1. Understand what this thing is for, and then consider the recommendations
To put it bluntly, the metal corrugated expansion joint is the "respirator" in the plumbing system. The pipeline expands by heat, shrinks by cold, or the pump vibrates or the foundation settles, and there will definitely be problems in head-to-head confrontation. It eats these displacements through the elastic deformation of the bellows, protecting the pipes and valves from being pulled and crushed.
Compared with non-metallic expansion joints, metal corrugated expansion joints have high pressure resistance, high temperature resistance and long life. However, it can't withstand excessive bending moment, so you have to calculate clearly when selecting the model. Don't place an order as soon as you hear "expansion joint"-first find out whether your pipe is going through steam, smoke, corrosive media or particulate matter.
The main steam pipe of the power station has a temperature of five or six hundred degrees and a pressure of more than ten MPa. If you install it with a general-purpose corrugated expansion joint, it will explode in two hits. What about that? Specialized high temperature axial expansion joints or external pressure single axial expansion joints must be used, and the bellows material must be Incoloy 800H or similar heat resistant alloy.
Okay, the principle is understood, let's get right to the point-which one is used for what scene?
2. Which one should be used for what working conditions? Check your seat by scene
I have sorted out six typical working conditions, and you can look for them according to your own pipeline conditions.
- High temperature steam/heat pipe: PreferredHigh temperature axial expansion joint(Product 8 of this site) orExternal pressure single axial expansion joint(Product 23). The external pressure structure can avoid the inner wall of the bellows directly contacting the high-temperature medium, and is suitable for large diameter and high pressure. If the pipeline is going to be complicated, you can also matchRotary compensator(Product 18) to absorb large angular displacement.
- Large diameter low pressure pipeline(For example, smoke and air duct):Metal rectangular expansion joint(Product 3) orUniversal corrugated expansion joint(Product 13) is enough. Low cost, large compensation, don't waste money to buy high-voltage models.
- Pipes requiring multidirectional compensation(For example, the equipment outlet vibrates greatly):Compound hinge transverse expansion joint(Product 24) orCurved tube pressure balance expansion joint(Product 30) can absorb both axial and lateral displacement without generating pressure thrust. Oh, yeah,Straight pipe pressure balanced expansion joint(Product 26) is also a good choice, suitable for arranging places with compact spaces.
- Corrosive media(Acid, base, brine): must be usedPTFE-lined hose(Product 2) orPTFE compensator(Product 16). The metal bellows is lined with a layer of PTFE, which is corrosion-resistant and temperature-resistant. Don't make do with a regular metal hose, it will pierce in two months.
- Vacuum environment(Air cooling island, vacuum pipe):Special hose for vacuum(Product 15) orDouble hinge expansion joint for air-cooled island vacuum pipeline(Product 21) with external pressure resistant reinforcing ring, will not suck deflation. Instability of common expansion joints under vacuum is a common accident.
- Direct buried pipeline(Underground Laying):Direct buried (fully buried) type expansion joint(Product 20) comes with its own external protective pipe and anti-corrosion layer, so it is directly buried in the soil without making an inspection well.
And guess what? Two days ago, a cement factory purchaser asked me: "I just use high-temperature air, can I use a rubber compensator?" I told him directly that it wouldn't work-rubber has a maximum temperature resistance of 200℃, and your air temperature is 350℃, and it will burn once installed. Honest and practical cement industry metal corrugated expansion joint (product 6), with the guide tube kind.
Third, type selection and pit avoidance: diameter, pressure and compensation amount must not be less
There are three cores of selection parameters, and one is followed by a pit.
Diameter (DN): It must be consistent with the inner diameter of the pipe, not large or small. Refer to our stationMetal hose size comparison tableWith standard diameters and flange sizes on it. Don't take it for granted, because pipe necking will lead to increased flow rate and increased erosion.
Pressure (PN): Pay attention to distinguish between nominal pressure and design pressure. The metal strength will decrease when the temperature is high. For example, the allowable stress of No.20 steel at 400℃ is only half that of normal temperature. Therefore, the high-temperature working condition should be selected according to the pressure after temperature correction. Our station hasMetal hose pressure standardYou can refer to it, but the high-temperature axial expansion joint should be calculated separately.
