1. How "fatal" are the desulfurization and denitrification conditions? — — Acid Dew Point, Negative Pressure and Thermal Displacement
Friends who have worked on desulfurization and denitrification projects know that this job looks like flue gas treatment, but in fact, the test of pipeline accessories is worse than that of steam pipelines. Let's talk about the acid dew point first-there is a large amount of SO₂ and SO₃ in the flue gas. Once the temperature falls below the acid dew point (generally between 110℃ and 150℃, which is related to the coal type and desulfurization process), the surface of the bellows immediately condenses to form dilute sulfuric acid, which is greeted by electrochemical corrosion and stress corrosion cracking. And guess what? Many projects began to leak after half a year. When they were disassembled, the bottom of the corrugated valley was full of cracks, which was the typical trace of dew point corrosion.
Let's talk about negative pressure. In the desulfurization system, especially in the inlet section, when the induced draft fan pumps, the negative pressure can reach-3kPa or even lower. If the metal expansion joint is not rigid enough or has a thin-walled design, it will be deflated directly. Last year, there was a case of a power plant in which a general-purpose corrugated expansion joint was used at the entrance of the desulfurization tower. As a result, the bellows collapsed at the instant that the fan was started, the welding joint was cracked, and it was shut down for three days for emergency repair. Tsk, the gain outweighs the loss.
Finally thermal displacement. From the original flue gas to the net flue gas, the temperature change drops sharply from 160℃ to about 50℃, the axial elongation of the pipeline moves by tens of millimeters, and there is gravity sinking in the vertical direction. These three mountains are pressed down. If the expansion joint selection is only cheap, the maintenance cost at the back will be enough for you to buy three good sets.
Second, the wrong model of metal expansion joint is selected, and it is only a matter of time before the weld cracks-how to choose general-purpose, high-temperature axial and compound hinges
Many purchases go directly to the general-purpose corrugated expansion joint and feel that everything is fine. However, in the desulfurization and denitrification condition, the general-purpose type is only suitable for bypass pipelines with low temperature and small displacement. The real main flue gas pipeline must be determined according to the displacement type.
- High temperature axial expansion joint: It is suitable for absorbing axial thermal displacement, and the working temperature can reach above 400 ℃. Note that it adopts multi-layer bellows and guide tube structure, which can resist high temperature creep. It is very stable when used in the raw flue gas heating section and the inlet and outlet of GGH.
- Compound hinge transverse expansion joint: When the pipeline has lateral displacement (for example, the horizontal pipeline is laterally displaced due to expansion) or the space is limited and cannot be naturally compensated by elbow pipe, the compound hinge absorbs the lateral displacement through two sets of hinge structures without producing blind plate force. Remember: it cannot absorb axial displacement, so it must be used in pairs or in conjunction with other types.
- Universal corrugated expansion joint: It is only recommended for branch pipes or clean flue gas pipes with temperature below 200℃, small displacement and low pressure. Don't go cheap to the main flue plug, it is only a matter of time before the weld cracks.
In addition, if the diameter of the pipeline exceeds 1.5 meters, large-diameter thick-walled expansion joints must be considered, and the wall thickness and wave height should be recalculated, otherwise the ripples will be crushed by self-weight.
Third, how to set up the expansion joint and the baffle door? — — Cooperation logic of desulfurization flue gas baffle door and double seal circular baffle door
In the desulfurization system, the baffle door is responsible for cutting off the flue gas passage, and the expansion joint is responsible for compensating the displacement. It is common for the two to cooperate poorly, and the door is not closed tightly or the expansion joint is stuck.
Let's startDesulfurization flue gas baffle doorIt is usually used in flue inlets and outlets and requires zero leakage. However, the baffle door itself is a rigid structure, and the thermal expansion is absorbed by the nearby expansion joint. Correct approach: Install an expansion joint in the front and rear of the baffle door. The front end absorbs the axial displacement of the flue, and the rear end absorbs the installation deviation and temperature change of the pipe. Don't install only one end, otherwise the door body will be deformed and the sealing surface will leak.
Look againDouble-sealed round flapper doorIt has two sealing surfaces with a blow-up gas in the middle, and is used in situations where absolute isolation is required (such as bypass flues). This kind of door is very sensitive to the axial displacement of the expansion joint-if the expansion joint is selected too large or the pre-compression amount is adjusted incorrectly during installation, it will hold against the door panel and cause it to not close in place. Standard operation: The expansion joint on one side of the double-sealed door shall be selectedCompound straight pipe bypass pressure balanced expansion jointIt can balance the pressure and thrust without transmitting axial force to the door body.
