How to understand the definition of metal corrugated expansion joint
The metal corrugated expansion joint is a section of telescopic "elastic joint" installed in the pipeline. It relies on the flexible deformation of the bellows to absorb the thermal expansion and contraction, equipment vibration and installation deviation caused by the temperature change of the pipe. It is also called compensator directly in the industry-it is exactly the same thing as the expansion joint, but it is called differently in different occasions.
Look at it literally. "Ripple" refers to the walls of the pipe made into a continuous wave, like the bellows of an accordion. This structure makes stainless steel and other metal materials change from rigid bodies to flexible bodies-don't underestimate this change. A steel pipe with a diameter of 500mm can be elongated to about 2.4mm per meter when heated to 200°C. Without this corrugated structure to "absorb" the amount of elongation, the pipe will abruptly break the equipment at both ends or crack at the weld.
The word "expansion" is even more straightforward, which means absorbing the displacement caused by thermal expansion. But many people tend to confuse metal corrugated expansion joints with metal hoses. Metal hose is a flexible pipe with corrugated entire pipe wall. It is mainly used to connect the inlet and outlet of equipment, absorb vibration and installation deviation. It is generally not long, two or three meters above the sky. The expansion joint is different. It has end tubes at both ends and bellows in the middle. As needed, it can also be matched with functional parts such as guide tube, tie rod and hinge. It is more like a "displacement management unit" in a pipeline system than a mere flexible connection.
Let's talk about the boundary with the fiber fabric non-metallic compensator. Non-metallic expansion joints are made of glass fiber cloth and silica gel cloth. The upper temperature resistance limit is usually 600-800°C, and the compensation displacement ability is large, but the pressure bearing capacity is weak. They are generally used in low pressure and large displacement conditions such as flue gas and air duct. However, metal corrugated expansion energy saving can withstand a higher pressure level, which is no problem from several kilograms to tens of kilograms, and the medium has a wider applicability-this determines that they each have a stall, and no one can replace anyone.
Core Structure Dismantling: Every Part Has a Reason to Exist
Look at the bellows first. This is the heart of the expansion joint, which directly determines the compensation ability and longevity of the product. Bellows are hydroformed or roll formed from thin-walled stainless steel plates, the wall thickness is generally between 0.5mm and 3mm, and the number of layers can be superimposed from one layer to multiple layers. The combination of wave height, wave pitch and layer number determines the elastic stiffness and fatigue life.
Both ends of the bellows are connected to the end pipe, which is a straight pipe section used to weld the pipe. The diameter and wall thickness of the end pipe should match with the pipeline system, otherwise the wall thickness will be misplaced during on-site welding, and the weld strength will not keep up, which will lead to accidents.
The guide tube is installed inside the bellows and inserted along the flow direction of the medium. It can significantly reduce media turbulence and decrease flow resistance in laminar flow state. The more critical function is to protect the inner wall of the bellows-the dust particles in the flue gas pipe and the condensed water in the steam pipe can be worn out in a few months after directly washing the bellows. With the guide tube, the medium grinds the wall of the tube, and the bellows is not worn.
The tie rod and the hinge are constrained in the direction of displacement. The tie rod restricts the bellows to only expand and contract axially, and does not bear the internal pressure thrust; The hinge limits the angular displacement, allowing the expansion joint to swing in only one plane. If you buy a straight pipe pressure balanced expansion joint, there is an extra set of bellows in it to bear the internal pressure thrust, which is equivalent to balancing the thrust, so that the fixed brackets of the pipes at both ends don't have to bear the huge blind plate force.
Working Principle and Mechanical Logic: Absorbing Everything by Deformation
Flexible deformation of bellows in exchange for safety of pipeline systems.
Axial displacement (length change), lateral displacement (up, down, left and right offset), angular displacement (axis angle change). Each wave of the bellows is a spring unit, and multiple waves connected in series can produce a large enough amount of deformation to absorb these displacements. The temperature of the system increases, the pipes elongate, and the bellows are compressed; The temperature decreases, the pipes are shortened, and the bellows are stretched. The cycle repeats, and the bellows has been subjected to alternating stresses.
