1. What is the DZ-type metal expansion joint: Start with the "DZ" in the name
DZ-type metal expansion joint is called "single axial expansion joint" in jargon. The letters are simple when taken apart: D stands for "mono" and Z stands for "axial". It means that an expansion joint only has one group of bellows, and the axial expansion and contraction of bellows mainly absorbs the thermal displacement of the pipeline.
One is the general-purpose corrugated expansion joint, which is a "panacea" that can absorb axial, transverse and angular displacements at the same time; The other type is the external pressure single axial expansion joint, which moves the bellows to the inside of the shell and bears the internal pressure thrust by external pressure, which is suitable for large diameter and low pressure conditions. DZ type does not have so much work, and its structure is that the bellows is directly connected to the pipeline, with accessories such as the upper end pipe, guide tube and tie rod, which is mainly durable.
The main steam pipeline of power plant boiler and the waste gas pipeline at the head and tail of kiln in cement plant have high temperature, coarse pipe diameter and large displacement. DZ-type metal expansion joints are common because of two words-"carrying and making". The wall thickness of the bellows can be increased casually according to the pressure, unlike the double hinge transverse expansion joint, which is limited by the structural space. Moreover, it does not have the function of balancing internal pressure thrust, so it has to rely on pipeline supporting piers or main fixed brackets to offset the blind plate force, so the cost is naturally lower.
2. Displacement compensation ability and stiffness: the most easily overlooked pits in type selection
How much displacement does the DZ type metal expand energy saving carry? There is no standard answer to this question. It depends on the wavenumber, number of layers and diameter of the bellows. For example, the DZ type of DN300 has a single-wave axial compensation of about 15 to 25 mm. If you need a compensation of 50 mm, you have to make four or even five waves. However, with every extra wave, the axial stiffness of the bellows is lower, and the bellows itself is easily compressed and unstable, so the more wavenumbers, the better.
In the displacement calculation, the axial displacement Δ x is the amount of thermal expansion, and the formula is Δ L = α · L · Δ T. α is the coefficient of linear expansion, carbon steel is about 12×10⁻⁶/℃, L is the length of the pipe between the two fixed brackets, and Δ T is the difference between the installation temperature and the maximum operating temperature. The calculated Δ L is the amount of axial compensation you want.
Lateral displacement and angular displacement are a little more troublesome. The lateral displacement Y and angular displacement θ cannot be directly derived from the thermal calculation, but depend on the pipe layout and the form of support and hanger. For example, in Z-shaped pipes, the lateral displacement of the short pipe in the middle section has to be absorbed by the expansion joints at both ends. At this time, DZ type is very difficult, because its transverse stiffness and angular stiffness are much greater than that of axial stiffness, and hard-carrying lateral displacement will make the bellows bear additional bending stress-which is why some working conditions clearly choose DZ type, and as a result, the bellows cracks in advance.
What's even more pitful is the stiffness. The axial stiffness K of the bellows determines an important parameter: the elastic reaction force F = K · Δ x. This force will act on the fixed bracket. Many people who select the model only look at the compensation amount, not the stiffness. As a result, the bracket design is small. As soon as the pipeline heats up, the bracket is directly deformed, and the insulation layer is squeezed out of shape. And guess what? All bellows manufacturers will mark the stiffness value on the sample, but the engineers who really take the stiffness value to calculate the bracket load can't pick three out of ten.
3. Guide tube, tie rod and nut: three supporting roles, three roles
The deflector is not an ornament. In pipelines with high medium flow rate and large dust content, such as the kiln tail exhaust gas of cement plant, dust particles directly wash the bellows trough, and it takes less than a year to wear them out. With the addition of the guide tube, the medium goes through the smooth channel in the tube, the bellows does not contact the medium, and the life of the tube is at least tripled. However, the premise is that the installation direction of the guide tube should be correct-the direction of the arrow points to the flow direction of the medium, and if it is installed backwards, it is equivalent to not installed.
The role of the tie rod is to constrain the bellows so that it deforms only in the direction it is designed to. Type DZ expansion joints usually have two or four tie rods, symmetrically arranged on the outside of the bellows. Both ends of the tie rod are nuts, which are generally locked before leaving the factory. At this time, the expansion joint is in a "rigid locking" state, which is easy to hoist and transport.
