After working in this business for a long time, someone always asks with a drawing, "What level is this expansion joint?" Tsk, you think it's the same as a hotel rating, just a star? In fact, the level of metal corrugated expansion joint can't be explained clearly by a sentence of "several levels". To understand it, you have to start with a few hard indicators.
The core dimensions of level division: pressure, temperature and displacement, which one has the final say?
No dimension alone has the final say. The three parameters of design pressure, design temperature and displacement are mutually coincident. If you only give a pressure, like 1.6MPa, is the temperature 200℃ or 500℃? Is the displacement 10mm or 50mm? With the same corrugation thickness, the allowable stress of the material will drop when the temperature is high, and the bellows will not be able to stand it when the pressure is topped again.
A DN300 steam pipeline, design pressure 1.0MPa, temperature 300℃, axial compensation amount 30mm. If you only look at the pressure, just choose a general-purpose corrugated expansion joint and you can use it. But if you raise the temperature to 550℃ and the pressure remains unchanged, the wall thickness, number of layers and materials of the bellows will all change. Therefore, the level is comprehensively evaluated by three parameters, and one less is a rogue.
Looking at the levels from the national standards and industry norms: How do the products of general-purpose, high-temperature, power station, cement and other industries correspond to different levels
The national standard GB/T 12777 "General Technical Conditions for Expansion Joints of Metal Bellows" is the foundation, but you don't expect to find out the classification of "first class" and "second class" from the national standard. The real level division is more reflected in industry applications.
General corrugated expansion joints are generally used in pipeline systems with normal temperature and low pressure, such as air conditioning water and fire pipes. The high-temperature axial expansion joint is specialized in dealing with high-temperature media, such as steam pipe network. The corrugated expansion joint used in power station industry should meet the harsh working conditions of boiler smoke duct and steam turbine exhaust pipe. The corrugated expansion joint of the cement industry has to be dust-proof and wear-resistant, and deal with the flue gas at the kiln tail.
Therefore, if you ask "What is the level of this expansion joint", it is better to ask "What industry and working conditions do you use it in". The industry norms have long framed the working conditions, and the selection follows the industry.
It is also called expansion joint. Why can some of them be installed in power station units, while others can only walk through room temperature pipelines? The key lies in the number of corrugated layers and material selection
The answer to this question, when taken apart, is money and craftsmanship.
The more ripple layers, the better. The number of layers is more, and the flexibility is greater, but the pressure resistance will decrease. Therefore, in high-pressure working conditions, thick-walled single-layer or few-layer bellows are used instead; Large displacement, consider multi-layer. Some power station units with large diameter, high pressure and high temperature use double or even three layers of corrugation, and the interlayer pressure has to be considered between layers. Room temperature pipeline, one layer is enough, cheap.
The material is even more straightforward. 304 and 316L are conventional models, and Inconel625 and 800H are high-temperature and corrosion-resistant, and the cost has doubled several times. Two days ago, I met a customer who insisted on using 304 as an expansion joint at 550℃. I asked him directly, "Are you going to let it burn for two months and replace it with a new one?"
So don't ask why the price difference is so much. The number of corrugated layers and materials will pull the level apart.
Practical selection: Is it enough to just look at design pressure? How does the configuration of guide tube, tie rod and direct burial affect the level judgment
Many people choose the model with a pressure gauge, which is naive. Whether the guide tube is installed or not directly affects the medium flow rate and bellows wear. For example, the flue gas pipeline has a large dust content. Without a guide tube, the bellows will be worn out in two strokes. Equipped with a guide tube, at least the root of the corrugation can avoid particle scour.
Look at the pull rod again. The tie rod is not a decoration, it is used to restrain the blind plate force of the bellows. Choose expansion joints with tie rods, such as compound hinge transverse type or curved tube pressure balance type, which means that the system has specific requirements for displacement, or high pressure and large thrust. Does this count as high level? Calculate, because the configuration is complicated, and the difficulty of design and manufacturing has increased.
There are also directly buried (fully buried) expansion joints, which are buried underground. You have to consider external pressure, soil corrosion and waterproofing. This level of expansion joint, outer sleeve, anti-corrosion layer, special sealing structure, all are indispensable. If you take a general-purpose type and bury it directly in the soil, it will be too late to cry if the pipe network leaks in two years.
Don't ignore installation and maintenance: whether the screw is removed or not and how to adjust the tie rod nut directly determines whether the expansion joint can reach the design level
If you choose the right thing, if you pretend wrong, it will be useless.
During transportation and installation, there are generally adjustable screws on the expansion joints, which are used to tighten the bellows to prevent transportation bumps and pre-deformation during installation. After installing the pipeline, will the screw be disassembled or not? Many people don't dismantle it, thinking they keep an extra insurance. Wrong! If it is not disassembled, the bellows will be locked, and no displacement can be compensated. The expansion joint is equal to a rigid pipe. Experienced masters know that the screw must be loosened after the hydraulic test and before it is put into operation.
How to adjust the tie rod nut? Look at the design drawings for this. The role of the tie rod is to distribute displacement, or to limit the blind plate force. When adjusting the nut, both sides should be symmetrically loose, and one side should not be more and the other side should be less, otherwise the bellows will be crooked, the stress will be concentrated, and the life will be greatly reduced. So don't bother. Every step of installation and maintenance is linked to "level"-it may be okay to install low-grade products at will, but it will be wasted if high-grade products are installed wrong.
In the final analysis, the grade of metal corrugated expansion joint is not as simple as attaching a label. It is the result of the joint action of working conditions, materials, structure and installation.