Using Rotary Unions With Thermal Oil
Rotary unions used with heat transfer oils require special attention during repair to ensure leak-free operation. Heat transfer oils tend to “coke” at high temperatures and deposit material that can settle in the rotary union and adhere to the sealing components. Due to this condition, the rotary union should be disassembled and cleaned thoroughly if a leak is detected. Care must be exercised to prevent marring the sealing surfaces when cleaning. All of the sealing surfaces are precision lapped to ensure effective seal performance and the fine surface finish must be preserved during the cleaning process. The rotary union should be depressurized and allowed to cool to room temperature prior to attempting any service or repair work.
Piping strains can force the rotary union out of alignment and result in leakage. Two pieces of flexible metal hose, installed perpendicular to each other should be used at each inlet and outlet connection of the rotary union. They should be properly supported so that piping loads are not applied directly to the rotary union. If a traveling loop is used, be sure to follow the recommendations of the hose manufacturer for length and proper installation.
The torque restraint is very important for rotary unions that are sealing heat transfer oil. Special care should be taken to ensure that the torque restraint does not apply significant side loading of the unit. This condition will accelerate the guide wear and will eventually cause seal misalignment.
Procedures for Starting New Hot Oil Systems
If the rotary union begins leaking soon after installation, disassemble and inspect the carbon seal rings for scratches or swirl marks on the flat or concave surface. These marks indicate particles are present in the oil and a more effective filtration system is required. Some of the contamination is typically evident within the rotary union and usually contains clues to determine the source of the contaminants. New piping or rolls frequently require flushing the entire system at a high flow rate to carry metal chips, grinding slag and other contamination out.
A filtration system capable of retaining all metallic and nonmetallic foreign material should be included in the system for continuous operation. These contaminants can build up in the system while it is in operation and they must be eliminated as they can damage the seal surfaces.
The ideal configuration is to install individual filters before each rotary union. The filter should remove particles 40-60 microns or larger in size. Pressure gauges should be mounted up and downstream of the filter to monitor the condition of the filter element. The gauges will allow the filter element condition to be monitored during operation.