1. The core basis of grading: pressure, temperature and compensation amount
After doing pipeline compensation for so many years, the most asked thing is: "How to divide the grade of metal expansion joints?" In fact, in the final analysis, there are three hard indicators: pressure, temperature and compensation amount. These three parameters basically lock in which level of product you should use.
Let's start with stress. The wall thickness, number of layers and wave height of bellows used in low pressure (0.1~0.6MPa) and high pressure (≥2.5MPa) are completely different from the same concept. For example, the general corrugated expansion joint has a general design pressure within 1.0MPa; However, the corrugated expansion joint used in the power station industry is 2.5MPa or even higher at every turn, so the corrugated pipe must be multi-layered, reinforced ring, and even armored. The pressure level directly determines the safety margin and fatigue life.
The temperature is tougher. 304 stainless steel can hold up to 100℃ at room temperature, but above 600℃, you have to use superalloys (such as Inconel 625), and you have to consider creep strength. Two years ago, a cement factory customer installed a high-temperature axial expansion joint on the kiln tail flue, with a nominal temperature of 800℃. As a result, ordinary 304 material was selected, and it cracked in three months-the wrong temperature grade was selected, which was equal to dry for nothing.
The amount of compensation is interesting. Axial compensation, lateral compensation and angular compensation, the compensation capabilities corresponding to different structural forms are far different. The straight pipe pressure balance expansion joint mainly absorbs axial displacement, while the compound hinge transverse expansion joint specializes in transverse displacement. The bigger the compensation amount is not the better. A large compensation amount means that the bellows is longer and has more wavenumbers, so the stiffness will come down and it is easy to become unstable. Therefore, when selecting a grade, the compensation amount must match the stiffness.
2. Looking at the grade from the standard: national standard, industry standard and manufacturer definition
When it comes to standards, many purchases say "just follow the national standard", but the national standard is actually very fine. The national standard for metal expansion joints is mainly GB/T 12777 (General Technical Specifications for Metal Bellows Expansion Joints), but don't expect this standard to cover all scenarios. For example, for metal corrugated expansion joints in cement industry, you have to refer to JC/T 967 (metal corrugated expansion joints for cement industry pipelines); Corrugated expansion joints used in the power station industry will involve DL/T 5185 (Technical Regulations for the Design of Bellows Compensators for Pipelines in Thermal Power Plants). The non-metallic expansion joint has JB/T 12235-2015, which is different from metal and non-metal, and the grading logic is different.
Manufacturer-defined grades are even more interesting. Domestic first-line brands, such as the products listed on our station, all have internal code names. For example, the general-purpose corrugated expansion joint is divided into PN0.6, PN1.0 and PN1.6 according to the pressure when it leaves the factory; However, it is also PN1.6. Different manufacturers may have differences in the number of bellows layers, the wall thickness of the end tube and the material of the guide tube. Therefore, it is not enough to only look at the "pressure level" of the manufacturer's trademark, but also the corresponding structural details.
For a desulfurization flue gas baffle door project, "Expansion joint grade: PN1.0, temperature resistance 200℃" is written on the drawings of the design institute. Results During on-site installation, it was found that there was a large amount of condensed acid in the flue gas, and 304 couldn't bear it at all. Finally, it was replaced with PTFE-lined hose, and the grade was redefined from the perspective of materials-this incident reminds us that the standard grade is only the foundation, and the "soft parameters" of medium corrosiveness and medium phase state are the real hidden grade lines.
3. Typical products corresponding to different grades: from general-purpose to power station/cement industry specific
Don't copy books about grading, it depends on the actual application scenarios. Let's pull out the product line and compare it:
- General scenario of low pressure and low temperature:Universal type corrugated expansion joint, metal hose, rubber compensator. The design pressure of these products is generally ≤1.0MPa and the temperature is ≤300℃, which is suitable for HVAC, water supply and drainage, and general industrial pipelines. Typical features are single-layer bellows, no guide tube or simple guide tube, cheap and frequent change.
- Medium-voltage and medium-temperature utilities:High temperature axial type expansion joint, external pressure single type axial type expansion joint, sleeve type pipe expansion joint. These products can have a pressure of 2.5MPa and a temperature of about 450℃, which are commonly used in steam pipelines and thermal pipe networks. Note that there will be a guide tube here, whose function is to prevent the high-temperature medium from directly washing the bellows-the specific function of the expansion joint guide tube, as specifically mentioned in our Q&A, is the key component to protect the bellows from high-speed fluid wear.
