1. Why should metal expansion joints be installed in heating pipes? What happens if you don't pretend
Heating pipes are simple to say. Once hot water is delivered, the whole building is hot. However, the pipe itself is metal, and it expands when the temperature is high. A 100-meter-long pipe can grow about 120mm when it heats up to 100℃. Think about it, if the pipe network of the whole building has no place to absorb deformation head-on, how big will the pipe stress be? It is no exaggeration to crack the flange, deform the bracket, twist the compensator itself into a twist, and even collapse the weld directly.
The bracket is responsible for "biting" the pipe at a certain point, and the expansion joint is responsible for absorbing the heat displacement between the fixed brackets. Both are indispensable, with only a bracket and no compensator, the pipeline is held, and the stress is all transmitted to the equipment; There is only a compensator without a bracket, the displacement direction is not controlled, and the expansion joint swings by itself, which is still broken. Installing metal expansion joints in the heating network is not an optional question, but a must-answer question.
2. Selection of expansion joint under heating conditions: Temperature, pressure and displacement calculation are the core
The heating pipe looks mild, but the selection can't be vague at all. The design temperature is based on the highest temperature of the medium, generally 150℃ or even higher. It doesn't look at the 70℃ or 90℃ when you are running, so you should leave a margin. The pressure is similar. For pipelines with a working pressure of 1.6MPa, the expansion joint should be checked according to the design pressure. Moreover, the pressure resistance of the bellows is directly related to its number of layers, wave height and wall thickness, and it is not the thick pipe that can carry the pressure.
What really makes a lot of newbies roll over is the displacement calculation. How to calculate the amount of thermal displacement? Is the length of the pipe section multiplied by the linear expansion coefficient and then multiplied by the temperature rise. But there is a pit buried here: the compensation algorithm is different for vertical pipes and horizontal pipes, pipe sections with elbows and straight pipe sections without elbows. Many people only know how to calculate axial displacement, ignoring lateral displacement and angular displacement. As a result, after the expansion joint is installed, the bellows works in a twisted state, and its life is greatly reduced. There are many elbows in the heating system, and the displacement of the actual pipe system is three-dimensional. When selecting the type, it is best to calculate the displacement of the whole pipe system clearly, and then decide how much compensation to choose the expansion joint.
3. Common structural forms of metal expansion joints: how to distinguish between general type, external pressure type and direct burial type
The three most common structural forms of heating pipe network, don't get confused.
General-purpose corrugated expansion joint, to put it bluntly, is a single-layer or multi-layer corrugated pipe plus two-end pipes, which has simple structure, low cost and large compensation, and is suitable for overhead or trench laying pipes. It absorbs displacement by the deformation of the bellows, which can absorb both axial displacement and a small amount of lateral displacement.
External pressure single type axial type expansion joint, bellows under external pressure, the outside of bellows under medium pressure during operation, unlike the general type, the inner wall of bellows directly bears pressure. This structure is not easy to produce column instability, and is suitable for working conditions with higher pressure or stricter stability requirements.
Direct buried (fully buried) expansion joint, specially designed for direct buried laying, with protective sleeves on the outside, and some with waterproof structures, is directly buried in the soil without setting an inspection well. There are more and more direct buried heating pipes, but when choosing the direct buried type, it should be noted that its outer protective sleeve must be able to withstand soil pressure and groundwater pressure, and the welding joint should be reliable during on-site installation, otherwise the groundwater will seep in and the bellows will quickly rust through.
General type of overhead pipe should be selected, external pressure type should be selected steadily for high pressure, and direct buried type should be selected for direct buried section.
4. Details easily overlooked in installation and maintenance: guide tube, pull rod, arrow direction
If you choose the right equipment, if you are sloppy in installation, it will be useless. The specific function of the expansion joint guide tube is to protect the inner wall of the bellows, isolate the impurities in the medium from the scouring of high-speed fluid, and reduce the flow resistance at the same time. When installing, the opening direction of the guide tube must follow the flow direction of the medium. If it is connected backwards, it is equivalent to not installing. The guide tube has a positioning function, and it can also reduce the wear of the inner wall of the bellows to a certain extent. Especially if the water quality is not good and there are particulate impurities in the heating system, this thing is especially important.
Tie rod this thing, many people think it is only for transportation protection. Not. For hinge-type and tie-rod-type expansion joints, the tie rod is a force-bearing member used to withstand the internal pressure thrust and prevent the bellows from being pulled off. The core function of the expansion joint tie rod is to constrain the displacement direction of the bellows and transmit the axial thrust to the pipe support. After installation in place, how to adjust the tie rod nut? You can't simply tighten it with a wrench, but you should adjust it according to the cold tightening amount of the design drawing, so that the pull rod can be in the position of force balance when it is hot. It is overscrewed, and the expansion joint has no compensation amount; It was loosened, and the internal pressure and thrust all hit the bellows.
Also, the direction of the arrow of the expansion joint refers to the direction of the medium flow, which usually coincides with the direction of the guide tube. This point is easily overlooked during installation. If the arrow direction is reversed, the cooperation between the bellows and the guide tube will be messed up, and some structures will even directly cause media backflow and scouring.
5. Quality control from fault cases: corrugated materials, welds and fatigue life
I met a customer two days ago, and the expansion joint on the heating pipe leaked after less than two heating seasons. When disassembled, there are severe pitting pits on the surface of the bellows, and there are also cracks in the heat-affected zone of the weld. The reason is not complicated: the corrugated material is 304 stainless steel, the chloride ion content in the heating water quality is high, and there is no effective anti-corrosion treatment.
Metal expansion joint for heating, 304 or 316L is generally used for bellows material. 304 is cost-effective, but if the chloride ions in the water quality exceed the standard, 316L or even better corrosion-resistant alloy must be used. This should be confirmed at the time of model selection, rather than remedied when something goes wrong.
How to control the quality of weld? Look at two points: First, the longitudinal weld of corrugated pipe must be 100% flaw detected, and unqualified products should be directly hit back; Second, the distance requirement between the weld joint and the bellows body is installed on the spot. According to the standard specifications, the welding heat cannot affect the bellows trough area, otherwise the fatigue life of the bellows will be immediately discounted due to local annealing. Fatigue life is calculated by the number of cycles. The temperature of the heating pipe network fluctuates all day long, and the expansion joint may go through several expansion and contraction cycles a day, and the design life requirement is generally more than 1000 times. The wave number, wall thickness and layer number of the expansion joint bellows you bought are wrong, and it will crack in less than two years.
Therefore, on the surface, the metal expansion joint for heating is a "several pleats added to the steel pipe". In fact, from design calculation, material selection to installation and maintenance, every step is particular. Choose a wrong parameter and save the purchase difference, which is not enough to compensate for a heating stop accident.