Why is it an expansion joint, not a corrugated compensator? — Different names, but do you really know the difference?
Expansion joints and corrugated compensators are the same thing at 99% of operating conditions. But if you write blindly in the bidding documents, the supplier will know at a glance that you are a layman. Most of the people in the industry who are used to calling "expansion joint" are old mages in electric power, heat and chemical industry, while the word "compensator" is more favored by HVAC and water supply and drainage circles. In essence, the thermal displacement of the pipeline is absorbed by the deformation of the bellows, but the metal expansion joint as appears particularly frequently in power stations and desulfurization projects-because it combines the advantages of high-temperature axial expansion joint and pressure balance expansion joint.
If you don't believe me, go to "Corrugated Expansion Joints for Power Station Industry" and "High Temperature Axial Expansion Joints" on our station. The number of corrugated layers and guiding structures of metal expansion joints as are all different. So stop worrying about the name. The key depends on whether the model you choose can withstand the temperature and pressure of the scene.
Look at these points in the selection: medium, temperature, displacement-how to choose general-purpose type, high-temperature type and pressure balance type?
Select the metal expansion joint as, and three parameters determine life and death: media composition, maximum operating temperature, and the amount of displacement to be absorbed.
Let's start with media. If it is a clean medium such as flue gas and steam, the universal corrugated expansion joint is completely sufficient. However, if the wet flue gas behind the desulfurization tower contains chloride ions and sulfate radicals, you have to consider lining PTFE hose or PTFE compensator, otherwise ordinary stainless steel will pitting and perforating in less than half a year. Two days ago, I met a buddy from a cement factory. I bought a general-purpose type cheaply and installed it in the kiln head. As a result, air leakage began in three months-there were dust particles in the medium, and no matter how good the 316L was, it couldn't bear it.
Temperature is a hard indicator. The upper temperature resistance limit of the conventional general-purpose type is 350℃. Beyond this number, high-temperature axial expansion joints must be installed, or non-metallic expansion joints (fabric fibers) must be used. For example, the temperature of the main steam pipe of the power station is 560℃. If you install it with the general type, the bellows will directly crack by thermal fatigue. The "High Temperature Axial Expansion Joint" on our station has specially designed multi-layer heat-resistant alloy corrugations for this kind of working conditions, and can also be installed with heat-insulating guide tubes.
The amount of displacement determines the structure form. Where the transverse displacement is large, use the double hinge transverse expansion joint; With large axial displacement, the straight pipe pressure balance expansion joint is the most stable; If the pipeline is cramped, the pressure balance expansion of curved pipe saves energy and space. The metal expansion joint as is popular because of its high integration-one product can absorb both axial and lateral displacement, saving the hassle of multi-stage compensation.
The most easily overlooked tie rod and guide tube during installation-if you install it wrong, it will leak within a year of use
Installing metal expansion joints as, 90% of field problems lie in two places: tie rods and deflectors.
Many people think that tightening the pull rod is done. Big mistake. The role of the expansion joint tie rod is to prevent the bellows from over-stretching or twisting during transportation and installation, but after the equipment is in place, the tie rod nut must be adjusted according to the design displacement-either loosened to the specified gap or removed. There is an article on our station called "Does the screw of the expansion joint need to be disassembled?", which makes it clear: If the tie rod is not loosened, the expansion joint can't move at all when the pipeline is thermally expanded, the bellows is hard supported, and the weld cracks. And guess what? I've seen people weld tie rods to death as reinforcements at the scene, which is called a desperation.
First, guide the medium to flow smoothly to avoid washing the bellows; The second is to reduce flow resistance. However, some installation teams try to save trouble, and install the guide tube in the opposite direction-the arrow should point to the medium flow direction. If it is misaligned, the high-speed airflow will directly hit the ripples, or it will wear out and leak in less than a year. A schematic diagram was specially drawn in "The direction of the arrow of the expansion joint" on our station. To put it bluntly, it is: the bell mouth of the guide tube must face the direction of the incoming flow, so don't turn it backwards.
as for the metal expansion joint as, its deflector is not the same as the ordinary model-because it has to take into account both high temperature and high pressure, the deflector has a thicker wall thickness and is lined with wear-resistant coating. If you install it backwards, it will wear out three times faster than you think.
