Start with a pipe: the core structure and working principle of metal bellows expansion joint
What is a metal bellows expansion joint? To put it bluntly, it is an "elastic joint" in the pipeline system. The core component is a section of corrugated, thin-walled metal pipe-corrugated pipe. These ripples are like the folds of an accordion, allowing the pipe to stretch, contract and bend, absorbing the thermal expansion and contraction of the pipe due to temperature changes, or offsetting the displacement caused by the vibration of the equipment.
When the pipe is elongated by heat, the corrugations are compressed; As cooling shrinks, the ripples are stretched. At the same time, transverse displacement and angular displacement can be absorbed by multi-wave or special structure. To put it bluntly, it is to exchange its own deformation for the safety of the pipeline. A few days ago, a power station customer asked me, "Is this thing the same as a spring?" Yes, it's essentially a spring, except that the spring is spiral and the bellows is annular.
But don't underestimate this design. The wall thickness, wave height, wave pitch and the number of layers of bellows directly determine the compensation amount and pressure bearing capacity. For example, we commonly useUniversal corrugated expansion joint304 or 316L stainless steel is generally used, with a single layer wall thickness of 1-1.5mm, andHigh temperature axial expansion jointHeat-resistant alloys or even multi-layer composite structures will be used to balance temperature resistance and flexibility.
What is the difference between it and ordinary metal hoses and compensators? Don't get confused
Many people take metal bellows expansion joints and metal hoses as the same thing – it's not even close. Metal hoses (like oursMetal hoseAndSpecial hose for vacuum) is mainly used for flexible connections when conveying media, allowing a small amount of displacement, but the main purpose is vibration reduction and easy installation. The bellows expansion joint is specially designed to absorb large thermal displacement, and the structure usually comes with attachments such as end tube, flange, tie rod or guide tube.
What about expansion joints and compensators? These two words are basically generic. We also explained in the question and answer: expansion joints and compensators are different names for the same thing. However, it is customary in the industry to include non-metallic products (such asNon-metallic expansion joint、Rectangular non-metallic expansion joint) is called compensator, and the metal bellows is called expansion joint. But in fact, rubber compensator is also a kind of compensator, and its materials are different. Have you figured it out?
Several common metal bellows expansion joints on the market: axial type, pressure balance type, double hinge type... How to choose?
Before choosing a model, you must first understand the common types. We divide into several categories by structure and compensation direction:
- Axial type: likeUniversal corrugated expansion joint、External pressure single axial expansion joint、Direct buried (fully buried) type expansion joint。 The axial type is the extension or contraction of the main absorption tube in the axial direction. Simple structure, low cost, suitable for straight line pipeline.
- Pressure balance type: likeStraight pipe pressure balanced expansion joint、Curved tube pressure balance expansion joint、Compound straight pipe bypass pressure balanced expansion joint。 These products have built-in pressure balancing elements, which can offset the thrust generated by the medium pressure on the fixed support, and are suitable for large diameter or high pressure pipelines.
- Complex hinge type：Compound hinge transverse expansion joint、Double hinge expansion joint for air-cooled island vacuum pipeline。 Direction is restricted by hinges and tie rods, allowing only lateral or angular displacements, commonly used for L-or Z-shaped pipe sections.
- Other special types：Large diameter thick wall expansion joint(for large flue),Sleeve type pipe expansion joint(mechanical compensation),Rotary compensator(for steam pipes, etc.).
How to choose? Look at the pipeline direction and displacement amount on site. If there is only axial displacement, use the axial type; If there are both axial and transverse directions, or if there is a need to reduce the thrust on the support, the pressure balanced type is preferred.
Power station, cement, desulfurization... What role does it play in different industries?
Different industries, working conditions are very different. TakeCorrugated expansion joint for power station industrySpeaking, the temperature of the main steam pipe can reach five to six hundred degrees, and the pressure is more than ten MPa. It must be usedHigh temperature axial expansion joint。 Moreover, the pipeline of the power station has a large diameter and many welds, so the expansion joint should be able to absorb the displacement in multiple directions. A system like an air-cooled island would useDouble hinge expansion joint for air-cooled island vacuum pipelineSpecially dealing with vacuum conditions and large displacements.
