Metallic gaskets rely on deformation or coining of the gasket material into the flange surfaces to provide effective sealing. Coining deforms and yields the gasket hence its reuse will not reliably achieve the required seal.

 As per API 6A section

"Ring gaskets have a limited amount of positive interference which assures the gaskets will be coined into sealing relationship in the grooves. These gaskets shall not be re-used."

Recommendation against reuse of gaskets is also specified in ASME PCC-1 (current edition)

Ring Joint Gasket

"Reuse of a gasket is generally not recommended. One current exception is large, grooved metal gaskets with facing layers (see Appendix C) that are reused in some instances after having been reconditioned and refaced in a manner consistent with the original product specifications. Use of gaskets so refurbished is not considered as gasket reuse in the context of the first sentence. For other gasket types, experience has clearly shown that only a new gasket will reliably provide the necessary plastic deformation and elastic recovery characteristics essential to achieve an effective seal. Visual or physical inspection of a used gasket for apparent damage is not sufficient to detect such sealing surface factors as workhardening, brittleness, or the affects of heat or interaction with the service fluid."

Below are recommendations from Flexitallic Gaskets

Ring joint gaskets are designed to have a limited amount of positive interference, which ensures that the ring type joint seats correctly into the groove on compression. Their reuse is not recommended for two reasons:

  • The initial seating of the gasket will be impaired
  • When the gasket is plastically deformed, work hardening of the external metal surface occurs. This may result in permanent damage to the groove.

Is it safe then to reuse the gaskets only for hydrotest purpose

Some fabricators consider that it is safe to reuse ring joint gaskets for hydrotest purpose. However this is not the case. On compression of the flange assembly, the gasket plastically deforms and is subject to strain hardening. It is important that the ring type joint be significantly softer than the flange groove so that the gasket plastically deforms and not the groove. The use of harder ring type joints can result in flange groove damage. 

As per ASME B16.20 section 2.3.1, It is recommended that ring-joint gaskets be of a lesser hardness than that of the mating flanges. The recommended maximum hardness values are as stated below:

Ring Gasket Material Maximum Hardness for Ring Gaskets
Brinell Rockwell “B” Scale
Soft iron 90 56
Low-carbon steel 120 68
4–6 chrome 1⁄2 Mo 130 72
Type 410 170 86
Type 304 160 83
Type 316 160 83
Type 347 160 83

Gaskets are therefore manufactured from softer material than the flange mating surfaces as they are expected to deform and flow into and fill any gaps and undulations in the flanges and yet provide sufficient strength to resist the pressure contained within.

The material from which a gasket is made is important. The material should have insufficient strength to withhold internal pressure. It must also be able to withstand operating temperatures and resistant to the service fluid which it will come into contact.