The three culprits of shortened life: corrosion, fatigue and overlimit-how to find out in advance?
Corrosion of this thing, to put it bluntly, is a mistake in selecting materials. The most common failure modes of metal expansion joint as in desulfurization and chemical scenarios are pitting and stress corrosion cracking. How to find out in advance? Look at the corrugated surface for needle-tip-sized rust pits, or blackening and peeling. If you have the conditions to do penetration testing regularly, if you have the conditions to stare at it with a magnifying glass, you can also see the doorway.
Fatigue is another chronic disease. The vibration of the pipeline, frequent start-and-stop, repeated deformation of the bellows, and the stress concentration part is easy to produce micro-cracks. Especially for straight pipe pressure balance expansion joints, if the bracket is not properly installed, the fatigue life will be cut in half directly. There is a case on our station: an air-cooled island project used a double-hinged expansion joint of air-cooled island vacuum pipeline. After two years of operation, it was found that there were cracks in the trough. Later, it was found that the frequency of hot and cold alternating was too fast, and the number of cycles was not considered in the design. Therefore, when selecting the model, we must mention the start-stop frequency of actual working conditions to the supplier, and don't just look at the temperature and pressure.
Exceeding the limit is when the displacement exceeds the design value. This problem is the most common in the cement industry-the deformation of the pipeline is considered small, and the expansion joint is dragged hard, and the bellows is directly unstable and bulging. Although the metal expansion joint as has good rigidity, you can't take it as a universal hercules. The way to prevent exceeding the limit is very simple: mark the initial position on the tie rod with red paint during installation, and measure whether the displacement has exceeded after half a year's operation. Once it is exceeded, the thermal expansion of the pipe system must be recalculated.
Industry case: Practical experience in power stations, cement and desulfurization scenarios-there is no need to step on some pits by yourself
The high-temperature axial expansion joint used on the main steam pipeline is most afraid of the guide tube falling off. During the overhaul of a 600MW unit, it was found that three pieces of the lining plate of the guide tube were broken, and the debris was stuck in the bellows, which wore through both layers of corrugations. The reason is that the expansion coefficient of the lining material is not calculated correctly, and the deformation can't be stuck at high temperature. After changing the metal expansion joint as, the guide tube was changed to an integral stamping and welding structure, and there was no problem again.
The temperature of the air duct of the kiln head and grate cooler fluctuates greatly. The rubber compensator was used at the earliest, and it was scorched within two months. Later, it was replaced with a rectangular non-metallic expansion joint, which has no problem in temperature resistance, but its sealing performance is poor and its air leakage is serious. Finally, a metal rectangular expansion joint with a non-metallic fabric composite layer is put on, which is not only high temperature resistant but also sealing. A reminder here: Don't confuse the installation directions of metal rectangular expansion joints and rectangular non-metallic expansion joints-the former mainly absorbs axial displacement, while the latter is better at angular displacement.
The net flue gas pipeline after wet desulfurization has both corrosiveness and scaling properties. Before I used ordinary 316L bellows, I changed it four times in three years. Later, the metal expansion joint as was used, the corrugated material was upgraded to 254SMO, the inner wall of the guide tube was lined with PTFE, and then it was washed regularly. It has not been leaked for six years now. The key is that when selecting the model, the technical parameters of Desulfurization Flue Gas Baffle Door and PTFE Compensator on the station were specifically referred to, and the pH value and chloride ion concentration of the medium were clearly written into the technical agreement.
At the end of the day, metal expansion joint as is not a mystery, but every link-selection, installation, maintenance-hides pits that make you spend more money. According to the above experience, at least half of the detours can be saved.