Will the bolt become tighter or looser as the temperature rises?

Thermal tightness is due to the uneven expansion coefficient of bolts and nuts when heated. The nut expands when heated and the screw stretches when heated, causing the tightening force to become smaller. ； If there is thermal tightness, there will be thermal looseness. ； Thermal loosening occurs when the nut shrinks during the cooling process, and the screw also shrinks, causing the tightening force to increase. This is usually done before the pipeline or equipment is completely cooled. ； Thermal loosening usually occurs before the device is shut down and overhauled, and thermal tightness usually occurs after the device is fed and overhauled. ；

When the bolt is heated, it will elongate under the action of pre-tightening force.

This post was last edited by lupg on 2022-3-28 17:45 1. Flanges, gaskets, bolts, and nuts have complex stress relationships. It is not appropriate to simply evaluate the state of the bolt when hot-tightening the bolt, but to balance the deformation coordination between the various components. 2. The state of the bolt after heat-tightening is tentatively given. First, a set of combinations is given to observe the simple relationship between different combinations affected by thermal expansion. 2.1. Carbon steel bolts and nuts + stainless steel flange tips: Carbon steel has a lower thermal expansion coefficient than stainless steel ； Cold preloading, flanges and bolts and nuts are not insulated, and the working temperature is higher than normal temperature 2.2. Carbon steel bolts and nuts + carbon steel flanges are cold preloaded, flanges and bolts and nuts are not insulated, and the working temperature is high pressure and normal temperature 2.3. Stainless steel bolts and nuts + carbon steel flanges are cold pretightened, neither flanges and bolts and nuts are insulated, and the working temperature is high pressure and normal temperature 2.4. The above combinations but the flange is insulated, and the bolts are not insulated 2.5. The above combination but the flange bolts are all insulated 2.6. The above combination is in windless condition 2.7. The above combination is in strong wind condition 2.8 ...... and so on 3. Under the above different combinations, the elastic modulus of the metal material, the resilience performance of the gasket, and the deformation of the connecting thread of the bolt and nut are not considered. The relationship is complicated enough 4. Give an example: Stainless steel flange + carbon steel bolt cold preloading without insulation for high temperature operation, without considering other factors: As the medium temperature rises, the expansion of the flange is higher than that of the bolt, which is equivalent to thermal tightening. ； On the contrary, under the same working conditions, the carbon steel flange + stainless steel bolt combination reaches equilibrium after heat tightening. When the internal medium cools down rapidly, the bolts will loosen and leak. 5. After metal components are stressed, they will tend to relax even if the temperature does not change. We have conducted experiments.: After tightening the bolts, remember the load value (measuring means are required). When you come back after a full meal, you will find that the bolts are loose. This slack has a certain proportion and needs to be tightened again or even three times to reach a relatively stable value. 6. Now we prefer to preload the bolt in place at one time. The preload load of the bolt is increased from the conventionally recommended 20% yield strength to 50% or even higher, and then the insulation is insulated without the need for a heat tightening process. 7. After selecting a reasonable load value, it is easier to achieve a reasonable and uniform bolt load with cold preloading, which is more reasonable than the hot tightening process that easily breaks the load uniformity. The slightly higher cold preload load takes into account the influence of other factors that are difficult to predict.

Cold tightness follows hot tightness, and the main reason is the large temperature difference between the installation temperature and the operating temperature. For details, please refer to the relevant requirements of SH3501 ； Taking into account the thermoplastic and cold-brittle properties of metal, firstly, the state of the material must be fully considered during design and the right material must be selected. ； Second, do not use the wrong bolts during installation. ； Third, even in the process of hot tightening and cold tightening, we cannot blindly "make miracles with great force" to avoid plastic deformation or cracking of the bolt until it breaks. @Fish in the desert

When the heat is tight, workers often bring a hammer.

