When choosing a non-metallic expansion joint, many engineers asked, "How much pressure can they withstand?" This question seemed simple, but if they were to choose according to the nominal pressure, they would probably go wrong. Nonmetallic expansion joint pressure is not the number printed on the ring belt, it is a systematic comprehensive indicator. Today we'll break it apart, crumble it into pieces and make it clear.
How Much Stress Does Non-Metallic Expansion Energy Saving Bear? First look at the essential difference with metal expansion joint
The metal corrugated expansion joint absorbs the displacement by the deformation of the thin-walled bellows. The bellows itself is a pressure-bearing element, and the pressure grade is indicated by PN. The non-metallic expansion joint is not this way at all-its main body is a fabric fiber band, and to put it bluntly, that layer of cloth does not rely on its own strength to resist compression at all.
How to carry the pressure in the circle belt? It depends on two things: one is the steel wire skeleton inside the ring belt, and the other is the structural constraint after the flange is compressed. You imagine a bulging balloon with a net pocket on the outside. The net pocket is what really limits the shape, and the balloon cloth only acts as a seal. The non-metallic expansion joint pressure system is highly similar to this logic.
This explains why the same non-metallic expansion joint, which is fine when used on a 30kPa flue gas pipe, bulges when changed to a 50kPa compressed air pipe-not because the pressure value is exceeded, but because the bolt preloading force on the flange pressure plate can't lock that thrust at all.
How to define pressure in standard: Design pressure and test pressure in JB/T 12235-2015
The current standard is JB/T 12235-2015 "Non-metallic Expansion Joint", which divides pressure into two completely different concepts: design pressure and test pressure. The design pressure is the highest pressure under normal working conditions, and the test pressure is the pressure applied during factory inspection. The difference between the two is a multiple relationship.
According to the standard requirements, the strength test pressure of non-metallic expansion joints is usually 1.25 times of the design pressure, and the holding time shall not be less than 30 minutes. The air tightness test pressure is equal to the design pressure, mainly depending on whether there is any leakage on the weld and flange surface.
Some people use the test pressure as the design pressure. Before leaving the factory, the pressure of 0.3MPa does not leak, thinking that this product can run under 0.3MPa for a long time-this is a gamble with your life. The test pressure is short-term loading, and the design pressure is the benchmark of the whole life cycle. The two concepts are 10 times different from the safety margin thinking.
The compressive strength is not determined by a layer of cloth: how do fiber layers, steel wire frameworks and flange structures work together to carry the compression
Take apart a non-metallic expansion joint (fabric fiber expansion joint), and you will find that the inside of the loop belt has something to do. The outermost layer is silicone rubber coated cloth, which is responsible for temperature resistance and corrosion resistance; The middle layer is a ceramic fiber blanket, which is responsible for insulation; Further inside is a PTFE film, which is responsible for airtightness-these layers together solve the problem of "no leak".
What really solves the problem of "not breaking" is the wire skeleton. The diameter and mesh density of the steel wire determine the radial tensile strength of the band. The flange structure is another decisive factor-the same ring belt, with a 6mm thick angle steel flange and a 10mm thick channel steel flange, allows the working pressure to be 50% different. The bolt spacing is encrypted from 150mm to 100mm, and the pressure rating goes straight up the next step.
Therefore, when selecting the model, don't just listen to the number of ring belt manufacturers, but take the flange and bolt specifications into consideration. The ring belt supplier and the expansion joint manufacturer are two different things. Many projects fail when they fail. The ring belt is bought and the flange is welded randomly.
Triangular relationship between pressure and displacement and temperature: pressure in flue gas pipeline does not exist in isolation
The most troublesome point of the pressure parameter of non-metallic expansion joint is that it is strongly coupled with displacement and temperature. The same product can withstand 0.2MPa at room temperature, but only 0.08MPa when the medium temperature is 300℃; The axial displacement is compressed from 50mm to 30mm, which allows the pressure to increase by 40% in turn-this is not the blind mark of the manufacturer, but the physical law that the material performance decays with temperature and the structural instability increases with displacement.
The dust content of flue gas will cause the wear of the ring belt, and the pressure bearing capacity of the wear belt will drop linearly. If you choose the type according to clean gas, after two years of operation, the dust grinds the fiber layer out of the steel wire and exposes it, so the original design pressure has no reference value.
The diameter of the pipe is 3 meters, the design pressure is 0.15MPa, and what is the axial thrust bearing by the end face of the non-metallic expansion joint? π ×1.5² ×150000=1060 kN, about 108 tons. This force is not eaten by the loop belt itself, but is transmitted to the pipe support through the flange. The bracket is not reinforced, the expansion joint is not broken, but the pipe is pushed away. This is why the pressure selection of non-metallic expansion joints must be accompanied by the bracket reaction force check.
On-site failures mostly occur during installation and commissioning: A guide to avoiding pits in pressure testing and operation misunderstandings
A while ago, I met a power plant customer. The non-metallic expansion joint was installed and suppressed. When it rose to 0.1MPa, I heard a "poof" sound, and the ring belt slipped out of the angle steel flange. When disassembled, the installer screwed the bolts one by one, and then directly boosted the pressure after pre-tightening them all for one round. The pressing force on the flange surface was completely uneven, and the pressure had not reached the design value, so the local instability became first.
The bolts are tightened in three passes in diagonal order, 30% torque in the first pass, 60% in the second pass, and 100% in the third pass. After screwing, check the flange clearance with a gauge, and the clearance deviation at any position should not exceed 0.5mm. After this step is done, let's talk about the stress test.
Another typical problem lies in the blind plate removal link after pressure test. After the pressure test is completed, once the blind plate is disassembled, the water or gas in the pipeline is relieved instantly, the pressure difference on both sides of the flange is too large, and the non-metallic ring belt is deflated by negative pressure-especially in the vertical installation position, and the ring belt is directly torn by its own weight plus negative pressure. The standard practice is: before removing the blind plate, open the exhaust valve or bypass valve to balance the pressure, and then slowly loosen the bolts.
The nominal pressure is only a reference value of the most surface layer. What really decides the success or failure is the structural design, installation quality and working condition matching degree. Remember these items, and then communicate with the equipment manufacturers. They will know that you are knowledgeable and dare not just quote you a price.