What exactly is a non-metallic expansion joint rubber ring? -Find out where it is in the structure
Non-metallic expansion joint, we call it in our stationNon-metallic expansion joint (fabric fiber expansion joint)This thing is especially used in the smoke and air ducts of power plants, cement plants and chemical plants. Its main body is a composite band of fabric and rubber, but many people don't know that what really separates the pipeline medium from the outside world is actually the rubber ring hidden at the connection of flanges. Yes, it isNon-metal expansion joint rubber ring。
Where is the rubber ring? To put it bluntly, it is a ring of elastic seals sleeved inside the flange bolt holes at both ends of the non-metallic expansion joint, or embedded in the groove of the flange sealing surface. Whether the high-temperature smoke and corrosive gas in the pipeline can escape depends on its face. Think about it, if the ring belt of the whole expansion joint is not broken, but the rubber ring is aging, and the smoke leaks out along the bolt hole, what is the difference between it and not installing it?
Rubber rings are made of various materials, such as silicone rubber, fluororubber, EPDM... how many degrees can they carry?
No, two days ago, a customer came with an old sample and asked, "What material is my rubber ring made of?" It was burnt at first glance, and it was impossible to tell it at all. Therefore, you have to find out the working temperature before selecting the material.
- Silicone rubber: Temperature resistance range-60℃ to +200℃, good elasticity, but oil resistance and acid and alkali resistance are average. It is no problem to use it on room temperature smoke and ventilation ducts, and it will be soft as soon as the temperature exceeds 200℃.
- fluororubber: Hard goods, able to reach 250℃, short-term peak can carry 300℃. It has strong resistance to acid, oil and solvent, and is the first choice for desulfurization flue gas and chemical media. The downside is that it is expensive and hardens at low temperatures.
- Ethylene propylene monomer (EPDM): Temperature resistance-40℃ to +150℃, first-class ozone resistance and weather resistance, commonly used in power plant air-cooled islands and outdoor pipes. But it can't hold up when it comes to oils or strong acids.
There are also polytetrafluoroethylene (PTFE) sealing gaskets, which are also available in our stationPTFE compensatorHowever, that is the overall structure, and pure PTFE is rarely used for rubber rings, because it is too hard to press tightly. Usually, a PTFE film is coated on the rubber surface to take into account both sealing and corrosion resistance.
Silicone rubber is cheap, but can it be replaced when fluororubber is used? Don't. The temperature exceeds the limit, the rubber ring hardens and cracks, and air leakage is a small matter. The pipeline stress changes to pull off the expansion joint is a big trouble.
Three pits that are easiest to step on during model selection: temperature exceeding limit, medium corrosion and wrong compression amount
Choosing non-metallic expansion joint rubber ring is difficult and simple. But the three pits below, I've seen too many people fall into them.
The first pit: the temperature exceeded the limit.There is an example of a cement plant-they use non-metallic expansion joints with a design temperature of 150℃ and EPDM for rubber rings. Results the actual operation of the kiln tail flue gas was 180℃, and the rubber ring became brittle in less than three months, and smoke emerged on the spot. It is only stable after changing the fluorine rubber. So don't just look at the design temperature on the drawing, but the actual peak temperature. It's not too much to leave a margin of 20℃, right?
The second pit: media corrosion.The desulfurization flue gas contains sulfuric acid condensate, and the pH value is as low as 2. With ordinary silicone rubber, it swells and deforms in a few weeks. Only the fluororubber or PTFE-lined scheme can hold up. Our station hasDesulfurization flue gas baffle doorThe matching seals are the same logic.
The third pit: the amount of compression is wrong.The compression amount of the rubber ring is not the greater the better. Some on-site masters tightened the bolts to death. As a result, the rubber ring was pressed into a pancake, and the elasticity was gone, but the air leaked. The standard compression rate is generally between 15% and 25%, depending on the hardness of the rubber. For example, fluororubber has high hardness and a compression ratio of 20%; Silicone rubber is soft, 15% is enough. How to control it? Use a torque wrench, which will be detailed below.
When should I change the rubber ring? Don't hesitate when these 4 signals appear on the scene
The non-metallic expansion joint rubber ring is a wear part, so don't expect it to last the same life as the main structure. If the following 4 signals appear on the scene, arrange replacement directly, don't delay.
- Signal 1: There is obvious smoke or water droplets seeping out at the flange connection.This indicates that the rubber ring has lost its sealing ability. Touch around the bolt hole with your hand, there are wet stains or dust and scale, and nine times out of ten the rubber ring is broken.
- Signal 2: The surface of the rubber ring is cracked, hardened or sticky.Take a look at the flange when shutting down for maintenance. If the rubber ring breaks when touched, or it sticks to the flange and can't be torn off, change it quickly.
- Signal 3: Frequent loosening of bolts.After the rubber ring is compressed and deformed permanently, the rebound force decreases and the bolt preload force attenuates. The bolt you tightened last week is loose again this week. Don't just blame the vibration. Check the rubber ring first.
- Signal 4: Non-metallic expansion joint belt abnormal displacement.The failure of rubber ring leads to uneven displacement constraint of pipeline, and the ring belt may be pulled and wrinkled. At this time, not only do you have to change the rubber ring, but also check whether the ring band is damaged.
How often do you change it? There is no fixed number. It is normal for desulfurization pipelines with bad working conditions to be changed once a year; Clean the ventilation ducts, and it will be fine for three or five years. It is recommended to check it together with the annual overhaul, and don't wait for a leak before stopping the production line.
Key points of changing the rubber ring: The strength of screwing and the cleaning of the sealing surface are more important than you think
Replacing the rubber ring looks simple, disassemble the old and install the new bolts, right? However, in practice, at least half of the early failures are caused by improper installation.
First, the sealing surface is cleaned.After the old rubber ring is removed, the residual glue, rust and dust on the contact surface of the flange must be removed. It is recommended to use a wire brush with solvent, and then wipe it dry with a rag. Think about it, there is a little bump on the flange, and the new rubber ring presses on it, the pressure is uneven, and the leak is there.
Second, bolt tightening sequence and torque.Don't just screw along. You have to cross twist diagonally, and gradually apply force in 2~3 times. For example, for M16 bolts, it is recommended that the torque be between 80~120N·m (depending on the width and hardness of the rubber ring). Use a torque wrench, don't rely on the feel. An old master told me that he had twisted it for 20 years and never believed in "feel", because it was far worse when it was cold and hot. I believe.
Third, apply grease before the rubber ring is installed?According to the situation. Viton rubber and EPDM can be thinly coated with silicone grease to prevent torsional damage during installation. However, when used in oxygen pipelines or strongly oxidizing media, greasing is strictly prohibited, which will have the risk of combustion. When unsure, consult the manufacturer.
Summary: The life of rubber ring directly determines the overall performance of non-metallic expansion joint
The rubber ring of non-metallic expansion joint is just a small circle, and the cost accounts for a low proportion of the whole expansion joint, but it is broken, and the sealing performance of the whole system is zero. Smoke leakage, uneven stress transmission, and even safety accidents are all chain reactions caused by this screw. When selecting the model, compare the temperature, medium and compression amount, stare at the cleaning and torque during installation, and grasp the four signals during maintenance-by doing these points, the non-metallic expansion joint can run safely and steadily for at least one maintenance cycle.
If you are unsure of the working conditions, or want to refer to the existing product informationrubber compensator、Rectangular non-metallic expansion jointFor the configuration of rubber ring, turn through the common questions and answers in our station at any time. For example, there are requirements for seals in the national standard JB/T 12235-2015 for non-metallic expansion joints.