- High pressure and high temperature working conditions:Corrugated expansion joints for power station industry, metal corrugated expansion joints for cement industry and directly buried (fully buried) expansion joints. The pressure of the main steam pipeline of the power station is 10MPa, and the temperature exceeds 600℃. It must be designed with multi-layer bellows + external pressure balance. The flue gas temperature at the outlet of rotary kiln in cement industry is high and dusty, so it is necessary to use wear-resistant guide tubes and high-temperature resistant alloys.
- Special Structure:Double hinge transverse type expansion joint, curved tube pressure balance type expansion joint, air-cooled island vacuum pipe double hinge expansion joint. These are in the "custom-on-demand" rating, there are no standard models, and they must be designed one by one according to the pipe stress analysis.
Can I use the general-purpose one for power station projects? Of course not, let alone that industry standards don't allow it, and the materials themselves can't bear it. The level is wrong, it either leaks or explodes, and there is no intermediate option.
4. Pits that are easy to step on when selecting grades: the influence of material, stiffness and guide tube
Material selection only depends on pressure and not temperature. The common mistake is the stainless steel bellows of PN1.6, which works at 500℃, and thinks it's okay-in fact, 304 starts to accelerate oxidation above 425℃, so it is recommended to use 321 or 347. Therefore, when selecting a grade, the material grade must be confirmed separately.
Ignore stiffness. The stiffness and calculation formula of bellows are explained in detail on our site, but many selection personnel only look at the compensation amount. When the compensation amount is large, the stiffness will be small, and the force feedback of the pipeline on the equipment will become larger, which may pull out the equipment joint. This thing of stiffness determines that the expansion energy saving can't be "soft" enough to absorb displacement without transmitting excessive thrust. If conditions permit, let the manufacturer provide the stiffness value, and then do the pipeline stress calculation.
The guide tube is reversed or not installed. The function of the guide tube is not only to guide the flow, but also to reduce the fluid excitation and prevent the deposition of particles. However, in some projects, in order to save costs, the guide tube was removed. As a result, the inner wall of the bellows was blown away by high-speed steam, and it leaked in one year. Conversely, the deflector cannot be thickened at will-too thick will increase the local stress and lead to a decrease in the fatigue life of the bellows. It's a balancing point.
Two days ago, I met a customer who made gas turbines, and chose a special vacuum hose for use in the exhaust pipe. The grade was enough, but there was no guide tube. As a result, the particulate matter in the flue gas wore the bellows out. Alas, such lessons are all too common.
5. Practical suggestions: How to determine the grade of expansion joint according to working conditions
After so much verbose, here are some steps that can be used directly:
Step 1: List the working condition parameters.Media, pressure (design/operating pressure), temperature (design/operating temperature), type of displacement (axial/transverse/angular) and amount of displacement, nominal diameter of pipe.
Step 2: Determine the pressure rating.According to the table of GB/T 12777, find the number of bellows layers and wave height corresponding to DN and design pressure. Note: The allowable stress will be reduced at high temperature, so the pressure level should be corrected according to the high temperature working condition.
Step 3: Select the structural form.According to the displacement type: axial displacement, select straight tube pressure balance expansion joint or general type; Lateral displacement selection compound hinge transverse type; Pressure balance type of angular displacement curve selection tube; For large diameter and low pressure applications, sleeve-type pipe expansion joint or multi-wave type can be considered.
Step 4: Check the material..Temperature Step 5: Confirm whether you want a guide tube.If the medium flow rate is> 10m/s and contains particles, or the medium temperature is> 400℃, a guide tube must be added. The material of the guide tube is generally the same or higher than that of the bellows.
Step 6: Find the manufacturer to confirm the stiffness.Especially in scenarios where the pipeline is sensitive to the thrust of the equipment, such as connecting steam turbines and compressors, the greater the stiffness, the greater the thrust, which may push the base of the equipment.
In fact, how to divide the grade of metal expansion joint? In the final analysis, it is these three dimensions: load (pressure + temperature + displacement), environment (corrosion + wear + scour), and function (safety + life). You sort out these three points, and you can't go wrong with your selection. If you are still worried, just talk to our engineer with the parameters. Don't slap your forehead to set the grade-after all, the cost of installing this thing and then dismantling it is not a little.