Specialized in manufacturing compensators, expansion joints, baffle doors

What are you most afraid of when choosing metal expansion joints? I am afraid that the customer will only say "Quote me the price of DN600" on the phone. After quoting, ask "why is it so expensive". Because the selection parameters are incomplete, manufacturers can only quote according to the most conservative and largest specifications. Can this price not be high?

What about that? If you want to buy a suitable expansion joint, don't rush to report the caliber. Find out the following 5 things.

Don't be in a hurry to report the caliber, first calculate how much displacement the pipeline should absorb

The core function of the expansion joint is to absorb displacement. The amount of displacement was not calculated clearly, and the rest was all for nothing.

The thermal expansion of a pipeline is generally calculated by multiplying the linear expansion coefficient of the pipeline by the temperature difference and then multiplying the length of the pipeline. The linear expansion coefficient of carbon steel is approximately 0.012 mm/m·°C, and the stainless steel is slightly higher. For example: a 30-meter-long carbon steel pipeline, when the medium temperature rises from 20°C to 200°C, the axial thermal expansion is 30×180×0.012=64.8mm.

This is not over, the ambient temperature during installation, equipment vibration, and displacement caused by pipeline foundation settlement have to be counted. If you only know the diameter of the pipe and don't know how to calculate it, ask the manufacturer for a "Quick Check Table of Pipeline Thermal Displacement" and check it. But only if you have to prepare the pipeline direction diagram and clearly mark the positions of the fixed bracket and sliding bracket.

Media, pressure, temperature: three-piece set that determines the material and number of layers of corrugated pipe

Looking at these three parameters together, the material, wall thickness and number of layers of the bellows are basically determined.

At this end of the medium, 304 stainless steel is enough for ordinary steam, hot water and compressed air; For the medium containing chloride ions, 316L is used; In this strong corrosion condition of desulfurization flue gas, corrosion-resistant alloys or non-metallic expansion joints are often selected. The composition of the medium must be written in full, don't just mention a "smoke". The content of SO₂, NO and dust in the smoke is different, and the corrosiveness is several times worse.

If the pressure is high, the wall thickness has to be added; When the temperature is high, the permissible stress of the material decreases, and the compensation amount is also affected. For example, a steam pipeline with a pressure of 1.6MPa and a temperature of 350°C usually uses 2 to 3 layers of bellows; At the same temperature but the pressure dropped to 0.1MPa, a single layer is enough. The more layers, the greater the stiffness and the higher the price, so there is no need to blindly pursue multiple layers.

On the other hand, don't leak the deflector. The flow rate of the gas medium is high, and the bellows are easily eroded without the guide tube. If the medium contains particulate matter, the anti-wear structure at the front end of the guide tube should also be considered.

Axial, lateral or angular displacement? Choose the universal or hinged type

If the displacement direction is not clear, the chosen structure must be wrong.

The pipeline mainly expands and contracts along the direction of the pipeline, that is, axial displacement. Choose general corrugated expansion joint, also called axial expansion joint. If the pipeline takes an L-shaped or Z-shaped bend, and there is displacement in both horizontal and vertical directions, a single axial type can't be handled at this time, so we have to consider the double hinge transverse expansion joint, or the use of two hinge expansion joints together.

Angular displacement is common at equipment connections, such as steam turbine exhaust port and fan inlet and outlet. Hinge type expansion joint or double tie rod type is a good choice under this working condition.

How to judge? To put it bluntly, just one sentence: telescope and contract along the direction of the tube to choose the general-purpose type; If the pipe is bent and the joint is deflected, you have to use one with hinges or duplex structure. Don't expect to eat all the displacements with a general-purpose type. That's called hard carrying, not compensation.

Tie rod, hinge or pressure balance? Constrained structural relationship support stress

The function of expansion joint is not only "telescopic", but also "controllable force".

The bellows will produce a huge thrust under the action of pressure. If this thrust is all thrown to the fixed bracket, how thick must the bracket be? Therefore, there should be a restraint structure inside the expansion joint to share-this is the meaning of the existence of tie rod, hinge and pressure balance.

The universal expansion joint does not come with its own constraints, and the internal pressure thrust must be carried by the main fixed bracket. Where the pipeline is complicated and the bracket is difficult to make, use straight pipe pressure balance expansion joint or curved pipe pressure balance expansion joint, which can offset the internal pressure thrust, and the force acting on the bracket is much smaller. It is often used on pipelines in cement plants and power stations.

