1. What exactly do bolts do in metal expansion joints? — Don't underestimate these fasteners
The metal expansion joint absorbs the thermal displacement of the pipe by the bellows, but the bellows itself will not grow on the pipe by itself. It has to be integrated with the pipe through end pipes, flanges or connectors, and then the bolts come into play. Don't look at it, it's just a few screws and nuts. If the expansion joint doesn't have them, no matter how well the bellows is designed, it will be useless.
Simply put, there are two things that the bolt does in the expansion joint: first, the flange of the expansion joint and the pipe flange are firmly pressed together to ensure that the seal does not leak; The second is to transmit the pressure thrust and blind plate force borne by the bellows to the pipe support. If you think about it, bellows expand and contract at the same time under high temperature. If the bolts can't hold it, and the flange loosens, the medium sprays out, that's no joke.
Therefore, the metal expansion joint bolt model is really not just a few screws and screw them on. The wrong choice can lead to air leakage or the whole expansion joint flying out of the pipeline. This kind of accident has happened many times in the steam pipe network.
Second, the basis for determining the bolt model: not the thread, but the force and displacement of the bellows
Many people choose bolts, and their first reaction is to look at the thickness and pitch of the thread. This idea is biased from the beginning. The core parameters of the bolt model are nominal diameter and thread specification, but what really determines how thick, long and high you should choose are the design parameters of the expansion joint itself: nominal diameter, design pressure, compensation amount, pipe thrust and medium temperature.
Take an example. The universal corrugated expansion joint is used on the air duct at normal temperature, and the pressure is only 0.1MPa, so the M16 bolt may be enough. However, it is also an expansion joint with a diameter of DN600, which is used in the steam pipe network. The pressure is 1.6MPa and the temperature is 350℃. The internal pressure thrust of the bellows can reach more than ten tons. At this time, M16 definitely won't work, at least M24 or even M30, and the number of bolts and the diameter of the distribution circle have to be calculated.
In addition, the force on the bolt is not only axial tension. When the lateral displacement is large, such as the lateral expansion joint of compound hinge and the expansion joint of large tie rod, the bolt will also be subjected to shear and bending. Therefore, the displacement, stiffness and elastic reaction force of the bellows must be calculated together when selecting the type. To put it bluntly, the bolt model is calculated by pipeline stress analysis, not measured with a caliper.
III. Common bolt materials and grades: from ordinary carbon steel to superalloy, what is the difference?
Bolt material, the most used on the market are carbon steel, alloy steel and stainless steel.
For normal temperature and low pressure occasions, such as ventilation ducts and flue gas ducts, carbon steel bolts of grade 4.8 or 8.8 are enough. Grade 8.8 is a high strength bolt with a tensile strength of 800MPa and a yield strength of 640MPa, which is cost-effective.
As soon as the temperature goes up, ordinary carbon steel won't work. Creep will occur above 200℃, and the preload force will soon drop out. At this time, you have to use 35CrMoA alloy steel, or 25Cr2MoVA, which can maintain strength at high temperatures. For example, corrugated expansion joints and high-temperature axial expansion joints used in power station industry, the working temperature is often above 500℃, and the bolt material has to lean on alloy steel or high-temperature alloy.
Stainless steel bolts are not a panacea either. 304 stainless steel has good corrosion resistance at room temperature, but in the temperature range of 400℃ ~800℃, carbides are easy to precipitate, resulting in intergranular corrosion and decreasing strength. Therefore, in high-temperature situations, it is more recommended to use high-temperature alloys such as Inconel X-750 and GH4169. Of course, the price is also good, with dozens of top carbon steel.
The material of the bolts and nuts should be matched. If the stainless steel bolt is matched with a carbon steel nut, the expansion coefficient of the two is different at high temperature, and it is easy to bite to death, and it can't be removed. Therefore, it is best to consider it in pairs when selecting models.
4. The most easily overlooked pits in type selection: gasket, preloading force, anti-loosening and installation sequence
Is it foolproof if you choose the right bolt model? Not. The following pits are often stepped on at the installation site of expansion joints.
The first pit: the spacer mismatch.The sealing surface of the flange of the expansion joint is pressed by the gasket, and the material and thickness of the gasket directly affect the preloading force of the bolt. Asbestos pad, PTFE pad, metal winding pad, each kind of gasket has different compression rebound characteristics. If you replace the PTFE pad with the pre-tightening force calculated by the metal winding pad, it may crush the gasket or it may not be sealed. Therefore, when the bolt is selected, the gasket must be set at the same time.
The second pit: preloading force depends on experience.Many installers screw bolts until they can't be screwed. That's not gonna work. The bolt is tightened too tightly, exceeding the yield strength and directly breaking; Screw too loosely, flange gap leaks media. The correct way to do this is to screw with a torque wrench according to the design preload, and at least two to three symmetrical screws. For example, the design preloading force of the 8.8-grade bolt of M30 may require 200kN. If you dry it in place at once, the flange will be unevenly stressed, and the bellows will also deform.
