How demanding are the working conditions of the main steam pipe?
The temperature is often five to six hundred degrees, the pressure is more than ten MPa, and the thermal displacement is complicated-axial, transverse and angular together. Under this working condition, if the expansion joint is selected wrong, it is really not a joke to collapse when the machine is started. Two days ago, a buddy from a power plant told me that they used ordinary goods for cheap. As a result, the bellows exploded directly just after being connected to the grid, and the whole unit tripped. So we have to understand the working conditions before talking about the selection.
Selection: Not price, but displacement and pressure
The axial displacement of the main steam pipe is particularly large, along with a combination of lateral and angular displacements. You have to judge: YesHigh temperature axial expansion joint, orDuplex hinge transverse type expansion joint If the space is constrained and the displacement is complicated,Corrugated expansion joint for power station industryOften the safer choice. Don't just buy a general goods as soon as you hear "high temperature". The corrugated material, wall thickness and guide tube design are all particular. Incoloy 825 or Inconel 625 is used when the strength of the ordinary 304 drops sharply above 500°C. The pressure level must also be calculated clearly. The main steam pipeline generally belongs to GC1 level, with a design pressure of more than ten MPa. The pressure resistance of the expansion joint cannot be lower than 1.5 times of the design pressure of the pipeline-this is the basic bottom line.
Displacement calculation. Many people only look at the axial direction and forget about the lateral and angular directions. However, the main steam pipe will generate lateral thrust due to thermal expansion. If the expansion joint cannot absorb the lateral displacement, the bellows will soon become unstable. OurCompound hinge transverse expansion jointSpecifically designed to absorb lateral displacement, whileThe high-temperature axial expansion joint is suitable for the working condition of axial direction. When selecting the type, it is best to throw the pipeline stress analysis report to the supplier, and let them match the type according to the actual displacement data.
Installing That Thing: Direction, Tie Bar, Pre-Stretch
Two days ago, a customer installed the expansion joint backwards, and the direction of the arrow was backwards. During the test run, the bellows bulged directly. In fact, the direction of the arrow must follow the direction of the medium flow, which is iron discipline. Why? Because the opening direction of the guide tube is downstream, it is strange that the medium is installed backwards and the root of the bellows is directly washed, so it is not bulging.
There's also the issue with the tie rod nut. Many expansion joints come out of the factory with tie rods and nuts for transport protection or pre-stretching. Once the installation is complete, exceptPressure balanced expansion joint(e.g.Straight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint) The tie rod constraint needs to be retained, and other types of tie rod nuts should generally be loosened after debugging. And guess what? In order to save trouble, some construction teams screwed the nuts tightly, the expansion joint lost its compensation function, and the pipeline stress was all suppressed on the bracket, and finally the fixed bracket was crooked.
Do you want to do pre-stretching? It depends on the design temperature. For the main steam pipeline, the working temperature exceeds 400℃, and the installation temperature is generally around 20℃, with large temperature difference and large expansion. If the design document calls for pre-stretching, you will have to pre-stretch at about 50% of the elongation. Note, however, that not all expansion joints can be pre-pulled, such asUniversal corrugated expansion jointUsually, pre-pulling is not supported, so check the manufacturer's instructions.
Don't underestimate the guide tube
The flow rate of main steam is high, and the high-speed steam entrains water droplets and impurities, which erodes the bellows extremely seriously. Without deflector protection, the bellows may blow out in a few months. OurUniversal corrugated expansion jointStandard guide tube, the lining thickness is generally not less than 3mm, and the material is the same as the bellows. Some bargains save the deflector in order to save cost, or use thin steel plates (1mm) to deal with it. You can ask the supplier: Do you dare to provide lifespan data? Do you dare to do an erosion test? The guide tube also has a rectifying function, which can reduce the vibration caused by eddy current.
In addition, the installation direction of the guide tube must be consistent with the flow direction of the medium, otherwise the air flow will directly impact the end of the guide tube, resulting in abnormal noise and vibration. If you hear a strange "whirring" sound in the pipe network, 80% of it is that the guide tube is installed backwards or falls off.
Common faults: cracking, instability, corrosion
Cracked bellows? Mostly, the fatigue life has reached, or the installation deviation causes additional stress. For example, the pipe support and hanger are stuck, which causes the expansion joint to bear extra bending moment. What about instability? The design pressure is considered low, or thin-walled corrugations with low stiffness are used. The pressure of the main steam pipe is high, and the number of layers and wall thickness of the bellows must be sufficient. Corrosion is another big head: the main steam condensate contains trace amounts of CO₂, acidic substances, and even chloride. Ordinary 316L is prone to stress corrosion cracking at high temperature. There was a power plant before, which used 316L. After two years of operation, there were a lot of cracks in the bellows, and it was replaced with Incoloy 800 to solve the problem. Therefore, the material selection depends on the quality and temperature of water vapor. Don't be superstitious about 304 or 316.
After the bellows is unstable, the uneven spacing of corrugations, "bulging" or "collapse" can be seen with the naked eye. Once discovered, it must be shut down and replaced immediately. The expansion joint is a consumable, and it won't carry it forever.
Maintenance and replacement cycles: Don't expect to do it once and for all
Generally, the design life is 10-15 years, but the actual life depends on the number of starts and stops and the fluctuation of working conditions. Frequent start-and-stop can lead to low cycle fatigue, and the life may be halved. How do you judge that it's time to change? Look at three points:
- Are there any strangulation marks or wear between the ripples?
- Are there any cracks on the surface (with penetration detection)?
- Has the deflector fallen off or shifted?
Don't buy three no products cheaply. The main steam pipe explosion is no joke. The expansion joint you choose must have a type test report, and the material must have a warranty. It is often said in the industry that "the money saved will have to be returned sooner or later", which is true at all.