What is the difference between pressure metal expansion joint and ordinary expansion joint?
Everyone in the dry pipeline industry knows that ordinary expansion joints (such as general corrugated expansion joints) are enough to deal with low-pressure and normal-temperature pipelines. However, once the medium pressure is 4 MPa, 6 MPa or even higher, and the temperature soars to 500 or 600 degrees, the ordinary "skin bag" can't bear it-the bellows will be flattened, the weld will crack, and the guide tube will fall off. At this time, the metal expansion joint must be pressed. To put it bluntly, the biggest difference between it and ordinary expansion joints is that the structure is specially designed for high-pressure working conditions, which can hardly carry the internal pressure thrust of the pipe system and absorb thermal displacement at the same time. You can just understand it as "expansion joints with armor".
Structural hardcore design: How do bellows, guide tubes and tie rods work together?
The core component of pressure metal expansion joint is still corrugated pipe, but the material, number of layers and wall thickness are different. For example, for corrugated expansion joints in power station industry, multi-layer stainless steel bellows are often used, with gaps between each layer to reduce stiffness, but the total wall thickness is larger than ordinary ones. The purpose is one: compression resistance and no wrinkles.
The role of the deflector is more critical. Many people think that the guide tube only diverts the flow. In fact, under high pressure conditions, it directly bears the erosion of the medium to prevent the high-speed fluid from directly impacting the inner wall of the bellows. We had a FAQ before that specifically talked about the specific function of the expansion joint guide tube, which will not be expanded here. But remember one sentence: there is no guide tube in high-pressure pipeline, and the life of bellows is directly cut in half.
Pull rod-this thing is the soul of pressure metal expansion joints. Ordinary expansion joints only rely on the bellows to carry the axial force by themselves. Under high pressure, the bellows will be crushed to death like an accordion. Therefore, straight pipe pressure balance expansion joint and compound hinge transverse expansion joint are equipped with tie rod or hinge structure. The function of the tie rod is to transmit the internal pressure thrust to the pipe flange by locking the nuts at both ends, so as to prevent the bellows from being stressed. If the tie rod nut is not adjusted properly during installation, or if the transport screw is lazy and not disassembled, the expansion joint will be scrapped. I have seen this situation more than once-the customer said, "We installed it, and the boiler water leaked as soon as it was installed." When I took it apart, the tie rod was locked and the bellows had no room to move at all.
Which working conditions must it be? Real cases of power stations, petrochemicals and cement
Let's talk about the power station first. The main steam pipeline and the reheating hot section pipeline have a temperature of 570℃ and a pressure of over 10 MPa. Ordinary expansion joints fail after half a month. We equipped an ultra-supercritical unit with a corrugated expansion joint for power station industry. The corrugated pipe material is Inconel 625, and a double-layer guide tube with inner lining insulation layer was designed. It took three years to test the fatigue life of 80%.
Look at the petrochemical industry again. The high-temperature flue gas pipeline of the catalytic cracking unit has a temperature of about 700 DEG C and contains particles and corrosive gases. In addition to high temperature and pressure, there are also wear problems in this kind of occasion. Metal corrugated expansion joints in cement industry are often used in flue gas pipelines at the head and tail of kiln, which not only compensate for axial displacement, but also bear the thermal shock caused by frequent driving and stopping. Last year, there was a cement factory customer. The original non-metallic expansion joint (fabric fiber expansion joint) rotted in one year. After changing the metal corrugated expansion joint in the cement industry, it lasted for more than two years. The price is double the cost, but do the general ledger – save the loss of two parking overhauls.
Vacuum pipes. For example, the double hinge expansion joint of air-cooled island vacuum pipeline has to bear negative pressure and prevent instability. Crushing expansion joints are common under internal pressure conditions, and suction deflation is also common under negative pressure conditions. Therefore, the selection must look at the direction of pressure.
Pits stepped on during type selection: wrong calculation of pressure grade, neglect of fatigue life, and reverse installation direction
The pressure rating is only the design pressure, and the hydraulic test pressure and impact pressure are not considered. The design pressure of a petrochemical project was 6.4 MPa, and an expansion joint with a nominal pressure of 6.4 MPa was selected. As a result, the hydraulic test reached 8 MPa, and the bellows was directly blasted. Later, a straight pipe pressure balance expansion joint with a nominal pressure of 10 MPa was changed before it passed. Therefore, when selecting the model, the pressure level should be at least one gear higher than the design pressure, leaving a margin.
Fatigue life is treated as an ornament. Many purchasers only know how to look at the number of bellows layers and wall thickness, regardless of the design fatigue life. International standards usually require 1,000 cycles, but thermal power units start and stop once a day, which is 365 times a year, and they will exceed in three years. If you design 1000 times, it will have to be changed in three years. The correct way is to clearly design the fatigue life with the manufacturer. For example, if it is required for 5,000 times, the thickness and number of layers of the bellows will change accordingly. The service life of the expansion joint is mentioned in the FAQ, and the core is to look at both the number of fatigue times and the corrosion allowance.
Install in reverse direction. The direction of the arrow of the expansion joint indicates the direction of media flow, not the direction of installation. A customer installed the expansion joint backwards, and the end of the guide tube faced downstream. The medium impacted the bellows in the reverse direction from the gap, and it wore out in three months. If you can't remember, remember one sentence: The big mouth of the guide tube faces the incoming flow, and the small mouth faces the outlet. Look carefully at the direction of the arrow of the expansion joint before installation.
Old problems in installation and routine maintenance: adjustment of pull rod and disassembly of screw
Let's start with the tie rod adjustment. How to adjust the expansion joint tie rod nut? The principle is: before installation, screw the tie rod nut to the limit position to ensure that the bellows is in free length (or cold tightness). Once the pipe is connected, loosen the locking nut on the tie rod to allow the tie rod to move freely. Otherwise, locking of the tie rod will limit the deformation of the bellows, which is equivalent to no expansion joint. Will the screw be removed? The transport screw must be removed after installation, but the tie rod itself is not removed – it is part of the permanent structure. To save trouble, some workers forgot to disassemble the transport screw after loosening the tie rod nut. As a result, the expansion joint was pulled off directly during the pressure test of the pipeline. Remember: the transport screw is "disposable" and should be thrown away after installation.
In terms of routine maintenance, pressure metal expansion joints are most afraid of two things: external corrosion and internal dust accumulation. External corrosion mainly occurs in chemical plants, and a layer of stainless steel skin or anti-rust paint can be wrapped around the bellows. Dust accumulation inside? For high-temperature flue gas pipelines, dust will accumulate in the bellows trough, affecting the compensation amount. Regular purging with compressed air from the guide tube injection port solves the problem. In addition, check whether the tie rod nut is loose every six months. After all, thermal expansion and contraction plus vibration, nut rebates are nothing new.
At the end of the day, pressure metal expansion joints are not universal parts, and every item has to be tailored. If you choose the right one, you don't have to worry about it for ten years; If you choose the wrong one, you will have to change it in half a year. Do you have a project in hand to put on a high-pressure pipeline now? Compare the above to see if there is any risk of stepping on pits.