Rotating unions

A rotating union is a unique product because it must be capable of containing pressure, sometimes very high, and withstanding corrosive fluids that can be at elevated temperatures, all the while rotating, sometimes at high speeds. This is possible through the use of almost perfectly mating seal faces. These lapped seals must rotate smoothly and easily, with minimum friction, to ensure long life and no leakage.

Rotating unions are stepped on, pulled at, overgreased, undergreased, overtightened, misaligned, overheated, and ignored. It's amazing that a device built with precision bearings and seals can be abused so much and still function.

A rotating union has a mechanical seal that must be free-floating and self-supporting. All connections should be made with flexible hose, not rigid pipe connections. If the flexibility of the union is restricted, seals and bearings will be damaged and fail.

High-temperature applications require a rotating union with extra bearing clearances to compensate for thermal expansion. Bearings may eventually anneal, affecting the seal.

Rotating unions have four major causes of failure: piping strain, misalignment, lack of attention, and improper repair. Piping strain is relieved using hoses. Misalignment occurs during installation and can be eliminated using proper procedures. Good maintenance overcomes lack of attention. Following manufacturer's instructions eliminates the problem of improper repairs.

Excessive flow rates can cause damaging water hammer when control valves close. Mechanical seal faces are overloaded and begin to leak. Always operate the rotating union to the specifications it was ordered under.

• Flexible hose must be used to connect each rotary union to its supply or drain piping. Install the hose to the union before it is mounted on a machine. Otherwise, the bearings and seals could be damaged.
• Use braided metal hose for applications involving hot fluids, such as hot water, hot oil, or steam.
• Use two-ply rubber hose for cooling water, refrigerants, air, and water.
• Important: Allow for an offset between the rotating union and rigid pipe. Permit a slight curve in the hose to allow it to float.
• Caution: A straight hose, when fully charged, becomes as rigid as solid pipe. This rigidity, plus the added weight of the fluid media and expansion due to temperature, can drastically shorten the service life of a rotating union.
• If valves or instruments are used, install them in the rigid pipe instead of the hose.
• Avoid sharp bends or twisted hose. Extreme bends or torsion loading can diminish or eliminate flexibility of the hose, resulting in shortened service life of the rotating union. Never use straight hose connections.

• Offset installations should be fitted with proper length hose to allow smooth curvatures and eliminate sharp, fracturing bends.
• Smooth curve and long hose installations provide maximum flexibility and service life.
• Sharp bends cause hose fatigue and fractures when corrugations are stretched and compressed. The use of one or two pipe fittings for proper installation ensures long, trouble-free service.
• The use of pipe unions aids installation and prevents twisting of hose. A twisted hose loses its flexibility and its service life is reduced.

Plant Engineering magazine extends its appreciation to Barco, Deublin Co., Duff-Norton Co., The Johnson Corp., and Rotary Systems, Inc. for their assistance in the preparation of this article.

Typical types of rotating unions