Rotary Joints and Unions

The terms are largely used interchangeably in industry, though 'rotary joint' is more commonly used for steam applications on dryer cylinders (often including an internal syphon for condensate removal), while 'rotary union' is more general and commonly used for hydraulic, pneumatic, and coolant applications. Both perform the same basic function: transferring fluid between a stationary line and a rotating shaft or cylinder.

As steam gives up heat inside a rotating dryer cylinder, it condenses into water that must be continuously removed to keep the cylinder heating efficiently and to prevent water hammer damage. A syphon tube inside the rotating cylinder picks up condensate and routes it back out through the rotary joint's separate return passage, which is why dryer cylinder rotary joints are typically multi-passage designs.

Mechanical face seals generally handle higher pressure and rotational speed with a longer service life but require more precise alignment and are more sensitive to contamination in the media. Packing seals are simpler and more tolerant of minor misalignment or contamination but wear faster at higher speeds and typically need periodic adjustment or repacking — choose based on your pressure, speed, and maintenance access.

Common causes include operating above the rated pressure or RPM, misalignment between the stationary and rotating shaft centerlines, particulate contamination in the media damaging the seal faces, or simply normal seal wear reaching end of service life. Most rotary unions are designed with replaceable seal kits so the union body doesn't need full replacement when seals wear out.

Only if it's a multi-passage design specifically built with isolated internal flow paths for each circuit — a single-passage union can only carry one media stream. Multi-passage unions are common where a rotating assembly needs, for example, separate hydraulic supply and return, or steam supply with a separate condensate return, through one compact rotating interface.