How bad is the working condition at the inlet of the desulfurization tower? — — Why can't ordinary expansion joints hold it
It is not an exaggeration to say that the entrance of the desulfurization tower is the "hell mode" of the expansion joint. The temperature fluctuates greatly here-it is common to soar from normal temperature to 150℃ or even 180℃ at start and stop. There are still undesulfurized SO₂, SO₃ and HCl in the flue gas, plus high humidity water vapor, which immediately condenses to form dilute sulfuric acid as soon as it touches the cold wall. And guess what? Ordinary carbon steel expansion joint in this environment, three days to give you corrosion out of the pit. What about using stainless steel? 316L can't hold it, and chloride ion stress corrosion cracking will make you have a headache to doubt your life. Two days ago, I met a customer. The general-purpose corrugated expansion joint for the inlet flue was not lined. It was perforated and leaked in less than two months, and hundreds of thousands were lost when it was shut down for maintenance. Therefore, if you want to save this money, you have to spit it out twice as much in the end.
Selection core: Should you use non-metal or metal? Rectangular or circular?
Don't be in a hurry to pat your head yet. The selection of flue expansion joint at the inlet of desulfurization tower depends on three hard indexes: medium temperature, corrosiveness and pressure. Pressure is generally low (slightly positive or negative), but temperature and corrosion are the main directions of attack.
Non-metallic expansion joints (we have them in our stationNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion joint) at the desulfurization inlet is simply the chosen one. Why? It is not afraid of acid-alkali corrosion, its temperature resistance can reach above 200℃, and its displacement compensation is extremely large-it can be eaten in axial, transverse and angular directions. Especially rectangular flues, withRectangular non-metallic expansion jointMuch more heart-saving than metal rectangular expansion. Although the metal has high strength, once the guide tube is corroded, the ripples are directly exposed. How long do you say it can last?
Then when is it necessary to use metal? There are only two situations: one is that the pressure exceeds the bearing limit of non-metals (generally not at the entrance of desulfurization tower), and the other is that there is an open flame or high-temperature particle impact on the customer's site, and the fabric coating can't bear it. But to be honest, 90% of the desulfurization tower inlets, non-metallic expansion joints with acid-resistant rubber or PTFE lining, is the optimal solution. We still haverubber compensator、Rubber PTFE compensator、PTFE compensatorCan be flexibly matched for different temperature segments. Direct conclusion: rectangular flue choose rectangular non-metallic expansion joint, round flue choose non-metallic expansion joint (fabric fiber type), and add metal frame insulation when the temperature exceeds 250℃.
Three most common failure modes and root causes in the field
Skin torn. It seems that the fabric is broken, and the root cause is mostly that there is not enough pre-compression during installation, or the inner diameter of the guide tube is too small, and the skin is cracked by the airflow impact. It is also possible that the tie rod is too dead, the expansion joint can't move, and it is broken by force.
Corrosion perforations. This is not just a matter of material selection. You choose 316L metal expansion joint, do you think you are generous enough? Wrong! The concentration of chloride ions in the flue gas is slightly higher, and 316L still cracks. Moreover, the heat treatment of the welded joints of bellows and end pipes of many metal expansion joints is not in place, and the sensitization zone is the corrosion breakthrough. Although the non-metal is stainless, if the inner PTFE film has needle holes, the acid will seep in and soak the reinforcing cloth, and it will not hold up. Therefore, when selecting non-metals, be sure to confirm the polytetrafluoroethylene (PTFE) used by the manufacturer, and don't try to be cheap.
The guide tube is detached or worn out. The function of the guide tube is to guide the smoke through smoothly and protect the bellows or skin. However, if the flue gas flow direction is not considered in the design, the arrow direction is installed backwards, and the end of the guide tube will directly bear erosion and wear out quickly. Also, the material of the guide tube is too thin, and the 1.5mm carbon steel sheet will rot in less than a year. The correct method is that the wall thickness of the guide tube is at least 3mm, and the material is weather-resistant steel or glass flake for corrosion prevention. During installation, the arrow direction must be consistent with the smoke direction (see the question and answer in our station "What is the arrow direction of the expansion joint").
Installation Details: Deflector, pull rod, pre-compression can not be wrong
No matter how well you choose your product, the installation is a mess, and it is useless. Let's talk about the deflector first-many scenes take the deflector as a supporting role, and forget it after installation. The guide tube should be flush with the inner wall of the pipeline and cannot protrude, otherwise vortex will be formed to aggravate wear. Moreover, it is best to add a ring of retaining ring at the end of the guide tube to prevent the airflow from blowing the skin directly.
The adjustment of the tie rod has a doorway. Don't come up and screw the nut to death! The tie rod of the expansion joint is used to limit over-stretching. When installing, the tie rod should be loosened so that the expansion joint is in a free state. If the pipe has cold tightness requirements, it should be pre-compressed or pre-stretched as per the design requirements. For example: the working temperature is 120℃ and the installation is at room temperature, then the expansion joint should be pre-compressed by an amount to offset the thermal expansion. How to tune it? Refer to the question and answer of "How to adjust the tie rod nut of the expansion joint" in our station. Each step is illustrated.
Pre-compression is an easy rollover point. After installing the non-metallic expansion joint, ensure that the skin is in a slightly relaxed state and cannot be tightened. Teach you a trick: press the skin with your hand after loading, and it feels like pressing an unfilled ball, then that's right. If it is hard, it means that the compression amount is not enough, and it will crack when it runs at temperature. In addition, the flange bolts must be symmetrically tightened, and the torque is uniform. Don't squeeze the gasket away.
Maintenance cycle and replacement judgment: Don't wait until it leaks before stopping
Many power plants are "broken and repaired". As a result, once the air leaks, the whole desulfurization efficiency drops, and the environmental protection data exceeds the standard before they panic. How to set the maintenance cycle? For the flue expansion joint at the inlet of desulfurization tower, it is recommended to inspect it every 3 months. Look at three points: whether there are bulging, seepage and aging cracks on the skin surface; Whether the guide tube bolt is loose; Did the tie rod nut rust. Especially when shutting down for maintenance, be sure to open and check the inside, and acid scale often rotts from the inside.
When do you have to change? When the skin has cracks greater than 30% thickness, or there is acid dripping, don't hesitate. Non-metallic expansion joints generally have a design life of 3-5 years, but it is normal to change them every 2-3 years. If you use the one from our stationNon-metallic expansion joint (fabric fiber expansion joint)Pay attention to retaining the flange and end tube when replacing, and only replacing the skin assembly can save half the cost. In addition, if the site space permits, it is recommended to install another line at the exitFlue gas baffle doorOrDesulfurization flue gas baffle doorIt can be isolated during maintenance without shutting down the whole system.
Don't believe any "maintenance-free" nonsense. Expansion joints are like tires. They burst sooner or later if they are not maintained. Looking at it regularly and changing it regularly is ten times more cost-effective than waiting for a missed downtime.