Why Bolt Tightening is Extremely Important in Equipment Maintenance
In equipment maintenance, bolt fastening is an extremely important thing. Many equipment failures are simply due to ignoring the bolt fastening during equipment maintenance. For example, the flange bolts of wind power towers are broken, petrochemical equipment leaks, etc.
Therefore, in this article, TorcStark will explain why bolt tightening is extremely important in equipment maintenance through the following aspects.
Accidents due to bolt-tightening problems
Through the following two cases, you will see major accidents caused by bolt-fastening problems. When fastening bolts, few people study the problem of choosing the solution, what tools to use, and how many turns to fasten. However, ignoring this small link will lead to various problems with the equipment. British UKOOA has made statistics that 81% of the 100 flange leakage accidents are caused by incorrect bolt load, that is, pre-tightening force. If these little bolts can be tightened well, the benefits will be considerable.
1. Wind turbine collapse due to broken bolts
Fans should be checked and maintained frequently during operation.
There is such a case.
The fan issued multiple alarms during operation and shut down quickly due to high vibration values. After the maintenance personnel dealt with it, it was reset and started again. After 24 hours, the wind turbine collapsed. The middle and upper sections of the tower, the fan nacelle, and the hub are laid flat on the ground, and the upper section of the tower is twisted and deformed in the middle. The connection between the gearbox and the hub spindle sleeve was broken, the gearbox coupling was broken, the blade was broken from the edge, and a large amount of filler was scattered on the ground.
2. CNC machine breaks down due to bolt fastening problems
A CNC high-speed end-face cylindrical grinder in a certain processing plant broke down. The amount of movement of the grinding wheel does not match the value displayed on the monitor, resulting in errors in processing. During the on-site disassembly and maintenance of the entire mechanism, the maintenance personnel found that the fixing devices of the two sets of guide rail balls were damaged, causing the balls to spill, and causing many scratches and small pits on the guide rails.
After technical analysis, inspection, and discussion, it was finally found that the parallelism of the two guide rails had an error of 14 microns. The reason for the error is that when the machine tool manufacturer fastened the cross linear guide rail with bolts, they did not crush it and adjust it evenly, which gradually caused uneven force on the linear guide rail, resulting in balls falling and failing.
Factory leakage pollutes the environment due to bolt-fastening problems
With the large scale of petrochemical plants and the rapid development of factory production, more and more people pay more and more attention to the problems of pipeline components and pressure vessels caused by improper tightening of bolts. Leakage not only causes material loss and environmental pollution but may also cause huge casualties.
In addition, because many factories have a large number of leaks that are difficult to find with the naked eye, the probability of various accidents in the production process has increased. Especially in recent years, major accidents such as explosions, fires, equipment damage, product scrapping, casualties, and environmental pollution caused by equipment leakage have never been interrupted, which has brought great harm to industrial production, economic development, environmental sanitation, and personal safety.
Obviously, many of these problems are caused by flange sealing problems. For example, VOC leakage in the petrochemical industry is difficult to detect with the naked eye. Because many factories do not use professional tools to fasten bolts for the purpose of saving costs, this is completely worthless.
Get the most out of bolt fastening
Factors that affect flange sealing include bolt pre-tightening force, gasket performance, sealing surface form and accuracy, mechanical properties of flanges and bolts, and operating conditions of equipment. Among them, the bolt preload is the most.
The feed and reaction product heat exchangers of the diesel hydrotreating unit in the reforming aromatics workshop of the refinery of the petrochemical company were installed and put into use during the overhaul in 1998. The reaction products are diesel, gasoline, hydrogen, hydrogen sulfide, and ammonia. An octagonal gasket is used between the tube box and the tube sheet flange and between the shell and the tube sheet flange. The operating temperature of the medium in this sealing part is 330°C. During the start-up and stop of the device, when the raw materials of diesel and jet fuel are switched, and when the operating pressure and temperature of the device fluctuate, the seal between the pipe box and the flange is prone to leakage and smoke. The petrochemical company then optimized the bolt tightening. After optimization, the “zero leakage” goal was achieved with good results.
