required 20° angle with the feed is advanced using a striker wheel and post assembly.

The striker assembly must be adjusted so that the striker leg connects with the teeth of the MM300e-LRK sprocket on each rotation.

When the cutting operation is complete the surface finish will need to be polished before the flange can be used. This can normally be done easily using emery cloth.

Find out more about Mirage Flange Facing machines by downloading our buyers guide. If you'd prefer to talk to us or enquire via email, please click here to get in touch.

Download the Flange Facing Machine Buyers Guide

Sources: http://www.smithgaskets.com/rtj-lens.php     http://goodrichgasket.com/Lens-Rings.aspx

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