I. What exactly is the axial expansion scale saying? Don't just look at the numbers, understand the definitions
People who engage in pipeline design probably have a metal expansion joint axial expansion meter in their hands. To be honest, many people directly choose the model with the maximum axial compensation amount in the table-the larger the number, the better? If you do this, you will probably have to rework it later.
Axial expansion, to put it bluntly, is how many millimeters the expansion joint can "expand and contract" along the axial direction of the pipeline. When the metal bellows expands under heat, the amount of deformation between peaks and valleys determines this value. However, the amount of axial compensation given in the table is not for you to bring it up. It is the limit value measured at a specific temperature, pressure and fatigue life. For example, a general-purpose corrugated expansion joint is marked with an axial compensation amount of ±50mm, which is usually measured at 20℃, 0.1MPa and 1000 cycles of life. If you change to the 300℃ steam pipeline, the actual available compensation amount may be cut by more than half.
So before looking at the watch, ask yourself three questions: What is the thermal displacement of the pipe? What is the working pressure and temperature? How many cycles are required for design life? I don't know these, but the numbers are symbols on white paper.
2. How much information is hidden in a table? Disassembly item by item from nominal diameter to axial compensation amount
In a typical axial expansion gauge of metal expansion joint, the vertical axis is the nominal diameter DN, and the horizontal axis is usually the parameters of wave number, axial compensation amount, stiffness coefficient and effective area. Let's take a DN200, wave number 4 axial expansion joint to disassemble.
- Nominal diameter: DN200, which represents the inner diameter of the pipe of 200mm, but the actual flow diameter of the expansion joint may be slightly smaller with the guide tube.
- wavenumber: 4 waves, meaning the bellows has 4 full peaks and troughs. The more the wave number, the greater the axial compensation ability, but the stiffness will decrease, and it is easy to become unstable.
- Axial compensation amount: Write ±40mm on the table, indicating a maximum stretch or compression of 40mm in one direction. However, note that many working conditions are both stretching and compressing, with a total stroke of 80mm? No, the actual allowable total displacement is the sum of the plus and minus values, and there must be a safety margin.
- stiffness coefficient: For example, 80 N/mm, meaning that a force of 80 N is required per 1 mm of compression. The smaller this value, the "softer" the expansion joint, but also the weaker ability to resist internal pressure thrust. If the stiffness is ignored in the selection, the force of the pipe fixing bracket will be miscalculated.
- Effective area: Used to calculate the thrust generated by internal pressure, the formula is pressure × effective area. This area is not the cross-sectional area of the pipe, but the area corresponding to the average diameter of the bellows. It is usually given in the table, and if it is not provided, it will be calculated by itself? Honestly ask the manufacturer for it.
In addition, some tables also indicate the fatigue life rating, such as "1000 times" or "10000 times". For the same expansion joint, the axial compensation scale of ±40mm corresponds to 1000 times of life, and the scale of ±20mm may reach 50,000 times. Which do you choose? Look at actual demand. Chemical pipelines start and stop dozens of times a year, and 1000 times is enough; Peak shaving of power plant pipelines is frequent, so you have to choose higher.
Third, the easiest pit to step on during model selection: isn't the greater the axial expansion, the better?
Two days ago, I met a customer who chose an expansion joint for a DN600 steam pipeline. When he came up, he asked, "Is there any axial compensation of ±100mm?" I asked him what the actual displacement of the pipeline was, and he said, "I don't know, buy a big insurance first". Tsk, that's the typical myth.
A larger amount of axial compensation means that the bellows is longer and the wavenumber is higher. Problems: First, the overall stability of the bellows decreases-the bellows with excessive slenderness ratio may suffer columnar instability and wave crest distortion when under pressure, resulting in leakage. Second, the stiffness is too low, and the internal pressure thrust is all borne by the tie rod or the bellows itself. If the tie rod is not equipped, the bellows will be pulled open into a "sausage". Third, the length of the guide tube has to be lengthened, the overall layout of the pipeline becomes complicated, and the cost doubles.
