Find out the concept first: metal bellows and expansion joints are really not the same thing
Many people confuse metal hose with corrugated expansion joint. In fact, metal corrugated pipe is a kind of structural part-for example, we often call metal hose and PTFE-lined hose. It is essentially a bendable pipe, which is connected by the flexibility of corrugations. The expansion joint is a functional part, which uses the elasticity of the bellows to absorb the displacement of the pipe and eliminate the thermal stress.
Open our product list, Metal hoses are one category, and universal corrugated expansion joints, high temperature axial expansion joints are another. The material, wall thickness and corrugation shape are different, and even some expansion joints are not even corrugated pipes. For example, non-metallic expansion joints (fabric fiber expansion joints) are made of composite fabrics. For a simple example: the metal hose under the gas stove mainly functions as flexible connection, not specifically used to compensate for large displacement; However, the corrugated expansion joints used in power station industry have to bear high temperature and high pressure and axial displacement of tens of millimeters, which is not of the same order of magnitude at all.
Are expansion joints the same as compensators, you ask? As we said in the FAQ, expansion joints and compensators actually refer to the same type of equipment in industrial pipeline systems, but they are called differently. But a metal bellows is not a compensator — it's a part, not a system.
Perform their duties: Under different scenarios, choose metal bellows or expansion joint?
The compensation requirement of the pipeline determines the selection. If it's just connecting vibrating devices or small displacements, metal hoses will suffice – such as vacuum-specific hoses for vacuum systems and PTFE compensators for corrosive media. However, if it is a long-distance steam pipeline or flue gas pipeline, the thermal expansion is often tens of centimeters, and the expansion joint must be put on.
Among our products, directly buried (fully buried) expansion joints are specially used for underground pipelines, external pressure single axial expansion joints are suitable for high-pressure applications, and double hinge transverse expansion energy saving absorbs multi-directional displacement. Two days ago, a customer asked: Which kind of rotary kiln export pipeline is used in cement factory? Direct on the metal corrugated expansion joint of the cement industry, and the guide tube has to be added for high temperature resistance.
Speaking of the guide tube-it is clear in the FAQ that it prevents high-speed airflow from washing the bellows body, saving the life of the tube from being compromised. The role of the guide cylinder is not only to guide the flow, but also to reduce the fluid resistance, but when installed backwards, it becomes a throttle hole. Alas, I have seen more than one or two cases of pretending to be reversed.
Three elements of type selection: pressure, temperature and displacement, one step is wrong, one step is wrong
Many people only care about the caliber, and as a result, the bellows cracks in a few days after installing it. The pressure level depends on the metal hose pressure standard (which is available in our FAQ). If the temperature exceeds 400℃, high-temperature axial expansion joint or heat insulation layer should be considered. Displacement is a technical activity: axial, transverse and angular displacement, and different types correspond to different products.
For example, the correct installation method of the expansion joint of the large tie rod emphasizes that the tie rod only bears the pressure thrust and does not absorb the displacement, so it is directly scrapped. There is another common pit: How to adjust the expansion joint tie rod nut? Pre-stretched or pre-compressed? Our question and answer explained thoroughly that the pre-deformation amount must be calculated according to the installation temperature before installation, otherwise it will either be broken or deflated in the hot state.
Tsk, there is something more outrageous-someone uses the expansion joint as a metal hose on a fixed pipe with little vibration, but the bellows doesn't deform at all, and its function is wasted. When it comes to choosing a model, don't rely on your feeling, you have to count.
Installation and maintenance: 90% of failures are man-made
The direction of the arrow of the expansion joint points to the flow direction of the medium, and when the guide tube is installed backwards, it becomes a throttle hole, and the pressure loss soars. The function of the tie rod is to restrain the internal pressure thrust, but will it be removed after installation? Our FAQ said: The screw for transportation must be disassembled, otherwise the bellows cannot expand and contract freely. In addition, non-metallic expansion joints and metal rectangular expansion joints are installed in completely different ways-the former is flanged to clamp fabric fibers, and the latter is welded or bolted.
Daily maintenance mainly checks whether there are cracks and corrosion on the surface of the bellows, especially pay attention to the dust accumulation at the bottom of the corrugated valley. For example, the expansion joint matched with the desulfurization flue gas baffle door has heavy acid dew point corrosion, so regular ultrasonic thickness measurement is recommended. The expansion joint next to the manual plug-in insulation door is often the first to break, because the dust wear is large-this is a common problem in the industry, and there is no way but to check it frequently.
By the way, the matching expansion joints of the electric plug-in insulation door and the circular baffle door (double seal) should be tightened symmetrically when installing, and the flange connecting bolts should not be tightened and loosened at the same time, otherwise the sealing surface will skew and leak.
How long is the lifespan? Don't listen to "maintenance-free for life"
Both metal bellows and expansion joints have fatigue life, and the number of cycles are usually designed between 1000 and 10000 times. Depending on the specific use conditions: high temperature and high pressure, frequent start-and-stop, vibration and shock will shorten the life. The service life of the expansion joint in our FAQ is generally recommended to be replaced in 5~8 years, but the double hinge expansion of the air-cooled island vacuum pipeline can save energy for longer, because the working temperature is low and the circulation is less.
So when should I change it? If there is obvious deformation, leakage, and peeling of the bellows surface, it has to be replaced. Don't think that you can make do with it. Once the bellows peels, it means that the material has been fatigued and failed, and if you carry it on, it will burst. By the way: the national standard for non-metallic expansion joints is JB/T 12235-2015, and the metal expansion joints refer to GB/T 12777. Remember to check the standard number when buying products. Some small factories do not produce according to standards, the wall thickness is discounted, the number of corrugations is less than two waves, and the price is low, but the life span can't even last for three years.
Do you choose metal bellows or expansion joints? Remember one principle: look for the hose for flexible connection, and look for the expansion joint for displacement compensation. When you are unsure, look through the product information on our website, and the working conditions and parameters of each type of product are clearly marked, or ask the technology directly.