First, when the power station pipeline "expands and contracts thermally", how does the metal expansion joint carry down?
High-temperature steam, hot water and even flue gas run in the power station pipeline, and the temperature soars to five or six hundred degrees at every turn. The pipeline stretches as soon as it is hot, and retracts as soon as it is cold. This expansion and contraction amount can reach tens of millimeters at least, and hundreds of millimeters at large. Hard carry? The weld cracked and the flange collapsed. What to do? Relying on electric metal expansion joints-to put it bluntly, it is a section of corrugated pipe, which uses the elastic deformation of the corrugations to absorb displacement.
But don't underestimate this bellows, it doesn't just roll up a sheet of iron. Under the working condition of power station, the expansion joint should bear internal pressure, thermal stress and bending moment pushed by pipeline at the same time. The material is usually a high-temperature resistant alloy such as 304, 316L or Incoloy 825, and the number of corrugated layers is calculated according to the pressure and displacement, from two to three layers to more than ten layers. The core logic is one: exchange flexibility for security. Bellows turn the thermal expansion and contraction of rigid pipes into their own elastic deformation, so that the pipeline system will not be "stuck" by itself.
Second, in the power station scenario, which metal expansion joints are the main force? - -From high temperature axial type to double hinge transverse type
The power station pipeline does not go straight to the end, but has straight pipe sections, turns and equipment interfaces. Different positions have different displacement directions, and the types of expansion joints selected are completely different.
- High temperature axial expansion joint: This is the most commonly used variety for power stations. The straight section of the main pipeline, with high temperature and high pressure, mainly absorbs axial displacement. Many of the "corrugated expansion joints for power station industry" in our station have this structure, with guide tubes to prevent high-speed steam from washing the corrugations.
- Compound hinge transverse expansion joint: Use this when the pipe needs to be turned or when the space is limited and the axial type cannot be freely expanded or contracted. Two sets of bellows are equipped with hinge structure, which specially absorbs lateral displacement and does not produce blind plate force. Air-cooled islands and desulfurization tower inlet and outlet pipelines are common.
- External pressure single axial expansion joint: The main steam pipeline with particularly high pressure is more stable with external pressure type, and the outside of the bellows is protected by an outer sleeve, so it will not be unstable.
- Straight pipe pressure balanced expansion joint: If there is no thrust next to valves and pumps, the pressure balance type can be used to offset the internal pressure thrust.
- Large tie rod/duplex hinge: There is also a kind called "double hinge expansion joint of air-cooled island vacuum pipeline", which specifically solves the displacement absorption of vacuum pipeline and prevents vacuum instability.
You see, the selection is not a pat on the head, you have to know how the pipeline goes, which direction it displaces, and what the pressure and temperature are. Two days ago, I met a brother who overhauled a power plant and said that they used to use the axial type for all the pipes, but as a result, two pipes were cracked at the turn. Tsk, choosing the wrong type is digging a hole for yourself.
Third, don't step on pits when selecting models: pressure, temperature, displacement... and attention to guide tube and tie rod
The selection parameters are simple, but there are pits everywhere in actual operation. Let's talk about pressure first: the design pressure must be greater than the highest working pressure of the system. Don't forget to add the hydraulic test. Many expansion joints collapse during pressure test. The temperature is more critical. Every time the bellows material rises by 100℃, the allowable stress may be reduced by half. If it is above 400℃, you have to use heat-resistant alloy. Don't use 304 hard to carry it for cheap.
The displacement calculation is even more interesting. You think it's just a thermal stretch? In fact, we should also consider the amount of cold tightness during installation. For example, the operating temperature of the pipeline is 500℃, and the ambient temperature during installation is 20℃. At this time, you pre-stretch part of the displacement, and the force of the expansion joint at high and low temperatures will be more uniform. Another easy overlook is the deflector-don't think it's just a "bushing". In addition to flow guidance, the guide tube can also reduce the direct scouring of the ripples by high-speed airflow and avoid flow-induced vibration. In our question and answer, we specifically talked about "the specific function of the expansion joint guide tube". Its two ends should be sealed well, otherwise the medium will rush into the corrugated gap and accelerate corrosion.
The pull rod is also learning. How to adjust the tie rod nut in the expansion joint of the large tie rod? When leaving the factory, it is generally pre-tightened. After installation, the locking nut should be loosened according to the actual displacement of the expansion joint, so that the pull rod can slide freely and only play a guiding role, otherwise the pull rod will limit the displacement. Last time, there was a project, and the installation team didn't dismantle the transportation fixing screw, but spot welded it directly. As a result, the trial operation stuffed the bellows. Do you think it was wrong?
4. Installation and maintenance methods that only veteran power plant drivers know
Expansion Joints Can't Hold Pipe Weight! It can only absorb displacement and cannot be used as a bracket. Therefore, guide brackets and fixed brackets must be installed on both sides of the expansion joint to ensure that the expansion joint only deforms along the design direction when the pipeline expands and contracts. Second, pay attention to the direction of the arrow when installing. The arrow on the expansion joint points to the flow direction of the medium. If the direction of the guide tube is reversed, it will recoil and wear out quickly.
Maintenance of this piece, many people think that the metal expansion joint is "maintenance-free", which is pulling. During the inspection, focus on whether there are cracks, corrosion pits, or obvious asymmetric deformation on the corrugated surface (indicating that the force is deviated). Then, whether the tie rod nut is loose, and whether there are any wear marks on the outlet side of the guide tube. The maintenance cycle of power plants is generally based on overhaul, but key positions, such as the expansion joint on the main steam pipeline, should be visually inspected every time the machine is shut down. After the hydraulic test, don't forget to release the accumulated water in the cavity, especially in northern winter, I have seen many frozen and cracked bellows.
Although the nameplate on the expansion joint is made of iron, it has the design pressure, manufacturing date and fatigue life number. Record the number of cycles of actual operation for each overhaul. If it is close to the design value, it is time to consider replacing it with a new one.
V. A real power station case: What have we learned from choosing the wrong expansion joint to shutting down for emergency repair?
Let's tell the truth. Last year, there was a biomass power plant with a 135MW unit. The design temperature of the main steam pipeline was 540℃ and the pressure was 9.8MPa. They chose an ordinary general-purpose corrugated expansion joint without a guide tube. As a result, half a year after it was put into operation, the corrugated pipe cracked, and the steam was directly sprayed out. It was shut down for 72 hours for emergency repair. After dismantling, it shows that the inner wall of the corrugation is washed by the steam flow, leaving only one layer of skin. The lack of the guide tube causes high-speed steam to directly beat the root of the corrugation. In addition, the material is 304, which has insufficient temperature resistance and fatigue cracking after softening.
Later, it was replaced with our "corrugated expansion joint for power station industry". The corrugated material was upgraded to Inconel 625, a guide tube was added, and the external pressure single axial structure was selected, and the pressure level was right. It has been more than two years now, and there is no problem at all. So you said that when choosing a power metal expansion joint, which one is cost-effective compared with the saved money and the non-stop loss? A non-stop power generation loss, equipment repair cost, and dispatching assessment, at least tens or millions.
The working parameters are given accurately, the structure is selected correctly, and the materials are not compromised. Everything else is false.