Some people think that the expansion joint with a tie rod can replace the fixed bracket. It wasn't. The tie rod constrains the displacement of the expansion joint itself, not the thrust of the pipeline. The bracket has to do what it needs to do, but the force is different.

High temperature flue gas, large diameter, vacuum pipeline: how to feed in special working conditions

Once the working condition is special, the standard products often can't stand it, so they have to be "added".

High-temperature flue gas, when the temperature reaches above 600°C, metal bellows are basically useless-material creep and oxidative corrosion will quickly shorten the life. At this time, the candidate non-metallic expansion joint (fabric fiber expansion joint) is more suitable, with high temperature resistance, good flexibility and certain sound absorption and vibration isolation effect. Non-metallic expansion joints are often used in desulfurization flues, which are used together with flue gas baffle doors.

Large-diameter pipes, such as flues or air ducts above DN3000, metal bellows are expensive to manufacture, heavy in weight, and laborious in construction and hoisting. Rectangular non-metallic expansion joints are more common schemes under such operating conditions. Of course, if the customer specifies metal, it has to use large-diameter thick-walled expansion joints, and the reinforcing rings and side plates have to be thickened.

The vacuum pipe is different again. Under vacuum conditions, the corrugated pipe should withstand external pressure, and its ability to resist instability is the key. Double hinge expansion joints are often used in air-cooled island vacuum pipelines, which have one more constraint in structure and good stability. The special vacuum hose is equipped with metal mesh sleeves outside the bellows, which is also the truth.

It is much more efficient to give the manufacturer a selection parameter table than to call back and forth

When communicating with the manufacturer, give the following information at once:

• Pipe diameter DN, connection mode (flange or welding)
• Type and value of displacement: What are the axial, transverse and angular displacements in mm/degree
• Name and composition of the medium, especially with corrosive components
• Design pressure MPa, Design temperature °C, Operating temperature °C
• Installation position indication (straight pipe section, near elbow, equipment interface)
• Arrangement of fixed bracket and sliding bracket
• Is there any thermal insulation layer and any outer sheath requirements

All this information is given, and the manufacturer can make the plan and quotation in half an hour. The time saved is much more efficient than calling back and forth to urge questions.

There is no "universal model" for metal expansion joints, only "fit or not". Only when the selection parameters are clear and the working condition boundary is clear can the manufacturer give you a reliable scheme. If you are not sure, send the parameter table directly to the manufacturer-let the engineer judge whether the scheme is valid or not.

How to weld metal expansion joint? This question looks simple, but in fact there are many pits. I met a customer two days ago. Within two months of installing the expansion joint I took back, the root of the weld leaked. When disassembled, the typical group pair edge misalignment exceeds the standard + the heat input is too large, which burns through the thin-walled bellows.

Everyone in this business knows that metal expansion joint welding is completely different from ordinary pipe welding. The bellows has thin walls, low stiffness and is sensitive to heat. If you don't pay attention to it, you will crack or burn through the weld. Today, we will explain these five key points thoroughly according to the actual working experience.

1. Look at the material before welding: the wrong selection of welding material will directly waste parts

Don't weld the pipe when you get it. Find out if the material is 304, 316L, Inconel or carbon steel first. The welding processes of these types of materials vary a hundred and eighty thousand miles.

Most of the general corrugated expansion joints and bellows in our station are made of 304 or 316L stainless steel. This kind of austenitic stainless steel is easy to weld, but attention should be paid to controlling the interlayer temperature, which is best if it does not exceed 150℃. Some high-temperature axial expansion joints are made of Inconel alloy, such as Inconel 625 or 800H. This material must be made of nickel-based welding wire, such as ERNiCrMo-3, and the heat input must be strictly controlled, otherwise the corrosion resistance of the weld at high temperature will be directly scrapped. Carbon steel is relatively solid, but preheating and post-weld insulation cannot be saved.

Before selecting welding materials, check the materials of bellows and connectors according to the product information. What materials are used for the product, the welding materials will follow the match, don't guess by experience.

2. Groove and assembly pair: Thin-walled parts are most afraid of edge misalignment

When the expansion joint port is paired with the connector, the bevel angle, blunt edge and clearance must be according to the process card. The wall of the bellows is thin, usually only 0.5mm ~3mm. When the groove is opened wide, it will directly melt through, and when the groove is opened small, the root will not melt through.

