1. What exactly does the expansion joint of the metal beater do? First distinguish the difference between it and ordinary corrugated expansion joint
The wave beater is the forming equipment for producing corrugated pipes, and the expansion joint of the metal wave beater is the corrugated compensator pressed out on this equipment. To put it bluntly, is it the same thing as a regular corrugated expansion joint? Not. The difference lies in the emphasis of the process: the expansion joint formed by the wave beater has a more regular wave shape and a more uniform wave pitch, and its fatigue life is usually higher than that pressed by hand or simple molds. Look at those corrugated expansion joints used in power stations and cement plants. Why do you emphasize beating machine molding? Because once the waveform is deviated, the stress concentration point will appear, and as soon as the pipe heats up, cracks start to grow from there.
Therefore, the first question in the selection is not to ask the price, but to ask: Is this expansion joint waveform molded as a whole by a wave beater, or is it welded in segments? The fatigue life difference between integral molding and welding can be two or three times, especially under high frequency thermal cycling.
Second, material and structure are the lifeblood: how to match stainless steel type selection, wave number design and temperature and pressure resistance
The material piece, the common 304, 316L, 321 and Incoloy 825, each has its own job. Flue gas pipeline with 316L resistance to chloride ion corrosion is fine; The cement industry has heavy dust and heavy wear, so it is necessary to consider the lining wear-resistant guide tube; High temperature steam pipe, 321 or Inconel 625 is more stable. Don't ask "304 is OK" as soon as you come up-you have to find out the three things of medium temperature, pressure and corrosiveness first. If you lack one, the manufacturer can't match the wavenumber for you.
The more the wave number, the greater the compensation amount, but the pressure resistance decreases instead, because each wave is sharing the pressure. If you are under high pressure, the wavenumber will have to be less; If the amount of compensation is large, it is necessary to lengthen the bellows or use a duplex structure. A customer asked about 1.6MPa steam pipeline, and insisted on a single expansion joint to compensate 100mm. This requirement itself was unreasonable, and finally it was solved by using a double hinge transverse expansion joint. Therefore, the selection should not only look at the compensation amount, but also report the pressure, temperature and displacement direction to the manufacturer together.
Third, the installation position is wrong, no matter how good the expansion joint is, it is useless: common mistakes and on-site cases
I met a customer two days ago, and the expansion joint was installed backwards. The direction of the guide tube should follow the flow direction of the medium, and he installed it in the reverse direction. As a result, the guide tube was directly lifted off by the airflow, and the bellows was eroded. It leaked in less than three months. This question is a cliche, but the scene is just repeated mistakes.
The fixing bracket wasn't done. If there are no fixed brackets at both ends of the expansion joint, the thermal expansion of the pipe will hard-pull the expansion joint, and the bellows will be stretched to the limit. Do you think it can not be broken? In a case of a cement factory, a non-metallic expansion joint was installed at the outlet of the fan. They thought that the expansion energy saving absorbed all the displacement, so they didn't install the guide bracket. As a result, the lateral displacement tore the fabric layer. Before installing the expansion joint, calculate the stress distribution of the pipeline first, which can't be saved.
When hoisting, the steel wire rope is directly sleeved on the bellows, which deforms the peak pressure. This kind of damage can't be seen on the surface, and it leaks when pressed. It is a cliche but always overlooked to protect the bellows part when installing it.
4. Don't just focus on price: how to calculate the general ledger of life, maintenance cycle and spare parts cost
What are purchasing most afraid of? The unit price is cheap, and you can change it after two years. Let's calculate the account: for a high-temperature axial expansion joint, the price difference between the cheap one and the good one may be only two or three thousand yuan, but the good one can last for 6 years, and the cheap one will crack in 3 years. Converted into the average annual cost, plus the loss of production shutdown and the labor cost of replacement, the cheaper one is twice as expensive.
The overhaul cycle is also practical. The corrugated expansion joints used in the power station industry are generally inspected with the overhaul cycle. However, if you leave less fatigue life margin when selecting the model, you may find cracks just after the overhaul, purchase spare parts temporarily, and the delivery date can't catch up-then you can only weld and repair, which will lead to a pile of potential safety hazards. It is recommended to ask the manufacturer directly when raising the demand: How many times is the design fatigue life? Is it according to EJMA standard or GB/T 12777? These two standards have different calculation methods for fatigue life, which directly affects the service life.
Spare parts costs are also factored in. Some manufacturers quote low prices, but the follow-up accessories are expensive. A tie rod nut quotes you dozens of dollars. Before signing the contract, write the price of wear parts clearly, so as to save you from arguing later.
5. Suggestions for the implementation of procurement: how to raise demand to manufacturers without being fooled
Give a list of purchases, fill it out according to it, and basically don't step on pits:
- Nominal diameter, wall thickness and material of pipe
- Medium name, temperature, pressure (indicate whether design pressure or operating pressure)
- Displacement: How many mm in axial, transverse and angular directions
- Installation Location: Indoor or Outdoor? Are there corrosive gases?
- Fatigue life requirements: how many cycles under design conditions
- Connection method: welding, flange or clamping?
These six items are filled in clearly, and it is difficult for manufacturers to fool you.
Two more details are easily overlooked. One is the stiffness value of the expansion joint, which directly affects the force of the pipeline bearing. If the stiffness is not written on the quotation sheet, it will be troublesome to calculate the pipeline stress later. The other is the material of the guide tube-under the high-temperature flue gas working condition, the thickness of the guide tube is not enough, and it will burn through in a few months. The bellows is directly exposed to the high-temperature flue gas, which is basically equivalent to scrapping.
If the working conditions are complicated, such as lateral displacement and axial displacement at the same time, don't pat your head to select the type, and let the manufacturer directly try with the lateral type of compound hinge or the pressure balance type of curved tube. Leave professional matters to professional people, and what you have to do is make the needs clear.