Let's start with standards: What exactly do EJMA and GB/T 12777 care about?
Many purchases ask, "Does your expansion joint meet the national standard?"-but if you ask him which national standard it is, he will probably not be able to say it. The standard of metal wave expansion joint is GB/T 12777, which is most commonly encountered in China, and EJMA in the United States. These two are not a two-choice relationship, but reference to each other: many clauses of GB/T 12777 are written with reference to EJMA, but they are not completely copied, such as the calculation of fatigue life and the dimensional deviation requirements of bellows after forming. Some details of the national standard are stricter than those of EJMA. On the other hand, if it is an export project or Party A designates EJMA, then you may not recognize it just by taking the national standard report. Therefore, before selecting the model, find out which standard is written in the project technical agreement, and don't wait for the goods to arrive at the site before arguing.
Core parameters: wave number, wave distance, effective area, stiffness
Just looking at the caliber is useless, I've said that more than once. Two days ago, I met a customer. The DN800 pipeline had to be equipped with a DN800 expansion joint. As a result, the compensation amount was simply not enough. Why? Because the core parameters in the standard of metal wave expansion joint are wave number, wave pitch, effective area and stiffness, these parameters directly determine how much displacement it can absorb. The more the wave number, the greater the compensation amount, but the more the wave number, the column instability pressure will decrease, not the more the better; Wave pitch affects the flexibility and pressure resistance of bellows; Effective area must be used when calculating thrust and blind plate force; The stiffness is directly related to the force of the pipe support. These parameters have corresponding calculation formulas and restrictions in the standard. Don't look at these when selecting the type, only look at the caliber. Nine times out of ten, there will be problems when installing them.
Material standards match working conditions: 304, 316L or Inconel?
If you choose the wrong material, the expansion joint is a time bomb. The working conditions of the three industries of power station, cement and desulfurization are very different. Power station steam pipeline, high temperature, 304 is ok, but if it is a supercritical unit, you have to use 316L or even Inconel 625. The metal corrugated expansion joint in the cement industry has large dust and may have sulfide corrosion. Ordinary 304 is easy to pit, at least 316L. Not to mention the desulfurization system. The flue gas temperature of wet desulfurization is not high, but the chloride ion concentration is high, and even 316L can't bear it. You have to use 2205 duplex steel or better nickel-based alloy. The standard has clear chemical composition and mechanical property requirements for these materials, but the standard will not choose materials for you, it only gives the qualified range. What material to really choose depends on the analysis of working conditions. Don't take a 304 to walk the world.
The Relationship between Structural Form and Standard
The standard does not dictate which structure you will use, but it will dictate what displacement each structure can withstand. The universal corrugated expansion joint mainly absorbs axial displacement. If you have to let it absorb transverse displacement, you have to look at the compound hinge transverse expansion joint. Power station pipelines often need to absorb axial and transverse displacements at the same time, so straight pipe pressure balance expansion joints have to be used, which rely on the combination of bellows to offset the blind plate force and avoid transmitting thrust to fixed supports. Desulfurization flue generally has low pressure and large size, so it is more cost-effective to use rectangular non-metallic expansion joint, but that is another standard system (JB/T 12235). Don't use non-metallic standards to set metal wave expansion joint standards. The displacement compensation mechanism of the two things is completely different. If the structure is wrong, it can't be compensated, or the force should be transmitted to a place where it shouldn't be transmitted. The pipe support can't stand it, and the weld cracks. This is how the accident came about.
Installation and Acceptance Criteria: Success or Failure in Details
Installing this chapter in the standard, many people don't read it at all. Pre-stretching, do you know what circumstances to do? The expansion joint needs to be pre-stretched when the pipe is installed below the design temperature, how much? There is a formula in the standard, which is calculated according to the actual temperature difference, not a pat on the head. There is also the direction of the guide tube. The arrow mark refers to the flow direction of the medium. If it is installed backwards, the guide tube will not work, and the bellows will be directly washed and worn out. Then there is the adjustment of the tie rod nut, which is most easily overlooked-the tie rod nut for transportation is locked, and after being installed in place, it should be loosened and adjusted to the specified cold and tight position. Some masters don't understand it, so they run with the locking nut. The expansion joint is completely ineffective, and the pipeline stress is all suppressed on the equipment. These details, valuable expansion joints are also useless. Therefore, don't only look at the appearance and size during acceptance. If the installation instructions require pre-stretching, you have to actually measure the stretching amount and keep a record, otherwise you can't tell if something goes wrong in the future.
Common Misunderstandings and Model Selection Suggestions
Take non-metallic expansion joint standard set metal wave expansion joint standard? Eight out of ten fell into this pit. Especially in the desulfurization project, the flue temperature is not high, and Party A chose non-metal cheaply. As a result, there are particles in the medium, and the fabric fibers are worn out, which leaks in three months. The reverse is also available-high-temperature pipes are made of non-metals, which are directly burned through. Therefore, the first step of standard selection is not to turn the sample, but to clarify the working conditions: temperature, pressure, medium, displacement type and cycle life, all of which are indispensable.
- Is GB/T 12777 or EJMA specified in the technical agreement?
- Is the bellows material verified for stress corrosion cracking? Did you choose the correct chloride content?
- Is there a calculation book for wave number, wave distance, effective area and stiffness? Who counts? Who is responsible for the miscalculation?
- Does the structural form match the displacement type? General type for axial displacement, compound hinge type for transverse displacement, straight pipe pressure balance type for pressure balance?
- The direction of the guide tube, the amount of pre-tension, the state of the tie rod nut, are they clearly written during the installation disclosure?
The standard is the bottom line, not the ceiling. Expansion joints that meet the standards may not be suitable for your working conditions, but those that do not meet the standards must not be required. It takes half an hour more to check the parameters when selecting the model, which is much worry-free than installing it and reworking it. Do you think this is the reason?