When it comes to lubricants, it’s important to select the optimal product to ensure proper gasket seating stress.
Anti-seize lubricants have three major benefits: They provide for uniformity of bolt-up assembly by ensuring a common friction factor for all studs, prevent galling of the stud and nut at elevated temperatures, and facilitate disassembly of the bolted connection when it comes time to take the joint apart.
In addition to these traditional functions, another parameter has become increasingly relevant. With more facilities now incorporating a hot retorque as a required step during start-up, it is important the lubricant’s K factor be consistent over a wide thermal range. Unfortunately, manufacturers of lubricants don’t test for this characteristic.
Warren Brown designed a test that specifically looked at the changes in the K factor from ambient to about 400 degrees F. Testing numerous products from different manufacturers with different metallic bases, he found nearly all anti-seize products get either more lubricious or less lubricious as they are heated. For example, some copper-based products become less lubricious as temperatures increase, where-as some nickel- and moly-based lubricants become even more lubricious as temperatures increase. Only certain moly-based lubricants maintain a consistent K factor as temperatures increase.
When hot retorquing, a consistent K factor is essential. If the K factor increases, hot retorquing the joint will result in less gasket stress than desired because the increased friction will use up more of the applied force. However, if the K factor decreases, the thread becomes “slicker,” so much more of the torque is converted to stud stretch, overloading the gasket and increasing the risk of yielding the stud. The K factor must remain constant in order to achieve the proper gasket seating stress.
For more information, visit www.lewis-goetz.com or call Advanced Sealing at (562) 802-7782.