Are expansion joints metal or not? Don't answer in a hurry
Is the expansion joint metal? Don't rush to answer "yes", and don't insist on "no". Everyone who has worked on pipelines knows that the expansion joint is not a single material at all. It is a family with various members, including all-metal, pure non-metal, and rubber and PTFE. You only ask "Is it metal?" It's like asking "Is the car burning oil?" The hybrid car shook its head when it heard it.
Expansion joints are both metallic and non-metallic. Metal expansion joints are the mainstream, but non-metallic expansion joints are better in some working conditions. If you only look at the material to draw a conclusion, nine times out of ten, you will step on a pit.
From bellows to non-metallic expansion joints, what is the difference in materials
The core of metal expansion joint is corrugated pipe, which is generally stamped with stainless steel and alloy steel, and absorbs the thermal displacement of pipe by elastic deformation of metal. The general corrugated expansion joint, high-temperature axial expansion joint and corrugated expansion joint for power station industry of this station are all representatives of this road. The bellows can be made into single layer or multi-layer, and the guide tube and tie rod can be added. The more complicated the structure, the stronger the compensation ability.
What about non-metallic expansion joints? Take rectangular non-metallic expansion joints and non-metallic expansion joints (fabric fiber expansion joints) for example. The main body is several layers of fiber fabric, with thermal insulation cotton sandwiched in the middle, and both ends are fixed with metal flange frames. Rubber compensator and PTFE compensator are more direct, even the bellows are not used, all depending on the flexibility of rubber or PTFE material.
As soon as the materials are changed, the performance logic completely changes. Metals rely on ductility to carry displacement, while non-metals rely on fabric deformation and internal structure sliding. Metals can carry hundreds or thousands of degrees of high temperature, but non-metals can't carry that high temperature, but they have good thermal insulation performance, are not afraid of acid-alkali corrosion, and even have a much longer life than metals in some media.
Metal expansion joint and non-metallic expansion joint, what problems do they solve
High temperature, high pressure and large displacement compensation. Steam pipeline, power station boiler air supply system, cement kiln head and tail, the temperature in these places is always five or six hundred degrees Celsius, and the pressure is not low. Only metal bellows can withstand it. For example, the metal corrugated expansion joint in the cement industry is specially designed for this working condition. There are also directly buried (fully buried) expansion joints, which have to bear external pressure and groundwater when buried in the soil, and non-metals can't do this job at all.
What about non-metallic expansion joints? It does not take the high-temperature and high-pressure route, and its home field is large-size, low-pressure, vibration isolation and noise silencing occasions. For example, the desulfurization flue has a cross-section of several meters wide. Do you try it with a metal bellows? Costs are frighteningly high and processing is difficult. At this time, the rectangular non-metallic expansion joint came on the field, which was easily done. Another example is the fan inlet and outlet. The rubber compensator can absorb vibration and reduce noise. PTFE compensator is the nemesis of strong corrosive media. The metal is directly wasted when it meets hydrochloric acid and concentrated sulfuric acid, but PTFE is unscathed.
See? No one is superior to anyone, only whether it is suitable or not. You take the metal expansion joint to catch the corrosive acid, and it will be perforated in three months; Take the non-metallic one to carry the high-temperature steam and burn it through directly. Wrong choice, no one can hold up.
Don't just look at the material when selecting the model. These working conditions are more critical
There is a big circle in front. In fact, the core is one sentence: don't worry about "whether the expansion joint is metal or not", first find out your own working conditions. What is the medium? How hot is the temperature? How stressful? Is the displacement axial, lateral or angular? How wide is the pipe size? Is there enough installation space for tie rods? These are the hard indicators that determine the selection.
Take an example. The same thermal pipeline, with a temperature of 200℃ and a pressure of 1.6MPa, has no problem choosing a metal expansion joint; However, if the medium is sulfur-containing flue gas, the temperature is not high, but it is easy to crystallize and block, then non-metallic expansion joint is more suitable. For example, when the installation space is limited, the rotary compensator and the lateral expansion of the double hinge save energy and a lot of space, but they are all metal. Do you want to use non-metal instead? There are no doors.
Stress is also a key watershed. Non-metallic expansion joints usually withstand low pressure, generally from several kilopascals to tens of kilopascals, and metal expansion energy saving bears several megapascals or even higher. Choosing the wrong material can lead to premature aging or pipeline cracking and leakage. Two days ago, I met a customer, who chose a non-metallic one for cheap. As soon as the operating pressure came up, the fabric bulged and scrapped directly. Therefore, the material is only the appearance, and the working condition is the essence.
Common Myth: Are expansion joints, compensators, metal hoses the same thing?
Many customers call the expansion joint compensator, which is no problem. The two are different names for the same type of equipment. But metal hoses are different. Some people always mix them together. Expansion joint is a section of flexible connection in the middle of rigid pipeline, welded or flanged at both ends, mainly absorbing displacement; A metal hose itself is a bendable tube for use in vibrations, deflections, or situations where frequent movement is required. The price, structure, and use are all different. There are metal hoses, PTFE-lined metal hoses and vacuum special hoses in this station. They are two product lines with expansion joints, so they can't be mixed.
Another myth is to use bellows as expansion joints. In fact, bellows is only a flexible component in the expansion joint, and there must be bellows in the metal expansion joint, but the bellows itself is not equal to the expansion joint. Similarly, a non-metal expansion joint also has a metal frame in it, and you can't say it is a metal expansion joint just because it comes with a metal frame. Therefore, the standard answer to the question "Is the expansion joint metal?" Is two words: look at the model.
Next time someone asks you if the expansion joint is metal, you directly ask him: Which kind are you asking?