What does a non-metallic expansion joint look like inside? Leave the terminology aside and dismantle how it works in vernacular

Don't make it complicated-a non-metallic expansion joint is a telescopic "soft connection"

Let's just be straightforward,Non-metallic expansion joint(Also called in the industryFabric fiber expansion joint、Non-metallic compensator) This thing is essentially an "elastic joint" for pipeline systems. When the pipeline is heated, it will extend, when it is cooled, it will shrink, and the equipment will vibrate when it runs. These displacements have to be digested somewhere, otherwise the pipeline interface will collapse and the equipment will be broken.

What is the difference from metal expansion joint? The core is just one word:Soft。 Metal bellows rely on the elastic deformation of stainless steel corrugations to absorb displacement, while the skin of non-metallic expansion joints is a flexible material made of two or more layers of fabric fibers, rubber and fluoroplastics. To put it bluntly, it is to replace metal corrugations with flexible materials. The advantage is that it can realize larger angle deflection and arbitrary displacement in three-dimensional space-this is not something that metal corrugated pipes can easily do.

The principle is one sentence: rely on the "wrinkling" and "stretching" of fiber cloth to digest displacement

Be thoroughPrinciple of non-metallic expansion jointWe have to see what it looks like first. A typical non-metallic expansion joint consists of the following parts:

• skin: Multi-layer fiber fabric + rubber/F4 film composite, is the core flexible element of the whole compensator.
• insulation layer: Close to the inner wall of the skin, usually ceramic fiber felt, used to insulate heat and prevent heat from burning the fibers directly.
• frame: Including flanges on both sides and a supporting skeleton in the middle to ensure the overall structural strength.
• guide tube: Mounted inside the pipe to protect the skin from the direct impact of high-speed media or particulate matter.

After thermal displacement, the skin does not deform by crests and valleys like a metal bellows-it has no ripples. It relies on the fiber cloth itselfcreaseAndInterlayer slipTo absorb axial compression, stretching, and lateral displacement. For example: when the flue gas pipe heats up and expands, the spacing between the pipes is shortened, the skin is compressed, and the fiber cloth is folded tighter; When the temperature shrinks, the skin is stretched and the wrinkles unfold. The whole process is like you repeatedly pinch a piece of canvas with your hands – it can withstand hundreds or thousands of bends, and the sheet metal cracks with fatigue after a few tries.

The principle is that simple, right? But don't underestimate this action of "pinching canvas". Under different temperatures and corrosive media, the material ratio of this "cloth" directly determines how long the expansion energy saving will last.

Why can non-metals "carry" high temperature and corrosion in power plants and cement plants?

That's a real question. Take the non-metallic expansion joint commonly used in the power station industry, for example, its skin is not a layer of cloth, butMultilayer compositeStructure:

• outermost layerIt is silicone rubber coated glass fiber cloth, which is resistant to temperature above 400 ℃, and at the same time is weatherproof and UV-resistant.
• Intermediate layerIt's polytetrafluoroethylene (PTFE) film, which is acid-and alkali-resistant, hardly reacts with any chemicals, and is used to block sulfide corrosion in smoke.
• inner layerThere is also ceramic fiber felt as heat insulation buffer to avoid heat transfer directly to the outer rubber coating.

Think about it, if you replace the metal expansion joint in this working condition-behind the desulfurization flue gas baffle door, the temperature fluctuates high and low, and the medium contains acid and humidity-the bellows can't do a few hot and cold cycles, and the weld will start to crack and pitting. However, the non-metallic expansion joint relies on the flexible composite layer, which not only absorbs the thermal displacement, but also withstands the corrosion, and its life can run for three to five years.

Its pressure bearing capacity is very weak, and it is generally only used in low-pressure ventilation and dust removal systems below 0.1MPa. When the pressure is high, the skin will be bulged, so there is no way.

Compared with metal expansion joints, its biggest advantage is actually "fault tolerance"

Anyone who has installed metal expansion joints knows that the installation accuracy requirements are ridiculously high. If the pipe is slightly tilted by a few millimeters, the bellows may become unstable, crack, or deflect and jam. Non-metallic expansion joints are built to eat this bowl of rice-it can withstand lateral displacements above ±50mm and angular deflections of ±30°. In other words: when you install it a little crooked, it can "patch it up" by itself.

And it is lightweight, and one person can drag the skin into place. Unlike metal expansion joints, which are dumb and heavy, and have to calculate the amount of pre-stretching, the installer makes a mistake and scraps them in minutes. But the cost is that the pressure is weak and it is not resistant to high temperature impact. Therefore, non-metallic expansion joints are generally installed inFlue gas baffle doorbehind,Cement kiln tail pipe、Dust collector inlet and outletThese places-low pressure, but large displacement and heavy corrosion.

If there are both metal expansion joints and non-metal expansion joints in the same system, then you have to find out the respective compensation amounts. Don't let the metal parts deadlift and tear the non-metal skin to pieces.

But it fears two things most: sharp objects and excessive temperature

• The medium temperature cannot exceed the skin limit.Usually silicone rubber fiberglass cloth is resistant to 350-400℃, and PTFE is resistant to about 260℃. Excessive heat can burn the fabric crisp or make the coating peel off-one excessive heat can end up scrapping.
• If there is particle erosion in the pipeline, a guide tube must be added.For example, the non-metallic expansion joint used behind the desulfurization flue gas baffle door, the guide tube is often made of 316L stainless steel, which guides the ash away and prevents it from hitting the skin directly.
• Don't let the bolts break the skin during installation.The flange surface of the frame must be polished smoothly, the gasket should be flat, and the bolt tightening moment should be even-otherwise the local stress will concentrate and the skin will quickly perforate.
• Regular inspections are a must.Fabric fiber has fatigue life, usually 3 to 5 years to change a wave, don't wait for it to leak before dismantling. Usually, see if there is any bulging, tearing and powdering on the surface of the skin. If you find that local damage can be patched, but if a large area is aged, you can directly replace the assembly.

How to set the standard? How to do maintenance? Some practical suggestions

National standardJB/T 12235-2015The test pressure and leakage rate of non-metallic expansion joints are clearly defined. Routine maintenance mainly looks at three points:

1. Appearance inspection: Is the skin surface intact, is there any bulge, tear, coating peeling?
2. Bolt tightening: Are the frame flange bolts loose? A loosening can cause the seal to fail.
3. Guide tube state: If the guide tube is worn out, it has to be replaced quickly, otherwise the particles will directly erode the skin.

Don't expose it to the sun when storing. Rubber and fluoroplastics are afraid of ultraviolet rays. It is best to lay flat in a cool and dry place. By the way, if metal expansion joints and non-metal expansion joints are used at the same time in the same system, be sure to set their respective compensation amounts-especially be careful not to let metal parts pull non-metallic parts, which is equivalent to pulling a rubber band with an iron chain, and the result can be imagined.

Don't buy products cheaply. What we maintain is the safety of the pipeline system and save hundreds of dollars. In case of an explosion, the loss of shutting down production is not a small amount.