How to choose non-metallic expansion joint material? Don't just look at the price, these parameters are the key
Two days ago, I met a customer, and when I came up, I asked, "How much is a meter of non-metallic expansion joint?" I asked him about the working condition parameters, and he said, "Quote first and then talk about it". As a result, after quoting the price, he was too expensive, so he turned to find a cheaper manufacturer. Three months later, the pipe leaked. When I removed it, I saw that the fabric layer was corroded into rags, and the rubber layer was hard and cracked. You say it was wrong or not?Material selection of non-metallic expansion jointsThis matter is really not decided by the price.
What exactly do you look at when you choose materials. Don't go routine, just talk about dry goods.
1. Common materials: fabric fiber, rubber, fluoroplastics, what work do they carry?
Fabric fiber(such as glass fiber cloth, aluminum silicate fiber cloth),Rubber(nitrile rubber, neoprene rubber, fluororubber, etc.),Fluoroplastics(Mainly polytetrafluoroethylene, aka PTFE). They do not work alone, but are used in stacked combinations.
- Fabric fiber layer— — Responsible for tensile resistance and temperature resistance. Aluminum silicate fiber cloth can carry above 1000℃, but it is afraid of water vapor and acid and alkali. Glass fiber cloth has a slightly lower temperature resistance (around 550 ℃), but the cost is low.
- rubber layer— — Mainly do air tightness and vibration reduction. For example, nitrile rubber is oil-resistant and solvent-resistant, but has poor ozone resistance; Neoprene has good weather resistance; Fluororubber is resistant to high temperatures (250℃) and strong corrosion, but it is expensive.
- Fluoroplastic layer— — PTFE (this site hasPTFE compensator) Resistant to almost all chemicals, low friction coefficient, but the upper temperature limit is 260 ℃, and the rigidity is large, not suitable for large displacement.
In the actual product,Non-metallic expansion joint (fabric fiber expansion joint)Usually multi-layer composite structure: inner PTFE or rubber for corrosion protection, middle fabric layer for bearing, outer rubber or fluoroplastic for protection. In the stationrubber compensatorAndRubber PTFE compensatorIt is a typical example-the former is mostly used in general water-vapor systems, while the latter is lined with PTFE to deal with strong acids.
2. Temperature, pressure and corrosive medium-three parameters directly determine the direction of material selection
Don't ask "which material is good", ask three numbers first: how many degrees? How much pressure? Is the media corrosive?
Temperature: ≤100°C is enough with ordinary rubber; 100-250℃ must use fluororubber or PTFE; If it exceeds 250℃, it can only rely on fabric fiber + multi-layer insulation layer, such as those used in flue gas pipelines of power stationsCorrugated expansion joint for power station industry(Although it is metal, the principle is similar) Temperature resistance is above 600℃. However, it should be noted that the maximum temperature of non-metallic expansion joints depends on the weakest layer-it is fine if you use aluminum silicate fiber for the outer layer, but once the inner rubber is overheated, it will carbonize, and the whole layer will be scrapped.
pressure: Non-metallic expansion joints are naturally weak in pressure resistance, and are generally used in low-pressure systems of 0.1-0.5MPa. If the pressure exceeds 0.5MPa, or there is negative pressure, a guide tube and a reinforcing ring must be added. The function of the guide tube has been mentioned in our FAQ-to reduce the direct erosion of the corrugation by the medium and reduce the resistance at the same time. Many people regard the deflector as an optional configuration, and this is the first big pit.
Corrosive media: The flue gas desulfurization system contains sulfuric acid and hydrochloric acid, so it can be fully lined with PTFE or fluororubber; Scenes containing hydrofluoric acid can't even hold up with fiberglass, and must be used with teflon-coated fabrics. In the stationDesulfurization flue gas baffle doorThe matching expansion joint is made of fluoroplastic composite layer.
3. The three easiest pits to step on in material selection
Pit 1: Deflector is ignored
Some manufacturers quote to save you the deflector, and the price is really low. However, when the medium contains particulate matter, there is no guide tube, and the fabric layer will be worn out within half a year. The guide tube can also prevent local overheating caused by airflow scurrying. The national standard JB/T 12235-2015 clearly requires that the guide tube must be designed under specific working conditions, so don't save this money.
Pit 2: Not enough installation space
Rectangular nonmetallic expansion joints such asRectangular non-metallic expansion jointIt requires sufficient flange spacing to ensure expansion and contraction during installation. Many people measured it on the spot and found that the space was 50mm smaller than the design drawing, so they hard-installed it. As a result, the compensation amount was simply not enough, and it was directly broken. The same goes for round ones, such asUniversal corrugated expansion jointThe reserved guide frame position for installation. Before selecting materials, confirm the pipeline layout, and don't wait until you enter the market before changing it.
Pit 3: The compensation amount was miscalculated
The amount of compensation for non-metallic expansion joints is limited, especially for angular displacement and lateral displacement. Many designers only count axial expansion and contraction, ignoring lateral misalignment. As a result, when the pipe is installed, it is thermally expanded, and the expansion joint is twisted and torn. What to do?When selecting materials, the three-dimensional displacement should be clearly markedLet the manufacturer design the layer thickness and wavenumber according to the actual displacement.
IV. Material difference between rectangular and round: How does structural shape affect material selection?
The circular expansion joint bears uniform force and is easy to seal, so the circular joint is preferred for high pressure or vacuum occasions. Rectangular expansion joint (such as this station'sMetal rectangular expansion jointAndRectangular non-metallic expansion joint) It is mainly used in rectangular cross-sections such as smoke duct and dust removal duct.
In terms of material, because the four corners of the rectangle have stress concentration, the fabric layer needs additional reinforcement. Usually, reinforcement layers are added at the corners or thicker high-temperature fibers are used. While round can use standard multi-layer winding process, the cost is lower. However, the fluoroplastic liner of rectangular non-metallic expansion joint is more likely to crack at the corner, so if the medium is highly corrosive, it is recommended to preferentially choose a round shape or use multiple rectangular splicing.
5. Take national standard JB/T 12235-2015 to inspect the goods
National standardJB/T 12235-2015("Non-metallic expansion joint") is a hard scale to judge whether the manufacturer's parameters are reliable or not. It clearly stipulates:
- Allowable deviation of material thickness (e.g. deviation of rubber layer thickness ± 0.5mm)
- Blasting pressure not less than 3 times the design pressure
- Fatigue life requirements (at least 1000 cycles)
- Minimum wall thickness of guide tube
Ask about the sealing structure —Material selection of non-metallic expansion jointsIt is not done after selecting the fabric. The sealing gasket and platen materials of the end flange have to be matched. For example, use PTFE gasket to seal, and the pressure plate must be used with stainless steel to prevent corrosion.
Low price is not unusual, and it is the ability to use it. If you choose the right material, an expansion joint will last five or six years without any problem; If you make the wrong choice, you will have to change it in half a year. Who loses and who earns?