What is a hinge metal expansion joint? -Don't think of it as an ordinary bellows
When many engineers hear "expansion joint", the first thing that pops up in their minds is the axially telescopic bellows. To put it bluntly, it can be pulled and compressed. But the hinge metal expansion joint is not this way at all. It carries one or several pairs of hinge structures-to put it bluntly, it is to install "joints" on the bellows. This joint limits axial displacement and only allows it to make angular deflections in the plane. Therefore, the core function of the hinge metal expansion joint is not axial expansion and contraction, but absorbing the lateral displacement and angular displacement of the pipe at the corner.
You may ask, wouldn't you just use the elbow? The elbow is rigid. When the temperature rises, the pipe expands thermally, and the elbow can't be pushed, so the stress is all held in the weld and the interface of equipment. The function of the hinge expansion joint is to turn this rigid corner into a flexible corner, so that the pipe can "take a breath".
Hinge metal expansion joints are generally composed of bellows, end tubes, hinge plates, pins and tie rods. The bellows are responsible for sealing and deforming, and the hinge system and tie rods are responsible for withstanding pressure thrust and guidance. Note that the tie rod here is different from the tie rod on the general expansion joint, which will be described later.
Which working conditions must it be? — — Hard requirements of rotation angle, large displacement, and space constraint
I have encountered many projects. The pipeline direction is L-shaped or Z-shaped, and the space is so small. The axial compensation can't be put down, and the general expansion joint can't bear the lateral force. At this time, the hinge metal expansion joint is the only solution.
Vacuum ducting system for air-cooled islands. The pipe at the top of the air-cooled island needs to turn from horizontal to vertical, and the thermal displacement is very large. The thermal expansion of the pipe alone can exceed 100mm. If the universal corrugated expansion joint is used, it can only absorb the axial displacement. Once the lateral displacement goes up, the corrugated pipe will become instable or fracture by fatigue quickly. The metal hinge expansion joint can easily absorb the lateral displacement of tens of millimeters-two hinge expansion joints are arranged in groups, which can convert the displacement into angular displacement. Each hinge only bears a part of the angle and has a much longer life.
For example, the kiln head and kiln tail pipelines in the cement industry have high temperature and large dust. The pipeline direction is often limited by the factory structure, and there is no way to make a long straight pipe section. Hinged metal expansion energy-saving crams into tight spaces while withstanding high temperatures and dusty airflow. The product list of this site has "Metal Corrugated Expansion Joints in Cement Industry"Specialized in dealing with such conditions.
There is also the main steam pipe and hot air duct of the power station, where the space is more precious. Sometimes you don't even have the position to install a double expansion joint. The hinge structure can walk against the wall or the linter, which is also its killer.
Hinge Expansion Joints vs Universal Expansion Joints: What's the Difference?
The general corrugated expansion joint (that is, what we often call the axial type) has no hinge in it, only the bellows and the end tube, which absorbs the three-way displacement by the bending of the bellows itself. But it has an Achilles heel: pressure thrust. Think about it, when the pressure inside the pipe is high, how much thrust is acting on the bellows? The thrust equals the pressure multiplied by the effective area of the bellows. The bellows of DN500 has a thrust of nearly 20 tons at 1MPa pressure. The universal expansion joint depends on the pipe bracket or the main fixed bracket to bear this thrust, and the bracket will collapse if you are not careful.
Hinge metal expansion joints are different. Its hinge and tie rod system is directly subjected to pressure thrust, which is transmitted to the pipe through the hinge plate, unlike the universal model that pushes it all to the external bracket. So the pipe where the hinge expansion joint is installed, the main fixing bracket can be made much smaller, and even can be cancelled in some occasions.
There is also lateral displacement capability. When the universal expansion joint absorbs the lateral displacement, the bellows will bend. This bending stress is very large, which easily leads to the early fatigue of the bellows. However, because of the guiding function of hinge, the bellows only has pure angular displacement, the stress distribution is uniform, and the fatigue life is usually several times higher.
