What exactly is a bidirectional metal expansion joint? — — What is the difference between it and ordinary general-purpose corrugated expansion joint and straight pipe pressure balance expansion joint
Don't rush through the parameter table. Let's find out what the hell this bidirectional metal expansion joint is. To put it bluntly, it is a compensation element specially used to "eat both ends" displacement-the thermal expansion and contraction of the pipeline, which can absorb deformation in both directions, and balance the pressure and thrust by its own structure.
What's the difference between it and ordinary general-purpose corrugated expansion joint? The universal expansion joint is a single bellows, without a tie rod, and all depends on the bellows itself to carry the pressure and thrust. Once the pressure is high, or the displacement is large, the bellows is prone to fatigue cracking. The two-way metal expansion joint, with its own pull rod and pressure balance element, can digest the thrust generated by the internal pressure of the pipeline by itself without transmitting it to the fixed bracket. This makes it particularly useful on high-pressure, large-diameter steam pipes.
Let's compare the "straight tube pressure balanced expansion joint" on our station-that thing also balances thrust, but it usually only absorbs axial displacement, and its structure is more complex, with a pair of balanced bellows at the end. The two-way metal expansion joint is strong in that it can absorb both axial displacement and certain lateral displacement (adjusted with hinge or tie rod), and has higher flexibility. Simply put: the straight pipe pressure balance type is "specializing all the way", and the bidirectional metal expansion joint is "all-rounder".
Disassembly of core structure: bellows, guide tube, tie rod and pressure balancing element perform their respective duties
A bidirectional metal expansion joint, when disassembled, has four core parts:
- Bellows: This is the soul, which is responsible for absorbing displacement. The material is usually stainless steel 304 or 316L, and 316Ti or Incoloy is obtained in high temperature conditions. Wave number, wall thickness and layer number directly affect fatigue life and pressure bearing capacity.
- guide tube: It is installed on the inner wall of the bellows, and its function is to guide the medium to flow smoothly, so as to avoid high-speed airflow or medium containing particles directly washing the bellows. Go to the question and answer on "The Specific Function of the Expansion Joint Guide" on our station, and it is very clear-without the guide, the bellows will be worn out in minutes. Note: The direction arrow of the guide tube must follow the flow direction of the medium. If it is installed backwards, it is equivalent to white.
- tie rod: This is the "brake system" of the bi-directional metal expansion joint. Both ends of the pull rod are fixed on the end plate by nuts to limit the axial deformation of the bellows within the set range. At the same time, it can also withstand the pressure thrust and prevent the pipe from crushing the bellows to death. But remember: the adjustment nut on the pull rod is temporarily fixed during installation. Don't screw it to death, otherwise the bellows can't expand and contract freely.
- Pressure balancing element: It is common to add a balanced bellows on each side of the bellows, or use a set of hinges and tie rod combinations. Its function is to counteract the axial force generated by internal pressure, so that both ends of the pipe are not subjected to thrust. This is particularly important at sensitive locations such as pump outlet and turbine interface.
These four pieces cooperate well, so as to ensure that the expansion joint will not have any moths during its service.
Hard parameters that cannot be avoided in type selection: temperature, pressure, displacement, medium corrosiveness, none of which can be missed
Two days ago, I met a customer and gave me a parameter table, only writing "DN600, working temperature 200℃, pressure 1.6MPa", and asked me to quote. I asked: What is the displacement amount? What is the medium? He was stunned. This is typical of stepping on a pit.
When choosing a bidirectional metal expansion joint, four parameters must be defined:
- Design temperature: Don't just look at the working temperature, but also consider the peak value of start-stop conditions. Temperature determines the material of the bellows, the material of the guide tube (ordinary Q235 will oxidize and peel at high temperatures), and whether it needs to be lined with thermal insulation cotton. For example, the temperature of hot air pipeline in cement industry often rushes above 500℃, so high-temperature axial expansion joint or corresponding high-temperature alloy must be selected.
- Design pressure: The higher the pressure, the more the wall thickness and the number of layers of the bellows, and the cross-sectional size of the tie rod should also be increased. At the same time, don't forget to check the load-bearing capacity of the pressure balance element, otherwise the fixed bracket will not be able to bear it under high pressure conditions.
- displacement amount: Including the amount of axial tension/compression, as well as possible lateral displacement. The compensation ability of bidirectional metal expansion joint for axial displacement is generally between ±50mm and ±200mm, depending on the wave number and wave height. Lateral displacement is realized by hinges or large tie rods. For example, the "compound hinge transverse expansion joint" on our station is specially transverse. If the displacement is not accurate, either the bellows will be stuck when installed, or the fatigue life will be cut in half directly.
- Medium corrosivity: Steam, flue gas, chloride-containing, sulfur-containing…Each medium has a completely different corrosion rate on the bellows material. For example, 316L or even duplex stainless steel must be used for desulfurization flue gas, otherwise it will be perforated in one year. Why don't you choose a rubber compensator, you ask? The flue gas rubber with a temperature above 200℃ can't bear it, and it has to be made of metal.
