"Our plant had excessive oil leakage and suspected oil evaporation on our paper machine drying cylinders, which work at speeds of 360 revolutions per minute with a temperature range from 110 to 125 degrees C and a centralized oil lubrication system. Operations personnel has faced this problem since the installation of 12 extra drying cylinders that use the same oil with the same viscosity (ISO VG 150). The sump temperature is about 72 degrees C. Before installation, evaporation was less, and there was no leaking. I believe they need to change to a higher viscosity or a synthetic alternative, but how do you choose the right lubricant in such a case?"

There are a few things to consider. First, industrial lubricants with a viscosity of ISO VG 150 generally should not evaporate much if they are exposed to temperatures up to 125 degrees C, as the flash point of these lubricants is above 200 degrees C. The loss of lubricant may be due to leaks, since there are more lines, connectors and components in the system.

For proper lubrication, the viscosity will depend on the bearing conditions, including the operating temperature, speed, load, bearing design and application method (circulation in this case). Adding drying cylinders to the system should not require a viscosity change if the lubrication parameters are maintained.

One possibility is that the operating temperatures increased after the system expansion. If this was the case, it certainly would drop the lubricant’s viscosity.

While the viscosity grade of the lubricant you are using is ISO VG 150, the most common viscosity grade for paper machines is ISO VG 220, which is the next higher grade in the ISO viscosity classification. With this in mind, you should verify that you have been using the right viscosity for the system. Even if the system was originally designed for an ISO VG 150 lubricant, it is possible to increase one ISO grade without compromising the lubrication quality. Of course, this may just be a “patch” to a problem that has a different root cause.

The sump size should also be calculated based on the system capacity and maximum pump flow rate. The sump should have a minimum volume in order to maintain a stable oil supply and allow the lubricant to remain in the sump at least five to 10 minutes. This permits the settling of eventual moisture or solid contaminants along with the dissipating of heat and the release of foam.

When a circulation system is expanded to include more machines or components, it may be necessary to enhance the sump capacity according to these new conditions. Also, be sure to confirm the new oil level when the system is in normal operation as well as after it has been shut down.

Lubricant manufacturers produce certain paper machine oils that are recommended for a variety of machine types. Check to see if your machine is suitable for a specific type of lubricant and verify that the one you are using is best for your application. 

Synthetics have a number of advantages, such as enabling you to increase energy efficiency or extend oil change intervals. While switching to a synthetic may offer some benefits, it would not necessarily provide the solution, as this would not attack the root of the problem. Therefore, the best strategy would be to determine the cause of the lubricant consumption and then confirm the optimum oil viscosity and formula for your application. Your conclusions should then allow you to take the appropriate actions.