I met a customer two days ago, and the non-metallic expansion joint on the flue gas pipe cracked in less than eight months. The manufacturer insisted that the working condition was overheated, so he asked him to change to a more expensive model. As a result, when we went to the scene, there were traces of acidic condensate next to the opening on the skin, and the inner layer of fluororubber had become hard and brittle, and it broke when it was broken-how is this overtemperature? It is obviously medium corrosion plus condensate infiltration, which has nothing to do with temperature.
I've seen this kind of thing a lot. The non-metallic expansion joint is cracked. On the surface, it is leaked, cracked and torn, but the failure mechanism is completely different. Today, I'll break these details apart and break them into pieces and make them clear. From investigation to repair, which pit manufacturers won't take the initiative to remind you, one by one.
First distinguish the rupture form, then talk about maintenance
Skin tearing, stitches unglued, flanges falling off. These three looks like air leakage and ash leakage, but the failure paths are completely different.
- Skin tear: The edge of the opening is hairy, and the fibers are stretched and broken, mostly because the displacement exceeds the limit or the pressure impact pulls the fabric layer apart. If there are wear marks at the tear, it is a missing guide tube or an offset installation causing direct media flush.
- Suture opening: Split along the lap seam to reveal the insulating cotton inside. This is basically because the adhesive process is not in place during manufacturing, or the temperature cycle during operation causes the adhesive layer to age-don't blame the glue in a hurry, first calculate whether the actual temperature peak exceeds the design value.
- Flange falling off: The skin around the bolt hole is pulled out of a tear-like hole, or the bead is deformed. In this case, the bolt preloading force is often uneven, and the stress is concentrated at several points, which tears the skin abruptly.
The fabric layer is worn out, the rubber is aged and cracked, and the stainless steel wire mesh is broken. These three things are dealt with differently. If a piece of skin made of the same material is worn out, the rubber will have to be changed in a whole circle when it ages-but if the wire mesh is broken, it means that there is a problem with the structural stress, and changing the skin alone is a cure for the symptoms but not the root cause.
Tracing back to the source: four factors, often superimposed
Temperature excess, pressure fluctuation, displacement overload, medium corrosion, any of these four factors can make the expansion joint scrapped. But in reality, it is rarely a single cause.
The acidic condensate seeped into the fabric layer, and on the surface there was only a small hole, and the inner layer was already crispy. Why? Because the flue gas temperature fluctuates, the condensate in the tube accumulates when the machine is shut down, seeps into the capillary pores of the skin fabric, and combines with the chloride ions in the thermal insulation cotton to form a corrosive micro-environment. The fluororubber coating is fine with temperature resistance, but not with this constant soaking.
How to reverse the cause of failure? Look at the fracture.
- The skin fracture has burning marks, yellow color and brittle-thermal oxygen aging, overtemperature.
- There are white or green crystals on the edge of the fracture-acid-alkali corrosion, check the composition of the medium.
- The broken end of stainless steel wire mesh has necking phenomenon-pulled and broken, displacement overload.
- The surface of the skin is blistered and delaminated, but the fracture is neat-the layers are peeled off and the glue fails.
This set of trace science is much more reliable than the "service life" said by superstitious manufacturers. The so-called design life is calculated according to the rated working conditions. How much temperature, displacement and corrosive medium you run in your actual working conditions, if you don't count it in your mind, it will be useless to change it how many times.
Hidden killers in installation and operation
To put it bluntly-many rupture accidents buried mines during the installation stage.
The flange surface is uneven. Non-metallic expansion joints have low stiffness, which does not absorb installation errors, but instead will amplify them to the skin. When the flange surface is warped by 3 mm, the local force on the skin doubles. The same is true for the uneven pre-tightening force of bolts. One side is tight and the other side is loose, and the skin on the tight side is fatigued first.
The problem of missing the guide tube is more concealed. The medium flows straight to the inner wall of the skin, and the wear rate is several times that of vertical scour. Especially for dusty smoke, there is no guide tube, and the skin life is directly cut in half.
