What exactly is a variable diameter non-metallic compensator? What is the difference with ordinary non-metallic expansion joint?
To put it bluntly, the variable diameter non-metallic compensator is an upgraded version of the non-metallic expansion joint (fabric fiber expansion joint) -one big and the other small, specially used to connect pipes of different diameters. Ordinary non-metallic expansion joints have the same diameter at both ends, which can only absorb displacement and vibration reduction; In addition to these tasks, the non-metallic compensator with variable diameter has to take into account the transition of variable diameter. Think about it, the flue suddenly shrank from 800mm of the fan outlet to 600mm of the desulfurization tower inlet. If an ordinary expansion joint is used to connect a big or small head in the middle, there will be a bunch of welds and leakage points. Direct on a reducing diameter compensator, hassle-free and reliable.
Many people confuse "reducing diameter" with "reducing diameter". The reducing diameter compensator is the same main body with one end large and one end small, and the inner fabric layer is continuous transition; Different diameter means that the diameter is changed by the flange, and the structure is still two independent parts. In our industry, the JB/T 12235-2015 standard clearly stipulates the technical requirements of non-metallic expansion joints, and the reduced diameter form is also included.
Structural decryption: fabric fiber plus reduced diameter, how can it withstand high temperature and corrosion?
When you disassemble a non-metallic compensator with variable diameter, you will know that its "core" is divided into three layers: the outer layer is glass fiber cloth coated with silicone rubber or fluororubber, which is waterproof and acid-proof; In the middle is the thermal insulation layer-ceramic fiber cotton or aluminum silicate cotton, which can be carried at 600℃ and 800℃. The inner layer is a corrosion resistant PTFE or PTFE coating. These three layers are molded or sewn into one piece, with a steel flange frame with reduced diameter.
Then how to handle the pressure? The tensile strength of the fabric itself is not low, and with multi-layer lamination, the working condition of 0.1~0.5MPa can be completely coped with. However, if the medium contains particulate matter, the inner layer must be thickened with a wear-resistant layer-which is why non-metallic compensators are more suitable for dusty gases than metal expansion joints. Two days ago, I met a customer. The temperature of the pipe at the tail of the cement kiln was 500℃ and the dust concentration was high. He insisted on using a metal expansion joint, and as a result, the guide tube was worn out in less than half a year. Later, it was replaced with non-metallic expansion joints (fabric fiber expansion joints), and it was fine for two years.
Real scenes: desulfurization flue, cement kiln tail, power station air duct-these places must be
Let's talk about desulfurization flue first. The flue gas temperature after wet desulfurization is 60~80℃, but it contains a large amount of SO₂ and HF, which is extremely corrosive. Even if the metal expansion joint is made of 316L or duplex steel, the weld is still prone to pitting corrosion. What about variable diameter non-metallic compensators? Lined with PTFE, the outer fabric is coated with fluororubber, which is not afraid of acid dew point corrosion at all. Moreover, the flue is often variable in diameter from the induced draft fan to the absorption tower, which can be done at once with a variable diameter compensator, and it can also absorb the thermal expansion and contraction of the pipeline and the foundation settlement.
Cement kiln tails are more typical. In the section from the outlet of the preheater to the waste heat boiler, the pipeline temperature is 400~500℃, and alkali metal steam contains alkali, chlorine and sulfur. When you look at the expansion joints in the cement plant, nine out of ten use non-metallic compensators-the metallic ones have long been damaged by high-temperature corrosion. Moreover, the diameter of the kiln tail pipe often changes. For example, the diameter of the fourth-stage preheater to the fifth-stage preheater should be reduced, and the non-metallic compensator with variable diameter has become the standard configuration. According to the data of our station, metal corrugated expansion joints in cement industry are mainly used in low-pressure cold air ducts of raw material mills and coal mills, and the high-temperature sections have to rely on non-metallic ones.
The same goes for power station air ducts. It is common for the diameter of the blower outlet to the air preheater to change from 2 meters to 1.5 meters. The combination of reducing diameter compensator + baffle door (such as flue gas baffle door or desulfurization flue gas baffle door) can not only adjust the air volume, but also absorb the thermal displacement. By the way, the baffle doors here are usually circular baffle doors (double seals) or single-axis double baffle doors. When installed with the reducer compensator, there must be enough flange space in the middle, otherwise disassembly and maintenance can be anxious.
Don't slap your head when selecting models: pressure, temperature, compensation amount, and standard JB/T 12235-2015
Temperature, pressure, amount of compensation. The upper temperature limit depends on the inner material and the insulation layer. Ordinary silicone cloth can only carry 250℃, while fluororubber cloth can reach 300℃. If a ceramic fiber cotton layer is added, the short-term peak value can reach 1000℃. In terms of pressure, the allowable pressure of non-metallic compensators generally does not exceed 0.3MPa. If it exceeds this number, it is necessary to consider using multi-layer thickened fabric or using metal expansion joints instead. The compensation amount includes axial, transverse and angular displacement. The reduced diameter structure will lead to the reduction of axial displacement capacity. When selecting the model, it should be discounted according to the product sample.
In terms of standards, the national standard for non-metallic expansion joints is JB/T 12235-2015. You can find that this standard specifies fatigue life, air tightness, flange connection size, etc. However, one thing should be noted: the standard has no mandatory requirements for the reducing taper of the reducing diameter non-metallic compensator, and the design of different manufacturers may be different. It is best to specify the number of PCDs and bolt holes for the big and small flanges in the contract, otherwise it will be embarrassing if you can't install them on site.
Installation and matching: How to match with baffle door and metal expansion joint? Is the tie rod nut removed or not?
The most common combination of variable diameter non-metallic compensators is used in series with baffle doors. For example, in the desulfurization system, the compensator is installed between the baffle door and the fan to absorb vibration and temperature difference. I have seen a project in which the flange between the compensator and the electric plug-in insulation door was directly hard connected, leaving no room for maintenance. As a result, the sealing surface of the baffle door was broken, and the compensator could not be removed. Later, the design was changed, and a short section was added in the middle, leaving 500mm space. Fortunately, it was.
When cooperating with metal expansion joints, the variable diameter non-metallic compensator is generally placed on the low temperature side or the corrosive medium side, and the metal one is placed on the high temperature and high pressure side. For example, the main steam pipeline of the power station uses metal expansion joints, and the diameter reduction of the smoke duct uses non-metal.
When it comes to tie rod nuts, many installers get the compensator and see tie rods at both ends, and their first reaction is to remove them. Wrong! The tie rod of the reducing diameter non-metallic compensator is used to limit the length and prevent twisting when transported and installed. After installation in place, **the tie rod nut must be loosened or removed**, otherwise the compensator will not be able to expand and contract freely, which is equivalent to a white installation. However, it cannot be completely disassembled, and a pair of diagonal nuts must be kept as limits to prevent the compensator from over-stretching. For the specific adjustment method, please see the Q&A "How to adjust the tie rod nut of the expansion joint" before our station.
One sentence summary: variable diameter non-metallic compensator is not universal, but without it many pipes really can't do
It solves the problem of pipeline compensation in the three-in-one scenario of high temperature, corrosion and variable diameter. Choose the right one, and save worry for five or six years; Choose the wrong one, and you will know how expensive regret medicine is when you remove it. Next time you encounter the reduced diameter nodes of desulfurization flue, cement kiln tail and power station air duct, don't hesitate to give priority to the reduced diameter non-metallic compensator. If the medium pressure exceeds 0.3MPa or the temperature exceeds 1000℃, then honestly put on the metal expansion joint + lining insulation layer.