How to choose metal expansion joint for chemical plant? Understand these 4 hard indicators first
How harsh are chemical plant pipelines? Temperature, medium, pressure, any of them can kill you. Two days ago, I met a customer, who bought a general-purpose expansion joint for cheap and installed it on the sulfuric acid pipeline. As a result, it was perforated and leaked in three months-it was a mess. Find out the bottom line of the working condition before selecting the model, and don't use the general-purpose model as a panacea. The 'universal corrugated expansion joint' in our station is suitable for conventional steam pipelines, but when it encounters strong corrosive media, it depends on 'PTFE-lined hose' or 'PTFE compensator'. This thing is acid and alkali resistant, but the upper temperature limit is stuck, and it hangs when it exceeds 200℃.
1. Material and structure: 316L is just the beginning, and high chloride ion gets 2205
After doing this for a long time, I found that many engineers heard about metal expansion joints, and their mouths were stainless steel 304. But how is it that simple in a chemical plant? In the environment where the chloride ion content exceeds 200ppm, 304 will be stress corroded and cracked in less than half a year. Stainless steel 316L is the starting price, and further up, the high chloride ion environment gets 2205 duplex steel. Not only the material, but also the number of bellows layers, wall thickness and wave number directly determine the fatigue life-how to choose between single layer and multi-layer? Single layer has good flexibility and low cost, but poor pressure resistance. The multi-layer wall is thin, the wave number energy is more under the same outer diameter, and the absorption displacement is more flexible. High pressure occasions? Then use 'external pressure single axial expansion joint', the outside of the bellows is compressed, and the stability is good. What about high temperature and large displacement? The 'high-temperature axial expansion joint' specializes in this, and the structure is insulated, so that the heat will not be directly transmitted to the bellows. Don't get confused. If you choose the wrong parameter, the whole pipeline may be wasted.
By the way, the stiffness thing has to be counted. We have a question and answer "Stiffness and Calculation Formula of Bellows" on our station, which is very thorough. Chemical plant pipeline vibration is large, too high stiffness will transmit stress, too low and easy instability. Check it according to the formula when designing to save trouble.
2. Displacement mode: axial, transverse, angular displacement, or combined displacement?
The way of absorbing displacement is particular. Not all expansion joints can do everything at once. If a pipe has only axial displacement of thermal expansion and cold contraction, then a single axial type is sufficient. However, the pipe system of chemical plants is complicated, with a pile of corners, corners and branches, which often need to absorb lateral displacement and angular displacement. At this time, the 'double hinge transverse expansion joint' comes in handy. Two hinge groups can withstand transverse displacement without blind plate force. If the pipe system has limited space and has to absorb a large amount of combined displacement, then the 'double straight pipe bypass pressure balanced expansion joint' can handle it. Another headache-large-diameter, high-pressure pipes, blind plate force can push the pipe support down. What to do? The 'straight pipe pressure balanced expansion joint' counteracts the blind plate force by built-in pressure balancing element, so the equipment does not have to bear additional thrust. Is that the truth?
III. Installation and fixation: How to match the tie rod, guide bracket and fixed bracket?
Choosing the right model is only the first step. If there is a problem with installation and fixation, it will still be useless. We have a question and answer "How to adjust the tie rod nut of the expansion joint", which makes it very clear: the function of the tie rod is to limit the over-stretching of the expansion joint and bear the internal pressure thrust at the same time. During installation, the nut cannot be screwed to death, and the margin for cold or hot tightening must be allowed. The distance between the guide bracket and the fixed bracket must also be calculated according to the design, otherwise the pipeline will be unstable and the expansion joint will sway. In addition, the role of the guide tube is to prevent scour, especially when the medium contains particles or the flow rate is fast. We have an article "The Specific Function of the Expansion Joint Guide Tube", which is very thorough. Simply put, it is to protect the inner wall of the bellows from being worn. Chemical plant pipeline vibration is large, don't forget to calculate the stiffness, refer to 'Bellows stiffness and calculation formula'.
4. Decision list: first look at temperature, pressure and medium, and then calculate the displacement
First confirm the design temperature, design pressure and medium type (corrosiveness, chloride ion content, whether it is flammable or explosive), then calculate the displacement (axial, transverse and angular displacement magnitude and direction) that the pipeline needs to absorb, and then choose the model. For example, the 'general corrugated expansion joint' is used for steam pipelines, the 'high-temperature axial expansion joint' is used for high-temperature flue gas, the cement industry has a specially customized 'metal corrugated expansion joint for cement industry', and the power station industry corresponds to the 'corrugated expansion joint for power station industry'. This kind of industry-specific model is more tasty than the general model-the manufacturer has already figured out the working conditions. If you are really unsure, ask the manufacturer for a case first, and don't guess yourself. There are also 'metal hoses', 'rubber compensators' and 'non-metal expansion joints' in the product library of our station, each of which has its own applicable scenarios, but the core part of the chemical plant is mainly metal expansion joints.
Alas, a last long-winded sentence: don't bother. How to choose metal expansion joint for chemical plant? To put it bluntly, four words: take the right seat. Each parameter corresponds to a product model, and there is no panacea. Looking at a few more cases and asking a few more why is much more hassle-free than patting your head.