Selection of metal expansion joints of pipes, overturning cases have seen too many. From leakage to shutdown to pipeline disconnection, this is not a scare. A while ago, a customer of a chemical plant took a general-purpose corrugated expansion joint model and wanted to install it on the high-temperature steam pipeline, and asked me if I could. I said you don't rush to buy first, find out five questions before placing an order.
First, what does the metal expansion joint of the pipeline do? Don't confuse it with a metal hose
Expansion joints and metal hoses look a little like each other, but they do completely different things. Metal hoses are mainly used for flexible connections, such as vibration damping connections of pump ports and compressor inlet and outlet, which mainly absorb installation deviations and a small amount of vibration. The core task of the metal expansion joint of the pipe is to absorb thermal displacement-the pipe heats up and expands by tens of millimeters, so you have to give it a "breathing" space.
What happens when you choose the wrong type? Let me tell you the truth. In a project, a metal hose was used on the steam main pipe for compensation. As a result, the corrugated part was axially compressed and killed, the weld cracked, and the steam was sprayed out like a cannon. Therefore, the classification of expansion joints in this station is particularly detailed: general-purpose corrugated expansion joints, high-temperature axial expansion joints, directly buried (fully buried) expansion joints... Each structure corresponds to a specific working condition. Find out the displacement type first and then talk about the selection. This order cannot be reversed.
Second, seat according to the type of displacement: axial, transverse, angular or combination?
Axial expansion, lateral offset, angular rotation, or a superposition of several. Choose axial type for axial displacement, such as external pressure single axial expansion joint. The advantage of external pressure structure is that the bellows bears external pressure and has better stability, which is suitable for large-diameter high-pressure pipelines.
The lateral displacement at the corner of the pipe is large. If you take the axial type to install it, the bellows will bend laterally, and the life will be cut in half directly. In this case, double hinge transverse expansion joint or double straight pipe bypass pressure balance expansion joint should be used. When it is necessary to bear internal pressure thrust and the space is limited, straight pipe pressure balance type and curved pipe pressure balance type are the first choice. There is a balance bellows inside the pressure balance expansion joint, which can digest the internal pressure thrust by itself, and the force of the fixed bracket is much less.
Large-diameter thick-walled pipeline, the thermal displacement is not large but the pressure is high, then large-diameter thick-walled expansion joint is necessary. And guess what? Many customers take a model and want to take all working conditions, which is the most prone to accidents. Choosing expansion joints is not about buying clothes, and you can't expect one-size-all.
3. Medium and temperature determine the material and structure of corrugated pipe
The metal expansion joints used in steam pipes, flue gas pipes and corrosive media are completely different. Corrugated expansion joints used in power stations have to withstand high temperature and high pressure, and the material of corrugated pipes is 316L or even Inconel; The metal corrugated expansion joint in cement industry should be wear-resistant and dust-resistant, and the guide tube should be thickened, otherwise it will be washed out in a few weeks; The expansion joint matched by the desulfurization flue gas baffle door is also resistant to acid and alkali corrosion, and the ordinary 304 can't last long.
Here we will focus on the deflector. The specific function of the expansion joint guide tube is to guide the high-speed fluid away and protect the bellows from being directly washed. When the medium flow rate is high or contains particulate matter, the guide tube cannot be saved, nor can the thickness be saved. Some customers remove the deflectors in order to save hundreds of dollars. As a result, the bellows wear out a year, and the labor cost of replacing them once is enough to buy ten deflectors.
4. Installation and constraint design are more important than the product itself
The expansion joint can't be installed when it is bought back, and it will break when it is installed, which is mostly a problem of constraint design. Many people only pay attention to the expansion joint to buy, neglecting such things as tie rod, hinge, pressure and thrust. There is one correct installation method of large tie rod expansion joint: the tie rod nut cannot be disassembled casually. Those tie rod nuts are adjusted when they leave the factory. Whether it is transportation protection or restraint reaction force, they have to be adjusted according to the drawings. If the wrong bellows is disassembled, it will be over-stretched and directly scrapped.
The role of the expansion joint tie rod is to restrain the internal pressure thrust and prevent the bellows from over-stretching. You bought a double hinge type. When installing, the hinge direction was 90 degrees wrong, and the scene was dumbfounded-the displacement direction was perpendicular to the hinge rotation surface, and the expansion joint couldn't move at all. Therefore, when selecting the type, report the direction of the pipeline, the position of the fixed bracket and the spacing of the guide bracket to the manufacturer together. Don't just give a caliber and pressure. If the bracket is not right, no matter how good the expansion joint is, it will be in vain.
5. Ask clearly before buying: What standards should be implemented? How to calculate the service life?
There are many customers who search for "Metal Expansion Joint Weight Table" on the Internet, but the weight is only for reference, and the key depends on the design fatigue life. The service life of expansion joint is related to the fatigue times of bellows, media corrosion and installation quality. Manufacturers will generally give design fatigue life, such as 1000 times or 5000 times, corresponding to specific displacement and pressure conditions.
Ask the manufacturer for the formula for calculating the stiffness of the bellows, and you can also check the thrust yourself. The formula is actually not complicated. The stiffness multiplied by the displacement is the elastic thrust. Coupled with the internal pressure thrust, we can calculate how much force the fixed bracket has to bear. These data are well known, and I have the confidence to communicate with the design institute.
Also, what is the difference between a metal expansion joint and a corrugated expansion joint? Actually, it's one thing, it's just a different name. However, non-metallic expansion joints (fabric fiber expansion joints) are another kind of thing. Fabric bands are temperature-resistant and corrosion-resistant but have low pressure-bearing capacity, so don't mix them.
What industry cases have you done? Power stations, cement, desulfurization or chemicals? Only when you have cases in the same industry can you have a bottom in your heart. The expansion joint, no matter how beautiful it is drawn on the drawing, dares not pat your chest without the actual working condition inspection.