Retrofitting Multijack Tensioners on a Combustion Gas Turbine Generator

Hydrocarbon Processing / January 2002

Consider this technology for difficult bolting applications

Global competition has been moving the hydrocarbon processing industry to ever-higher levels of efficiency and production rates. This has led to an increased demand on both equipment and manpower to produce higher reliability and shorter turnaround times. One major problem faced when dealing with high-temperature and pressure equipment is its bolting and unbolting. This is due to the very large torque required to properly seal the joint and the difficulty of its application, especially in the field.

One of main gas turbine generators in a large refinery has been notoriously known for being very hard to bolt and for developing leaks when bolted. A number of methods are available to help with bolting problems, but for this particular application none could be used. Multijack tensioners (MJTs) seemed to be very promising to help in alleviating some of these problems. Consequently, some replacement bolts were specified, designed, manufactured and installed on the unit to see if they could help with the problem.

The basics. Fig. 1 shows a sketch of a typical bolted connection. When the bolting is tightened, the main body of the bolt (or the stud if there is a stud and two nuts) is elongated. the increase in bolt length introduces tensional stresses into the bolt or stud. At the same time, the distance between the nuts decreases by the same amount. This introduces compressive stresses in the flange faces, which hold the flange together and prevent leaks.

The tensional stress in the stud is produced by the rotation of the nut or bolt. Essentially the bolt is a tool that converts the torque applied into stress in the stud or bolt. This induced stress is called preload. The torquing value required to achieve a certain preload varies and depends on many factors. This includes the friction coefficient between the nut and bolt, the cross-section of the bolts as well as many other factors. By far the biggest contributor to the required torquing value is the bolt or stud cross-sectional area. Fig. 2 and Table 1 show the different values recommended from one gas turbine manufacturer for casing bolts. These torquing values are representative of most other manufacturers as well as many local and indsutrial standards.

As can be seen from the table and the plot, the torquing value required to set a bolt to its proper preload is a very nonlinear function of the bolt diameter. While it only takes 369 ft lb to seat a bolt to 45,000 psi preload when its diameter is 1 in., it takes 3,313 ft lb (8.9 times as much!) to set a 2-in. diameter bolt to the same preload. In fact, the required torque is a third power of the bolt diameter if other parameters are kept constant.

Hand tools can be used to apply the required torque with relative ease and accuracy for smaller diameter bolting. However, as bolting diameter - and consequently torquing requirement - gets higher, methods for applying the required torque get more and more extreme. For example, it is quite common to use large sledgehammers and fieldmade "tools" to bolt and unbolt large joints. In fact, sometimes so much force is required that cranes and/or other heavy equipment have to be used to tighten large nuts or free seized bolts.

Apart from providing totally inaccurate preloading to the joint and possibly causing joint failure, these extreme methods have major safety concerns especially when the bolting is in confined or awkward areas. In fact, there are many reported cases of serious injuries as a consequence of trying to bolt or unbolt a large bolt or stud. In addition, the inaccurate, uneven and often insufficient bolting stress will sometimes lead to serious joint leaks that will be a major problem.

A number of methods have been proposed and used to help in solving or easing the large-diameter bolting problem. Some of the most common are:

• Bolt heating: In this method, the studs have a specially designed heating cavity in their center. The principle is to heat the bolt or stud using special heating rods and tighten it while it is hot. The rods are later removed and the preload will gradually develop as the bolt cools to the casing temperature. This methods, although helpful in many cases, requires special studs and heating elements and has a large torque requirement. In addition, torque accuracy can be limited in many cases, though it is generally better than standard bolting practices.

• Hydraulic stud tensioning: This bolt tightening method relies on stretching the bolt stud a certain amount to induce the proper preload in it. After that, the bolt or nut is tightened by hand until it touches the flange face. The bolting is complete when the hydraulic tension is removed and the nut holds the preload in the bolt. This method can be used fo rmore than one bolt at the same time, which can be very advantageous especially when bolting gasketed flanges. On the other hand, this method can be inaccurate (especially for short studs), and it can't be used for all types of bolting. In addition, special hydraulic tensioning devices and associated pumps and auxiliaries have to be purchased and maintained.

• Hydraulic bolting: It is identical to normal bolting except that the normal torque is applied using a special hydraulic mechanism. Again, this method works in many applications but can't always be used, especially when there are space limitations. Apart from not having a high accuracy of preload, they suffer from some of the same limitations of normal bolting including thread galling. And like hydraulic stud tensioning, there is the upfront cost of the hydraulic mechanism. This can be significant.

Multijack tensioners. Multijack tensioners (MJTs) are a relatively new method of trying to deal with the problems described in the previous section. MJTs are special patented design fasteners that replace existing bolts or nuts. The main idea behind MJTs is very simple: to tighten a number of smaller jacks instead of one large bolt or nut. Fig. 3 shows a cross-section of an MJT that will help to visualize how it works.

The figure shows a cross section of just one of the many forms of MJTs. The main parts are common to all MJTs and they are:

• A number of high-compressive strength jacking bolts that are tightened when the bolts are installed. these jacking bolts are embedded in the body of the MJT and push against the bottom washer.

• A special hardened washer located between the bolt and the flange body. The jacking bolts push against this washer when they are tightened, creating a gap between the bolt body and the washer.

• Some have internal threading that holds the stud like a standard nut. When the jacking bolts are tightened, the nut is pushed away from the flange. This causes the stud to be pulled to the proper bolt preload.

The advantage of MJTs is that the torque required to achieve the target preload using the jacking bolts is much less than that required to preload the big nut or bolt. Table 2 shows the jacking bolt torquing required verses the original bolt for one particular application. In addition, the last column shows the torque advantage, which varies from 26:1 to 273:1 for larger diameter bolting!

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