How to choose a metal expansion joint? Understand these 5 steps and buy the right goods for less money
Two days ago, I met a customer and bought a batch of general-purpose corrugated expansion joints and installed them on high-temperature steam pipelines. As a result, they exploded within three months. After asking, he found out that he only looked at the nominal diameter, not the actual temperature at all. This happens a few times a year. How to choose a metal expansion joint? Don't just look at the price, you have to follow the following five steps, and the probability of wasting money can be reduced by more than half.
Step 1: Understand the working conditions of the pipeline thoroughly-pressure, temperature and displacement must be missed
Many purchases ask "How much is DN200" as soon as they come up, and the price is too expensive after quotation. But when you tell him, "Is your pipe pressure 1.6MPa or 2.5MPa? Is the temperature 300℃ or 600℃?", the opposite side often froze. The first pit of selection is here:Does the pressure depend on the design pressure or the maximum working pressure?The design pressure is usually 10% ~20% higher than the highest working pressure, but the selection of expansion joints should be based on the highest working pressure, and the safety factor should be sufficient. Don't blindly choose bigger, it is not cost-effective to double the cost.
The temperature is even worse.Medium temperature plus thermal radiation and pipe outer wall temperature, the actual corrugated pipe may bear a temperature of 50~100℃ higher than the medium temperature. For example, the steam pipe medium is 450℃, but there is no heat insulation near the bellows, and the heat radiation directly bakes it, and the surface temperature of the bellows can reach 550℃. At this time, you take the ordinary 304 material to carry it, and the intergranular corrosion will accelerate, and it will crack in half a year. High temperature operating condition preferredHigh temperature axial expansion jointOrExternal pressure single axial expansion jointThe material is heat-resistant alloy.
Don't just focus on axial expansion and contraction.The thermal expansion and contraction of pipelines not only exist in axial direction, but also in horizontal and vertical directions. After installing at many sites, it is found that the expansion joint is twisted into a twist because only the axial vector is counted. Therefore, before selecting the type, you must get the pipe system stress analysis report, or at least draw a sketch to mark the displacement in all three directions.
Step 2: Don't confuse the structure type-choosing the wrong is equal to buying for nothing
There are many patterns of metal expansion joints on the market, but there are only a few core structures. Using them in the wrong place is a disaster.
- High temperature and high pressure steam pipeline: Decisive external pressure single axial expansion joint or high temperature axial expansion joint. Why? The universal corrugated expansion joint is easy to lose stability in the bellows under high temperature, the external pressure structure can disperse the stress, and the guide tube has its own heat insulation layer, so the service life is directly doubled.
- Large diameter smoke duct: For example, the desulfurization flue of a power plant has a diameter of two or three meters.Metal rectangular expansion jointOrNon-metallic expansion joints (fabric fiber expansion joints)。 Rectangular expansion joints absorb a large amount of displacement, and non-metallic joints can also resist acid and alkali corrosion. However, it should be noted that the national standard JB/T 12235-2015 for non-metallic expansion joints stipulates that the fatigue life should meet the design requirements.
- Need to absorb lateral displacement: There are many turns in the direction of the pipeline and large lateral displacement, withCompound hinge transverse expansion joint。 It has two hinge structures that absorb large lateral displacements, but note that it does not absorb axial displacements.
- High-pressure pipeline to prevent blind plate force: For example, before and after the pump outlet and the valve, if there is blind plate force in the pipeline, selectStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint。 This expansion joint has a balanced bellows inside, which can cancel out the blind plate force, otherwise the pipe support will be pushed down.
And guess what? Some small factories take general-purpose corrugated expansion joints to connect with pressure balance conditions, but as a result, they are directly broken after installing them for pressure test. If you choose the wrong structure, you can't make up for it no matter how much money you spend later.
Step 3: Materials and Connection Methods – Saving money is not on materials
The media is corrosive, so don't think about saving money with 304. For example, in the desulfurization flue gas environment, the chloride ion concentration is high, and the conventional 304 pits and perforates in less than three months. That's the time to goCorrugated expansion joint for desulfurization flue gas baffle doorThe material is 316L or even duplex steel, or simply lined with PTFE. The PTFE-lined metal hose and PTFE compensator are particularly good for dealing with strong corrosive media, but note that the PTFE-lined metal hose is not resistant to negative pressure, so it should be used in vacuum conditionSpecial hose for vacuum。
How to choose the connection method?The flange is convenient to connect, easy to overhaul and disassemble in the future, and is suitable for low-pressure large-diameter pipelines. Butt welding connection has good sealing performance, suitable for high pressure and small diameter. However, if you use butt welding with low pressure and large diameter, the on-site welding volume is large and the construction period is long, which saves flange money and spends more installation cost. On the other hand, if you use a flange for high pressure and small diameter, the flange gasket is easy to leak, so it is better to direct butt welding. Remember one principle: don't spend 2000 to repair the air leak in order to save 200 yuan of flange money.
Step 4: Accessories and Installation Details That Are Easy to Overlook
Some installers welded the expansion joint to the nozzle when they came up, and they didn't even know if the direction of the guide tube was installed backwards. The direction of the arrow of the expansion joint usually refers to the direction of media flow, not the direction of displacement. The function of the guide tube is to guide the medium to flow smoothly and prevent high-speed fluid or particulate matter from directly impacting the bellows. If it is installed backwards, the guide tube will become a baffle, and the medium will directly hit the bellows, which will wear as fast as cutting. Especially for working conditions containing granular media, such as ash transportation pipelines in power plants, guide tubes must be installed.
How to adjust the tie rod nut?Pre-stretching and pre-compression are realized by it. Adjust the nut according to the displacement given by the manufacturer during installation. Don't forget to remove the limit screw after installation. Some construction teams don't dismantle the limit screw, and the expansion joint can't play a role at all. The pipeline is thermally expanded and hard, and finally the weld joint cracks. AndDirect buried (fully buried) type expansion jointThere must be a moisture drainage pipe buried underground, otherwise the groundwater will seep in and the bellows will not know if it is corroded.
BTW, does the screw of the expansion joint need to be removed? The answer is: the limit screw must be removed, but the tie rod nut cannot be moved. The function of the tie rod is to prevent the bellows from over-stretching, not for fixing. For details, see the article "How to Adjust the Tie Rod Nut of the Expansion Joint" on this site.
Step 5: Avoid fakes and low-price traps
Those stainless steel expansion joints on the Internet that are cheaper than steel prices have a high probability of 201 pretending to be 304. How to tell? Regular products haveMetal expansion joint weight tableFor reference, for example, the general corrugated expansion joints of DN200 and PN16 have a standard weight of about 15~18 kilograms. If the weight quoted by the seller is only 10 kilograms, it will be directly blackened, and the wall thickness will shrink. In addition, look at the wave height and wave pitch of the bellows, and take a caliper to measure it to know if you have cut corners.
Working conditions with high fatigue life requirementsFor example, frequent start-and-stop of power plants and rotary kiln pipelines in cement plants, don't buy general corrugated expansion joints. UsefulCorrugated expansion joint for power station industryOrMetal Corrugated Expansion Joints in Cement Industry。 After structural strengthening and wave number optimization, the design life of these corrugated pipes is 2~3 times that of the general type. The selection is not to look at the beauty of the sample book, but to see whether the application scenario can live up to it. You bought the general-purpose model cheaply, but the result was to change it once every two years. The total cost was higher than buying the special-purpose model.
How to buy metal expansion joint? In the final analysis, it is: understand the working conditions thoroughly, choose the right structure, do not save materials, keep an eye on accessories, and avoid fakes. After five steps, he basically wouldn't step on a pit.