Introduction: Why can't the boiler pipe be separated from the metal expansion joint
The matter of thermal expansion and contraction looks simple, but it is a big deal to put it on the boiler of a power station. Boiler body, air flue, steam and water pipe, which one is not a temperature difference of several hundred degrees? If the pipeline does not give room for expansion and contraction, it will be pulled hard, and the flange will leak at least, and the equipment support will be directly cracked at least. Power station boiler metal expansion joints do just that-absorb displacement, cut vibration, and release thermal stress. If you choose the wrong model or install it incorrectly, you will wait and see. The bellows will crack, the weld will leak, the maintenance period will be delayed again and again, and the power generation will suffer.
First, look at the working conditions first and then talk about the model: temperature, pressure and medium determine everything
Some people ask "Is there an expansion joint of DN800?" This question can't be answered. Expansion joints are not randomly picked according to the caliber of standard parts. You have to find out what medium, temperature and pressure go in the pipeline first.
In the high-temperature flue gas section, when the temperature goes up at 700℃, ordinary 304 stainless steel bellows can't bear it, so high-temperature resistant alloys, such as nickel-based alloys such as Inconel 625, or heat-resistant materials specially equipped with high-temperature axial expansion joints, have to be used. As for steam pipelines, the pressure is high and the temperature is high, so the number of bellows layers should be calculated clearly-the number of layers is not enough, and the pressure is not enough; There are too many layers, and the compensation amount is affected. When it comes to the import and export of the desulfurization tower, the environment is even more headache. The flue gas contains SO₂ and chloride ions, which is very corrosive. Ordinary stainless steel is pitted and perforated within two years. At this time, either choose a corrosion-resistant alloy or add an anti-corrosion layer. High-temperature axial expansion joints commonly used in power station industry and corrugated expansion joints used in power station industry are used in this way.
To put it bluntly, the more complete the working condition parameter table is, the more accurate the manufacturer's selection. Temperature, pressure, medium, displacement, installation position, don't save any.
Second, the structural form is not determined by patting the head
A lot of engineers like to ask "which expansion joint is best", and honestly, there is no best, only the most suitable. The structural form depends on how the pipeline goes and in which direction it is displaced.
The straight pipe section is simple, and the axial expansion joint directly absorbs the expansion and contraction along the pipe direction, which is the most common. But what about the pipe bend? The axial type is not strong, so it is necessary to use the double hinge transverse expansion joint or the curved tube pressure balance expansion joint to absorb the transverse displacement. If it is a large-section rectangular flue, such as the inlet and outlet section of desulfurization tower, the circular expansion joint can't be installed at all, so metal rectangular expansion joints must be installed, and all four sides can absorb the displacement. In the case of vibration in the air-cooled island vacuum pipeline, the ordinary structure can't bear the repeated lateral swing, so it is necessary to use the double hinge expansion joint of the air-cooled island vacuum pipeline. The two hinges share the displacement and have much better stability.
Also consider the reaction force. The advantage of pressure balance expansion joint (straight pipe pressure balance type, curved pipe pressure balance type) is that the internal pressure thrust cancels itself, and the force will not be transmitted to the fixed bracket. If your pipe bracket is designed to be economical, then don't hard on the ordinary axial type, otherwise the thrust is fully pressed on the bracket, and the bracket will have problems sooner or later.
3. Guide tube, liner and baffle door matching: Don't ignore the details
The expansion joint body is selected correctly, and the matching components are not worry-free. The guide tube looks like a tube, but its actual effect is much greater-it guides the high-speed flowing medium past, preventing the medium from directly washing the bellows, and the life of the bellows can double. Especially in the air flue, the dust particles are very abrasive, and without the guide tube, the bellows can be worn out in a few months.
In addition, the flue gas baffle door and the desulfurization flue gas baffle door are often arranged in cooperation with the expansion joint. As soon as the baffle door is opened and closed, the thermal displacement of the pipe changes. If the expansion joint is not coordinated with the position of the baffle door, the pulling stress will be concentrated on the bellows when opening and closing. Choosing an electric plug-in insulation door or a manual plug-in insulation door also depends on the frequency of operation and maintenance space-electric is labor-saving but expensive, while manual is cheap but has to leave a place that people can reach.
4. Several points that are easiest to overturn in installation and operation and maintenance
No matter how well the expansion section is selected, it will be useless if the installation goes wrong.
Let's start with pre-stretching. The difference between the design temperature and the installation temperature is how much, and the pre-stretching amount is calculated according to this. If you are not cold tight when it should be cold tight, the force of the bellows during operation will not match the design working condition, and the fatigue life will be directly discounted. There is also the tie rod nut-it is locked during transportation, and it should be loosened according to regulations after being installed in place. Many people forget this step, and the expansion joint becomes a "dead joint", which can't be compensated by any displacement. When and how much nuts are removed, the installation instructions are clearly written, that is, no one reads them.
Many people think that the direction of the arrow on the expansion joint is the direction of airflow. In fact, it is often the mark of the medium flow direction or installation direction. If the bellows is connected backwards, the stress state will change. Whether the on-site welding joint should be misaligned and whether the adjustment gap should be reserved, these details have to be kept an eye on.
I met a customer two days ago, saying that the bellows cracked after less than a year of use. Go back and check the installation records-no pre-stretching was done at all, and the tie rod nut was not loose. Alas, can the expansion joint be blamed for this?
Conclusion: Some practical advice for procurement and engineers
When selecting the metal expansion joint of power station boiler, remember these items: First, fill in the working condition parameter table, and don't miss any of the temperature, pressure, medium composition, displacement direction and installation position. Second, let the manufacturer select the model according to the power station standard. Don't take the universal corrugated expansion joint hard sleeve, use the high-temperature axial type for the high-temperature section, use the metal rectangular expansion joint for the rectangular flue, use the double hinge for vibration, and use the pressure balance type for high thrust. Third, check the product name and corresponding parts. Metal hoses, non-metallic expansion joints and rectangular expansion joints have their own uses, so don't mix them. Finally, make a judgment based on the on-site maintenance space-the distance around the expansion joint must be allowed for people to go in and work, so that the maintenance will be enough for you.
The expansion joint looks inconspicuous. If it is used well, it will ensure your stable operation. If it is smashed, it will cause you trouble every day. It is much more cost-effective to take some time to understand the selection thoroughly than to remedy it after an accident.