I met a friend from a design institute two days ago. They usedSingle-axis double-flapper doorAdd the general-purpose expansion joint, but the door panel is stuck during debugging, and finally it has to be disassembled and re-welded. So remember: for expansion joints near baffle doors, preference is given to the pressure balanced type or the large tie rod type.
4. Guide tube, tie rod, screw-these details are not clear, and they have to be disassembled after installation
guide tubeThere are several articles written in the function book. In the desulfurization condition, the most important thing is "diversion + protection of the bellows from being washed by particles". If there is not enough gap between the guide tube and the bellows, the guide tube directly presses against the bellows after thermal expansion, and wear and fatigue are accelerated together. Correct practice: The inner diameter of the guide tube should be 10~15mm larger than the inner diameter of the bellows, and it must be consistent with the flow direction of the medium — —Arrow direction of expansion jointThat means the direction of flow.
tie rodNot for adjusting displacement. Many people twist the tie rod nut to press or elongate the expansion joint, which is a big mistake. The function of the tie rod is to limit the excessive deformation of the expansion joint during transportation and installation. After installation in place, the tie rod nut must be loosened to allow the bellows to expand and contract freely. If you forget to loosen it, it is equivalent to turning the expansion joint into a rigid tube, and the stress at the weld joint is concentrated.
As forscrewIf it is the transport screw that comes with the factory, it must be removed after installation. Some workers try to save trouble and don't dismantle it. As a result, as soon as the pipeline heats up, the screw directly pushes the expansion joint to death, and the bellows bulges and is scrapped. There was a case before, a cement plant denitrification project suffered a big loss because of this.
The specific function of the guide tube of the expansion jointIt not only prevents scouring, but also prevents the bellows from being deflated under negative pressure conditions. So don't remove the deflector yourself.
5. Three common "death-seeking" operations in on-site installation and correct treatment methods
Quest for Death 1: Forced Counterpart。 There was a deviation in the pipeline installation, and the worker took a crowbar to hard break the expansion joint until the bolt holes were aligned. This operation causes the bellows to twist and internal stress to concentrate. Correct method: Use padding iron or adjust the pipe support, so that the natural gap is within 3mm and then install the expansion joint.
Death 2: No supports or fixed brackets。 The desulfurization flue has a large diameter and thin wall, and the expansion joint itself cannot bear the weight of the pipe. The fixing bracket and guide bracket must be designed separately. I have seen someone use the expansion joint as a support point, and after three months, the flange surface warps and leaks.
Seeking Death 3: Ignoring Cold Tight Pre-Compression。 When the pipe temperature is low during installation in winter, if it is not pre-compressed (or pre-stretched) as designed, the expansion joint will exceed the design compensation amount when the operating temperature rises in summer. Correct practice: Calculate the difference between the installation temperature and the design temperature, and lock the pre-compression amount with the tie rod positioning bolt according to the method in "How to adjust the tie rod nut of expansion joint".
Having said so much, there is actually one word: construction according to the drawing. The amount of pre-compression, the tightness of the tie rod and the position of the bracket marked on the drawing should not be lazy.
6. From selection to operation and maintenance, a compressed checklist
Finally, make a practical list for your peers, and follow it to avoid 90% of the pits:
- Acid dew point temperature, minimum negative pressure and maximum thermal displacement (axial/transverse/angular displacement) of flue gas must be obtained before selection
- Primary flue preferenceHigh temperature axial expansion jointOrCompound hinge transverse expansion joint； For bypassUniversal corrugated expansion joint
- Front and rear expansion joints for baffle doorsPressure balance typeSo as to prevent the thrust from affecting the door body
- The arrow of the guide tube must point to the flow direction of the medium, with a sufficient gap of more than 10mm
- The transport screw must be removed after installation; The tie rod nut is loosened to the free state after installation
- The bracket shall be arranged according to the drawing, and the fixed bracket shall be placed at both ends of the expansion joint
- Cold tightening pre-compression amount should be set according to the design value. Do not rely on feeling
- During the operation and maintenance period, check the appearance of the bellows once a year, focusing on whether there are corrosion pits in the trough and whether there are wear and perforations in the guide tube
Desulfurization and denitrification projects use metal expansion joints. If you choose the right one, you will not worry about it for ten years, but if you choose the wrong one, you will plug leaks every day. I hope this article can give you who are working on the project something real.