I answered a phone call two days ago, and the first sentence was: How big is your maximum metal expansion energy saving? I asked back: What is the diameter of your pipe on site? The other side was stunned and said that he hadn't measured it yet, so he wanted to ask about the upper limit first. This kind of question, to be honest, is quite common. However, anyone who has actually done several power plant desulfurization projects or cement kiln tail renovation knows that asking the caliber often means that the demand has not been sorted out.
1. What exactly does the largest metal expansion joint mean?
There is not only one dimension of caliber. Some people think that the largest is the thickest pipe diameter. In fact, the "big" of the metal expansion joint is also reflected in the compensation amount, pressure resistance grade and diameter combination.
Take the main steam pipeline of the power station as an example. The diameter of the high-temperature axial expansion joint used may not be particularly exaggerated, but the compensation amount is always tens of millimeters, the internal pressure thrust is not good, and all the brackets have to be reworked. However, the requirements of wall thickness, corrosion resistance and wear of large-diameter thick-walled expansion joints in desulfurization systems are completely different. It is also "big", one is thick, and the other is able to carry. Which one you really need, you have to think clearly first.
Second, what kind of working conditions can really use "Big Mac"?
Smoke duct of power plant, kiln head and tail of cement industry, import and export of desulfurization tower-these places have a diameter of two or three meters or even larger, with high temperature, low pressure and complicated displacement, and ordinary expansion joints simply can't stand it. You will know when you go to the scene and take a look. The pipe is taller than a person, and the expansion joint is installed in it, like a city gate.
Under such working conditions, metal rectangular expansion joints or duplex structures composed of multiple bellows are often required. For example, the double hinge transverse expansion joint, with an intermediate pipe in the middle of two bellows, is specially used to absorb transverse displacement, which is particularly practical in smoke ducts. There is also an expansion joint matched with the desulfurization flue gas baffle door. The medium is sulfur-containing flue gas, which is extremely corrosive, and the material selection is completely different from that of ordinary power plants. These experiences are accumulated bit by bit at the specific project site.
Third, what is the difficulty in building a large expansion joint?
Stiffness and fatigue are the first barrier. The rigidity of corrugated pipe directly affects the force of pipeline support-if the coefficient is large, the support will be made in vain, which is a waste of money; It's too small, and the bellows itself will be pulled out first.
Internal pressure may cause column instability and plane instability. At this time, the role of the guide tube is very critical. The guide tube not only protects the bellows from being directly washed by the medium, but also improves the flow field and helps to resist internal pressure.
But is the thicker the deflector, the better? Not. A client asked this two days ago. There must be enough gap between the guide tube and the bellows, otherwise the thermal expansion will hold each other, and the bellows will have problems first. How to leave this gap depends on the temperature and caliber, and it can't be determined by patting your head.
4. Type selection is not to look up tables, but to calculate comprehensive accounts
Many people say "come to the biggest" as soon as they come up, but the biggest may not be the most suitable.
Straight pipe pressure balance type expansion can save energy and eliminate internal pressure thrust, so it is not necessary to build such a large thrust bearing, but the structure is complicated and the price is high. General-purpose corrugated expansion joints are inexpensive, and can require that the pipe has sufficient fixed brackets to absorb internal pressure thrust. On the one hand, the equipment saves the expensive pipeline; on the other hand, the equipment saves the expensive pipeline. This account has to be calculated clearly.
Calculate the thermal displacement, pressure and fatigue life first, and then determine the structural form. For example, the rotary compensator is suitable for large displacement compensation of steam pipe network, and the sleeve pipe expansion joint is very useful under low pressure and large diameter working conditions. Each has its own applicable scenario, not the bigger the more advanced, matching working conditions is the king.
5. There are more pits for installation and maintenance than you think
The big expansion joint is not installed and left alone. When leaving the factory, there are generally tie rods or transportation fixings, which must be removed according to the instructions after installation, otherwise the compensation function will be completely wasted-just like wearing a plaster bandage and making people run, they can't move at all.
Last month, at a project site in Yunnan, the direction of the arrow of the expansion joint was reversed, and the medium flowed against the ripples. As a result, it leaked in less than half a year. The arrow is clearly marked, but it is not paying attention. Another is pre-stretching or pre-compression, which is not determined by a pat of the head, and has to be accurately calculated according to the installation temperature. After installation, check the welding slag and splash on the bellows. These things are the starting point of fatigue cracks, and cleaning them up can save a lot of follow-up trouble.
6. Don't be superstitious about "the biggest" and think clearly about your true needs first
If you are staring at a super-large diameter project, it is recommended to list the six items of medium, temperature, pressure, displacement, connection method and space restriction first, and then check them with the manufacturer's engineers one by one.
We have done corrugated expansion joints, air-cooled island vacuum pipeline double hinge expansion joints and external pressure single axial expansion joints for power station industry. Each structure was not thought up out of thin air, but was "forced" by on-site working conditions. The metal rectangular expansion joint is specially designed for the rectangular section of the flue duct. The metal corrugated expansion joint in the cement industry is optimized for the high-temperature dust working conditions at the head and tail of the kiln. The desulfurization flue gas baffle door should take into account both sealing and corrosion prevention.
So, how big is the maximum metal expansion energy saving? The answer is often: as long as the logistics can be transported to the site, it can be built. The question is do you need it?