1. Find out the working condition parameters first, so that the compensation amount can be bottomed
The amount of compensation for the bypass flue expansion joint is never a forehead-fixed number. The elastic modulus and fatigue life of bellows are directly determined by the three hard indexes of temperature, pressure and medium corrosivity. As soon as the temperature goes up, the elastic modulus of stainless steel drops-for example, the elastic modulus of 304 at 300℃ is only about 70% of that of room temperature, and the compensation design is not corrected. The rigidity of the bellows in hot state is not enough, and it is only a matter of time before it becomes stable. What about the stress? Under high positive or negative pressure conditions, the wall thickness and wave height of the bellows must be adjusted, otherwise the internal pressure thrust will flatten or bulge the bellows. The medium contains sulfur and chlorine? Then stress corrosion cracking should be considered, and the fatigue life safety factor should be kept at least 2 times when the compensation amount is designed.
Installation location. Horizontal flue and vertical flue, dead weight displacement is far different. The vertical corrugated pipe should additionally bear the self-weight compression of the pipe section, and the compensation amount must be superimposed on this; In the horizontal section, the constrained stiffness of lateral displacement should be considered. Two days ago, I encountered a case in a cement factory. They used a horizontal flueCorrugated expansion jointIt was directly installed in the vertical section, and as a result, it was pulled off after two months of production-the fundamental reason was that the additional displacement of self-weight was not counted.
2. The structure selection is wrong, and the compensation amount is useless no matter how accurate it is
Different structures of the expansion joints in the bypass flue behave very differently. Non-metallic expansion joints (i.e.Fabric fiber expansion joint) Suitable for large displacement and corrosion resistance, but the pressure resistance is limited, generally not more than 0.1MPa. If you use it in the negative pressure section of the desulfurization bypass flue, no problem; However, in the positive pressure section, it is common for the non-metallic layer to be burst. Conversely,High temperature axial expansion jointCan withstand high temperatures above 600℃, but must haveguide tubeProtection, otherwise the high-speed flue gas directly washes the bellows, and the bellows will soon wear and leak.
Corrugated expansion joint for power station industryPay more attention to axial compensation and fatigue life, often thousands of cycles;Metal Corrugated Expansion Joints in Cement Industry
- More emphasis is placed on wear resistance and high temperature resistance, and the compensation amount tends to be more redundant. If you choose the wrong type, no matter how accurate the compensation design is, it is useless-just like taking the suspension of a sports car off-road, no matter how beautiful the parameters are, it can't withstand the impact.
Then how to choose? Look at temperature + pressure + displacement direction. Select axial type for large axial displacement; Which need to absorb lateral displacement, upperCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 When the bypass flue space is limited,Compound straight pipe bypass pressure balanced expansion jointIt can save space, but the price is also a little more expensive.
3. Engineering details: Do you do cold pre-stretching? How to judge the displacement direction?
Should cold pre-stretching be done or not? The answer is simple-it depends on whether your thermal displacement direction is elongation or compression. If it is elongated, pre-stretch part of the cold state, and the bellows can work near the equilibrium position in the hot state, and the fatigue life can be doubled. If it is compressive displacement, pre-stretching is harmful. Judgment method: Calculate the thermal elongation of the pipe from the installation temperature to the highest operating temperature, and the direction will be clear naturally.
Tell a real lesson. The design temperature of the desulfurization bypass flue of a power plant is 180℃, the installation temperature is 20℃, and the calculated axial compensation amount is 120mm. The engineer was lazy and didn't do pre-stretching, so he directly pressed zero pre-tightening and installed it. As a result, after three months of operation, the wave pitch of the bellows appeared uneven, and then the weld between the corrugations cracked. Finally removed and checked, the actual displacement was 30mm larger than the calculated value-because the thrust displacement caused by opening and closing of the flue baffle door was not taken into account. And guess what? The rework cost is enough to buy three new sets of expansion joints.
Therefore, when calculating the compensation amount, not only the thermal displacement should be considered, but also the thermal displacement should be set asideFlue gas baffle doorOrDouble-sealed single-axis circular baffle doorThe displacement margin at opening and closing. Many people don't realize this-when the flapper door acts, pressure fluctuations in the flue can be transmitted to the expansion joint, causing additional axial or lateral displacement. If the compensation amount is not enough, the wear of the sealing surface will be aggravated, and the air leakage rate will go up.
4. How to match the compensation amount with the baffle door? Leave an allowance to avoid grinding the sealing surface
The bypass flue is usually installedDesulfurization flue gas baffle doorOrSingle-axis double-flapper doorTo cut the smoke. The compensation ability of the expansion joint cannot only be counted as the thermal expansion of the pipe, but also must leave room for the action of the baffle door. For example:Double-sealed single-axis circular baffle doorDuring the full opening and full closing process, the sealing frame will have a slight deformation displacement, about 5~10mm. If this displacement is hard eaten by the expansion joint, and the sealing surface is rubbed repeatedly, it will leak in less than half a year.
When designing the compensation amount, the moving displacement of the baffle door is included as an additional load. The axial compensation amount is increased by 10~20mm margin, and the transverse compensation amount is increased by 5~10mm. In addition, between the expansion joint and the baffle doorguide bracketSo that the displacement direction can be controlled. Otherwise, the bending moment generated by the expansion joint will directly act on the baffle door housing, and the deformation of the door shaft will be troublesome.
5. One-stop installation and debugging can keep the theoretical compensation amount
On-site installation is the easiest to discount the theoretical compensation amount. First, the bolts are tightened too tightly. The bellows or non-metal ring belt of the expansion joint absorbs the displacement by flexible deformation. If the connecting bolt is screwed to death according to the standard of the foundation anchor bolt of the equipment, the bellows can't move at all. Standard practice: the bolt is screwed just enough to eliminate the gap, usually controlled at 60% ~70% of the design value with a torque wrench.
Second, there is a lack of guide brackets. The thermal displacement direction of the flue pipe system is guided by the guide bracket. Without the guide bracket, the expansion joint may be subjected to axial compression, lateral bending and torsion at the same time-this is called non-design bending moment, and the bellows will soon crack by fatigue. I have seen a project before where the owner only installed two guide brackets on the 30-meter-long pipe section of the bypass flue to save money. As a resultCorrugated expansion joint for power station industryThe trough crack appeared two months after commissioning.
Third,Expansion joint tie rod nutNot in place. The role of the tie rod is to limit the excessive displacement of the expansion joint and prevent the bellows from being crushed or pulled off. During installation, the tie rod nut must be adjusted to the designated position according to the design drawing-generally the nut position corresponding to the cold pre-tension amount. The adjustment is offset, and the tie rod will either not work in the hot state, or it will stick the bellows. Remember:How to adjust the tie rod nut of expansion jointFor this problem, you must refer to the manufacturer's instruction, and then lock it after measuring the size with a caliper on the spot.
Alas, say a thousand words and ten thousand, no matter how accurate the compensation amount is calculated on the drawings, if the site is installed wrong, everything will be wasted. Therefore, it is necessary to communicate well with the installation team in the design stage.Compensation amount of bypass flue expansion jointThe verification steps are written into the construction plan-including cold dimension recording, guide bracket position review, and bolt tightening moment inspection. These hard indicators are stared at, and the amount of compensation can really land.