The third pit: not loosened.The expansion joint vibrates when it runs, especially the pipe section connecting the fan and pump. The bolts will loosen after a period of time, which is the result of metal fatigue plus vibration relaxation. Therefore, elastic washers, double nuts or anti-loose nuts should be used in high-temperature occasions, and spot welding should be done if it doesn't work. Don't expect the thread glue. When the working temperature of the expansion joint is high, the thread glue will be fully carbonized, so it is useless at all.
Pit 4: Installation order reversed.When the expansion joint is installed, the tie rod bolt, the transport support bolt and the fastening bolt are each of their own functions. When some manufacturers ship goods, the nut on the tie rod is locked to prevent the bellows from shaking during transportation. However, after installing the pipeline system, the tie rod nut must be loosened to the design position, otherwise the expansion joint is useless. If you don't figure out which is the tie rod nut and which is the mounting bolt, all are tightened, and the bellows will be directly damaged.
5. Talk about the actual matching in combination with the products of this station: what bolts should be matched with general-purpose type, high-temperature type, direct burial type, etc
Just talking about theory, it may still be a little drifting. Take several types of products in this site to actually talk about.
Universal corrugated expansion joint: This type is used in general pipelines, and the pressure and temperature are not high. The matching bolts are conventionally 8.8-grade high-strength bolts, made of 35CrMoA or 40Cr, and the surface is blackened or galvanized. M16 to M20 for small nominal diameter (DN50 to DN200) and M24 to M30 for large nominal diameter (DN1000 or more). Depending on the design pressure, M20 below 0.25MPa is generally enough.
High temperature axial expansion joint: Operating temperatures often exceed 450°C, even to 600°C. In this case, the bolt material must be upgraded to 25Cr2MoVA or GH4169 superalloy. The preloading force should also be allowed a margin according to the high temperature relaxation amount, otherwise the flange will leak after running for a month. The power station industry of this station uses corrugated expansion joints, which are matched with this kind of high-temperature bolts all the year round. Customers feedback that it is no problem to use three or five overhaul cycles.
Direct buried (fully buried) type expansion joint: This thing is buried in the ground, so the maintenance is very troublesome, and the bolt selection can't be made do. The directly buried expansion joint is in the soil corrosion environment, and the bolts must be made of corrosion-resistant stainless steel and cathodic protection. In addition, because it can't be maintained on the ground, the number of bolts is usually matched with one more turn, which is counted as redundancy. Once one is broken, it will not affect the overall seal.
Rectangular expansion joint: Flue gas ducts and air ducts are used more, the flange is a rectangular frame, and the bolts are distributed along the four sides. The pressure in this working condition is low, but the cross section is large and the force is uneven. The bolt should be selected longer to pass through the insulation layer conveniently. The matching gaskets are also exquisite. The flue gas baffle door and rectangular expansion joint are dusty, so the gaskets have to be high-temperature and wear-resistant ceramic fiber pads.
VI. Overhaul and replacement of bolts: when should they be replaced? How to judge failure?
Are the bolts disposable? Strictly speaking, high-strength bolts have been used once, removed and reinstalled, and their performance will be compromised. However, it is impossible to replace all of them on the actual site, so the key parts must be replaced.
How to judge that the bolts should be replaced? Three signals.
First, the tightening torque drops significantly. You tighten it with a torque wrench and find that the nut has turned several times, but the torque has not gone up yet, which means that the bolt has stretched and is close to yielding. This kind of change directly.
Second, the threads are corroded or bitten. Especially the bolts of high-temperature steam pipelines, when removed, the threads are covered with oxide scale. The friction coefficient of this bolt has changed, and the preloading force control is not accurate, so it must be replaced.
Third, disassemble and assemble more than twice. The same bolt, you remove and install it again, and then remove and install it again, its fatigue life has almost been consumed. Especially when there is vibration, it will break next time.
In addition, as a reminder, it is best to change bolts in groups, not just the broken one. The stiffness of the old and new bolts is inconsistent, which will lead to uneven force on the flange. The old one is not broken but has been loose, the new one is tightly screwed, the flange is deformed, and the bellows is unlucky.
For routine maintenance, it is recommended to check the pre-tightening force of the expansion joint bolts at least once a year. Focus on the bolts near the peak position, where the stress is the greatest. If the nut is found to have loose marks, mark it, observe it for a week after re-tightening, and change it if it is loose again.
After all, the job of metal expansion joint bolt model looks simple, and the door inside is no less than choosing bellows. Before selection, list the working temperature, pressure, displacement, media corrosion and installation environment clearly, and then ask the manufacturer for the bolt specification table. Don't pat your head. In case you are unsure, ask the manufacturer's engineer directly, which is better than reworking afterwards.