O-Ring Compounds
By: Dennis Pearson
Vice President/General Manager

There are several o-ring options to consider.

Barco Rotary Unions are available with three different o-ring compounds which should be considered based on the media that will be passing thru the union. Proper o-ring material selection is critical in order to achieve the maximum service life of the union.

Most rotary unions use an o-ring as a seal between the stationary seal and the casing. The o-ring must seal the same fluid, pressure and temperature that the mechanical face seal is designed for. Compatibility with the fluid is critical for the o-ring to perform it’s function.

The o-ring must allow for axial movement of the stationary seal so that the sealing face is always mated with the rotating seat. If the o-ring prevents movement of the seal, the seal “hangs up” and face separation occurs.

To make a tight seal, the o-ring must be compressed or squeezed. The amount of compression is designed into the mechanical seal based on the original size and shape of the o-ring. Incompatibility with the fluid or temperature can cause the o-ring to “compression set” and loose the initial shape or to “swell” becoming larger than normal. Compression set can cause leakage past the o-ring. Both compression set and swelling can lead to seal hang up.

The general all purpose o-ring material is Buna N. This compound is excellent for general purpose water applications for temperatures up to 225ºF. Buna N has excellent compatibility with petroleum oils and ethylene glycol solutions. Do not use Buna N for brake fluid, MEK, acetone or trichloroethylenes. The Barco elastomer code for this compound is –01.

For higher temperature applications, ethylene propylene or EP o-rings can be used for temperatures up to 350ºF. If the union is used for intermittent steam service EP should be specified. Besides water and steam, EP can also be used for brake fluid, dilute acids and alkalies, MEK and acetone. Ethylene propylene cannot be used for any petroleum products. This is important to keep in mind when rebuilding a rotary union. Also note that you must use only silicone grease to lubricate the o-ring at installation. The elastomer code for EP is –04.

Viton is the third choice of materials for o-rings. Viton has the widest chemical compatibility and can be used up to 400ºF. Specify viton for heat transfer oil applications. Viton cannot be used for MEK and acetone. Use the elastomer code –06 to specify this compound.

Remember :
Knowledge of both the product and the application will help assure that the correct union will run at its full potential.
If you are not sure which o-ring is best for a particular application, call a Barco Application Engineer.

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