In the line of dry pipe compensation, who doesn't have many cases of using the wrong compensator? Two days ago, a customer complained to me, saying that the skin expansion joint at the entrance of the desulfurization tower leaked after less than half a year. When I looked at the photo, the temperature was 450℃, and the pressure was pulsed. In this working condition, the gods couldn't save it with the skin.
Many people think that skin and metal expansion joints are compensators. Wouldn't it be over to just choose one and install it? That's really not what happened. From structure to temperament, the difference between these two guys is greater than a tractor and a sports car.
Skin and metal expansion joints, how different is the structure behind the name?
Skin expansion joints (that is, non-metallic expansion joints and fabric fiber expansion joints), to put it bluntly, are stacked with multiple layers of flexible composite materials-the outermost layer of silicone glass fiber cloth, the middle layer of thermal insulation cotton, and the inner layer of polytetrafluoroethylene or stainless steel wire mesh, layer by layer like a thousand-layer cake. It absorbs three-dimensional displacement but has a pitiful stiffness.
Metal expansion joints are different. Single-layer or multi-layer stainless steel bellows rely on the plastic deformation of the corrugations to absorb displacement. You press it with your hand, and it doesn't move at all. There are common corrugated expansion joints, compound hinge transverse expansion joints and straight pipe pressure balance expansion joints. Corrugated expansion joints used in power stations and cement industries are basically of this kind.
Structure determines character. Skin is "soft kung fu", metal is "hard kung fu", and it is an accident to use it backwards.
From the working conditions: temperature, pressure, medium, what are they good at?
Let's start with the temperature. The limit of the skin is generally around 600℃, and if it is higher, thermal insulation layer must be added; What about metal expansion joints? The temperature resistance can be 900℃ or even higher, such as high-temperature axial expansion joints, which specialize in this. However, it should be noted that the temperature resistance of metal expansion joints is related to the material. The upper limits of operating temperatures of 321 and 316L are different. Don't blame the bellows if you choose the wrong one.
Talk again pressure. The design pressure of the skin usually does not exceed 0.1MPa, and it is no problem to run the smoke in the large flue. If you put a skin on the steam pipe, you're looking for trouble. The metal expansion joint can withstand high pressure. The design pressure of the corrugated expansion joint used on the main steam pipeline of the power station is more than ten MPa at every turn, and the external pressure single axial expansion joint can also remove most of the pressure thrust.
The media piece is more interesting. The skin has a natural advantage to acidic corrosive media-the PTFE lining can hold dilute sulfuric acid in the desulfurization tower; Metal expansion joints are most afraid of chloride ions, and 316L still pits when it encounters wet chlorine. The non-metallic expansion joint connected behind the desulfurization flue gas baffle door is because of its corrosion resistance and wear resistance.
Installation and maintenance pits: Why is the skin afraid of scratches and the metal afraid of fatigue?
The skin is the thing that is most afraid of sharp things when installing it. Once the outer layer of fiberglass cloth is scratched, the insulation cotton absorbs moisture, and the whole compensator is wasted. Therefore, when installing the skin, the splash of cutting and welding on the side has to be blocked.
What about metal expansion joints? Fear of fatigue. Each time a displacement is absorbed, the bellows undergoes a stress cycle. The number of cycles counts dead. If you overload it or exceed the displacement standard, the bellows will crack sooner or later. Before, there was a user who felt that the metal expansion joint was strong, so he screwed the tie rod nut to death and pulled it hard. As a result, it cracked after three months of use.
The stiffness of the metal expansion joint interacts with the pipeline system. The stiffness is too great for the bracket to eat; The stiffness is too small, and it can't compensate for the displacement. This depends on the stiffness data provided by the manufacturer to check. The FAQ in our station has the stiffness and calculation formula of bellows. Turn it over before selecting the type.
Displacement quantity and stiffness problems that are easily neglected in model selection
When choosing skin or metal expansion joint, the first thing is not to look at the price, but to calculate the displacement. The skin single wave can absorb the displacement of tens of millimeters, and it can absorb the three-dimensional displacement of axial, transverse and angular directions at the same time; Generally, a single bellows of metal expansion joint can only absorb a certain amount of axial displacement. The transverse displacement depends on the transverse expansion joint of compound hinge, and the angular displacement depends on the universal hinge.
The second thing is to calculate the stiffness. The skin stiffness is extremely low, and the thrust on the pipe support is almost negligible; No matter how small the stiffness of metal expansion joints is, there is a base, especially for large-diameter thick-walled expansion joints, which can be frighteningly stiff. If your pipe bracket is designed according to flange connection, it is suddenly replaced with metal expansion joint, and the bracket will deform directly.
Flange connection moment. The flange moment of the skin expansion joint is very soft, so just tighten the bolts; The flange moment of the metal expansion joint is very large, so it is necessary to use a torque wrench to pre-tighten it diagonally several times during installation, otherwise the flange surface will leak steam.
Desulfurization, Cement and Power Station: Selection Logic in Three Typical Scenarios
In the desulfurization system, it is best to use non-metallic expansion joints at the inlet and outlet of the flue gas baffle door. Although the flue gas temperature of wet desulfurization is only 50-80℃, it contains a lot of SO₂ and moisture, which is extremely corrosive. The metal expansion joint is easily corroded by acid dew point here, but the PTFE in the inner layer of the skin can carry it. Besides, the desulfurization flue has a large diameter, a low positive pressure, and the skin is completely sufficient.
The cement industry is not quite the same. The rotary kiln head and grate cooler are high in temperature and dust, so the high-temperature axial expansion joint is the positive choice. Metal corrugated pipe is resistant to high temperature and wear, and has a long service life. However, it should be noted that the vibration of the cement line is large, and the fatigue life of the bellows must be checked in place. Don't take the hard top of the universal corrugated expansion joint, but the transverse expansion joint with compound hinge must be used.
The operating conditions of power stations are the most complicated. Metal expansion joint for main steam pipeline and reheating hot section pipeline with high pressure and high temperature; Rectangular non-metallic expansion joint for smoke duct, large diameter, low pressure and low temperature. The air-cooled island vacuum pipe also has a special double-hinge expansion joint, which is another set of logic. There is no universal goods, only whether it is right or not.
Skin and metal expansion joints are never the relationship of who replaces who. The selection depends on the temperature, pressure, medium and displacement of your site. Knowledgeable people look at the working conditions first, then the products, and finally talk about the price. If you ask the price first, nine times out of ten you will have to step on a pit.