Also known as rotary joints, these items are designed to allow fluid flow from a stationary supply to a rotating component on a machine or installation.

Rotary unions are often located on the end of a rotating roll shaft or rotary cylinder, where cooling is required for the removal of heat during a manufacturing process.

Fluid is forced into the roll or cylinder via the rotary union inlet port and passage where heat is absorbed and carried away with the fluid as it exits the rotary union through the outlet passage and port.

High fluid flow rates and lower fluid inlet temperatures will be more effective at removing heat from the application.
Rotary unions comprise an internal seal arrangement (often dynamic or mechanical) often used with a ball bearing. These seals permit rotation without fluid leakage within the union housing.

Various designs exist to suit many different applications and fluid types, such as oil, water & air. Port sizes are chosen to match mating components and pipework.

The seals and bearings are often replaceable items within a rotary union because long operating durations cause these parts to deteriorate.
To answer the customers question, I would need to check the function of the fastener, since Deublin hasn’t a publicly available drawing to study the rotary union in question, it is difficult to say exactly what the purpose of the bolt is for, although it seems unlikely it is provided for the function of providing a better seal. It’s possible the fastener is used to provide access for bearing lubrication if a grease nipple is not provided or preferred.

The seals in this case may be exhibiting a short service life for other reasons. The rotary union is possibly underrated and the design selection process was flawed or the machine has since been upgraded to run above the original specification.

The factors affecting the life of rotary union seal kits include temperature, speed, pressure, flow rate, fluid compatibility, and mounting design & installation. Running a rotary union on a machine or installation with operating parameters outside of the rotary union manufacturer’s specification will certainly cause premature seal and bearing failure.

Understanding the customers application with its specific data along with the actual mode of failure of the seals may help locate any issues and provide a solution. Please provide additional information including photographs.