Two days ago, a customer asked with the drawings, saying that he wanted to change the expansion joint, but he didn't know how to report the parameters. When he asked, he threw over a blurry picture of the scene. I've seen this way too many times. The expansion joint looked like a bellows, but if the parameters were not selected correctly, it would bury a mine if it was installed. Today, break it apart and crumble it into pieces to make it clear-what are the detailed parameters of the expansion joint? Remember the following 7 items, and you can also be a half-expert.
1. Pipeline diameter and connection method: first fix DN, everything else is easy to say
There's no room for negotiation on caliber. DN50 is DN50, DN2000 is DN2000, the wrong measurement is not a joke. Especially large-diameter thick-walled expansion joints, which can't be used when made, and scrap iron can't be sold at a high price.
The flange connection is convenient to disassemble and assemble, but there are more leakage points with more flange surfaces; Welding the connection once and for all, but it can be difficult to cut off the expansion joint during overhaul. Choose flange or welding, ask your on-site maintenance worker's opinion, it is more effective than flipping the design specification.
2. Compensation amount: axial, horizontal and angular, don't just stare at one number
When many people report the parameters, they say "compensation amount 50mm"-axial 50? Horizontal 50? How many degrees in the angular direction? Something completely different.
Axial displacement is the pipeline expanding and contracting along the axis direction, lateral displacement is the pipeline running sideways, and angular displacement is the pipeline turning at an angle. The universal corrugated expansion joint mainly absorbs axial displacement, while the compound hinge transverse expansion joint specializes in dealing with transverse displacement. If it is at the corner of the pipeline, it has to rely on the angular direction.
If you think about it, as soon as the main steam pipe heats up, displacements in three directions tend to exist simultaneously. Only staring at one number report, who will carry the remaining two directions?
3. Pressure and temperature: four numbers, none of which can be less
Design pressure, test pressure, operating temperature, instantaneous temperature-these four items are the lifeblood of expansion joints.
The design pressure is the maximum pressure that the bellows can bear. The test pressure is generally 1.5 times the design pressure, and each unit must be hit before leaving the factory. The working temperature determines what material the bellows uses, instantaneous temperature, how much the temperature can soar when you drive and stop in your factory, you have to tell the truth. Before, I met a cement factory customer, the working temperature was 200℃, and the instantaneous temperature rushed to 450℃. The direct material of 304 bellows was deteriorated, so it was better to change it to 316L. You said that this parameter is not complete, who dares to do it for you?
4. Corrugated material and number of layers: How to choose 304, 316L and Inconel
This piece of material, 304 is cheap, 316L is resistant to chloride ion corrosion, and Inconel alloy carries high temperature. For corrosive medium in desulfurization pipeline, it is correct to choose 316L; The gas turbine exhaust temperature is five to six hundred degrees, and Inconel can withstand it.
The number of layers is interesting. Under the same pressure capacity, multi-layer thin-walled bellows have longer fatigue life and better flexibility than single-layer thick-walled bellows. However, one more layer will make it more difficult to process, and the cost will rise. Many of the corrugated expansion joints used in our power station industry are multi-layered, but the cement industry is uncommon-the working conditions are different.
5. Stiffness and fatigue life: These two determine how long your pipeline can last
The rigidity of the bellows is too great, the thermal deformation of the pipe can't be absorbed, and the force is all transmitted to the support; The stiffness is too small, and it can't hold up the internal pressure. How do you give this number? Generally, it is calculated by the manufacturer according to the EJMA standard, and you will have the measured stiffness value on the parameter table.
There is a concept in the design of fatigue life called "equivalent cycle times". Designing 1000 times does not mean that it can be used 1000 times-if the actual displacement in each operation is smaller than the design value, it may be okay to use it tens of thousands of times; On the contrary, if it exceeds the design displacement, it may crack hundreds of times. This is not a math problem, it is an empirical account.
6. Guide tube, tie rod and pressure balance: accessory parameters really can't be saved
What does a deflector do? When there is high-speed airflow or particulate media in the tube, the corrugations will be directly washed away. A guide tube was added, and the wear and tear were all carried on it, and the bellows were clean.
The function of the tie rod and the nut is to limit the displacement of the bellows, which is used to adjust the pre-deformation during installation, which really needs to be adjusted. But note that the tie rod is not designed to absorb lateral displacement-that's what hinges and gimbals do.
There are also internal and external pressure balance expansion joints, such as straight pipe pressure balance type, whose core function is to eliminate blind plate force. If you choose an ordinary expansion joint, install it near the fixed bracket, and directly overturn the bracket during the pressure test of the pipeline, then you will really cry without tears.
7. Implementation standards and special working conditions: Act according to rules and do not step on red lines
For metal corrugated expansion joints, look at GB/T 12777 in China, and EJMA for design calculation. Non-metallic expansion joints follow JB/T 12235-2015, and there are rigid regulations on materials and test methods.
There are more rules for special working conditions. Flue gas baffle door and expansion joint in desulfurization system, corrosion resistance is the first priority; The expansion joint of the cement kiln tail, high temperature and wear resistance is the core; The vacuum double hinge expansion joint used in air-cooled pipelines of power stations should also consider the vacuum degree. Like a mountain, professional manufacturers are used to which industry, you better find deep cultivation in this field.
After so much verbose, the core is one sentence: What are the detailed parameters of the expansion joint? Pipe diameter, compensation amount, pressure and temperature, number of material layers, stiffness life, accessories, standards-seven things, each falling on the paper, the manufacturer can make something reliable for you. When you select the model next time, sort out these seven parameters before calling, and the efficiency will at least double. "