Large bolts/nuts that are more than one inch in diameter are often difficult to install and disassemble. Super bolt multi-push pretensioner technology can help users solve many problems such as the installation, disassembly and precise pre-tightening of large bolts/nuts.

1 Overview

The multi-push pre-tensioner can be installed with only a common torque wrench. It does not require traditional bolt fastening methods such as sledge hammering, heating, hydraulic stretching, and hydraulic wrenches. Really save time, effort and money. This article will expand the principle of multi-push pre-tensioner technology, detail its technical characteristics, and specifically share some cases of applying this technology on the valve.

2 technical principle of multi-push pretensioner

People usually use ordinary nuts and bolts to connect parts, but if the diameter of the nuts and bolts is greater than 1", it will be very difficult to use a torque tightening method to generate sufficient bolt load to tighten or loosen the bolts. The torque required for conventional screws and nuts increases in three dimensions with size. T=(C)D3

Where T = torque, C = constant, D = bolt diameter. For example, a 1" nut requires a mounting torque of 365 lb.ft, and a 4" nut requires a torque of 37850 lb.ft.

To overcome this major problem, there are many bolting solutions available on the market today. Unfortunately, these solutions can cause many serious problems, including work injuries, inaccurate preload, thread wear, difficulty in disassembly, equipment costs and extended downtime.

The design of the multi-push pretensioner is very useful, it can directly replace the traditional nuts and bolts. These pretensioner units can be screwed onto a brand new or existing bolt, stud, screw or shaft. The main thread is used to place the pretensioner on the bolt or stud and against the hard washer and bearing surface. Once in place, the actual preload of the bolt or stud is achieved by a simple hand tool to tighten the jackscrew that surrounds the main thread one turn. The jackscrews evenly transmit the preload to the main thread and, thus, to the connector. The main thread can be locked by pure tension.

As shown in Figure 1, a strong top thrust (axial) is created by tightening the jackscrews. This top thrust acts directly on the hardened washer. The jacking screw has a small friction diameter, so that a relatively high torque can be achieved with a relatively small torque. The load is transferred to the nut body that has been manually mounted to the main thread. A hard washer is used to transmit thrust while protecting the flange face. The (axial) top thrust of many jackscrews together with the reverse force of the main bolt head creates a strong clamping force on the flange. The top thrust (axial) of the jackscrew produces a force of the same magnitude on the main bolt. In this way, the pre-tightening of the entire large bolt can be achieved by applying a relatively small torque to each of the jacking screws.

(figure 1)

Top ten advantages of 3 multi-push pretensioners

3.1 Only manual tools are required, and a standard torque wrench can be used to produce a higher bolt tension than any conventional fastening method.

3.2 Save time and effort: Don't worry about the number of large bolts that need to be installed. It is easy to install and disassemble. It can be operated by many people at the same time. The initial stage of installation can also be accelerated by pneumatic wrench.

3.3 Fast fastening: As long as the torque is correct, even in the vibration, pulse, or reciprocating equipment, the multi-stage thrust pre-tightening bolt will never loosen, greatly reducing the downtime caused by the above problems.

3.4 Pure tension fastening: Because it is fastened with pure tension, it avoids the clasp and thread lock, and no need to worry about picking up the bolt.

3.5 Safety of use: It avoids the dangers often caused by many traditional fastening methods and can be installed in various unsuitable positions, such as over-the-top installation, the top of large equipment and the installation of nuclear islands with harsh environment.

3.6 Pre-tightening accuracy: precise tightening control is possible. This is critical for a gasket with a gasket that maintains a uniform bolt tension on the flange.

3.7 Suitable for small spaces: It is also possible to design various multi-stage thrust pre-tightening bolts that meet the space limit according to the customer's special use environment. And because the installation tool only requires manual tools, even if the space is small, it does not affect the installation.

3.8 High overall elasticity: The multi-stage thrust pre-tightening bolt has the characteristics of high elasticity, which can increase the elasticity of the bolted joint. This connection can withstand high temperature and impact more effectively.

3.9 Avoid bolt breakage: When the push bolt is tightened, the nut body is slightly deformed, and the stress concentrated on the first few threads is released. Accurate load control and stress relief characteristics avoid screw breakage.

3.10 Economical and practical: Push bolts are simple mechanical parts that are economical and reusable compared to other fastening methods.

