Spring turnaround season is right around the corner and to prepare you will need plenty of lubricants and to know the benefits of selecting the optimal product. Your gasket supplier should be able to help you select the right one.
Anti-seize lubricants have three major benefits:
- They provide for uniformity of bolt-up assembly by ensuring a common friction factor for all the studs
- They prevent galling of the stud and nut at elevated temperatures
- The facilitate disassembly of the bolted connection when it comes time to take the joint apart
In addition to these traditional functions, an additional parameter has become increasingly relevant. With more facilities now incorporating a hot retorque as a required step during start-up, it's importnat that the lubricant's K factor be consistent over a wide thermal range. Unfortunately manufacturers of lubricants don't test for this characteristic.
Warren Brown of The Equity Engineering Group designed a test that specifically looked at the changes in the K factor from ambient to about 400-degrees. Testing numerous products from different manufacturers with various metallic bases, he found that nearly all anti-seize products get either more lubricious or less lubricious as they are heated. Here's a sampling of what he measured:
- COPPER BASED– Starts well with a K factor of .16; but becomes less lubricious as the temperatures increase.
- NICKEL BASED – Begins higher (.17-.18); but becomes more lubricious as the temperatures increase.
- MOLY BASED 1 – Starts low (.13); and continues to become even more lubricious as the temperatures increase. The K factor drops by 50%!
- MOLY BASED 2 – Starts well with a K factor of .16; and maintains a consistent K factor as the temperature is increased.
The Results of the Sampling
When hot retorquing, a consistent K factor is essential. If the K factor increases (as with the copper-based product shown), 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 that 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.
In summary, using the right lubricant is essential to insure proper gasket seating stress. For more information on bolt anti-sieze performance in a process plant environment, click here to download a full technical paper.