What exactly is a guard plate? Don't confuse it with bellows
Many people who are engaged in equipment procurement only stare at the bellows when they mention the expansion joint-the material, wave number, number of layers and wall thickness, and they are clear. But when asked about the guards, eight out of ten couldn't tell. This part of the guard plate usually refers to the guide tube inside the expansion joint, the wear-resistant liner plate or the external protective cover. It directly faces the medium scour, high temperature and high pressure, which is equivalent to putting a layer of armor on the bellows. Take usCorrugated expansion joint for power station industrySpeaking, there are many particles in the flue gas pipe, and there is no guard plate, so the bellows will be worn out in a few months. Do you think the weight of the guard is important? But the reality is that many people only care about the parameters of the bellows when selecting the model, and don't know the material, thickness and connection method of the guard plate.
Block the medium from directly impacting the bellows, guide the airflow to pass smoothly, and by the way, it can also reduce vortex erosion. But just such an "armor", if you choose or install it incorrectly, the equipment will still be scrapped in advance.
How to choose the material? Stainless steel 304, 316L, 321? Or the more temperature resistant Incoloy 825?
This depends not only on the temperature, but also on the medium. Two days ago, I met a customer of a cement factory. The flue gas contained sulfur and dust, so I chose an ordinary 304 guard plate, which was corroded and perforated in half a year. Later, we put on ourMetal Corrugated Expansion Joints in Cement IndustryThe matching wear-resistant and corrosion-resistant guard plate has been used for more than two years without any problem. Why? Because there are sulfide in the flue gas, 304 can't withstand that corrosive environment, so you have to use 316L or higher grade alloy.
When the temperature exceeds 600°C,High temperature axial expansion jointThe guard plate also has to consider the thermal expansion gap. Think about it, metal will expand at high temperature. If the guard plate is stuck to the bellows, once the heat expands, the guard plate will directly jam the bellows, and the whole expansion joint will be wasted. Therefore, you have to leave a reasonable gap when designing. How to calculate this gap? It must be determined according to the thermal expansion coefficient of the guard plate material and the actual working temperature. Also, something likeDesulfurization flue gas baffle doorThe nearby expansion joint has not only sulfur, but also chloride ions in the medium. 304 can't bear it at all. Let's honestly go to duplex steel or nickel-based alloy.
Don't just look at the upper temperature limit. For example, 321 stainless steel has higher temperature resistance than 304, but it still pits in chlorine-containing environment. Although Incoloy 825 is expensive, it is cost-effective under strong corrosion conditions-after all, the labor and downtime loss of changing the guard plate is far greater than the price difference of materials.
Doorway in Guard Design: Length, Thickness and Fixing Method
If the guard plate is too short, the medium directly impacts the root of the bellows, which is equal to no protection; Too long adds weight and cost, and may interfere with the internal flow channel of the pipe. For example,Corrugated expansion joint for power station industryFor flue gas pipelines, the length of the guard plate should generally cover the whole length of the bellows plus the balance of 50~100mm, but the specific calculation depends on the medium flow rate and particle size.
Thickness is also not the thicker the better. You likeDesulfurization flue gas baffle doorThe nearby expansion joint, the pipeline itself may also be accompanied by vibration. If the guard plate is too thick, it will increase the rigidity and affect the flexible compensation ability of the expansion joint. Usually 3~6mm is enough, and 8~10mm can be achieved in severe wear conditions, but it should be considered overall when designing.
The fixing methods are insert type, welded type and bolted type. The plug-in type is the most hassle-free, directly inserted into the flange mouth, but easy to loosen; The welding type is reliable and can be cut during maintenance, which is time-consuming and laborious. Such asManual plug-in insulation doorIf the guard plate of the pipe next to it is welded to death, it will take half a day to cut it once, and the worker master can scold his mother. The bolt connection type is convenient for disassembly and assembly, but the bolt head exposed in the flow channel will form spoiler and accelerate wear. Therefore, the real professional approach is to choose according to the frequency of maintenance: those that need to be replaced frequently are connected with bolts, and the bolts sink into the inner surface of the guard plate; Welding for long-term immobile working conditions.
Differentiated configuration of guard plate under different working conditions
You likeDouble hinge expansion joint for air-cooled island vacuum pipelineThe guard plate should take into account both vacuum sealing and low temperature resistance. Under vacuum conditions, if the guard plate leaks, the vacuum degree of the whole system can't be kept. Moreover, the air-cooled island is in the north, and it is minus 30 or 40 degrees in winter. Ordinary carbon steel guard plates will be brittle and cracked as soon as they are frozen, so materials with good low-temperature impact toughness, such as 16MnDR or 304L, have to be used.
Compound hinge transverse expansion jointThe guard plate is more troublesome, because this expansion joint has to absorb multi-directional displacement, and the guard plate is designed to not interfere with the bellows. If it is slightly deviated during installation, the guard plate will hold against the bellows during operation, resulting in local instability. Tsk, these details are clearly written in the design materials, but they are often overlooked during on-site installation.
AndDirect buried (fully buried) type expansion jointThe guard is buried in the ground, and it can't be replaced even if the rust is worn. Therefore, the guard plate itself should be externally anticorrosive, coated with epoxy coal pitch or coated with anticorrosive layer. Some manufacturers try to save trouble and directly use ordinary carbon steel. As a result, it rotts from the outside to the inside in two years, and the whole pipe has to be dug out.
The easiest pits to step on during installation and maintenance
The guard is installed in the reverse direction-the arrow points in the direction of the medium flow. This common sense question of principle is still wrong. Think about it, if the guard guide tube is installed backwards, the medium directly impacts the trough of the bellows, which is equal to no protection. In a previous project, the installation team reversed the direction of the guard arrow, and the bellows was torn after three months of operation, and the rework cost more than 100,000 yuan.
When adjusting the expansion joint tie rod nut, is the reserved gap between the guard plate and the bellows sufficient? During an overhaul, the guard plate deformed the top of the bellows because the thermal expansion was not counted correctly. The pull rod plays a limiting role. When adjusting the nut, ensure that the guard plate does not contact the bellows under all working conditions. Generally, leave a gap of 2~5mm during cold installation, which is calculated according to the thermal displacement.
Check the wear degree of the guard plate regularly, and don't wait for it to be worn through before replacing it. At that time, the bellows may have become partially unstable. How to check? The simple method is to measure the thickness of the wear-prone part of the guard plate with a thickness gauge, or simply install a few wear indicator holes. Especially those with high dust conditions, such as flue gas pipelines in cement plants and power plants, it is recommended to check them every six months. Before the guard is worn out, the bellows may carry for a while, but once the guard fails and the bellows are exposed to high-speed particle flow, the instability rate will accelerate exponentially.
So, don't underestimate this steel plate. The metal expansion joint guard plate is selected and installed correctly, and the equipment will run for ten or eight years without any problem; If you choose the wrong one, you will have to remove it and replace it in half a year. Next time you buy expansion joints, remember to ask more questions: What material is the guard plate made of? How thick? How to fix it?