Start with the name: metal bellows, expansion joint, compensator, what is the difference between these names?
Two days ago, I answered a phone call, and the customer came up and said, "I want to buy metal bellows." After chatting for half an hour, I realized that what he wanted was actually an expansion joint-the kind used to absorb the thermal displacement of the pipe. This is so common. Metal bellows and expansion joints, which sound like brothers, are actually a layer away from being "assembled".
A metal bellows is just the core elastic element, just like a spring is the heart of a shock absorber. The expansion joint (also called compensator) is a complete assembly assembled by bellows plus end pipe, flange, tie rod, guide tube and other accessories. Look at the product list on our site. There are both "metal hose" (pure corrugated pipe plus braided net), "universal corrugated expansion joint" and "high-temperature axial expansion joint"-different names, naturally different uses. You taste, you taste.
Structural "twin brothers": bellows are skeletons, expansion joints are complete equipment
Inside the "straight pipe pressure balance expansion joint" is a multi-section bellows combination, which is matched with a tie rod to balance the pressure thrust; However, "double hinge transverse expansion joint" uses two bellows and hinge structure to realize transverse compensation. When selecting the type, if you only stare at the parameters of the bellows, not the overall structure, the installation site will be blind.
The relationship between bellows and expansion joint can be understood as follows: bellows is the skeleton, and expansion joint is the whole body equipment. You can't work with a skeleton, you have to wear armor and weapons, isn't that the truth?
Don't step on pits in model selection: which one should be used in different working conditions?
Working conditions determine the model, don't buy the universal model for cheap. For example, the desulfurization flue gas pipeline of power station has low temperature but strong corrosion. It is recommended that "desulfurization flue gas baffle door" cooperate with "non-metallic expansion joint (fabric fiber expansion joint)". There is much high-temperature dust in the cement industry, and "metal corrugated expansion joint in cement industry" is even more manufactured. Steam pipeline is high temperature and high pressure, so it is necessary to use "external pressure single axial expansion joint" or "high temperature axial expansion joint" to avoid columnar instability. There are also buried pipelines. Don't use the ordinary model blindly. You have to choose the "directly buried (fully buried) expansion joint"-this thing can be buried directly in the soil, and the anti-corrosion and waterproof sealing have been specially optimized.
Then you have to use non-metal for high-pressure steam pipes? I'll blow it up for you in minutes. Therefore, don't fight metal, and don't blindly believe in non-metal. Matching and combining is the real skill.
Life and installation: Wrong direction equals white installation
The direction of the arrow on the expansion joint refers to the flow direction of the medium. If the guide tube is installed backwards, the medium is rushed into the bellows, and the life span will directly hit the bone. The tie rod nut is not decorative, and the transport screw can only be removed after pre-stretching or pre-compression is adjusted according to the drawing. Otherwise, the compensation amount returns to zero when the tie rod pushes the bellows dead when the machine is started. Two days ago, another customer asked, "Does the screw of the expansion joint need to be removed?" The answer is that it must be removed! Not dismantling is equivalent to letting the bellows do useless work.
After installing the pressure test, you have to check the gap of the guide tube. If the gap is too small, it will easily get stuck, and if it is too large, it will lose the diversion function. These details are explained thoroughly in our question and answer on "The Specific Function of Expansion Joint Guide Tube". If you are interested, you can turn them over.
Nonmetallic vs. Metallic: When is it time to change tracks?
Metal bellows are pressure-resistant and temperature-resistant, but when encountering strong acid and alkali or requiring a large compensation amount, "non-metallic expansion joint (fabric fiber expansion joint)", "rubber compensator" and "polytetrafluoroethylene compensator" are more suitable. For example, in the desulfurization flue of power plants, the flue gas contains sulfur and the temperature is low, so it is easy to corrode and perforate with metal, and a package of non-metallic fabrics can solve the problem. There is also a "rectangular non-metallic expansion joint" that specializes in square flues, and metal can't do so much compensation.
Conversely, high-pressure steam piping with non-metal? That's a joke about your life. Therefore, engineering selection depends on the medium, temperature, pressure and displacement direction-if one parameter is wrong, the whole set of equipment will be useless.