WHAT IS A GASKET? WHY ARE THEY USED? WHAT ARE THE CONSIDERATIONS?

A gasket stores energy, like a spring, and seals two mating surfaces that fail to perfectly meet.

• Wikipedia: “Fills the space between two objects, generally to prevent leakage between the two objects while under compression. Gaskets save money by allowing less precise mating surface machining and allowing the gasket to accommodate for the flange irregularities....”
  wikipedia.org/wiki/Gasket

The application dictates which sealing solution to use.  Although your application may not resemble the sketch, there are common terms found across sealing systems using gaskets.

PLANNING A GASKET SYSTEM

As a member of a sealing system, which is made of the flanges, the gasket and the fasteners, a gasket is one of the system elements used to construct a sealing solution.   Some requirements that the gasket must meet include, but are not limited to: conforming to the mating surfaces to achieve a seal, chemical resistance to the medium being sealed, and ability to withstand all other environment conditions like temperature, cavity pressure and compressive stress.

Additional Considerations:

• Zero leakage through the gasket.


• Accommodate surface finish conditions of flanges.


• Reduce and/or control flange or port distortion.


• Accommodate thermal expansion and contraction.


• Possess adequate recovery.


• Minimize torque loss caused by creep relaxation.


• No re-torque.


• Transfer heat, as desired.


• Close tolerance on compressed thickness.


• Meter fluid.


• Provide acoustic or thermal isolation.


• Possess anti-stick properties.


• Pass customer verification testing


• Manufacturing / Handling
  
  
  
  • Robotics
  
  
  • Cleanliness
  
  
  • Visibility for sensors
  
  
  • Fit
  
  
  
  

GASKET MATERIAL: Minimum and Maximum Load to Seal

Before planning gasket material selection, it’s best to plan the sealing system overall.  The compression of the gasket material is dependent upon the other members of the assembly.  They work together as a system and can be cost optimized as a system.

Gasket compression is caused by fastener clamping force and flange design.  The flange surface area determines the flange pressure distributed across the gasket surface.   The fastener size, spacing and retained fastener force produces applied flange pressure to compress and seal the gasket.   The amount of load required to seal is affected by the surface finish of the joint system.  Any flange surface irregularity and flexibility produce flange contact pressure variation across the gasket surface and potential leaks.  The gasket must be resilient and creep resistant enough to maintain an adequate portion of the applied flange force across its entire surface to remain sealed.  It’s important that the gasket have enough strength to resist crushing under the applied load and maintain its integrity during static and dynamic conditions.

Gasket Material Selection

There are thousands of specialized engineered gasket materials available globally from large and small material development companies.  No single material has the capability to serve all applications.

2500+ engineered gasket material options could be discussed, and to simplify, rubber provides the core technology of most gasket materials, due to its barrier and flexibility properties.   One thing all sealing solutions have in common: they all rely on the flexible rubber to do the sealing.

Then, it’s a matter of how the rubber is modified to meet performance requirements.  As great a material that rubber is, it does have failure modes.  ASTM provides standardized accelerated aging test systems for rubber, including chemical and temperature compatibility rankings.

Engineered composites, based on rubber, are used to achieve sealing and durability performance.     Rubber and fiber fillers are blended in proprietary recipes by material development companies to enhance gasket performance.   The result is a material impervious to leakage that also retains its shape under load over time.   Without the addition of fiber and fillers, rubber is a poor gasket material, as it extrudes under load, resulting in eventual leak paths and fastener load/torque loss.

SIMPLIFICATION: All Gasket Materials Fit into one of Three Compression Families

To create structure, we organize materials into three families, based on their compressibility.

 



Compressibility Family
Material Construction


Soft
Cellular Elastomers-sponge and foam


Medium
Solid Rubber Elastomers-sheet, extruded, or molded rubber, no cells or bubbles, no fillers


Firm
Reinforced Rubber Elastomers-many engineering fillers exist to reinforce rubber

GASKET MATERIAL IDENTIFICATION & SELECTION RESOURCES

Please contact PGC about these resources below, which are tools to increase development speed:

Chemical Resistance Database – Program provides compatibility of rubber used in different gasket materials to the chemical media it will be exposed to.

PGC Competitive Material – Program for comparing competing material to recommend options for cost reduction or performance improvements.

ASTM F104 – Program used to identify gasket materials by deciphering the F104 callout and compare to other gasket material specifications i.e. Military Specifications.

PGC Laminated Composites – Program deciphers an ASTM F868 callout on laminated composite gasket materials.

PGC Obsolete Materials – Program searches database for obsolete gasket materials and shows material components to select a comparable available material.

COMPRESSION & BLOWOUT ANALYSIS TOOLS

Material selection leads to material performance testing.  Depending on the end application, the testing can vary.  The following experiments may support verification and prediction of application success.

Compression Set Resistance – the ability of an elastomer to return to its original thickness after a compression load is released.

Load Retention / 3D Creep - Program that provides creep relaxation data for fiber-based gasket materials.  This data directly correlates to torque/load retention.

Load Compression – Program used to determine compression of material under a range of clamp loads.

Compressed Stack-up Tolerance is a program to calculate the compressed thickness range of a gasket material to be used in an application where stack up tolerances are critical.

Pressure Blowout Predictive Study- Program calculates the ability of a gasket material to withstand specific internal pressure with Joint Friction and Hoop strength calculations.

STATIC & DYNAMIC STRESS ANALYSIS

What applications/case studies could we use this in?

Flange Model Analysis (FMA) – Program used to calculate “minimum load to seal” by using flange, bolt, and environmental data to theoretically determine the best materials for the application.

FEA Minimum Pressure - This program was designed to determine the best material for sealing an application using Finite Element Analysis with results leading to modify the assembly to lower cost.

Fuji Stress Analysis – This process measures static clamping force and displays the pressure distribution map.   Digital analysis of the image can be performed to provide “real time” data to determine material selection or if enhancements are needed to affect a seal.

FlexiForce Analysis – A method of measuring dynamic stress real time. By selecting the appropriate sensor, we will be able to analyze the dynamic stresses in an assembly over time and select the technology to last the service life of the assembly.

Services and capabilities- PGC project manages to deliver the project vision through 250 sources of specialized non-metallic material technologies

PGC Development Center for rapid training, development and prototyping

• Highly specialized polymers: Concepts to application engineering support


• Solutions for complex projects: Proven discovery, diagnostics, development, prototyping, testing, and manufacturing processes

A LONG-LASTING SEAL

Stress maps show that the combined action of elastic properties and adhesion assure a long-lasting seal. Larger contact areas support an effective sealing barrier against potential flange surface imperfections, that could result in leakage paths.  Unevenness in the flange hardware can increase greatly the stress in the gasket or even crush it.   Concentrated loads under bolt heads are common in bolted joints.

Significant gasket stress relaxation can result in compromised sealing if a gasket no longer fills a gap with enough spring force.  Whenever a flange bolt is tightened, elongation of the bolt occurs. This stretch provides compressive load on the gasket.   Bolt torque loss should be measured, and re-measured, after life cycles that include thermal cycling, to identify loss of bolt load and compressive seal-force loss.

CONCLUSION

This concludes an overview of several system issues.   PGC supports customer solutions.  Often, PGC engineering advice and training, along with fast analytics help customers to identify their choices for each specific application, considering the system issues.