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Selecting Metal O’Rings

 

There are three major factors to consider  in selecting a Metal O’Ring.

 

I.        Operating Conditions

II.      The flange or gland in which the seal is to  be used.

III.  The O’Ring or seal.

 

I.     Operating Conditions

       

    1.  Temperature (both systems

         & seal)

    2.  Pressure- magnitude &

           duration.

    3.  Fluid- viscosity

    4.  Corrosiveness of medium to

           be sealed.

    5.  Temperature-stable or

           cycling

 

II.       Flange Assembly

        

     1.  Thickness and rigidity

     2.  Material

     3.  Groove finish

     4.  Hardness

     5.  Parallelism

     6.  Method of joining

     7.  Flange load and location

     8.  Dimensional tolerances

     9.  Flange separation under

          pressure or thermal shock

 

III.    Seal Design

 

      1.  Material

      2.  Plating or coating and 

            thickness

      3.  Dimensional tolerances

      4.  Amount of compression

            required

 

Design of Metal O’Rings

  

For most applications, Metal O’Rings made of 300 series stainless steel or high nickel alloys are recommended.  (Rings made of tube size .125” OD and smaller are made most frequently of 321 SS.)  For high temperatures, Inconel X 750 is suggested.

 

Metal O’Rings can be made to any shape with limitations only on the corner radii.  No radius should be smaller than the minimum shown in the chart below.

 

Minimum Radii– Shaped Rings

Tube Size

(inches)    .035  .062  .094  .125

Min. Radii

(inches)    .125  .250  .500  1.000

 

There are some practical limitations on the minimum and maximum cross-sections for various diameter, i.e., large diameter rings would be extremely flimsy if made from a very small cross section tubing.  Also, there is a minimum size ring that may be formed economically out of any given size tubing without crimping or  distorting the material.

 

Also available are round and shaped rings to fit contours which are not in the same plane.

 

Serious consideration should be given to the cost saving in manufacturing, labor and inspection that can be achieved by using Metal O’Rings for your sealing  requirements.

 

Ring Finishing

 

Tool marks resulting from machining must be parallel to the axis of the tube, i.e. circular tool marks only.  Spiral, radial, and transverse tool marks lead to compression of the O’Ring seal.

 

Although the practice is not recommended, in some instances with a rougher finish, a heavier walled tube and heavier plating may be used to obtain a seal.

 

The surface finish required to make a perfect seal is a function of the hardness of the surface, the wall thickness of the O’Ring, the character of the machined surface and viscosity characteristics of the fluid.

 

It is often desirable to apply an electroplate or coating to the Metal O’Ring.  This coating will flow (when the ring is compressed) into any minute scratches or imperfections in the sealing surfaces.  In some applications, this enables the use of flanges which might otherwise be rejected due to poor finish.  It also serves as a lubricant to prevent galling in screw type applications.  Plating or coating improves the ability of a Metal O’Ring to seal at higher pressures.

 

A variety of coatings may be used for special applications.  Coatings such as nickel, silver, gold, Teflon.

 

Bolt Loading

 

The bolt load is that force required to overcome the force necessary to compress the O’Ring or the flanges will separate.  Therefore the bolt load must cause a definite

compression of the O’Ring seal.

 

The flange must be designed so that no deflection occurs due to internal pressure.  After the O’Ring is installed and the system is pressurized, the sealing force results from compression of the O’Ring.  

The table below give the pounds of force required per linear inch of the O’Ring circumference to install O’Rings of various size tubing and wall thicknesses of stainless steel.

These values may vary some either way due to the effects of variables encountered.  These values are to be used only as a guide.

 Material: Stainless Steel

Tube Size Wall Thickness Compression Load

(inches)      (inches)           (lbs. Per linear in.)

  1/32           .006               450

 1/16           .010               525

 1/16           .012               760

 3/32           .010               250

 3/32           .012               385

 3/32           .020               1250

 1/8             .010               170

 1/8             .012               320

 1/8             .020               975

 5/32           .010               135

 5/32           .020               675

 3/16           .012               165

 3/16           .020               420

 3/16           .032               1450

 1/4             .012               100

 1/4             .032               1300

 

High Pressure Sealing

 The Pressure Equalized Metal O’Ring is vented and capable of sealing fluid pressures, gas pressures and high vacuums.  However, in a specific application, the limitation in pressure is a result of excessive deflection of the contacting surfaces of the O’Ring and the O’Ring groove due to expansion or contraction, insufficient bolt loading, under designed flanges.  This step should be considered if systems pressures are in the order of 200 psi or over.

When sealing at high pressures it is advisable to use the smallest ring cross-section practical.  It is better to use a larger cross-section in ring diameter when maximum resilience and lower flange loading is desired.  If an attempt is made to achieve both objectives it is necessary to compromise.

 Sealing at Extreme

Temperatures

When using at sub-zero temperatures or above 900°F, flanges, bolts and O’Rings must be of the same material.

 When sealing at high temperatures, the ring may be pressurized to compensate for the loss in strength of the ring at elevated temperatures.  Maximum temperatures for the following coatings or platings are:

 Material    Temperature

  Silver            1500°F (816°C)

  Teflon           500°F (260°C)

  Nickel           2200°F (1204°C)

  Copper          1900°F (1038°C)

  Gold              1900°F (1038°C)

 

 -450°F to 200°F—Stainless steel or Metal O’Rings, non-pressurized, with a coating for volatile liquids or gases.  For the extreme cold temperature, flanges, bolts and O’Rings must be of the same material.

 200°F to 550°F—Stainless steel, pressure equalized for pressures above 500 psi, Teflon coating for volatile liquids or gases.  

550°F to 800°F—Inconel, pressure equalized, silver plated for volatile liquids or gases.

 800°F to 1300°F— Inconel X,  pressure equalized with silver plating.  Mating parts must be of similar material.

 Above 1300°F— Special design requirements and recommendations are required for extreme temperatures.  

Sealing Fluids & Gases  

When sealing against heavy fluids coating is generally not required.

When used with volatile liquids, coating is recommended.  This is also true with gases such as helium or hydrogen.  

Heavy Liquids -  requires thin wall, no coating, pressure equalized Metal O’Ring above 500 psi … examples: tar, heavy lubricating oils, heavy hydraulic oils, plastics.

 Medium Liquids -  medium wall, coating is optional, pressure equalized above 500 psi … examples: jet fuels, thin hydraulic oils.

 Volatile Liquids -  heavy wall, coating is required, pressure equalized above 500 psi … examples: liquid propellants, steam, refrigerants.

Gases -  heavy wall, coating is required, pressure equalized above pressures of 500 psi.  This is also the requirement for high vacuum sealing … examples: air, helium, flourine.

 

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