Rotary Joints
What is a Rotary Joint?
A rotary joint, sometimes referred to as a rotary union, provides a mechanical seal between a stationary supply pipe and a rotating drum or cylinder to permit the flow of heat transfer media into and/or out of the rotating roll. Heat transfer media typically used with rotary joints include steam, water, thermal oil, hydraulic oil and coolant.
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How It Works
With conventional rotary joints, a spring-loaded mechanical seal is used to keep the fluid or gas medium from being dispersed into the atmosphere. The spring is used in the joint to maintain the initial internal seating, and the joint is pressure-sealed once in operation. Low pressure or vacuum service relies on the spring force to create the seal. Conventional rotary joints are either self-supported with the use of an internal carbon bushing, or external-supported using rods or a mounting bracket.
Balanced seal rotary unions (joints) are generally used with fluid service (e.g. water, coolant, etc.), but can be used with steam and other gases. The balanced seal technology relies on external spring pressure to create a positive seal within the rotary joint. The operating pressure of the media has little, if any, affect on the seal load. Balanced seals are typically flat face seals and the rotary unions are usually supported by ball bearings.
The seal ring is the main wearing part of the rotary joint. During operation, seal rings can experience internal loading forces from fluid pressure, as well as friction from contact between the sealing faces. Factors related to the machine speed, temperature, and media used can also affect the life of the seal ring. To a large extent, a rotary joint is only as good as its seal package. Poor quality seals naturally wear out more rapidly, leading to an increase in maintenance, downtime, and repair cost. When a seal fails, media escapes and the efficiency of the entire heating or cooling process suffers. All of this makes the actual replacement cost of the seal just a small portion of the total cost of a seal failure.
Flow Configuration
Single-Flow, Dual-Flow, or Multi-Passage:
In a single-flow joint, the medium flows in only one direction through the joint. A single-flow joint is used where the medium enters at one end of a drum or cylinder and exits the other end, making a single pass through the joint.
In a dual-flow joint, the media enters and exits through the same joint. It is most often utilized in applications where syphon or distribution pipes are used. A dual-flow joint seals between both media, as well as between one of the media and the surrounding atmosphere.
In multi-passage joints the various types of media make multiple passes through the same joint. These
directions of flow can be in the same direction or opposing directions with any combination thereof.
Read about the history of the rotary joint.