When actually working, many people are planted in the wrong side. The specification requires that the amount of misalignment should be controlled within 10% of the wall thickness. What do you mean? For example, if the wall thickness is 2mm, the amount of misalignment cannot exceed 0.2mm. Beyond this number, the root of the weld seam is prone to unfusion, and the surface looks like it is welded. In fact, there is delamination inside, and it will leak when it is pressed.

When assembling, use special fixtures, do not hold your hands and fix them by spot welding. The bellows itself cannot bear the concentrated load, and if it is hard broken when assembling, the corrugation deformation will be wasted.

3. How to choose the welding method: argon arc welding is standard

Thin-walled bellows are all argon arc welded, which is the iron law. The heat input of argon arc welding is controllable and the arc is stable, which can ensure the back shaping. Thick-walled parts such as large-diameter thick-walled expansion joints can be filled with argon arc bottoming + manual arc welding, which is efficient, but the bottoming layer must be argon arc welded.

Note that when filling and cover welding, the arc should not directly face the bellows, and try to be as close to the connecting side, so as to reduce the damage of high temperature to the bellows. The welding current should be adjusted according to the material and thickness. 304 stainless steel, about 1mm bellows, current control at 50~70A. Inconel material is lower, and you have to use the back to protect the gas.

There is no cookie cutter routine for welding methods, but there is one principle: the smaller the heat input, the better, the faster the better, and the thinner the wire, the better.

4. Welding sequence and deformation control: longitudinal seam first and then annular seam

What are bellows most afraid of? Welding splashes and arc scratches. Splash on the ripples is a corrosive starting point. Therefore, before welding, the bellows surface must be blocked with protective cloth or white iron sheet.

In the welding sequence, the longitudinal seam is welded first, and then the circumferential seam is welded. The longitudinal seam is the weld of the bellows itself. If there are drawings, it must be argon arc welded, and it should be smoothed after welding. In circumferential welding, symmetrical welding or segmented desolding is done to reduce local heat buildup. Sectional desoldering, each section is 200~300mm, and the next section is welded after one section is cooled down.

During on-site welding, protective gas should be passed through the inside of the expansion joint. Some people say that it doesn't make sense to save money, but as a result, the back shaping is a mess, and the oxide scale falls off one by one. Remember, the back side is not well formed, and the weld life is directly folded in half. The back of the stainless steel bellows is protected with argon gas, and the purity is not less than 99.99%.

5. Post-welding inspection and pressure test: Don't skip that step

A weld appearance inspection is only the first step. Biting edges, surface pores, crater cracks, these can be seen by the naked eye is definitely unqualified. But what is more critical is internal defects, which must be nondestructive tested according to standards.

For corrugated expansion joints used in power station industry and expansion joints used in pressure pipelines, the welds generally require 100% RT or UT. RT is filming, UT is ultrasound. Conditionally, use phased array, which is more efficient. We provide a complete report on the welded expansion joints in our station, as long as the customer requests.

The same goes for pressure testing. The pressure test of metal expansion joint should be carried with a restraining device to prevent excessive extension of bellows. When the pressure is tested, it is low first and then high, and the pressure is increased gradually. If the pressure gauge has been calibrated, the measuring range should be 1.5~2 times the test pressure. During the hydraulic test, the internal air should be discharged, otherwise the pressure will be unstable.

How to weld metal expansion joint? In fact, the core is one sentence: treat the bellows as a precision part, and every parameter is true. There is no shortcut to welding, but according to the above five steps, the scrap rate can be reduced by more than half. Don't bother, the cost of rework is much higher than the meticulous effort when welding.

Two days ago, I met an engineer who was doing heating network design. He asked me with the drawings, "What is the unit of angular displacement of the expansion joint? Why do some of the drawings write ° and some of them write mm?" I saw that on his expansion joint selection sample, the angular displacement column was marked with "±5°", while the other company gave it with "±12mm". If you compare these two directly, something will go wrong. Today, break this parameter apart and crumble it into pieces to explain clearly.

What exactly is angular displacement? First distinguish the difference between it and lateral displacement and axial displacement

What the expansion joint does in the pipeline, to put it bluntly, is to absorb displacement. There are three kinds of displacement: axial displacement is the expansion and contraction along the axis of the pipe, transverse displacement is the misalignment perpendicular to the axis, and angular displacement-is the relative rotation between the flanges at both ends of the expansion joint. You can think of it as the pipe "folded at the waist", and the size of this folded corner is the angular displacement.

