Opening glue is not as simple as "glue doesn't work"-talk about the worst failure of non-metallic expansion joints
Two days ago, a power plant customer called, and his tone was very urgent: "I just changed the non-metallic expansion joint for three months, and the skin interface was opened again. Did you buy a fake?" I asked him what the temperature of the flue gas was and if there was any acidic medium, but he hesitated and couldn't say it. I have seen too many similar situations-when many people see the opening of the skin interface of the flue expansion joint, their first reaction is that the quality of the glue is poor and the manufacturer cuts corners. But the truth is often not that simple. Skin opening has nothing to do with glue sometimes.
Let's break it up and talk about it today. If you also encounter similar problems, don't rush to report for repair or replacement. Spend five minutes troubleshooting these three hidden killers, and you may save tens of thousands of dollars.
1. Temperature, corrosion, mechanical stress: Who is the real murderer who tears open the skin interface?
The skin of non-metallic expansion joints (also called fabric fiber expansion joints) is usually composed of multiple layers of composite materials-fluororubber, silicone rubber, polytetrafluoroethylene, glass fiber cloth, etc., and the layers are bonded by high-temperature vulcanization or high-strength adhesives. It stands to reason that these materials are not bad in temperature and corrosion resistance, but as long as the working conditions cross the boundary slightly, the interface will collapse first.
The first killer: hyperthermia.The design temperature of many flue gas pipelines is 180℃, but in actual operation, the boiler load fluctuates, the flue gas at the inlet of desulfurization tower is short-circuited, and the temperature may soar to 250℃ or even higher. The temperature resistance limit of fluororubber layer is generally about 230℃. If it exceeds this number, the adhesive will soften and then carbonize first, and it is only a matter of time before the glue will be opened. You check your DCS history curve, are there any periods of frequent overtemperature?
The second killer: chemical corrosion.The wet flue gas in the desulfurization flue contains sulfuric acid and sulfurous acid, and the pH value can be as low as 2. If the corrosion-resistant coating on the outer layer of the skin is slightly damaged, the acid seeps into the interlayer gaps, the adhesive is slowly dissolved, and the glue is rotten from the inside out. Touch the section of the glue opening with your hand. If it is sticky, black and sour, it is basically caused by corrosion.
The third killer: mechanical stress.This one is the easiest to overlook. If there is misalignment, excessive cold drawing, or the brackets on both sides of the expansion joint are not leveled during pipeline installation, the skin will have to bear additional shear and torsion. The adhesive layer at the interface has been pulled for a long time and cracked by fatigue. You stand on the side of the expansion joint and measure the flange plane with a horizontal ruler. If the height difference between the left and the right exceeds 2mm, you don't need to check anything else. Stress is the culprit.
2. On-site "three-step pulse sensing method": 1 look, 2 listen and 3 test to quickly locate the type of glue opening
Without any high-end instruments, you can judge it by seeing and listening to your eyes. If you don't believe me, you want to try?
Step 1: Look.Clean the area around the expansion joint and look at the skin surface with a flashlight. If the whole circle of glue is cracked horizontally, there is a high probability that it is over-temperature or high-temperature aging; If it is a local strip disengagement, and the location is on the side of the acid medium scour (usually the flue gas inlet side), it is in all likelihood corrosion; If the glue opening is jagged and there are tear marks on the edges, it is basically caused by mechanical stress.
Step 2: Listen.If there is negative pressure in the flue gas pipe, there will be a "hissing" inhalation sound at the glue opening. Take a long-handled screwdriver, stick the cutter head near the skin interface, and listen to the ear on the wooden handle to distinguish the specific location of the leakage point. This trick has been tried and tested in the hands of power plant masters.
Step 3: Measure.Measure the surface temperature of the skin with an infrared temperature measuring gun, focusing on the position of the adhesive line at the interface. If the temperature at a certain point is significantly higher than that in other areas, it means that the layer has broken and leaked, and the high-temperature smoke gas directly washes into the inner layer of the skin. In addition, prepare a piece of litmus paper, dip it in some condensate water to measure the pH value. If it is less than 3-do you still think it is a problem with glue?
3. Don't change if you can make up: two low-cost on-site repair schemes (with pit reminder)
After finding out the cause, if the main body of the skin is still intact and the damaged area does not exceed 30% of the total area, the on-site repair means can be used to prolong the life.
