It is the most flexible and the most vulnerable. The wall thickness of bellows is often only a few tenths of millimeters to a few millimeters, but it has to bear the displacement compensation of the whole pipe system. When the scene is bad, it is bad to scold the material, but the real murderer is nine times out of ten scouring. Want to Solve Metal Expansion Joint Scour? First, understand the following five questions, which works better than changing the model ten times.
1. How exactly does scouring happen?
Not all wear is called scouring. Particles in the medium, excessive flow rate, gas-liquid two-phase flow, and sudden change in flow direction-at least two of these four factors account for at least two, and the bellows wall surface will be repeatedly cut. To put it bluntly, this is the high-speed friction between the medium and the metal surface, which is the same as polishing with sandpaper. If you touch the flushed bellows with your hand, the surface is shiny, and you can even see the metal texture, then it doesn't run.
In some working conditions, the flow rate is not high, but the medium is steam with water, and the water droplets hit the trough at high speed, which still cuts iron like mud over time. What is more hidden is the sudden change of flow direction, such as the elbow immediately behind the expansion joint, and the medium directly turns against the bellows, which is equivalent to opening a sandblasting machine locally.
Second, how long does it take to scouring to destroy an expansion joint?
Three months, or even less. The wall thickness of the bellows is inherently thin. Once it is washed and thinned, the pinhole leaks first, and then it tears apart along the trough. Many scenes thought that the material was not good, so they changed 316L and 254SMO, but it still leaked. The roots are washing away, not corroding.
So the deflector is not an optional accessory, it is a shield for the bellows. The expansion joint without the deflector is equivalent to going to the battlefield without body armor. When selecting, if the medium has particles, high flow rate and gas-liquid two phases-any of these three words appear, the guide tube must be matched.
Third, how to prevent scour in design?
Add the guide tube, change the material and change the structure, all three tricks are indispensable. The guide tube guides the high-speed medium into the inside of the tube, so that the bellows avoid direct flushing, which is the first line of defense. When the dust content of the medium is large, you can consider surfacing cemented carbide on the inner wall of the pipe. Don't be reluctant to give up this cost. The replacement cost of an expansion joint is enough for surfacing for several years.
If the working temperature is high, choose high-temperature axial expansion joint or external pressure single axial expansion joint, and move the bellows to a position that is not directly impacted by the medium. The external pressure structure is particularly easy to use. The bellows is outside, the medium goes through the internal channel, and the trough can't be touched on the scour path at all. Two days ago, I met a cement factory customer. The temperature of the smoke duct was 800℃. It used to be an ordinary axial type, and it wore out in three months. Later, the external pressure single axial expansion joint was changed, and it took more than a year to see that the bellows was intact when disassembled.
4. What can be done by on-site operation?
Don't think everything will be fine if you put it on. Controlling flow rates, avoiding running at critical flows for long periods of time, checking bellows troughs for bright spots or pits during shutdowns — these details are more important than anything else. And guess what? Many scour accidents are caused by incomplete pipeline purging in the early stage of driving and welding slag stuck in the gap of the guide tube. The welding slag tumbles inside at a high speed, which is equivalent to filing the bellows every day.
When the expansion joint is installed, the direction of the guide tube must be consistent with the flow direction of the medium. The arrow is clearly marked, but someone is acting backwards. The consequence of reverse installation is that the medium is directly poured into the interlayer between the bellows and the guide tube, so it is not called washing, but cutting.
5. How to avoid stepping on pits when selecting models?
Calculate the medium first, then check the flow rate, and finally determine the structure. Flue gas pipeline uses non-metallic expansion joints or rectangular non-metallic expansion joints. The erosion resistance depends on the skin material. Don't just look at the thickness of the frame. The skin is the layer that directly contacts the medium. For general-purpose corrugated expansion joints or metal hoses for liquid pipelines, focus on the length and wall thickness of the guide tube-the front end of the guide tube should extend beyond the turbulent zone of the medium, and the wall thickness should be at least the same as that of the connection tube.
For power plant desulfurization system, corrosion-resistant expansion joints matched with desulfurization flue gas baffle doors have to be used. The concentration of chloride ions in desulfurization slurry is high and there are many particles, which ordinary stainless steel can't bear at all. Non-metallic or fluorine-lined structure is the correct solution. All in all, don't take the general structure to harden the bad working conditions. The first principle of type selection is to choose the structure under any working condition.
VI. Handling sequence of scour accidents
First stop the machine for inspection, then judge the degree of damage, and then decide whether to repair the weld or replace it as a whole. If the bellows is not perforated but thinned obviously, replace it directly-the strength and fatigue life of the thinned bellows are discounted, and the welding repair will only leave the stress concentration point. If only the guide tube is worn out, you can only change the guide tube, and the cost can be saved by half.
Remember, the expansion joint is the weak link in the pipe system, but it is because of its weakness that it protects the whole pipe system for you. When it is broken, I think of replacing it. It's better to ask more questions when selecting a model: Will this medium be washed? What is the flow rate? How big are the particles? — — This sentence can save hundreds of thousands of parking losses.