To a company that routinely cleans and reuses stud bolts, a recommendation to use new studs may seem a little bit over the top. Why should new studs be used when the used studs still look new? Isn’t that just a senseless waste of money? Why throw away a perfectly good stud? In this age of recycling, isn’t it better to reuse than replace?
Remember the age-old idiom, “You can’t judge a book by its cover”? Well, here is a perfect example: You can’t judge a stud by its looks. It is true that pre-used stud bolts can be cleaned up. They can even be wire-brushed to look like new, but the appearance of a stud is not the most critical of its attributes. What is most important is its performance, and the data shows that the frictional drag on a used stud is very unpredictable.
Several years ago, a larger refinery performed a field test to investigate the issue of frictional drag on a used stud. This particular refinery had hundreds of heat exchangers. On the day of the test, they replaced the floating head gasket on each exchanger and tightened the connections. Because there wasn’t enough room on the floating heads, no hardened washers were used. Other than that, best practices were used; the refinery’s team lubed everything properly and used a calibrated clicker torque wrench on both. The only difference was the condition of the studs. After torqueing, they measured the actual stud stress.
The results proved that in addition to using an optimal gasket type, the secret to long-term sealing in heat exchangers depends on how one deals with relaxation in the joint. In order to manage relaxation, it is critical to load the gasket to achieve a high seating stress. This needs to be done reliably to a high, predetermined value.
So, if an engineer determines that a gasket needs 20,000-psi seating stress to give long-term reliability — and that 600 footpounds of torque (for example) is required to achieve that load — he or she has to be able to depend on the fact that, when the studs are torqued to 600 foot-pounds, they will actually generate the bolt stress he anticipates.
And that is the problem with used studs. Due to the almost microscopic rolling and galling on the thread surfaces, the stud no longer converts torque into stress in a predictable manner. The relationship between bolt torque and gasket load is broken. Because of this, the use of new studs should be required whenever an exchanger joint is opened.
Now, there are a couple exceptions to this rule that should be recognized as standards. First, if a stud is being tensioned (not torqued), then it’s fine to reuse them, as torque doesn’t come into play. Second, if a person is willing to run the threads on both the studs and nuts with a tap-anddie, thus renewing the threads, then the fasteners can safely be reused.
The argument is sometimes advanced that used studs are better than new studs because they have been work-hardened. The simple response to that argument is that work-hardening doesn’t matter. What matters is the ability of the stud to deliver a predictable load to the gasket, because that is how leak-free performance is achieved.
The proof of this approach is easily seen. Using new studs to optimize the gasket load is an important component of the solutions employed to eliminate exchanger leaks from refineries — something many considered impossible until recently.
For more information and to download the full results and complete technical notes of the study, visit http:// info.lewis-goetz.com/new-vs-old