What exactly does the platen do in the non-metallic expansion joint
Let's start with the most common misconception. Many brothers who are engaged in purchasing think that the non-metallic expansion joint pressure plate is just a flat steel bar. What is there to pick? If you think so, there is a chance that something will go wrong later.
The non-metallic skin — that is, the flexible band of the fabric fiber expansion joint — was pressed against the edge of the flange. Without the platen, the skin can't hold. The smoke, hot air, and dust in the pipes had to leak out through the cracks. The environment of the flue at the tail of the power station boiler is shut down if it leaks, and it will lose hundreds of thousands of electricity bills if it stops.
The cross-sectional shape, bolt hole distance and edge chamfer of the pressure plate directly affect the sealing effect. Take the most common example on the spot: rectangular non-metallic expansion joint, how to splice the pressure plate at the corner of the four corners? Many installation teams directly cut four straight plates at a 45-degree angle and put them together hard. As a result, the skin wrinkled into a ball at the corner, and it wore out after two months of running. The correct way is to cut the arc transition section separately, and the skin cutting also follows the arc line. The processing accuracy of the head here is one order of magnitude higher than that of the straight edge section.
If the material is not selected correctly, no matter how cheap it is, it is throwing money in vain
How to choose the material of the platen? It depends on the working conditions, not just the price list. I have said this many times, but every time a customer still takes a carbon steel plate and asks: Why is the expansion joint next to our desulfurization tower rusting out in less than two years?
Think about it, the temperature of the net flue gas after wet desulfurization is 60 or 70 degrees, the humidity is close to saturation, and there are still unremoved chloride ions and sulfite radicals in it. The corrosion rate of carbon steel in this environment, the laboratory data is 0.3 to 0.5 mm per year, and the field is actually faster than that. 2mm carbon steel platen, two years rust wear is completely normal. If you change to stainless steel, you have to pick the brand-304 still pits in front of chloride ions, so you have to use 316L or even 254SMO. Some people say that with aluminum alloy, light is light, but the strength is there. Under large-diameter working conditions, the preloading force can't be pressed tightly at all, and it is only a matter of time before the skin loosens.
The surface treatment of the platen is also particular. Hot-dip galvanizing, Dacromet, and spraying anticorrosive paint have their own applicable scenarios. In outdoor open-air conditions, the thickness of hot-dip galvanized layer should reach more than 85 microns, and it must be checked according to GB/T 13912. Dacro coating has good salt spray resistance, but poor wear resistance, and it will be exposed when bumped during assembly-if you finish assembling and repaint it, the adhesion at that position will never catch up with the original coating. The most fearful thing is not to choose the wrong surface treatment, but to choose is equal to not choosing: the coating will fall off as soon as it is touched, and it will start to rust when it is installed.
Three True Death Methods of Pressing Plate Failure
The bolts slipped loose and the skin tore through the edges. Many people know that non-metallic expansion joints vibrate at high frequency when they run, but few people realize how much vibration threatens the platen bolts. When there are no anti-loosening measures for the pressure plate bolts, the vibration of the fan at 3000 rpm is transmitted, and the nut grinds its teeth while withdrawing, and it can be loosened one or two times in three months. As soon as the pre-tightening force drops, the skin begins to shake, the cloth layer and the rubber layer rub each other, and the edges quickly grind out burrs, and then one seam becomes two seams, and finally the whole tears. Anti-loosening washer, double nut, thread glue, choose the same, cost a few dollars, can save tens of thousands of skin.
The pressure plate is not stiff enough, and the skin bulge is bent by wind pressure. In some sites, the bolt spacing of the platen is pulled to 300mm or even wider, and the middle span is too large, and the platen bulges with the skin. The bending stress of the skin in the bulging area is four or five times that of the normal area. Under the repeated impact of wind pressure, the fiber layer is broken layer by layer. If you look at those expansion joints that bulge after one year of operation, nine times out of ten, the cross section of the platen is selected small, or the bolt arrangement is too thin.
Thermal expansion deforms the top of the platen. After the pipeline is heated, the linear expansion coefficient of the skin is much larger than that of the metal platen. When the temperature rose, the skin stretched longer, but the platen didn't stretch so long, and the sides of the joint pulled at each other-the force was not small at all, enough to tear the weld apart. There have been several cases in the hot air pipeline of cement industry, where the weld of the pressure plate is cracked laterally, the crack is extended to the bolt hole, and the whole pressure plate is scrapped. You said that this kind of thing can't be found until it is removed. When the inspection sees that the skin is everted, it is already a certainty that it will be shut down for maintenance.
Installation and maintenance are all details
In fact, there is a recommended practice in the national standard for the bolt tightening sequence when the pressure plate is installed: twist symmetrically from the middle to both sides, and apply force in three times. The first time is twisted to 40% of the preloading force, the second time is 70%, and the third time is 100%. You asked what is the specific preload force? It should be on the design drawings. If not, it should be calculated according to the bolt specifications and skin pressure grade. If it can't be calculated, try it on the spot-but at least ensure that all bolts are evenly stressed, and don't tighten one end and loosen the other.
Can the platen be reused or not? The bolt hole is not elongated after removal, the pressure plate is not obviously deformed, and there is no rust crack, so it can be reused normally. But if it has rusted into fish scales, or there are visible cracks on the edges, change it when you need to. Don't dig into the cost of such small pieces. The loss of one leak is enough to buy dozens of pressure plates.
Finally, let's talk about the most overlooked point-the edge of the platen in contact with the skin, be sure to smooth it. Burrs and acute angles look unremarkable, but the cuts they make in the skin accumulate chronically. The fabric fiber layer was repeatedly cut, and the surface could not be seen at first. After half a year, it was disassembled and inspected, and the inside of the skin had been worn out in a large area. The concealment of this problem is that you won't know what day it started to break, and by the time you find out, the entire circle belt is basically scrapped. During the operation and maintenance inspection, focus on whether there is a gap on the edge of the pressure plate and whether the skin slides out from the bottom of the pressure plate. It is more intuitive than anything else to shine on the seam between the pressure plate and the skin with a flashlight. Alas, to say a thousand words, the seemingly inconspicuous part of the pressure plate is often the key to deciding that the non-metallic expansion energy saving can't be used for a long time. The next time you select a model, it is worth more than anything to spend ten more minutes going through these questions.