First, let's find out a question: why are ordinary non-metallic expansion joints not enough, and we have to emphasize "insulation"?
Two days ago, I met a customer, and he asked, "Is your non-metallic expansion energy-saving insulation?" I said, what kind of insulation did you mean? He said, "If there is stray current in the pipeline, you can't let the current run along the pipeline, otherwise there will be problems with the equipment."
Tsk, this demand is actually quite common, but many people equate "non-metal" with "insulation".Non-metallic expansion joint (fabric fiber expansion joint)The main body is made of glass fiber, silicone cloth, polytetrafluoroethylene and other materials, which are indeed non-conductive. However, the problem is that there are usually metal parts at the flange connection, or if metal bolts and gaskets are used during installation, the current may "bridge" through the flange surface or fasteners. The true insulating expansion joint is designed to cut off all possible conductive paths from the structure-including insulating cushions between flanges, insulating sleeves for bolts, and even, in some cases, insulating partitions inside the compensator.
In other words, ordinary non-metallic expansion joints have only material insulation potential, whileNon-metallic insulated expansion jointIt is to implement this potential into every detail. So, on what occasions must this "enhanced version" be used?
Application scenario: Do you think only power plants need it? Wrong.
The most common is, of course, the power station industry-the ones in our stationCorrugated expansion joint for power station industryIt is mainly made of metal, but when it comes to smoke duct, dust removal pipe or desulfurization system, the temperature is high, the medium is corrosive, and it is afraid of current interference, so non-metallic insulating expansion joint becomes the first choice. For example, at the inlet and outlet of the electrostatic precipitator in the power plant, there will be static electricity accumulation in the pipeline, which may cause safety accidents when directly conducted to the equipment. At this time, the current has to be "intercepted" by the insulating expansion joint.
For example, in the industries of electrolytic aluminum and chlor-alkali chemical industry, there are often stray currents in the pipelines near the electrolytic cell-the current does not go through the wires honestly, but runs around through the pipelines, brackets and flanges. And guess what? An expansion joint without insulation treatment may become a "leakage channel" of the whole pipeline, which can accelerate corrosion at least and ignite and explode at worst.
There are also ventilation ducts of rail transit and subway platforms-the electromagnetic environment in the subway is complex, and the pipeline system also requires grounding and isolation. To be honest, many design institutes will only choose ordinary nonmetallic compensators at first, and wait until the on-site debugging finds that the current interferes with the signal system before they go back and change the insulation type, tossing back and forth the construction period and cost.
By the way, we still haveRectangular non-metallic expansion jointAndrubber compensatorThese products often have certain insulation ability in rectangular pipelines or low-voltage systems, but if you have clear insulation requirements (such as withstand voltage grade and insulation resistance value), make it clear to the manufacturer before placing an order, so don't take it for granted.
Structural dismantling: how exactly is it "insulated"?
Let's talk about the subject first- -with the ordinaryNon-metallic expansion joint (fabric fiber expansion joint)Similarly, the ring belt part is a multi-layer composite material: the outermost layer is a silicone rubber or fluororubber coated cloth, which is temperature-resistant and corrosion-resistant; There is a reinforcing layer of glass fiber or ceramic fiber in the middle, which is tensile; The inner layer may also have a PTFE film, which is anti-adhesion. These materials themselves have a resistivity of more than 10^12 Ω · cm and are absolute insulators.
The key is in the flange part. Ordinary non-metallic expansion joints, flanges are made of carbon steel or stainless steel frames, which are compressed by bolts. The current will go all the way through the bolts, flanges and pipes. The insulation type will add a layer of insulating gasket (usually PTFE board or epoxy glass cloth board) to the butt surface of the flange, and add insulating sleeves and insulating gaskets where the bolts pass-essentially, all metal contact points are "wrapped" with a layer of non-conductive material.
If the pipe medium itself is charged (such as transporting dust gas with high static electricity), then the compensator may need to be installed with a conductive layer and grounded to conduct static electricity into the earth instead of allowing it to accumulate on the band. At this time, "insulation" and "static conductivity" sound contradictory, but in fact, they should be designed separately according to the working conditions-that's why you can't make do with general products.
So you see,Non-metallic insulated expansion jointNot simply "changing a cloth", but systematic insulation design.
Guide to selection and avoiding pits: Don't step on these three common pits
Pit 1: Only look at the temperature, not the insulation grade.Many people directly choose high-temperature resistant non-metallic expansion joints as soon as they see the on-site temperature of 400℃. But if the insulation voltage you ask for is 10kV, ordinary silicone cloth can't hold it at all-the insulation performance of silicone rubber will decrease at high temperature. It is necessary to confirm the temperature resistance and voltage resistance cross curve of the material, or use the way of thickening the insulation layer. In our stationHigh temperature axial expansion jointThe combination of materials used may be able to solve the problem, but it depends on the insulation index.
Pit 2: Save trouble when installing, so I changed the bolts myself.The on-site construction team found it difficult to screw the insulating sleeves and gaskets equipped in the factory, so they replaced them with ordinary galvanized bolts, thinking that "it's okay to have so many bolts and a few less insulation anyway". And the result? Current exclusively takes shortcuts, and those few ordinary bolts will waste the whole insulation design. Remember: Fasteners for insulating expansion joints must be used in a full set, not one less will work.
Pit 3: Take "non-metal" and "insulation" as the same thing, and ignore the flange material.Some projects are cheap, so choose ordinary non-metallic expansion joints and add insulating gaskets on the spot. However, the flange of ordinary non-metallic expansion joint is painted with carbon steel. After a long time, the paint layer is damaged, but it is still conductive. For real insulation expansion joints, the flange surface should be sprayed with Teflon or coated with rubber, or simply use stainless steel flanges to match insulation parts, so as to be reliable for a long time.
Have you measured the insulation resistance on site? According to JB/T 12235-2015 standard, non-metallic expansion joints should be tested for insulation resistance and withstand voltage before leaving the factory-if the manufacturer has not done it, then you have to have a question mark in your mind.
Standards and Practices: What does JB/T 12235-2015 say?
This national standard covers the basic technical requirements of non-metallic expansion joints. Although there is no separate chapter on insulation performance, dielectric strength is mentioned in materials and test methods. In practical applications, the power industry usually cites DL/T 5154 or IEC standards to supplement the insulation requirements.
A nuclear power project requires that the insulation resistance of the expansion joint is not less than 1000M Ω (measured with a 500V megohmmeter), and there is no breakdown at 1.5 times the rated voltage. This is not a generic indicator for non-metallic expansion joints, but rather an additional requirement that the insulated type must meet.
Therefore, when you get the manufacturer's product information, don't just look at the words "non-metallic expansion joint", but ask specifically: What is the insulation resistance of your model? Is the test report attached? Is there a third party type test report? If the other party is faltering, then change the house.
Finally, let's talk about a practical case
Last year, there was a cement factory customer. The pipeline at the end of their kiln was often perforated due to current corrosion. Changing ordinary non-metallic expansion joints twice didn't solve the problem. Later, we recommended a custom insulation type-that is, puttingMetal hoseAndNon-metallic expansion jointCombining the advantages of insulation and isolation at both ends,PTFE compensatorMake the lining. Used it for over two years and never had any problems again. Is it worth the money?