Advancements in engineering design and analysis tools have allowed many new products to be developed for maintaining and operating gas turbines. They are not only safer, but they also allow for faster and easier operation as well as cost savings. Some of these enhancements have become best practices in the industry.
One specific area of improvement is seen in critical fastening applications. A critical fastening application is any bolted joint that requires accurate and uniform bolt loading.
Heat and Slug Wrench Method
In early years of turbomachinery, it was common practice to tighten a nut and bolt through thermal expansion. This process, sometimes referred to as “heat and beat”, was done by heating the stud or bolt to cause the material to expand in length. You tighten the nut with a slug wrench and a large hammer.
Bolt Torque Method
Another method used to tighten fasteners is to apply torque. It is common to see stud sizes well above one inch in diameter, a mechanical torque wrench used by a single person cannot supply enough torque.
To achieve high torque values on studs of larger size, the hydraulic torque wrench was developed. Using hydraulic pressure, it transmits far more torque than a mechanical torque wrench. This enables better bolt loading of large studs.
Hydraulic Bolt Tensioning Method
An alternative to stud heating and hydraulic torque wrenches that has become accepted as a best practice is hydraulic bolt tensioning. Benefits include repeatability and accuracy, as opposed to the high variance in pre-load that exists with both the heat and slug wrench method, as well as the bolt torque method.
A standard hydraulic bolt tensioner fits over the stud and nut. Its threaded adapter grips the exposed threads above the nut.Using hydraulics, the stud is stretched, and the nut can be freely turned down to the flange face.
The problem of friction between threads is another area that is resolved by bolt tensioning. Friction develops in threads of the nut and bolt, as well as when the nut is being tightened onto the flange itself. This tends to gall the material. As a result, the nut can stick and needs to be cut off.
Even if the nut does not gall and stick, friction remains a problem. The amount of friction can vary from bolt to bolt which leads to preload variation. Friction also tends to cause the bolt to wind up, which relaxes over time and loses its initial clamping force. Hydraulic bolt tensioning eliminates this issue.
For applications that do not have enough spacing between the studs for a tensioner, or there are not enough threads exposed above the nut for a standard tensioner to grip onto, a hydraulic rod tensioner can be used. The hydraulic rod tensioner was developed for applications with a very tight radial foot print.
The hydraulic rod tensioner needs a foot print no bigger than that of the original heavy hex nut. This kind of tensioner allows the stud to be as close as possible to one another, which is often the case with many of today’s flange designs.
The perforated nut makes it possible to tension studs that were originally done with torque and therefore did not have the stud length to allow proper thread exposure above the nut for the standard tensioner to grip onto.
There are many different types of hydraulic bolt tensioners available on the market. Various supplier have developed tools which can be used in many different situations. As well as off-the-shelf items, there are specialty designs.