What about that? The correct way to do this is to calculate the thermal displacement of the pipe first. Thermal elongation formula of straight pipe segment: Δ L = α × L × Δ T. α is the linear expansion coefficient of steel about 0.012 mm/m·°C. L is the length of the pipe section, and Δ T is the difference between the operating temperature and the installation temperature. For example, a 30-meter-long carbon steel pipe, with a temperature rise of 250℃, has a theoretical elongation of 0.012×30×250=90mm. This 90 mm requires an expansion joint to absorb, but the expansion joint usually absorbs only a portion, and the rest is shared by the pipe natural compensation or other compensator.
Therefore, the bigger the axial compensation amount is not the better, but just enough, while ensuring the balance of stiffness, stability and life. If you insist on limits, you have to chooseHigh temperature axial expansion jointOrExternal pressure single axial expansion jointThey are specially designed for large compensation and high-pressure working conditions, but they are also expensive.
4. How to use this table under different working conditions? Take the example of power stations and cement industries
Look at the power station first. The main steam pipeline of a thermal power plant has a working temperature of 540℃, a pressure of 10MPa, a pipe diameter of DN350 and a straight pipe section length of 50 meters. The thermal elongation is calculated to be about 300 mm. With such a large displacement, a singleUniversal corrugated expansion jointDefinitely not-no matter how big the axial compensation is, it is not that long, and the wall thickness of the bellows at 10MPa is very thick, and the stiffness is extremely great, so it is almost impossible to expand or contract. So it is generally divided into a plurality of compensation segments, each compensation segment uses 1-2Corrugated expansion joint for power station industry, with cold drawing or pre-drawing installation. At this time, the axial expansion gauge selects the compensation amount under the maximum tension force, and the cylindrical stability of the bellows under high pressure should be checked. Usually, the double hinge structure with tie rods is chosen instead of the pure axial type.
Look at the cement industry again. High temperature fan outlet on cement production line, flue gas temperature 350-400℃, large dust, pipe diameter DN2000. The displacement is mainly axial, but the amount of thermal elongation is not large (because the pipe section is short), plus the dust scour is serious. At this time the candidateMetal Corrugated Expansion Joints in Cement IndustryThe axial compensation amount on the table is usually only ±20mm, but the emphasis is not on the compensation amount, but on the wear-resistant design of the deflector and the bellows material (heat-resistant stainless steel 254SMO or Incoloy). If you only look at the axial expansion gauge and choose a small caliber according to the compensation amount, the dust wear will wear through the wave crest in three months.
You see that? The "usage" of the same table in different industries is completely different. Power stations look at pressure and fatigue, cement look at wear and temperature resistance. The table is just a starting point, and the final selection must be combined with the working condition parameters.
5. Axial expansion gauge is not a panacea: don't forget to cooperate with other compensators
In a pipeline, the displacement direction is often not simply axial. Lateral displacement, angular displacement, and even torsion can't be carried by axial expansion joints alone. For a chemical plant project last year, the customer insisted on usingUniversal corrugated expansion jointThe lateral displacement of the L-elbow of the pipe is absorbed, resulting in deformation and tearing of the side of the bellows. Later changedCompound hinge transverse expansion jointWith two axial types, the problem is solved.
Therefore, the axial expansion gauge is only the first level of model selection. When space is constrained or displacement is complicated, you have to match:
- Curved tube pressure balance expansion joint-Used to absorb axial and lateral displacement at the elbow without blind plate force.
- Straight Pipe Pressure Balanced Expansion Joint-the best choice to avoid thrust on straight pipe sections.
- Rotary compensator— — Used in long-distance directly buried pipelines to absorb large displacements.
- Non-metallic expansion joints (fabric fiber expansion joints)— — Low temperature, large displacement, low pressure pipeline, low cost.
In addition, don't forget that the parameters of the axial expansion gauge are measured under ideal conditions, and the actual installation error, bracket settlement and temperature difference gradient will make the actual displacement deviate from the theoretical value. Therefore, it is recommended to leave a margin of 10%-20% when selecting, but don't exceed 30%, otherwise the bellows is too long and will cause problems.
The Metal Expansion Joint Axial Expansion Gauge is a tool, not a Bible. Reading tables, calculating displacements, and matching other compensators are the real skills.