There is a contradiction that must be weighed-stiffness versus fatigue life. The corrugated pipe with low stiffness is easy to deform, has strong ability to absorb displacement, and the load transmitted to the fixed bracket is small. However, the corrugation is too soft, and it is easy to become unstable when subjected to internal pressure. Moreover, the deformation per wave is too large, the stress is high, and the fatigue life is shortened. High stiffness and long life, but large elastic force and large system load. Therefore, the design should select the wave height and layer number of bellows according to the medium pressure, temperature and cycle times, which is a delicate job.
The larger the amount of single-wave displacement, the fewer cycles are allowed. For example, an expansion joint with a design cycle life of 1,000 times can be used for less than three years in a system that starts and stops once a day. Therefore, if the system starts and stops frequently during the selection, it is necessary to improve the fatigue grade of the bellows, instead of just looking at whether the displacement is enough.
Common types correspond to engineering: the consequences of wrong choice are very direct
General-purpose corrugated expansion joints are most used, which are suitable for occasions with small pipeline displacement and medium pressure and temperature, such as general hot water pipes and low-pressure steam pipes. It has simple structure, low cost, and can absorb a little in axial, transverse and angular directions.
High temperature axial type expansion joints are used in steam main pipes and hot air lines, and operating temperatures exceed 500°C and even up to 800°C. It is filled with high-temperature resistant insulation cotton inside, and the guide tube is also thickened to prevent high-temperature media from directly radiating the bellows. It should be noted when selecting the model that the working temperature of this expansion joint refers to the medium temperature, not the ambient temperature.
Straight pipe pressure balance type expansion joint, specially to solve the problem of blind plate force of large diameter high pressure pipeline. The larger the pipe diameter and the higher the pressure, the greater the internal pressure thrust-the pipe of DN800 has a blind plate force of up to 50 tons at 1.0MPa pressure. If all these forces are borne by the fixed brackets at both ends, the brackets must be made into concrete piers. A set of bellows is added inside the pressure balanced expansion joint to balance the thrust by itself.
Compound hinge transverse expansion joint is suitable for lateral displacement compensation of Z-shaped or L-shaped pipelines. It consists of two bellows plus an intermediate pipe and a hinge, which only allows lateral displacement and cannot be expanded or contracted axially. It is used in the air-cooled island vacuum duct, the smoke duct of power plant and other applications that require large lateral displacement. What happens if you choose the wrong one? The most typical mistake is to use the general-purpose hard to carry large lateral displacement, and the bellows is laterally unstable and cracks as soon as the machine is turned on.
Selecting Three Easy Pits
Install the guide tube in reverse direction. The deflector must be installed along the flow direction of the medium, and the direction of the arrow is the flow direction of the medium. When the direction is reversed, the medium directly washes the gap between the bellows and the guide tube, and the wear is aggravated and the flow resistance becomes larger. In engineering, I have seen too many cases of installing the expansion joint backwards on the spot-it is not that the workers are not careful, but that the logo of the equipment is not eye-catching enough when it leaves the factory.
Improper tie rod adjustment. In the shipping state, the tie rod nut is locked to prevent the bellows from deforming during transportation. Once installed in place, loosen the tie rod nut to the free length specified by the design. Some construction teams try to save trouble, or don't know to loosen it, so they run directly in a locked state, which means that the expansion joint becomes a rigid body, and the displacement all makes the pipeline and bracket eat, and the system will soon have problems.
The bellows material does not match the medium. 316L stainless steel has weak resistance to chloride ion corrosion. It is used in desulfurization slurry pipeline, and stress corrosion cracks in a few years. For medium containing high chloride ions, 254SMO or nickel-based alloy should be used. The medium contains sulfur, and the carbon steel bellows can't bear the high-temperature sulfur corrosion at all. The more expensive the material is, the better, the more suitable the better. In the cement industry, the tail gas of hot air ducts has high sulfur content, so we have to choose sulfur-resistant alloy materials instead of ordinary 304.
Each of these details is experience gained from practical lessons. Although the corrugated expansion joint is only one component in the pipe, it is the last line of defense against the deformation of the pipe. Choose the right, install the right, and use the right, and this system can run smoothly for ten or twenty years; If any link goes wrong, the price will be shutdown for maintenance or even equipment damage.