Understanding this is critical. Many people confuse angular displacement with lateral displacement, which is actually two different things. The lateral displacement is translation and the angular displacement is rotation. For example, a piece of equipment is lifted up by 20mm because of thermal expansion. If the pipe connecting it turns a corner, the expansion joint may actually bear an angular displacement of several degrees, instead of a simple 20mm lateral displacement. You just report a "lateral displacement of 20mm" to the manufacturer, and people really can't choose a model for you.

Is the unit of angular displacement only degrees (°)? Why do some drawings say millimeters?

The standard unit of angular displacement is degrees (°), which is nothing to contend with. But why do some drawings say millimeters? Because in many manufacturers' samples, angular displacement is expressed by "equivalent lateral displacement". What do you mean? It is to convert this angular displacement into the arc displacement at one end of the bellows.

The arc length is equal to the radius times the radian. If the radius of rotation of the bellows is 500mm and the angular displacement is 2°, the corresponding arc length is 500×2× π/180≈17.5mm. But there is a big pit here: different expansion joint structures have different radii of gyration at all. The same 2° angular displacement may be several times different in millimeters between a single hinge expansion joint and a large double hinge transverse expansion joint.

So when you see mm written on the drawing, don't rush to convert it. You have to find out where this mm comes from, whether it is the equivalent lateral displacement calculated by the manufacturer according to its own product structure, or it refers to the swing arc length of a certain point of the bellows. If you take this mm directly as an angular displacement to select the model, then you can't install it on the spot.

Generation of angular displacement from bellows structure: Why are single hinges different from double hinges

Angular displacement is not produced for nothing, it has to be guided by hinge structure. Single hinge expansion joint, that is, a hinge is installed at both ends of the bellows, which can only rotate in one plane, and its ability to absorb angular displacement is very limited. The compound hinge transverse expansion joint has two sets of hinges and a section of pipe in the middle. The bellows can undergo more complicated deformation and absorb large angular displacement and transverse displacement.

For example, the double hinge expansion joint used on the vacuum pipeline of the air-cooled island has a more specific structure. The two hinges cooperate with the bellows to deal with the thermal expansion of large-area horizontal pipelines. You see, for the same angular displacement, the magnitude that a single hinge and a double hinge can absorb is completely different. Therefore, when selecting a model, don't just look at an "angular displacement of 2°", but you have to confirm what structure it is. If you give the manufacturer an angular displacement of 4 °, the double hinge can be easily handled, but if you take the single hinge to carry it, the bellows will become unstable sooner or later.

How to read angular displacement values? Common unit misunderstandings and conversion traps in engineering type selection

Get a copy of the parameter table of the expansion joint. The angular displacement column says "±6°", and you can read it directly as "plus or minus 6 degrees". However, the pipeline layout drawings issued by some design institutes will mark the angular displacement as "6mm/m", which is easy to be confused. 6mm/m is actually the slope, which when converted into an angle is arctan (0.006) ≈ 0.34°. Don't think of it as a simple 6mm displacement.

Think of the unit of angular displacement "°" on the sample as radians. If anyone thinks of 5° as 5 rad, the selected expansion energy saving is twisted into a twist. Remember, the default unit of angular displacement in engineering is degrees, not radians, or millimeters.

Then how to check whether the tonnage given by the manufacturer is correct? Let me tell you a way: you first calculate the total angular displacement required for the pipeline, then look at the allowable angular displacement of each bellows on the manufacturer's sample, and then multiply it by the number of sets of hinges. If it is a single hinge, a bellows carries an angular displacement; If it is a duplex hinge, each set of hinges shares a portion. Don't expect one bellows to eat all the angles.

Type selection practical combat: the cooperation relationship between angular displacement and tie rod and hinge structure, and how to raise capital to manufacturers

The tie rod and hinge are the "seat belt" of the expansion joint. The tie rod is responsible for bearing the internal pressure thrust and preventing the bellows from being pushed open by the pressure; The hinge is responsible for constraining the direction of displacement, so that the angular displacement goes honestly along the designed plane. Without tie rods and hinges, a bellows alone can't absorb angular displacement alone.

Therefore, when selecting and raising capital, you need to provide the manufacturer with these things: pipe diameter, design pressure, design temperature, medium, and the size and direction of angular displacement. The angular displacement must be clearly written as "degrees" and explain which plane it is rotating. It is best to attach a simplified diagram. Don't just say "need to absorb angular displacement". The manufacturer is not a god and can't guess the direction of your pipeline.