The universal type is like a rubber band, deadlifted and pulled; The hinge type is like a hinge, and it is in one plane how it turns. There are many rules but it is very stable.
The most overlooked details of selection and installation (especially pins and tie rods)
When choosing a hinge metal expansion joint, many people only focus on the material and wavenumber of the bellows, but it is often the details of the hinge system that really decide whether and how long it can be used.
Let's start with the pins. The pin is the rotating core of the hinge, which bears the entire pressure thrust and part of the lateral force. Many manufacturers use ordinary carbon steel pins for cheap, but in high temperature or corrosive environment, the pins are quickly worn or even stuck. The consequence of jamming is that the hinge fails and the bellows is forced to bear additional stress. The correct approach is: the material of the pin shaft should be at least stainless steel or alloy steel, with surface hardening treatment and self-lubricating bushing. In our station. "Double hinge expansion joint for air-cooled island vacuum pipeline"It is a typical high-demand product, and the pin strength has been specially calculated.
Let's talk about the pull rod. The role of the tie rod is to transmit the thrust between the hinges while limiting the axial displacement of the bellows. Many people screw the tie rod nut tightly, and as a result, the expansion joint becomes a rigid body, completely losing the ability to compensate. The correct installation method is: during installation, the clearance specified by the factory should be reserved for the tie rod nut. This clearance value generally corresponds to the cold tightness or thermal displacement direction of the pipeline. You flip through the FAQ in the site. "Function of expansion joint tie rod"and"How to adjust the tie rod nut of expansion joint", there are specific operations inside. Let me emphasize here: the tie rod is not used to lock, but to guide and limit.
Connecting weld between hinge plate and pipe. Many on-site installations can be welded twice casually, and as a result, the load-bearing will be torn as soon as it comes up. The hinge expansion joint should adopt full penetration weld and do non-destructive testing. Don't save that money. If you save it, you will have an accident.
Real case: How did it carry down in power stations, air-cooled islands and cement lines
Tell me an example of a power station. For the main steam pipeline of a 2×300MW turbine unit, a compensator needs to be arranged at the corner of the pipe gallery. The height of the space is only 1.2 meters, and the axial compensator can't fit at all. Finally selected "Corrugated expansion joint for power station industryThe matching scheme in the "double hinge transverse expansion joint" is arranged in pairs on both sides of the L bend. It has been put into operation for 8 years, and it is shut down for inspection once a year. There are no cracks in the bellows, and the wear of the pin is within 0.3mm, which fully meets the design life.
The air-cooled island side is more typical. An air-cooled island of a power plant in the north, the ambient temperature is-30℃ in winter, and the pipe exhaust temperature is above 100℃ in summer, with a temperature difference of 130℃. The pipes need to be diverted from horizontal rows to vertically distributed vacuum pipes, and the space is packed with steel structures. They use our "double hinge expansion joint of air-cooled island vacuum pipeline", a group of four hinges, which decomposes the lateral displacement of 350mm into the angular displacement of three planes. It hasn't been changed for ten years, depending on the careful design of pins and tie rods.
On the cement line, a 5000t/d production line, kiln tail waste gas pipeline. The temperature is 350℃, the dust concentration is high, and the pipeline should bypass the dust collector. It turns out that the general-purpose expansion joint will wear out the air leakage in one year. Replace with "Metal Corrugated Expansion Joints in Cement Industry"The high-temperature-resistant hinge type in, with wear-resistant guide tube, has not been changed for three years now. The action of the guide tube is specifically talked about in the FAQ station ("Specific Function of Expansion Joint Guide Tube"), in fact, it is to prevent the dusty airflow from directly washing the bellows and protect the life.
Therefore, the structure of the hinge metal expansion joint looks more complicated than that of the general-purpose type, but when used in the right place, it is more than a little worry-free. Next time you encounter a pipeline project with corners, limited space and large displacements, don't rush to pile general-purpose expansion joints. Think about the hinge scheme first-if you choose the right one, it will stop in the next few years.