The coking tower and pipeline flanges in the coking workshop of the refinery are prone to leakage due to large daily temperature changes (from room temperature to above 300°C). The flange of a coking plant near the distillation line used to leak every few months. Since the bolts were fastened with the pre-tightening force control technology, no leakage occurred during the next 3 years of operation.
The oil slurry steam generator heat exchanger of the 3 million tons/year heavy oil catalytic cracking unit of the petrochemical company. In the past, in order to prevent leakage, disc springs were installed under the nuts on the tube side and shell side flanges of the oil slurry generator, and then tightened with a hydraulic wrench to set torque. However, the seemingly perfect fastening has hidden dangers. In the following years, there have been many major accidents such as leakage and fire of the two oil slurry generators. Later, the bolt fastening technology was used for transformation to solve the leakage problem, and no accidents due to bolt fastening problems occurred in the biennial operation cycle.
Essentials of bolt fastening: tensile yield and preload calculations
The bolt-tightening process is generally divided into 4 steps:
1. Determine the bolt tensile yield.
2. Calculate the axial preload (0.7 to 0.9 times the tensile yield).
3. Determine the torque value corresponding to the optimal preload load based on data and experience.
The first two steps of the fastening process are look-up table calculations, basically, there is no big problem. The first step is to determine the tensile yield of the bolt, which is related to the material of the bolt, and the relevant data can be found in the mechanical manual. There are two types of calculation of the pre-tightening force in the second step: the first calculation of the pre-tightening force of steel structures, the international authoritative standard is VDI2230. Calculation of the pre-tightening force of the pressure vessel and pressure pipeline flange of the second type of sealing gasket.
There are various errors in the last two steps due to the difference in the selection of fastening tools and methods, which brings many hidden dangers to the safety of the equipment.
In order to solve the above-hidden dangers, the fastening process has been continuously improved and developed in recent decades, from the initial manual fastening to pneumatic, electric, and hydraulic fastening, great progress has been made. (The commonly used tools are pneumatic/electric torque wrenches, hydraulic wrenches, bolt tensioners, etc. These tools can set accurate torque values.)
The traditional fastening methods such as hammering with a sledgehammer and manpower pulling the lever are completely judged by human feeling. The size of the torque cannot be measured, and there is no precision at all. The fastening effect can only be hoped for by the personal experience of the operator.
Although the initial tightening process of pneumatic, electric, and hydraulic wrenches can set the torque to turn the bolts, there is an unbalanced load because the reaction arm is needed to balance the driving force. As a result, the actual frictional resistance when different bolts are tightened is quite different, and the actual frictional resistance when each bolt is tightened cannot be accurately measured, so the value of the pre-tightening force is naturally unknown. In addition, to tighten or loosen the bolt’s torque, the bolts may be damaged due to the torsional force that the screw bears during the fastening process.
As for hydraulic stretching and heating stretching. Because it is necessary to overstretch the bolt, the estimated value of overtension and the influence of manual tightening of the nut. Coupled with the influence of various application factors, working conditions, bolt and nut fit tolerance, verticality, and friction coefficient, the accuracy of bolt fastening becomes difficult to control and unsatisfactory.
For different bolts of the same flange. If the pre-tightening force is uneven, the bolt with a relatively small pre-tightening force will become the Achilles’ heel, or it will leak from this point, or the bolt will loosen or break due to the large alternating stress, causing the equipment to vibrate aggravated, or even damaged.
The problem of bolt fastening is one of the important causes of leakage and other accidents. In the development of fastening technology in recent decades, people are actively discovering and improving the shortcomings of existing technologies, such as the accuracy, life, and ease of use of existing products. On the road of bolt fastening, TorcStark is also one of them. Like other well-known companies, it is also actively developing new products and technologies. Through cooperation and exchanges with various factories, TorcStark improves its products and technologies, and strives to protect the environment and reduce factory losses.