The universal corrugated expansion joint itself does not absorb angular displacement, so it can only be carried by the flexibility of the bellows itself, and its life will drop suddenly if it is carried too much. If you want to absorb angular displacement, choose a compound hinge transverse expansion joint or a special hinge expansion joint. If you want to save that little money, it will be more troublesome to replace the corrugated pipe if it cracks in the future.

What is the unit of angular displacement of the expansion joint? The standard answer is one word: degrees. However, understanding the structural logic behind it, the conversion trap and the cooperation relationship with the hinge tie rod will prevent the selection from overturning. Next time I see mm written on the sample, ask more "how to convert it", which is much more reliable than stuffy selection.

Are expansion joints metal or not? Don't answer in a hurry

Is the expansion joint metal? Don't rush to answer "yes", and don't insist on "no". Everyone who has worked on pipelines knows that the expansion joint is not a single material at all. It is a family with various members, including all-metal, pure non-metal, and rubber and PTFE. You only ask "Is it metal?" It's like asking "Is the car burning oil?" The hybrid car shook its head when it heard it.

Expansion joints are both metallic and non-metallic. Metal expansion joints are the mainstream, but non-metallic expansion joints are better in some working conditions. If you only look at the material to draw a conclusion, nine times out of ten, you will step on a pit.

From bellows to non-metallic expansion joints, what is the difference in materials

The core of metal expansion joint is corrugated pipe, which is generally stamped with stainless steel and alloy steel, and absorbs the thermal displacement of pipe by elastic deformation of metal. The general corrugated expansion joint, high-temperature axial expansion joint and corrugated expansion joint for power station industry of this station are all representatives of this road. The bellows can be made into single layer or multi-layer, and the guide tube and tie rod can be added. The more complicated the structure, the stronger the compensation ability.

What about non-metallic expansion joints? Take rectangular non-metallic expansion joints and non-metallic expansion joints (fabric fiber expansion joints) for example. The main body is several layers of fiber fabric, with thermal insulation cotton sandwiched in the middle, and both ends are fixed with metal flange frames. Rubber compensator and PTFE compensator are more direct, even the bellows are not used, all depending on the flexibility of rubber or PTFE material.

As soon as the materials are changed, the performance logic completely changes. Metals rely on ductility to carry displacement, while non-metals rely on fabric deformation and internal structure sliding. Metals can carry hundreds or thousands of degrees of high temperature, but non-metals can't carry that high temperature, but they have good thermal insulation performance, are not afraid of acid-alkali corrosion, and even have a much longer life than metals in some media.

Metal expansion joint and non-metallic expansion joint, what problems do they solve

High temperature, high pressure and large displacement compensation. Steam pipeline, power station boiler air supply system, cement kiln head and tail, the temperature in these places is always five or six hundred degrees Celsius, and the pressure is not low. Only metal bellows can withstand it. For example, the metal corrugated expansion joint in the cement industry is specially designed for this working condition. There are also directly buried (fully buried) expansion joints, which have to bear external pressure and groundwater when buried in the soil, and non-metals can't do this job at all.

What about non-metallic expansion joints? It does not take the high-temperature and high-pressure route, and its home field is large-size, low-pressure, vibration isolation and noise silencing occasions. For example, the desulfurization flue has a cross-section of several meters wide. Do you try it with a metal bellows? Costs are frighteningly high and processing is difficult. At this time, the rectangular non-metallic expansion joint came on the field, which was easily done. Another example is the fan inlet and outlet. The rubber compensator can absorb vibration and reduce noise. PTFE compensator is the nemesis of strong corrosive media. The metal is directly wasted when it meets hydrochloric acid and concentrated sulfuric acid, but PTFE is unscathed.

See? No one is superior to anyone, only whether it is suitable or not. You take the metal expansion joint to catch the corrosive acid, and it will be perforated in three months; Take the non-metallic one to carry the high-temperature steam and burn it through directly. Wrong choice, no one can hold up.

Don't just look at the material when selecting the model. These working conditions are more critical

There is a big circle in front. In fact, the core is one sentence: don't worry about "whether the expansion joint is metal or not", first find out your own working conditions. What is the medium? How hot is the temperature? How stressful? Is the displacement axial, lateral or angular? How wide is the pipe size? Is there enough installation space for tie rods? These are the hard indicators that determine the selection.