Compensation amount: Axial, transverse and angular, calculated according to the length of pipe arrangement and thermal expansion coefficient. The formula is not complicated: Δ L = α · L · Δ T. However, many novices only count the axial direction and ignore the lateral displacement-as a result, the expansion joint is installed and twisted to deadlift, and the bellows cracks. If the pipe has a 90° bend, the lateral compensation amount must be presented separately.
By the way, the guide tube (see Q&A 7 for the specific function of the expansion joint guide tube) is also very critical. It can not only protect the bellows from erosion by high-speed media, but also reduce the flow resistance. However, the direction cannot be reversed-the direction of the arrow must be consistent with the flow direction of the medium (Q&A 10 is shown).
And pull rods. The role of the expansion joint tie rod (Q&A 16) is to withstand the thrust generated by internal pressure and prevent the bellows from over-stretching. But after installation, how to adjust the tie rod nut (Q&A 12)? First loosen the transport screw (does the screw of the expansion joint need to be removed? It must be removed! Q&A 13), then adjust the tie rod nut to the pre-tensioned position, and finally lock it tightly.
Fourth, improper installation is equivalent to buying for nothing: those things in the direction of pull rod, guide tube and arrow
Installation issues I've seen too many rollover cases. Tell me three of the worst ones.
The first one: the transport screw is not disassembled.Some engineering teams try to save trouble. The expansion joint is bought and connected directly to the pipe, and the transportation screw is still screwed. As a result, the bellows can't move after the temperature is heated, and the flange is pulled to deform.Does the screw of the expansion joint need to be removed?The answer is that it must be completely dismantled, otherwise it will lose its compensatory effect.
Second: the deflector tube is installed reversely or not.In the high-temperature dusty flue gas pipeline, if there is no guide tube, the dust will directly hit the ripples and wear out in half a year. Pay attention to the small head of the guide tube facing the flow direction to avoid the backward flow of media into the corrugated gap.
Third: the tie rod nut is misadjusted.The correct installation method of the expansion joint of the large tie rod (Q&A 20) clearly states that the amount of pre-stretching should be calculated according to the installation temperature. Install the pipes for summer in winter and stretch them in advance; Install pipes for winter in summer and compress them in advance. When the nut is locked, both sides should be symmetrical to prevent the bellows from twisting caused by eccentric load.
What is the direction of the arrow, you ask? There is usually an arrow on the expansion joint body pointing to the direction of the medium flow. If it is installed backwards, the deflector will take a ride, generating vortex and noise, and in serious cases, the bellows will be damaged.
5. How to choose if the budget is limited? Common Industry Recommendation Portfolio
Not budgeting is not about buying the cheapest, but about using steel on the edge. I'll give you a few combination suggestions by industry.
- Power station industry: For main steam and reheat pipesCorrugated expansion joint for power station industry(Product 5), withRound Flap Door (Double Seal)(Product 7) Do isolation. For smoke exhaust systemsMetal rectangular expansion joint(Product 3) +Flue gas baffle door(Product 4). Don't use circular expansion joints on the flue, the rectangular section better matches the shape of the air duct, saving flange material.
- Cement industryFor high temperature fan outletMetal Corrugated Expansion Joints in Cement Industry(Product 6), with wear-resistant guide tube. For blanking tubeManual plug-in insulation door(Product 11) as maintenance isolation. If the budget is tight, the tube cooler section can be usedRectangular non-metallic expansion joint(Product 10) Alternative to metal, half cheaper, but pay attention to the temperature resistance limit.
- Desulfurization system: Wet desulfurization flue corrosion is serious, recommendedDesulfurization flue gas baffle door(Product 19)rubber compensator(Product 9) orRubber PTFE compensator(Product 12). If the temperature is above 200℃, it must bePTFE compensator(Product 16).
- Steam pipe networkFor long-distance directly buried pipelinesDirect buried (fully buried) type expansion joint(Product 20), for indoor overhead pipingUniversal corrugated expansion joint(Product 13). If the amount of compensation is insufficient, addRotary compensator(Product 18) -It can hold three axial expansion joints one, isn't it fragrant?
Don't just focus on the price when selecting models. Buying the right one time is much less worry-free than swapping around. If there are any uncertain working conditions, directly turn the product information of our station, and the model parameters are all above.