What exactly are metal expansion joints used for? Find out the three problems it solves first
The pipe should be elongated when heated and retracted when cold. This is the natural law of thermal expansion and contraction, and no one can change it. If the pipeline is rigidly connected, there is nowhere to release this expansion and contraction, and the stress will be all held in the interface between the pipe wall and the equipment, ranging from leakage of the interface to deformation or even tearing of the pipeline. The metal expansion joint is the flexible segment specifically designed to absorb this displacement, which is its first use.
In addition to thermal displacement, vibration and mechanical displacement during operation of equipment must also be managed. Think about the high-temperature and high-pressure occasions of power station boiler outlet, cement kiln head and tail, and flue gas desulfurization system. The expansion joint is not optional, but standard. For the corrugated expansion joints used in the power station industry in our station, the flue gas temperature is 500 to 600 degrees, and some are even higher. Under this working condition, the ordinary rubber compensator can't bear it at all, so it must be fitted with metal corrugated structure. Without the expansion joint, the pipe and equipment interface would have been torn apart in a few days.
How to set the specifications? Don't just look at the path. None of the six parameters can be missed
Many customers come up and ask, "Is there any expansion joint of DN500?" This question is too general. The DN500 is only the nominal diameter, which is the size of the pipe interface, but the specifications of the expansion joint are far more than that. The same DN500, used on normal temperature water pipes and used on 500℃ flue gas pipes, are completely two things.
- Path DN, that's the basics, that's right.
- The design pressure, the wall thickness and the number of layers of the bellows are directly determined by it, and even multi-layer corrugated structure should be used in high pressure applications.
- Design temperature, temperature determines material selection. 304 stainless steel is resistant to three or four hundred degrees no problem, and you have to use 316L or heat-resistant alloy to go up. Like the high-temperature axial expansion joint, the corrugated material and the guide tube design have to change accordingly.
- The amount of compensation, that is, how many millimeters of axial expansion and contraction, how many millimeters of lateral offset and how many degrees of angular angle, directly determines the wavenumber of the ripple.
- Different media, flue gas, steam, corrosive gas, different media, the internal guide tube and corrugated contact surface anticorrosion treatment is completely different.
- Connection mode, flange connection, welding connection or clamp connection, how to install it on the spot is how to choose.
Only when these six parameters are gathered together can the manufacturer design for you. If you dare to quote if you lack a parameter, you are gambling on your luck.
Type selection is not parameter selection, but structure selection
Look at what displacement you want to absorb first. If the pipeline has only axial expansion and contraction, choose general corrugated expansion joint or external pressure single axial expansion joint. The corrugation of external pressure type is outside, and the medium goes through the inner pipe, which has better anti-instability ability, and is suitable for high-pressure large-diameter pipelines.
If the pipe has lateral displacement or angular displacement, it is necessary to use a double hinge transverse expansion joint or a double straight pipe bypass pressure balanced expansion joint. The hinged type restrains the axial force through the hinge structure and specifically absorbs lateral deflection. The bypass pressure balance type has balance ripples inside, which can offset the blind plate force generated by the medium pressure, and is suitable for the occasions where the pressure is high and the compensation amount is large.
There's no way to make natural compensation for space constraints? The directly buried (fully buried) expansion joint is directly buried in the soil, without setting a compensation well, thus saving space and cost. There is also a rotary compensator, which absorbs displacement by rotation, has small friction resistance, and is suitable for long-distance directly buried steam pipelines. When selecting a model, tell the technician about the actual working conditions, and they know better than you what structure to use.
Use determines life and death: Expansion joints in different industries live completely different lives
It is also a metal expansion joint, which is used in power stations and cement plants. The design concept is very different. In the tail flue of the power station boiler, the flue gas contains sulfur and the temperature fluctuates frequently. Besides absorbing displacement, the expansion joint has to withstand low-temperature corrosion and wear. For the corrugated expansion joint we made for the power station, the guide tube should be thickened, and the corrugation should be equipped with wear-resistant bushings, otherwise it will be worn out in two or three years.
Problems in the cement industry In turn, the dust content of the kiln tail flue gas is huge, the wind speed is also fast, and the dust accumulation and wear inside the expansion joint occur at the same time. At this time, the design of the guide tube is very critical. If the angle is wrong, ash will accumulate inside, paste the ripples to death and lose the compensation function.
What about the desulfurization system? There is gypsum slurry in the medium, and the concentration of chloride ions is high, which is extremely corrosive to stainless steel. In this kind of working condition, high nickel alloy or PTFE-lined scheme, such as PTFE compensator and PTFE-lined metal hose, is specially designed to deal with this strong corrosive medium. There is also an air-cooled island vacuum pipeline. During operation, there is negative pressure in the pipe. The expansion joint not only has to absorb thermal displacement, but also has to bear external pressure, so that the double hinge structure has sufficient stability. With the same name, if you change your industry, the design logic will all change.
Installation and pre-displacement: All the previous lessons, at the end, look at this shivering
When the expansion joint is delivered to the site, it is not just welded. First, for the convenience of transportation when leaving the factory, the tie rod or transportation bolt is fixed, and it must be loosened after being installed in place, otherwise the expansion joint can't deform freely. Second, if the temperature difference in the pipeline installation environment is large, pre-displacement should be done during installation. What do you mean? For example, the operating temperature of the pipeline is 300℃, and the ambient temperature during installation is only 20℃. When the expansion joint is cold, it should first stretch a pre-bias. When the pipeline heats up and pulls to the working temperature, the expansion joint just returns to the middle position of the design. In this way, both cold and hot states are in the optimal operating range, rather than only being in the extreme position in the hot state.
Third, the spacing of guide brackets must be according to the design requirements. The first guide bracket at each end of the expansion joint should be close to it, and the second guide bracket should be farther away, so that the displacement can be concentrated on the expansion joint and no additional stress is placed elsewhere in the pipe. Many problems on the spot are not that the quality of the expansion joint itself is not good, but that the pipeline constraints in the back section are not done correctly.
The difference between the cheap and the expensive is invisible
The price of expansion joints on the market can be several times different. Some people choose cheaper ones to buy them. When they are installed, there will be no accident in that year, but they will start to leak in the next year. What's the difference? Materials. Stainless steel plates for bellows, genuine materials and refurbished materials, the spectrum can be known as soon as a dozen. Whether the solution treatment is done or not after corrugation molding directly determines the corrosion resistance of the material. There is also the wall thickness of bellows. The national standard allows negative tolerance, but the life gap between negative tolerance and positive tolerance may be half.
The welding process is more critical. The longitudinal seam welding of corrugated pipe is the core process. If the welding is not good, it will crack after hundreds of fatigue cycles. Therefore, when buying expansion joints, let the manufacturer report the specific grade, wall thickness, wave number and design fatigue life of the corrugated material. These parameters are written in the contract, and the spectrum is sampled when the goods arrive. Don't just look at the thickness of the shell. No matter how thick the shell is, it will be useless if the ripples can't work. Understand the specifications and uses of metal expansion joints, and it's not too late to place an order.