Starting with a power plant repair: Why did they give up metal expansion joints?
I received a phone call in the middle of the night last month from the maintenance monitor of a power plant in Shandong. His voice was filled with fireworks: "Brother, that unit jumped again. The 'click' at the interface of the flue gas pipe is scary. The metal expansion joint cracked after less than three months. The maintenance window is only 48 hours. Can you get a set of non-metallic ones?"
In fact, the "non-metal" he said is what we often call a non-metal compensator, also called a non-metal expansion joint and a fabric fiber expansion joint. Someone mentioned that the set of metal bellows was selected at that time-the high-temperature flue gas contained a large amount of sulfide, which formed an acidic corrosive liquid after condensation, and then encountered metal fatigue, which was simply "chronic suicide". But it is cheap to purchase the picture over there. What is the result? One downtime loses hundreds of thousands, enough to buy dozens of non-metallic sets.
"If I knew earlier, I should have listened to you. Your rectangular non-metallic expansion joint is used on the pipe sections in front and behind the desulfurization flue gas baffle door. The brother factory has not changed it for five or six years."
Non-Metallic Compensators vs Metallic Expansion Joints: What's the Difference?
Stainless steel bellows pop up in many people's minds as soon as they hear "compensator". In fact, this is a misunderstanding. The metal expansion joint is pressure-resistant, and it is a little weak when it meets corrosive media, high-temperature dust, or large displacement rectangular flue.
To put it bluntly, the non-metallic compensator is laminated with flexible materials (fluororubber, silica gel, polytetrafluoroethylene, glass fiber cloth, ceramic fiber, etc.) to make a "soft connection". It can absorb axial, lateral and even angular displacement, and it has minimal thrust on the pipe-because it is not rigid to begin with.
Under the same caliber, the spring stiffness of metal expansion joint may be thousands of N/mm, while the stiffness of non-metal compensator is often only tens or even several N/mm. What does that mean? Pipe supports and fixing points can be designed to be lighter and less material efficient.
Moreover, in terms of temperature resistance, metal expansion joints are generally several hundred degrees, but ceramic fiber layers are used in non-metallic compensators, and with heat insulation cotton, some of them can withstand more than 1000℃. Tsk, which do you say is better for a flue gas system?
Taking apart a non-metallic compensator: what exactly is in it?
Last month the workshop happened to be assembling a batch of non-metallic expansion joints (fabric fiber expansion joints), and I will show you the internal structure.
The outermost layer is anticorrosive fluorine tape or silicone tape, which is waterproof and acid-and alkali-resistant; The inner layer is a reinforcement layer, usually stainless steel wire mesh or fiberglass cloth, used to withstand pressure; Further inside is the insulation layer-ceramic fiber felt or aerogel felt, which keeps the high temperature out; The innermost part is sometimes lined with a film of PTFE compensator liner to prevent the medium from adhering.
Our common rubber compensator is also a kind of non-metal, which mainly depends on rubber elastic deformation to absorb displacement, but its temperature resistance and corrosion resistance are worse than those of composite type. Therefore, the selection depends on the working conditions: if it is room temperature water and low pressure, rubber PTFE compensator will do; If it is high temperature smoke and acidic environment, fabric fiber multilayer structure must be applied.
Two days ago, I met a customer who insisted on installing the rubber compensator at the outlet of the desulfurization tower. As a result, it bulged in less than a month. I asked him, "Why didn't you tell me earlier?" He said, "I thought it was all the same." Everything?
How was the smoke pipe that almost had a big problem solved later?
There is another case that impressed me particularly. The flue gas pipeline of the sintering machine head of an iron and steel plant has a large temperature fluctuation (250℃ → 400℃ going back and forth), and the medium contains sulfur and dust. The original metal rectangular expansion joint burst three times in a year. The third time, the high-temperature smoke sprayed out and almost burned the cable tray next to it.
Later, it was replaced with a rectangular non-metallic expansion joint. The structure was designed according to JB/T 12235-2015, with high-strength metal flanges at both ends and six layers of composite fibers in the middle. When installing, a guide tube was specially added (the specific function of the expansion joint guide tube is to protect the internal fibers from being directly washed by high-speed dust), and there is no problem after more than two years.
And guess what? Later, Party A changed the inlet and outlet of the flue gas baffle door of the whole plant to non-metallic compensators. Even the pipe next to the manual plug-in insulation door was replaced.
Selection Notes: In what cases must you use a non-metallic compensator?
After more than ten years in this business, I have summarized several iron laws:
- High temperature and large diameter(For example, a round or rectangular flue with a diameter of more than 1 meter) -Metal expansion joints are too heavy, too expensive, and difficult to process.
- Corrosive media(Desulfurization, denitrification, chemical tail gas) -Metal can't withstand acid dew point corrosion.
- Large displacements need to be compensated frequently-Non-metal has good flexibility, and can be pre-compressed or pre-stretched to accommodate installation errors.
- Pipeline thrust sensitive(For example, connected to FRP or plastic pipe) -Non-metal has small load on the end, so it is not easy to break the equipment.
Of course, in high-pressure and high-temperature steam pipelines, non-metallic pipelines are beyond their capacity, and high-temperature axial expansion joints or metallic pipelines have to be used. But remember, there is no universal compensator, only the right choice.
From standard to landing: JB/T 12235-2015 What should ordinary users think?
As mentioned in the question and answer of the national standard of non-metallic expansion joints, JB/T 12235-2015 is the main technical basis in the industry. But many people have big heads when they look at the standards, which are all terms and parameters.
When you get the product description, focus on three data — —Design pressure, design temperature, compensation amount (axial/transverse/angular)。 For example, the standard requires that non-metallic compensators must be tested for airtightness, and the leakage rate should not exceed the specified value. When the goods you ordered arrive, you can ask the manufacturer to provide the factory inspection report, which has the specific values of pressure and displacement.
In addition, the flange connection size and bolt hole spacing are also specified in the standard. If you install rectangular non-metallic expansion joints on existing pipes, be sure to measure the distribution of flange screw holes. Don't buy them back and can't install them, which will be embarrassing.
In fact, saying this is not to let you memorize the standards, but to give you an intuition: the products of regular manufacturers will be clearly marked on these three data, and they can correspond to your on-site working conditions one-to-one. If the merchant says "universal type, all can be used", you turn your head and leave. The universal corrugated expansion joint is said to be "universal", and the non-metallic compensator is customized according to the project.
Okay, so much verbose, let me send a final word: Don't let the "hard" of metal hurt you, and don't let the "soft" of non-metal pit you. Choose the right one, and the pipeline is as stable as Mount Tai; Wrong choice, constant phone calls in the middle of the night. If you're still struggling, send me the working parameters and I'll take a look for you.