Find out exactly how much pressure your desulfurization flue is under
Don't come up and just look at the nominal pressure on the product sample, that's a pit. Two days ago, a customer asked with the parameter table, saying that he chose a 2.5MPa metal expansion joint, but it leaked after half a month. After checking, I found that there is usually negative pressure in his flue, but the positive pressure can soar to 0.8MPa at the start and stop instant. In addition, the flue gas temperature suddenly rises from 80℃ to 200℃, and the actual equivalent pressure acting on the bellows has long exceeded the design value. So the first and most critical step is to get the real working condition pressure.
How to do it? Flip the design drawings to find "Maximum Working Pressure" and "Design Positive/Negative Pressure Value". If not, look for the running record. Note: Desulfurization flue is usually divided into positive pressure section (such as absorption tower inlet) and negative pressure section (such as induced draft fan outlet), and the expansion joints of the two sections are completely different. There's another point that is easy to overlook-temperature. Every time the flue gas temperature increases by 100℃, the allowable stress of metal materials decreases by about 20% ~30%. That is to say, the expansion joint with the same nominal pressure can actually bear the pressure at high temperature will be greatly reduced. When you calculate, you have to multiply the temperature reduction factor. Don't just look at the normal temperature data.
When you are unsure, take the design pressure ×1.25 as the selection benchmark, and the safety factor of ×1.1 if the temperature exceeds 200℃. This basically won't overturn.
Choose the right expansion joint type according to pressure: metallic or non-metallic?
The pressure is determined, then choose the material. The most headache in desulfurization flue is corrosion-the flue gas after wet desulfurization contains sulfide and chloride ions, and the temperature fluctuates. Choose the wrong type, and a change can make your flesh hurt for half a year.
Pressure ≥0.1MPa, or frequent alternation of positive and negative pressures— Use metal honestly. Specific to the products in our station:Corrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement Industry、Universal corrugated expansion jointOrMetal rectangular expansion jointAll right, the key depends on the shape and space of the pipe. The metal bellows has strong pressure resistance and good fatigue resistance, but the disadvantage is that it is sensitive to corrosion. Therefore, stainless steel (316L or 254SMO) should be selected for the flue lining, or simply usePTFE-lined hose— — It is lined with a layer of polytetrafluoroethylene, which is resistant to acid and alkali corrosion, and the pressure level can also reach 1.6MPa, which is suitable for strong corrosive media.
Pressure ≤0.05MPa and substantially negative pressure or slightly positive pressure-Non-metallic expansion joints are more cost-effective. OurNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion joint, inside with silicone rubber, fluororubber coated fabric, temperature resistance to 250℃, the price is only 1/3 to 1/2 of metal. But there is a pit: if the negative pressure is too large (for example, it exceeds-5kPa), the non-metallic layer will be flattened and deformed, resulting in seal failure. Therefore, when non-metal is used in the negative pressure section, an internal support ring or guide tube must be added.
The pipeline needs frequent disassembly and maintenance, or the vibration is large and the installation space is limited. At this time, you can considerrubber compensatorOrRubber PTFE compensatorBut the pressure cannot exceed 0.6MPa and the temperature cannot exceed 120℃. In addition, circular flues with high pressure and large diameter are sometimes usedStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion jointThey balance the blind plate force and reduce the thrust on the bracket, but the price has gone up.
Check these pressure-related parameters before installation
Don't be in a hurry to weld the equipment when it arrives. Take out the drawings and physical objects, and check them one by one according to the following list. If you leak one, you may deform or leak.
- Nominal pressure: The PN value on the equipment nameplate must be ≥ your design pressure × temperature reduction factor. For example, if you calculate that it needs 0.8MPa, then choose at least PN1.0.
- Test pressure: Usually 1.5 times the nominal pressure, used when doing hydraulic pressure tests. Note: During the test, the guide tube should be installed properly, otherwise high-pressure water may flush it off.
- Pressure bearing capacity of guide tube: Many expansion joints are equipped with guide tubes (such as oursSpecific Function of Expansion Joint Guide TubeAs mentioned in), it mainly plays the role of diverting flow and protecting bellows. However, if the design wall thickness of the guide tube is too thin, it may deflate under the impact of positive pressure. Check the wall thickness and fixing method of the guide tube, which must be able to withstand at least 1.2 times the working pressure.
- Pre-tightening force of tie rod nut adjustment: When the expansion joint leaves the factory, the tie rod nut is usually loose for easy installation. However, after installation, the preload force should be adjusted according to the working conditions. How to tune it? The question and answer "How to adjust the tie rod nut of the expansion joint" is very detailed-the preloading force generally takes 20% ~30% of the working thrust, which limits the displacement of the expansion joint too much, and can't protect it too small. If you don't have a torque wrench in your hand, rely on your feeling: after the nut is screwed until it can't be screwed by your hand, go back half a turn to one turn (see the design drawings for details).