Take an example. The same thermal pipeline, with a temperature of 200℃ and a pressure of 1.6MPa, has no problem choosing a metal expansion joint; However, if the medium is sulfur-containing flue gas, the temperature is not high, but it is easy to crystallize and block, then non-metallic expansion joint is more suitable. For example, when the installation space is limited, the rotary compensator and the lateral expansion of the double hinge save energy and a lot of space, but they are all metal. Do you want to use non-metal instead? There are no doors.

Stress is also a key watershed. Non-metallic expansion joints usually withstand low pressure, generally from several kilopascals to tens of kilopascals, and metal expansion energy saving bears several megapascals or even higher. Choosing the wrong material can lead to premature aging or pipeline cracking and leakage. Two days ago, I met a customer, who chose a non-metallic one for cheap. As soon as the operating pressure came up, the fabric bulged and scrapped directly. Therefore, the material is only the appearance, and the working condition is the essence.

Common Myth: Are expansion joints, compensators, metal hoses the same thing?

Many customers call the expansion joint compensator, which is no problem. The two are different names for the same type of equipment. But metal hoses are different. Some people always mix them together. Expansion joint is a section of flexible connection in the middle of rigid pipeline, welded or flanged at both ends, mainly absorbing displacement; A metal hose itself is a bendable tube for use in vibrations, deflections, or situations where frequent movement is required. The price, structure, and use are all different. There are metal hoses, PTFE-lined metal hoses and vacuum special hoses in this station. They are two product lines with expansion joints, so they can't be mixed.

Another myth is to use bellows as expansion joints. In fact, bellows is only a flexible component in the expansion joint, and there must be bellows in the metal expansion joint, but the bellows itself is not equal to the expansion joint. Similarly, a non-metal expansion joint also has a metal frame in it, and you can't say it is a metal expansion joint just because it comes with a metal frame. Therefore, the standard answer to the question "Is the expansion joint metal?" Is two words: look at the model.

Next time someone asks you if the expansion joint is metal, you directly ask him: Which kind are you asking?

Two days ago, the equipment supervisor of a thermal power plant called me, saying that the non-metallic expansion joint at the outlet of their induced draft fan was leaking, and the noise at the scene was so loud that the air volume was obviously not enough. He asked me if I wanted to replace it with a new one directly. I asked him not to rush to change it, and checked it in order, and finally solved the problem for less than 500 yuan. I have seen too many similar cases. Today, I will write the investigation ideas clearly. If you encounter them, just follow them.

Air leakage is no trivial matter: energy consumption and corrosion will come to your door

Let's talk about the harm first, otherwise you don't take it seriously. The most direct impact of non-metallic expansion joint leakage is the increase of fan power consumption. If a part of the sealed flow rate of the flue gas system is missed, the fan has to increase the power to make up. A simple comparative data: for every 1% increase in air leakage rate, fan power consumption increases by about 2% to 3%. It is normal for the air duct matched by a 300,000-kilowatt unit to consume hundreds of kWh of electricity an hour. The extra electricity bill in one year is enough to replace several expansion joints.

Even worse is corrosion. When the flue gas leaks out, along with the condensed water, sulfur dioxide and chloride ions are all attached to the surrounding pipelines and equipment surfaces. We have seen a case where the pipe bracket was next to the air leak point. In less than half a year, the corrosion layer of the bracket peeled off like fish scales. If you really have to wait until the equipment is thoroughly corroded before repairing it, the cost is not to be stopped by another expansion energy saving.

Find the leakage point first: Look at it with a flashlight, and you will know it with the smoke method

There is no need to guess the location of the air leak, it is just those few places.

First, the skin surface. The skin of non-metallic expansion joint is a fabric fiber layer, which is washed and aged in high-temperature smoke for a long time, and is most prone to tiny cracks and even holes. Use a flashlight to stick to the skin slowly, and see that the light transmitting point is the leakage point. When the crack is fine and can't be seen, take a piece of paper and rub it on the surface of the skin. The place that is sucked is the air leakage point.

The second place, the flange seal. Where the flange surface is connected with the steel pipe, the sealing gasket is aged and deformed or not compacted during installation, so the gap will come out. This is not easy to find by looking outside. Take a smoke pipe and walk along the flange to see where the smoke drifts.

