Nonmetallic Expansion Joint Rubber Ring: Practical Experience in Type Selection, Failure and Maintenance
Many people think that the non-metallic expansion joint rubber ring is a ring of rubber pads, which is not worth a few dollars. In fact, it is the "goalkeeper" of the whole expansion joint-sealing the gas or dust in the pipe under high temperature, corrosive media and repeated displacement. We often call non-metallic expansion joints (such as fabric fiber expansion joints and rubber compensators), the main body is a flexible fabric layer, and it is often the inconspicuous rubber ring that really bears the sealing burden. If the rubber ring is chosen wrong, the whole expansion joint will be white, with leakage, shutdown and replacement in one stop, and the cost will rise.
How to pick rubber ring materials? This is not about slapping the head
The three hard indexes of working temperature, medium composition and pressure grade must be met at the same time. Take the desulfurization flue gas baffle door as an example. The equipped rubber ring should be able to hold wet SO₂ at about 150℃. Ordinary nitrile rubber can't last for more than half a year, SO fluororubber or silicone rubber must be applied. For example, the non-metallic expansion joint used in the cement industry has many dust particles and the rubber ring is wear-resistant. Two days ago, I met a customer who used the rubber ring of the universal corrugated expansion joint on the directly buried (fully buried) expansion joint. As a result, the groundwater level was high and it was soaked in three months. Do you think it's a loss or not?
Also don't ignore the trace components in the medium. The swelling effect of ammonia and acidic gas on the rubber ring is particularly hidden-the surface looks unchanged, but the volume expands by 10%, and the sealing surface leaks when squeezed. Therefore, it is best to take out the media composition analysis sheet before selecting materials, and don't just listen to the supplier saying "general type".
Don't step on this pit: the hardness and compression ratio of rubber ring is not the greater the better
In order to save money, some manufacturers randomly find an O-ring with a hardness of 70 to fool. After installation, the steel ring will either be too tight to deform, or it will leak when it is too loose. The correct method is to refer to JB/T 12235-2015 standard and calculate the cross-section size of rubber ring according to the design displacement of pipeline. We have measured that for a rectangular non-metallic expansion joint with an inner diameter of 500mm, the leakage rate can be controlled below 0.1% when the rubber ring section diameter is 12mm and the hardness is 60±5 Shore A. This data is not slapped on the forehead, but measured by repeated suppression.
And the compression rate-not the tighter you press, the better. If the compression ratio is too high, the permanent deformation of the rubber ring will be accelerated, and the resilience will be lost in half a year; The compression rate is too low, and air leaks directly when it shrinks at low temperature. The experience value is 15% ~25%, depending on the rubber ring material and flange surface roughness.
Several ways to die when rubber rings fail-guess which one is the deadliest?
The first is thermal aging, the surface is cracked and hard, and it breaks as soon as it is broken; The second is chemical corrosion, especially swelling caused by ammonia or acidic gas, and the seal fails after the volume becomes larger; The third type is mechanical scratch. For example, the flange bolt is deviated during installation, and the rubber ring is cut out by the flange edge. But the most hidden thing is fatigue fracture: the pipe vibrates tens of thousands of times a day, the rubber ring bends repeatedly, heat generates inside, and finally cracks from the root. We've counted after-sales records, and 90% of rubber ring replacements are due to fatigue issues, and they tend to happen between the 8th and 12th months of operation. Many people think that the material is not good, but in fact, they are "exhausted" by vibration.
There are tricks for maintaining rubber rings
Don't wait until it leaks before replacing it. Sweep the sealing surface of the rubber ring with an infrared thermometer every two months. If the temperature fluctuates by more than 15℃, it means that there may be local gas blowing. Stop the flange bolt immediately and pre-tighten it again. In addition, don't pry with metal tools when disassembling and assembling, soak them with plastic crowbars or hot water before operating. Experienced masters will apply a thin layer of molybdenum disulfide grease on the inside and outside of the rubber ring, which can not only reduce friction, but also delay oxidation. This action is done every time it is disassembled and assembled, and the life can be extended by 30%.
Incidentally, although the rubber rings of the rubber compensator and the non-metallic expansion joint look similar, the installation torque is much worse. The former generally requires 80~100N·m, while the latter is easy to compress and deform the flange when the torque exceeds 60N·m because the flange is thin, resulting in local overpressure failure of the rubber ring.
When is it time to replace a new rubber ring? Three signals
First, there is black powder residue on the flange surface after shutdown, which is the wear or aging debris on the surface of the rubber ring; Second, the smell of the medium can be smelled during operation, indicating that the seal has failed; Third, touching the sealing surface with your hand feels sticky or brittle, indicating that the rubber molecular chain has been broken. Remember an iron rule: non-metallic expansion joint rubber rings are consumables, don't expect them to last as much as metal expansion joints for ten years. Under normal working conditions, changing once a year is the safest rhythm.