4 multi-push pretensioner on the valve

4.1 Ball Valve Application Case

The super bolt multi-push pretensioner has been used on ball valves for many years. Most of the time, these pretensioners were suddenly installed during a reinstallation. A major practical and innovative strategy for using superbolts is to reduce the amount of repair time spent on downtime.

Part of the planned downtime is to take the ball valve offline and then use the shortest downtime to do some turbine maintenance. A customer's plan is to replace the standard nut on the original ball valve that needs to be installed and disassembled with a hydraulic tensioner, and replace it with a super bolt multi-push pretensioner before stopping. In order to achieve this goal, the customer purchased a batch of multi-push pre-tensioners of (145) 4-1/2” – 8 tpi three months before the planned downtime, and for a period of several months. The bolts on both sides of the valve were all assembled using 2-3 days of neutral time. During the spring shutdown, 4 workers (two workers on one side) put all these 4-1/2" pretensioners in two and a half. Disassembled within hours! It saves 2-3 days compared to the previous hydraulic method. This will also save time when reinstalling the valve. Because this is a pumped storage facility, the factory will sell electricity to supplement its other steam power plants during the peak hours of the day. Therefore, it is necessary to minimize downtime for these facilities. Use super bolt products to help them achieve this. Significant savings are achieved by reducing downtime. The super bolt pretensioner is not only accurate but also reusable, providing continuous loading on all studs, which also saves time. Initially, the elongation of these studs needs to be monitored by an extensometer. After seeing the repeatable results of the super bolt pretensioner after each installation, the remaining studs do not need to be further monitored. This is different from the hydraulic method.

As shown in Figure 2, the bolt axial stress in this case is 55,000 psi. The preload is 813,000 LB and the torque of each jack screw is 190 ft.lb.

(figure 2)

4.2 Quick valve application case

Regular inspections of valves can affect production efficiency and result in reduced production. Preheating the bolt can take a long time, especially if the amount of elongation required for the bolt is not possible. Therefore, it is sometimes necessary to repeat the two or three pretensions on the bolt. Can't achieve fast, simple and reliable results.

Super bolt high temperature mechanical pretensioners have been able to replace expensive cap nuts since 1990. They are easy to use and ensure that the valve is precisely pre-tensioned every time. Existing holes in the bolt can be reused. In order to release the pretensioner more quickly, the holes can be heated briefly to reduce the initial torque loss.

As shown in Figure 3, a customer installed 40 SH-M80x6/W super bolt multi-push pretensioners for their quick valves.

(image 3)

4.3 Turbine Globe Valve Application Case

Previous problems include steam leaks and bolt thread seizures. The steam leak is due to the fact that when the bolt is installed using the torque method, the bolt is not sufficiently stretched and preloaded, resulting in insufficient preload. Threading causes the bolt and nut to be repaired or replaced.

Nuts up to 4 inches in diameter are very prone to thread wear or seizure when installed with a heating rod or hydraulic wrench. Another possible disadvantage mentioned by steam turbine engineers is that the surface of the thread decarburizes when the stud is heated, which increases the chance of stud fatigue fracture and stress corrosion cracking. There are three studs on this valve that have not been ultrasonically tested due to cracks in the threaded portion. It takes a lot of effort and time to remove the stud and replace it.

Solution: Replace all studs with super bolt multi-push pretensioners. The stud is directly stretched using a multi-push pretensioner. Since the stud is not forcibly "screwed" into the valve housing, much friction is reduced and it is more convenient to replace the stud in the future.

As shown in Figure 4, the bolts for the two valves were installed using two workers at the same time. It took a total of three hours to install. This installation time even includes a timeout to check the amount of stud elongation. The stretching of the studs can be repeated. The torque of each jacking screw on the final multi-push pretensioner is only 155 ft. lbs. This torque is expected to achieve a stud elongation range of 0.019" to 0.024".

Disassembly: After 18 months of operation, the stud removal is also easy and quick. It took only two hours to disassemble. The pretensioner can be reinstalled after re-cleaning and lubrication, and the pre-tightening effect is as it is.

(Figure 4)

5 Conclusion

Due to its special principle and technical characteristics, the super-bolt multi-push pre-tensioner has a significant advantage in the installation and disassembly of larger size bolts. There are usually larger bolts on the valve. In the case where the traditional method is not ideal, it is recommended to use the super bolt multi-push pretensioner to solve the problem.

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