What about the cement industry? High temperature and dust. For flue gas pipeline at outlet of rotary kilnMetal Corrugated Expansion Joints in Cement IndustryIt is usually equipped with heat-resistant lining and guide tube to prevent dust accumulation and high-temperature ablation. AndFlue gas baffle door、Desulfurization flue gas baffle doorUse together to achieve isolation and switching.
Desulfurization systemMore troublesome-the flue gas after wet desulfurization contains acidic condensate, which is extremely corrosive. At this timePTFE-lined hose、PTFE compensatorIt is more suitable, but the metal bellows expansion joint will also choose corrosion-resistant alloy (such as C276) or lined with PTFE. At the same time, the air duct section of the desulfurization system is large, so it is often necessary to useMetal rectangular expansion jointOrRectangular non-metallic expansion joints to absorb thermal displacement.
The easiest pits to step on during model selection: stiffness, guide tube and tie rod. Which of these parameters do you look at?
Stiffness. The rigidity of the bellows determines its reaction force to the pipe support. The stiffness is too large, the pipeline thrust is large, and the support is easily damaged; If the stiffness is too small, the bellows will easily become unstable. How to calculate? We have a special Q&A talkStiffness and Calculation Formula of BellowsBut to put it simply: stiffness is directly related to wave height, wall thickness and elastic modulus of material. When selecting the model, check the stiffness curve provided by the manufacturer to ensure that it is within the allowable range.
Deflector tube. Many people think that the deflector is dispensable-a big mistake.Specific Function of Expansion Joint Guide TubeIt is to protect the inner wall of the bellows from high-speed medium scour, especially in gas-solid two-phase flow (such as dusty gas flow in the cement industry). Without the deflector, the bellows may wear out in a few months. In addition, the guide tube can also reduce the medium flow resistance and noise. Therefore, when the medium flow rate is high or contains particles, it must be mixed.
Tie rods and nuts. The tie rod is not intended to adjust the length, but to restrain the bellows from over-stretching or compressing during installation.Function of expansion joint tie rodIs to prevent accidental deformation during transportation and installation. After site installation, the tie rod nut needs to be adjusted to the correct position (seeHow to adjust the tie rod nut of expansion joint) -Some are removed, others are loosened to the designated gap. Many projects are planted in this detail.
Some practical suggestions for installation and maintenance, especially attention to the adjustment of expansion joint of large tie rod
Let's start with the general principles when installing. First, the factory pre-stretch or pre-compression amount must be checked. When manufacturers leave the factory, they usually pre-deform the bellows according to the pipe installation temperature (for example, the axial expansion joint is marked with "cold tightness"). Strictly follow the marks during installation, and don't install it directly to save trouble. Second, the guide bracket cannot be saved. Especially for axial expansion joints, guide brackets must be provided at both ends to prevent the corrugated pipe from being twisted due to lateral instability of the pipeline. Third,The direction of the arrow of the expansion joint refers toMedia flow direction, don't pretend backwards.
Focus on talkingCorrect installation method of expansion joint of large tie rod。 Large tie rod expansion joints (belonging to the double hinge type or pressure balance type) are commonly used to absorb lateral displacement. When installing, first loosen all the nuts on the tie rod to make the bellows in a free state, then adjust the pipe counterpart, and finally tighten the tie rod nut to the specified gap according to the requirements of the drawing. Here is a formula: loosen first and then tighten, adjust first and then lock. Some masters try to save trouble and screw the nut to death at once. As a result, as soon as the pipe heats up, the bellows can't move, and the end pipe is directly cracked.
For maintenance, check the bellows surface regularly for cracks, corrosion pits, and wear. For corrosive environments, regular thickness measurements are recommended. There is also a point that is easily overlooked: whether the load of the spring support hanger is balanced. If the pipeline settlement causes the expansion joint to be unevenly stressed, the support and hanger should be adjusted in time.
Alas, having said so much, in fact, selecting and installing an expansion joint can make the pipeline system run safely for more than ten years. On the contrary, blind selection or violent installation will have to be changed once every six months. I hope this article will help you avoid some detours.