Why do non-metallic expansion joints need separate standards? — — Start with the purpose
Two days ago, I met a customer who was doing a desulfurization project. When I came up, I asked, "Aren't all expansion joints the same standard? Why do you always emphasize the separate standard of non-metals?" Hey, that's a point. Metal expansion joints and non-metallic expansion joints, although both called expansion joints, work in completely different places.
Metal expansion joints, as we often sayUniversal corrugated expansion joint、Corrugated expansion joint for power station industryIt mainly relies on stainless steel bellows to absorb displacement, high pressure resistance and high temperature, but it is not very useful when it encounters corrosive smoke, large dust or large compensation. At this time,Non-metallic expansion joints (fabric fiber expansion joints)It comes in handy-it is made of flexible materials such as fiber fabric, rubber and PTFE as loop belts, which are corrosion-resistant and wear-resistant, can absorb multi-dimensional displacement, and is light to install.
Think about it, the desulfurization flue gas in the flue of the power plant is not high in temperature but contains acid and humidity. The metal bellows will corrode through in a few years, but the fabric fiber can carry it. Can the standard be the same if the use is different? Therefore, the country specially issued aJB/T 12235-2015 Non-Metallic Expansion JointsThe technical threshold of such products has been set to death.
What exactly are the hard indicators stipulated in JB/T 12235-2015?
The full name of this standard is "Non-metallic Expansion Joint", which was released in 2015. It is the only special machinery industry standard for non-metallic expansion joints in China at present. It divides the hard indicator into several pieces:
- Displacement compensation capability: There are clear test methods for compensation amounts in axial, transverse and angular directions. The standard stipulates that the product must be tested for displacement cycle before leaving the factory, and there should be no leakage or permanent deformation.
- Sealability: That's the bottom line. The standard requires airtightness test. The medium is generally air or water, and the pressure is usually 1.5 times the design pressure. Hold the pressure for 5 minutes, and there shall be no visible leakage.
- Temperature resistance grade: The temperature resistance range of non-metallic materials must be clearly marked. For example, how much the fiber part of the fabric can withstand, how much the rubber part can withstand, and how much the PTFE lining can withstand-can't be vague.
- Fatigue life: Although non-metallic expansion joints are not fatigued for tens of thousands of times like metal bellows, the standard also stipulates the requirement that at least 2000 displacement cycles are not damaged. Is it enough? The flue operating condition is only opened and closed a few times a year, which is completely enough.
Material and structure: fabric fiber, rubber, PTFE, how to distinguish in the standard?
The core of the material combination of non-metallic expansion joints is the "loop band"-that is, the middle layer of flexible body. The standards are divided into three broad categories based on the material:
Category I: Fabric Fiber Expansion Joints.This is the most common, such asRectangular non-metallic expansion jointAndNon-metallic expansion joint (fabric fiber expansion joint)。 The loop belt is made of multi-layer fiber cloth (glass fiber, polyester fiber, aramid, etc.) composite with silicone rubber or fluororubber coating in the middle. The standard requires that the breaking strength of fiber cloth and the adhesive strength of coating must be inspected from the factory.
Class II: Rubber Compensator.Likerubber compensator、Rubber PTFE compensatorThe main body is rubber elastomer, and the inside is reinforced by cord cloth. The standard draws a red line on the oil resistance, acid resistance and heat aging resistance of rubber-for example, after aging in hot air at 100℃ for 168 hours, the tensile strength cannot decrease by more than 20%.
Category III: PTFE Compensator.Which isPTFE compensatorAndPTFE-lined hoseThat kind. Tetrafluorine material is extremely chemically inert, and its welding strength and penetration resistance are specified separately in the standard. Because PTFE cannot be bonded, hot air welding is required, and the weld strength must reach more than 80% of the base metal.
Inspection, Installation and Service Life: Details of Standard Landing without Pit
No matter how beautiful the standard is, it is useless to land in a mess. Let's talk about a few places where it is easy to step on pits:
Inspection link.JB/T 12235 requires two necessary inspections before the product leaves the factory: appearance inspection (no bubbles, no wrinkles, no damage) and dimensional inspection (length, width, flange parallelism). Sampling items include airtightness test and tensile strength. When you receive the goods, you can randomly draw a piece of water to see if it leaks. If there is no condition for water, at least check whether the appearance and size are correct with the drawings.
Installation link.The most afraid thing about non-metallic expansion joints is "installation stress". It is clearly stated in the standard: you can't forcibly stretch or compress the loop belt during installation, otherwise the life will be directly discounted by 50%. Such asElectric plug-in insulation doorOrManual plug-in insulation doorNext to the non-metallic expansion joint, before installation, confirm the cold position of the pipeline, and then adjust the length of the expansion joint so that it can be connected in a natural state.
Service life.Many people ask, how many years does non-metallic expansion save energy? In fact, the standard doesn't write a specific number of years, but it gives a concept of "equivalent full cycle times". According to the displacement frequency, temperature and medium corrosiveness of your pipeline system, convert it into the number of displacement cycles to judge. Generally, it is fine to use it for 5-8 years on the flue, provided that the material selection and installation are not wrong. If the temperature exceeds the standard for a long time, the life span will drop off a cliff-for example, if the design temperature of silicone rubber is 200℃, you will be done in half a year if you hard top it to 250℃.
Note when selecting: Standard differences between non-metallic expansion joints and metallic expansion joints
Why can't you put the standard of metal expansion joint on non-metal?
The standard mainstream of metal expansion joints isGB/T 12777-2019 General Technical Specifications for Metal Bellows Expansion JointsIt mainly talks about the design pressure, single wave compensation amount and fatigue life of bellows. While the JB/T 12235 of non-metallic expansion joints basically does not involve pressure design (because non-metals themselves do not withstand pressure and are usually only used in low-pressure flues), it is more concerned with:
- Corrosion resistance: There is no such special requirement in the metal standard, but the acid-base immersion test is required in the non-metal standard.
- Vibration and Noise: Non-metal has vibration reduction function, and there are recommended indicators of damping performance in the standard.
- Flanged connection: Metal expansion joints are mostly used for welding, while non-metallic expansion joints are mostly connected with flange bolts. The standard requires the sealing surface and bolt torque of flange.
So, don't get mixed up when making a selection. You have a desulfurization flue with a temperature of 80℃ and a pressure of 0.01MPa. You have to choose a metal expansion joint, which is a knife to kill chickens and is easy to corrode. In turn, dare to install fabric fiber expansion joints on high-pressure steam pipelines? That just blows away. When selecting a type, look at the parameter table to see whether it is executedJB/T 12235StillGB/T 12777This distinction itself determines the applicable scenario of the product.
Standards are dead, working conditions are alive. Understanding the standard is only the first step. A truly reliable manufacturer will combine your medium, temperature and displacement direction to match the appropriate materials.