Also, don't forget to check the grade of the flange bolts. Many ordinary 8.8-grade bolts used in the field resulted in fatigue fracture at high temperature, and the expansion joint flew out directly. It is recommended that the desulfurization flue should use at least grade 10.9 and be coated with anti-bite coating.
What should I do if the pressure is abnormal in operation? Three-step method for quick troubleshooting
If the flue pressure gauge suddenly alarms, or you hear an abnormal noise near the expansion joint, don't panic. Check in this order and 80% of the problems can be solved on the spot.
Step 1: Check Flap Door Status。 The most common cause of pressure abnormalities is not the expansion joint itself, butFlue gas baffle door(includingDesulfurization flue gas baffle door、Round Flap Door (Double Seal)、Double-sealed single-axis circular baffle doorEtc.) not opened in place or stuck. If the baffle door is not fully opened, throttling will be formed in the flue, resulting in an increase in upstream pressure and a decrease in downstream pressure. Go to the scene and manually disc the baffle door axle to see if there is any jamming. If the baffle door is not closed tightly, it will also cause smoke to flow back, causing the expansion joint to bear reverse pressure. Therefore, priority should be given to the baffle door when checking.
Step 2: Observe whether the surface of the expansion joint is bulging。 The bulging of the metal expansion joint indicates that the internal bellows has been plastically deformed, which may be overpressure or fatigue. Bulging of non-metallic expansion joints, usually partially shedding of fabric layers or collecting dust to prop up. Either way, as long as it bulges, it will be shut down. Emergency measures: Fix the bulge area with clamps or temporary steel plates, and replace it after the furnace is shut down.
Step 3: Confirm whether the guide tube is detached and blocked。 If the guide tube falls off, it will be washed into the downstream pipeline by the airflow, causing blockage and instantly soaring pressure. How to judge? Listen to the sound: If you hear the "bang bang" sound of metal collision, the deflector will drop nine times out of ten. Or see if there are foreign objects in the flue through the observation hole. Emergency treatment: If the guide tube falls inside the expansion joint, you can temporarily cut the shell and take it out, but pay attention to safety. The flue gas temperature is high and toxic, so you must wait until the furnace is shut down to cool down before operation.
If the above three steps are checked and there is no problem, it may be that the pressure gauge itself is broken. Try changing the watch.
Tips for Extending Life: Maintenance Points Under Pressure Fluctuations
Even if the choice is right, it will still be short-lived without maintenance. Pressure fluctuation of desulfurization flue is common, and start-stop, load change and baffle door adjustment will bring impact. How to carry it?
Frequency of daily inspection: At least once a week, see if there are corrosion spots on the surface of the expansion joint, if the fabric layer is fluffed, and if the tie rod nut is loose. Focus on checking the pressure bearing direction-look at the weld in the positive pressure section and whether the non-metallic layer is flattened in the negative pressure section. OurNational standard for non-metallic expansion jointsIt is required to do a comprehensive inspection every quarter, but if the actual working conditions are bad, it is recommended to shorten it to every month.
Attention to damage to metal and fabric layers by corrosive media: The flue gas after wet desulfurization is very acidic, and the metal bellows is easily pitted. Even if 316L is used, it can't be held when the chloride ion concentration exceeds 200ppm. Solution: First, check the weld seam regularly, and repair welding in time if micro-cracks are found (pay attention to using the same material welding electrode and stress relief treatment); Second, for non-metallic expansion joints, check whether the coating blisters and falls off. If it is partially damaged, it can be repaired on the spot with silicone rubber, but large-scale damage can only be replaced. Besides, ourPTFE compensatorAndPTFE-lined hoseIt has the best corrosion resistance, but it is expensive and suitable for use in the most corrosive sections.
Replace seals regularly: The sealing gasket at the connection between the expansion joint port and the flue, replaced every two years. The material is reinforced with graphite or PTFE, which is temperature-resistant and corrosion-resistant. Don't use asbestos sheets. The Environmental Protection Bureau found that it would be fined, and the sealing performance after aging was also poor.
Avoid flattening of non-metallic expansion joints caused by excessive negative pressure: As mentioned earlier, non-metallic expansion joints are particularly prone to flattening in the negative pressure section. Solution: When selecting the model, the manufacturer is required to build in a support ring (such as a spiral wire ring), or install a limit rod outside the expansion joint. If it has been installed and slightly flattens, you can adjust the tie rod nut, tighten it appropriately, and increase the preload force to resist negative pressure. But don't pull too tight or it will limit the displacement.
Pressure selection is not just a number, but temperature, corrosion and fluctuation frequency have to be taken into account. Following the steps above, you can basically avoid 90% of the pits. If you are still unsure, contact the manufacturer directly to provide the working condition parameters and let them help you calculate-don't be embarrassed, professional selection service is much more reliable than patting your own head.