If one of these four parameters is less, the follow-up will be an accident.
Those pits at the installation site: how serious are the consequences when the direction of the arrow is reversed, the tie rod is not dismantled, and the pre-stretching is not done?
A bidirectional metal expansion joint was installed in the steam pipeline of a power plant, and the workers installed the arrow of the guide tube backwards. As a result, after three months of operation, the bellows was torn by high-speed steam, and the steam sprayed out on the spot and almost hurt people. The direction of the arrow refers to the direction of the medium flow, which is basic common sense, but some people always ignore it.
The locking nut on the tie rod is not loosened. Many manufacturers screw the nuts at both ends of the tie rod to death to keep the expansion joint compressed for the convenience of transportation when they leave the factory. Once installed in place, the nut must be loosened to the designated position (usually leaving a pre-stretched amount of clearance). If you don't loosen the nut, the bellows can't move at all, and the stress caused by thermal expansion and contraction of the pipe is all suppressed on the weld, and the consequence is that the pipe bursts.
Pre-stretching is not done. Especially for high-temperature pipelines, the ambient temperature and operating temperature during installation may be one or two hundred degrees different. If the expansion joint is not pre-stretched for a section according to the calculated value, when the pipeline heats up, the expansion joint will be directly compressed to the limit, and it will fail soon. Let's stand on the question and answer of "How to adjust the tie rod nut of the expansion joint" in detail, and novices suggest turning it over first.
Do not use expansion joints to forcibly adjust pipe installation errors during installation. Many people's pipes can't match their mouths, so they rely on pulling the expansion joint to make up the distance. This is called forced displacement, which will make the bellows generate extra stress and greatly reduce its life. The right thing to do is to readjust the pipe support.
Daily maintenance and fault judgment: how to find out in advance when the guide tube falls off and the bellows is fatigued cracked?
Bidirectional metal expansion joints rarely require routine maintenance, but regular inspections cannot be saved. And guess what? Many factories throw expansion joints there as "iron bumps" and don't replace them until they leak.
What is most likely to go wrong? The guide tube fell off. Because one end of the guide tube is fixed in the tube, and the other end is the free end, if the medium flow rate is high or there is vibration, the weld of the guide tube at the free end will crack and fall off. The fallen debris is stuck in the pipeline, which can block the pipe at least and damage the downstream equipment at worst. How to find out? Listen to the sound-If there is a sudden noise or flow fluctuation in the pipe, it is likely that a section of the guide tube has fallen off.
Usually occurs at trough locations. Early cracks are difficult to see with the naked eye, but they can be tested with penetration detection or ultrasound. It is easier to observe the bellows surface for corrosion pits or discoloration. In addition, if the thread of the tie rod is found to be worn or deformed, it means that the expansion joint is running over-displacement, and you must quickly check whether the pipe support has been displaced.
If it is a double bellows balanced construction, one of the bellows leaking will cause the other to take full pressure and will soon break too. Therefore, usually pay more attention to the pressure gauge (if any) of the balance chamber at both ends, or feel the temperature by hand-hot and cool at the same time, then something must be wrong.
Case: From power station steam pipeline to cement industry hot air pipeline, how to use bidirectional metal expansion joint is the right way
Let's talk about a few practical cases.
Steam pipeline of power station: Main steam pipeline of a thermal power unit, design temperature 540℃, pressure 10MPa, DN500, axial displacement 80mm. The bidirectional metal expansion joint, with guide tube and tie rod, is selected on site, and the material is 316Ti, lined with thermal insulation cotton. It has been in operation for three years without a single problem. Key point: The pre-stretching amount is calculated as 40mm according to the formula. When installing, pull it in place with a hydraulic jack, and then lock the outer nut. In addition, all brackets adopt guide brackets to ensure that the pipeline does not deflect.
Hot Air Pipeline in Cement Industry: The flue gas temperature of cement kiln head is as high as 450 ℃, containing dust. With a common general-purpose corrugated expansion joint, it wears out in a year. Later, it was replaced with a bidirectional metal expansion joint, but two points should be paid attention to: First, the guide tube must be thickened, and the material must be heat-resistant steel or stainless steel, otherwise the guide tube will be worn out first; The second is to install a cyclone dust collector in the front section of the expansion joint to reduce the dust content. In this way, the life span is extended to more than three years. In addition, the cement plant has great vibration, so it is necessary to add anti-loosening nuts on the tie rod, otherwise it will loosen during operation.
If the displacement you want is particularly large (for example, the axial direction exceeds 200mm), and a two-way metal expansion joint can't be handled, you can consider using two in series with a fixed bracket in the middle. Or go directly to the "compound straight pipe bypass pressure balance expansion joint" on our station, which can achieve large compensation in a narrow space.
Selection is a meticulous job, the parameters are right, the structure is understood, and the installation is not wrong. An expansion joint can be used for more than ten years without any problem. Conversely, it will be scrapped in three months, and the pipeline will be damaged. So don't be too long-winded, read the above points a few times, and take less detours to really save money.