Scheme 1: Cold compensation bonding method.Suitable for linear opening of small areas (
Option 2: Local skin replacement method.If the opening length exceeds 300mm, or the skin has been bulged and layered, you can only change this piece. The method is to cut the damaged skin section along the inner side of the flange bolt hole with a utility knife, coat the newly cut skin with the same specification with high-temperature vulcanized glue, and then fix it on the original flange with stainless steel pressing strips and bolts. There is a pit here: the overlapping surface of the new and old skin must be staggered at the original interface position, at least 100mm, otherwise the stress will be concentrated at the junction of the new and old skin, and it will take less than a month to open again.
Both options cost less than one-tenth of the cost of replacing the entire expansion joint. But only if-you have to get rid of the hidden killer in front first. If you do not solve the problem of overtemperature, corrosion or stress, it is also a white patch.
4. Take an extra 5 minutes to select the model, saving the maintenance cost in the next year-Product configuration suggestions for flue gas conditions
Many projects choose a general-purpose one in order to save hundreds of dollarsRectangular non-metallic expansion jointJust put it on the flue. Wait until the skin is opened to regret it, and it costs tens of thousands to change. In fact, most pits can be avoided by considering a few more parameters when selecting.
First of all, where the flue gas has high acid content or high humidity (such as the inlet and outlet of desulfurization tower), the skin material should be selected with PTFE lining, such asPTFE compensatorOr non-metallic expansion joints lined with PTFE. The PTFE layer can resist strong acid, and the friction coefficient is low, so it is not easy to stick ash and scale.
Secondly, in the working condition of large temperature fluctuation, don't use a single layer of silicone rubber skin for cheap. Although the temperature resistance of silicone rubber reaches 250℃, the adhesive force will rapidly decay after repeated hot and cold cycles. Suggested choiceNon-metallic expansion joint (fabric fiber expansion joint)This product usually employs a multi-layer composite structure: inner stainless steel wire mesh reinforcement, intermediate aluminum silicate cotton insulation, and outer fluororubber seal. The high temperature stability is good, and the service life of the expansion joint can be doubled.
In addition, the form of flange interface is also critical. Many cases of glue opening are actually due to the insufficient width of flange strips and too large bolt spacing, which leads to uneven local stress on the skin. When selecting the type, the manufacturer is required to make the width of the pressing bar more than 40mm, and the bolt spacing should not exceed 120mm. If conditions permit, install an additional circleguide tube(There are expansion joint guide tubes in our product warehouse), which can prevent smoke from directly washing the skin interface.
Don't just look at the parameters on the product sample, be sure to ask the manufacturer to provide the same working conditionsNational standard for non-metallic expansion jointsType test report as required in JB/T 12235-2015. You're buying the actual performance, not the numbers on the promotional page.
5. A real case: the desulfurization flue skin interface of a power plant was opened, and finally it was found that this matter was forgotten during design
Last year, there was a desulfurization system of 300MW units, which was put into operation for less than half a year.Glue opening of skin interface of flue expansion jointThree times. The first two manufacturers changed the skin for free, but the third time only lasted two months. Party A is angry and asks us to do troubleshooting. After walking around the site, it was found that the installation position of the expansion joint was just 3 meters downstream of the inlet elbow of the desulfurization tower. After the flue gas passed through the elbow, the flow velocity was unevenly distributed, and the local flow velocity was as high as 18m/s. The skin interface was washed away by the flue gas containing high-speed acid droplets like sandpaper polishing. When the design institute laid out the pipeline, it did not consider the wear margin at all, nor did it set the wear-resistant lining plate.
The root cause of the problem has been found-it is not that the glue is not good, nor is it that the working conditions exceed the standard, but that the flow field analysis was forgotten during design. Later, we installed a deflector at the inlet side of the expansion joint to guide the high-speed fluid to the center of the pipeline, and the flow rate at the skin interface was reduced to 8m/s. Since then, there has been no opening of glue. You say it was wrong or not?
So, encounteringGlue opening of skin interface of flue expansion jointDon't rush to scold the manufacturer. Check the temperature curve, measure the pH of the condensate, and see the flange alignment. Go through the three steps in this article, and there is a high probability that you can find out the real murderer. If you are not sure, you can send the site photos and working condition data, and I will help you check it.