When you get to the scene, how do you adjust the nut? If it is pure absorption axial displacement, loosen the nut at one end, leave a gap corresponding to the displacement amount, and lock the other end. For example, if you need a compression amount of 30mm, leave a gap of 30mm on the stretch side. If it is a working condition with lateral displacement, a gap should be left on both sides, and the specific amount should be calculated according to the displacement component. After adjusting the nut, the nut has to be tightened another half a turn-to prevent loosening caused by running vibration. As mentioned before, the adjustment of the tie rod nut directly determines that the expansion energy saving cannot work according to the design displacement. Wrong adjustment and the bellows will either be crushed to death or pulled off.
4. Suggestions on the selection of DZ-type metal expansion joint under different working conditions
The high-temperature steam pipeline is DZ type, and the material is stainless steel 321 or 316L. If the working temperature exceeds 550℃, high-temperature alloy must be used. The high-temperature axial expansion joint and bellows of this station adopt multi-layer thin-walled structure. The more layers, the better the flexibility and the longer the fatigue life. Of course, the wall thickness can be thicker, but the stiffness will become larger, and the load of the fixed bracket will increase, which is not cost-effective.
The problem with flue gas pipes is not temperature, but corrosion. The clean flue gas at the outlet of the desulfurization tower, although the temperature is low, contains acidic condensate, which causes severe stress corrosion to the bellows. It is recommended to choose 316L or 254SMO as a material, and the guide tube must be equipped. Referring to the corrugated expansion joint used in the power station industry of this station, aiming at the inlet and outlet of dust collector and the inlet and outlet of induced draft fan of coal-fired power station, the usual practice is to install an armored ring outside the corrugated pipe to prevent the corrugated pipe from jamming due to external dust accumulation.
The exhaust gas pipeline at the tail of the kiln in the cement industry is high in temperature, large in dust, and accompanied by alkali metal corrosion. When the metal corrugated expansion joint in cement industry of this station deals with this kind of working condition, the guide tube is made of wear-resistant alloy steel, and the bellows material is Incoloy825 or C-276 alloy. Here is a detail: the temperature changes quickly when the kiln tail system is started, and the DZ-type expansion joint is best pre-stretched and installed in the cold state-the pre-displacement amount is half of the designed axial compensation amount, so that the bellows changes from the stretched state to the compressed state in the hot state, and the fatigue life is the best.
5. The most common mistakes in installation and maintenance
When is the transport tie rod removed? The answer is: before the expansion joint is installed and the pipeline system is hydraulically tested. Many people forget to dismantle it. As soon as the pipeline heats up, the bellows can't expand and contract, and it is forcibly stretched. The bellows is deformed at the light and the weld cracks at the worst. If the removed tie rod is to be kept for spare use, be sure to mark it properly and don't mix it with the formal tie rod.
How to do pre-displacement? There are two kinds. Axial pre-displacement is to compress or stretch the bellows into place with a jack or bolt tensioner and then fix it with a temporary gasket. Transverse pre-displacement is usually used on Z-shaped pipes by pulling the pipe into place with an inverted chain and then locking the expansion joint tie rod.
What if you find water in the bellows? Check whether the guide tube is damaged first, and then check whether the drainage hole is blocked. If the accumulated water is not treated, the bellows will be directly swelled and cracked by ice once it freezes in winter. There are two kinds of abnormal noise: one is the sound of airflow washing, indicating that the guide tube is loose; The other is the sound of metal friction, indicating that the tie rod nut is loose and the bellows is rubbing against the end pipe. When the tie rod is loose, the clearance is readjusted and locked, and the bellows body is cracked and the whole unit is replaced-don't think about repair welding, the bellows is a thin-walled part, and the thermal stress of repair welding will directly lead to accelerated cracking in the surrounding area.
The DZ-type metal expansion joint looks simple, but it is not easy to use. When selecting the model, understand the three things of displacement, stiffness and medium characteristics, and with correct installation and maintenance, it can steadily carry you ten years of displacement; On the contrary, the selection time you save will have to be spent on shutting down the furnace for maintenance sooner or later.