1. Have you ever experienced overheating or tightness in hydrocracking high-pressure equipment or pipelines? Our experience is: The pressure test after installation must be passed once before hydrogen can be introduced. This is a typical example. Since hydrocracking can do it, other devices with mild conditions can do it better. 2 Provide two curves: 2.1 One part is the load distribution of each bolt during the pretightening process. Round after round of tightening, the bolt loads are finally within the required range. 2.2. The other part vividly reflects the bolt load range. The elliptical shape indicates the unevenness of each bolt load. When the pretightened bolt load is at a low level, as the temperature rises or the pressure fluctuates, the load area deviates and approaches the minimum load. When the load zone exceeds the minimum load, leakage occurs. 3. The phenomenon observed by desert fish friends reflects the current situation of many operators. After this thermal tightness, the load area may be moved to the right, but the ratio of the long and short axes of the ellipse is larger, which means that the bolt load tends to be more uneven and more likely to cause leakage in subsequent operations. Of course, those with more experience will get better results. 4. Nowadays, various petrochemical companies have gradually promoted more reasonable methods in major overhauls, investing costs but also achieving good results. Although the propaganda caliber is different, the principle is to use a higher load and achieve even distribution.

lupg friends, judging from the training materials, three months ago, we should still belong to the same barrel of oil. Let me share my opinion. In 15 and 16 years, I worked in a refinery doing maintenance work, including one major overhaul and one minor repair. I was responsible for the daily maintenance of four sets of heavy oil, diesel, gasoline and cracking units. ； Among them, the heavy oil reactor and high-pressure heat exchanger are the focus of every maintenance. ； 1, 7, or 8 years ago, constant torque value fastening technology and equipment were fully popularized. However, some equipment is very old, and many equipment can no longer verify the original fastening data. At the same time, some old comrades do use "strength to create miracles" during maintenance management and do not pay attention to scientific fastening methods. ； 2. Due to maintenance quality or daily maintenance problems, high-pressure bolts often "seize" and can only be cut off and replaced. New and old bolts may also have uneven loads. ； 3. During the driving process, the system heats up. Pressure fluctuations will also have a certain impact on the sealing system, which may cause loosening. ； Our general approach: 1. Fastening with constant torque value ； Of course, we won’t go into details about the conventional operations such as tightening sequence. ； 2. Heat tighten and still tighten according to the fixed torque value. At this time, you will find that the bolts do become loose. Practice has proved that heat tightening is indeed necessary. 3. In the diesel hydrogenation unit, there is a diaphragm-sealed high-pressure heat exchanger imported from Italy. The manufacturer has a complete set of tightening sequences and torque requirements. It is super-torque tightened, so no heat tightening was performed in the end.

friend: 1. Let’s first shake hands with the colleagues we once had: handshake 2. Sealing leakage is the most troublesome problem for users. This is not due to technical difficulty, but to fixed thinking and lack of money. 3. Dating back to my generation or the previous generation, the relationship between flange sealing has been basically clear. We have promoted it for at least 20 to 30 years, but it was covered up by the word "poor". Because of poverty, it is acceptable in the thinking of management decision-makers * * A small fire alarm, but it is difficult to accept the slightly higher cost to solve the installation quality. 4. Times are developing, and thinking is updated. Now from the top to the grassroots, the awareness of spending money to buy high-quality services and quality is gradually established. Also benefit from * * The general environment in terms of safety and environmental protection has changed. 5. Another: Note the fixed torque value and super-torque value you mentioned. If there are effective measurement methods to obtain the details of the tightening process, you may realize the limitations of using torque as the basis for judgment. We are fortunate enough to be exposed to and experience the detailed changes in the bolt tightening process outside the door sill. We understand that the "super" of super torque is only a relative comparison with the conventional torque value, not the original meaning of the word "super". 6. To install "high-quality services", foreign companies will first enter and gradually open up the domestic market, and the country is quickly following suit. I hope that domestic enterprises can support this big market, so that friends like you and other "petrochemical construction technology and management" people can have a greater space to display their talents. 7. Construction is a hard and technical job. To get into this profession, you must be able to smoke and drink well: lol