And the stent is stuck. Don't rush to scold the quality when checking-look down to see if the pipes and brackets are competing. When the pipeline expands, the bracket gets stuck, and the displacement that should be absorbed by the expansion joint becomes a deadlift. In this case, the skin is "torn" and has nothing to do with the quality of the expansion joint itself.
Anti-rupture Strategy for Type Selection and Design
The most common mistake to make when selecting a model is to take the hard-sleeved working condition of the general model.
The same non-metallic expansion joints, circular and rectangular non-metallic expansion joints have completely different force characteristics. The circular structure has uniform circumferential stress, which is suitable for pipeline system; The four corners of rectangular (that is, rectangular non-metallic expansion joint) are stressed, and the skin is most likely to crack first at the corner-the corner should be extra strengthened or the number of skin layers should be increased when selecting the model.
Differences in working conditions should also be taken into account. The expansion joint of flue gas desulfurization system should focus on acid corrosion prevention, the outer layer of the skin is coated with fluororubber, and the inner layer is coated with polytetrafluoroethylene film as an isolation layer; The expansion joint of the cement kiln tail should be resistant to high temperature and abrasion, the density of insulation cotton should be high, and the outer sheath should be reinforced with stainless steel wire mesh.
The number of skin layers, fluororubber/silicone rubber coating, insulation cotton density, these parameters should be matched with medium temperature and corrosiveness. It is much more useful to leave enough displacement margin in the design stage than to install the limit screw afterwards-the limit screw only prevents over-displacement, but the number of cyclic fatigue times that the non-metallic expansion joint itself can withstand is limited. If the margin is not enough, it will crack sooner or later.
Emergency treatment and standard repair process after rupture
Use high-temperature-resistant tape or metal patch for temporary plugging, which can last for a few days, but this is not a long-term solution. Plugging just seals the surface, and the internal failure mechanism is still there.
According to the inspection requirements of JB/T 12235-2015, the skin can be replaced if the metal frame is not deformed and the internal insulation cotton is not condensed or coked. The criteria for judgment are simple:
- The metal frame is deformed more than 5mm, or there is rust and perforation-replace it as a whole and don't repair it.
- Insulation cotton is damp and coked, and the plate forms a block-it is necessary to change cotton and skin, and only changing skin is equivalent to leaving the root of the disease inside.
- The damaged area of the skin is less than 30% of the width of one side, and the frame is intact-it can be replaced locally, but the overlap seams should be staggered.
During repair, the torque of the strip bolt should reach the design value, be symmetrically tightened, and the force should be applied two or three times, and it should not be screwed to death at one time. Or fix here and crack there.
Daily inspection and life management
To kill the risk of rupture in the bud, it is enough to stare at three positions:
First, the wear points at the folds of the skin. When the expansion joint works properly, the skin will have slight wrinkles. These locations are the stress concentration areas and are the first areas to be worn down. Shine it with a flashlight. If the fabric fibers are exposed, it means that it is grinding.
Second, corrosion marks on the sealing surface of the flange. If there is any trace of rust dripping around the bolt hole, it means that the seal has failed and condensate is seeping in-as mentioned earlier, the inner layer may be completed.
Third, whether the body has any abnormal vibration. The non-metallic expansion joint itself does not have a vibration source. If it feels high-frequency jitter, it means that the pipeline is excited by airflow or the bracket is loose. This vibration will repeatedly bend the skin and accelerate fatigue.
Measuring the temperature distribution with infrared thermal imaging camera and checking the flange gap with a gauge are all easy ways to land on the spot. Infrared photo, which skin temperature is on the higher side-indicating that the insulation layer has become thinner; Which block is on the low side-indicates that cold air from outside has leaked in.
Non-metallic expansion joints are consumables, don't use them as permanent parts. Establish a ledger to record the temperature change, displacement and maintenance records of each start and stop, which is more efficient than regular disassembly and inspection. When the data accumulates to a certain amount, you can predict its failure cycle and replace it before it breaks.
This expansion joint thing, no one cares about it when it doesn't ring or cry. Wait for it to crack, leak, shut down, know the distress. A ten-minute look is better than a three-day shutdown.