The third place, the beading. The bead is used to press the skin tightly on the flange frame. When the bolts on the bead are loose or the bead itself is deformed, the skin will come out of the groove, and the air leakage is not small. None of these locations are difficult to check. It takes ten minutes.

Emergency treatment: Patches and bolts have their own specifics

Find the leakage point, and decide whether to fill or replace it depending on the situation.

Small cracks or pinholes on the skin can be patched with high-temperature repair tape or silicone cloth, which can last for a while. Note that the surface is cleaned before repairing, and the dust and oil stains are not wiped off. The tape is also a white sticker, and it will fall off after a few days. This method is suitable for temporary repair, and it is no problem to last until the planned shutdown for maintenance, but it is not a long-term solution.

If there is an air leak caused by a loose bolt, don't screw it when you come up. Let's first see if there are any signs of displacement on either side of the flange-thermal expansion will pull the bolts loose, especially in the thermal power plant case I mentioned earlier. The normal practice is to loosen the bolt, adjust the natural position of the expansion joint, and then tighten it symmetrically according to the standard torque. Direct hard screwing may deform the pressing strip, or tear the skin folds, which is really the worse the more you repair it.

Radical cure plan: change when you should, don't make up for it blindly

What circumstances don't make up? The skin is aging and hardening in a large area, the surface cracks are densely packed, or the flange and bead have been deformed. At this time, it is not something that can be solved by repair. It's safer to replace it directly with a new one.

Rectangular non-metallic expansion jointIt is suitable for rectangular air ducts, such as the inlet and outlet of induced draft fan and the inlet and outlet of desulfurization tower;Non-metallic expansion joint (fabric fiber expansion joint)It covers circular and special-shaped pipes, and the fabric fiber skin itself is the core sealing layer and displacement compensation layer.

Don't just look at the interface size when selecting a model. The three parameters of medium temperature, working pressure and displacement amount determine the number of layers and material structure of the skin. For example, if the flue gas temperature is 200 °C or less, a conventional combination of silica gel cloth + glass fiber is sufficient; If the smoke temperature exceeds 350℃, fluororubber or ceramic fiber layer should be considered. The pressure is simple. Generally, the pressure of flue gas duct is not high, but it should be noted that the skin is easily deflated under negative pressure conditions, so a supporting skeleton has to be added.

Don't forget matching inspection: lax baffle door will aggravate air leakage

Expansion joint air leakage is sometimes its own problem, but half the probability is related to supporting equipment. Two days ago, I met a customer. The expansion joint was replaced with a new one or leaked. After checking it, the problem was in the upstream and downstream baffle doors.

The desulfurization flue gas baffle door or ordinary flue gas baffle door is not closed tightly, which causes the system pressure fluctuation during operation, and the skin of the expansion joint is repeatedly agitated, and fatigue cracks appear. Double-seal structures such as circular baffle doors (double seals) and double-seal single-axis circular baffle doors, although the sealing grade is high, the wear of the sealing surface must be checked regularly. It is best to open and close the manual plug-in type insulation door or the electric plug-in type insulation door once when the machine is shut down for maintenance to see if there is any sticking. The whole air duct is a system, and the expansion joint is just one of the links. It protects the duct, and you have to protect it.

Routine Maintenance: Spend Your Effort at Usual Times

At the end of the day, non-metallic expansion joints shouldn't wait until there is an air leak to attract attention. During daily inspection, take a flashlight to see if there are any scratches or cuts on the surface of the skin, especially pay attention to whether there are any places scratched by the insulation iron sheet. The torque of the flange bolt is checked every six months. Don't be too troubled. The standard requires that the torque value be as much as it is. Don't rely on the feel. The fixing bolt of the guide tube is also easy to loosen. After falling off, the high-temperature smoke directly washes the inner layer of the skin, and the wear speed is more than ten times the normal.

According to JB/T 12235-2015 standard, in the inspection items of non-metallic expansion joints, the skin condition, flange connection and the integrity of the guide tube are listed. If you do maintenance in the front, it is very common for an expansion joint with passing quality to last three or five years, and the air leakage problem has nothing to do with you. If we wait for the air leak to deal with it, then the money for repairing it is almost enough to buy a new one.

So, what should I do if I encounter air leakage in non-metallic expansion joints? First, reassure yourself and go through it in the above order: find the leakage point, assess the severity, deal with it urgently, change it when you need to, check the supporting equipment, and then adjust the daily maintenance strategy. In most cases, there is no need to go to war at all.