Specialized in manufacturing compensators, expansion joints, baffle doors

What role does the deflector play in the expansion joint? Why diameter deviation is more critical than you think

The guide tube inside the expansion joint looked like a cylinder, and many people treated it as an ordinary bushing. However, after working in this line for a long time, you will understand that the diameter deviation of the guide tube directly determines that the expansion energy saving can't run a maintenance cycle safely. The primary task of the guide tube is to divert the flow-allowing the medium to pass smoothly through the inside of the bellows, reducing the impact of turbulence and vortices on the corrugations. Secondly, it separates the high-temperature medium from the bellows to avoid the bellows from being directly heated and overheated. Another function is easy to overlook: the guide tube can hold the impurities and welding slag washed down by the tube wall, preventing these things from getting stuck in the gaps of the corrugations.

Therefore, once the diameter of the deflector is ridiculously deviated, trouble comes. The diameter is large, and the gap between the bellows and the inner wall is not enough. As soon as the bellows are compressed, the guide tube directly presses against the corrugated valley, and it will wear out in a few cycles. The diameter is small, the gap is too large, the medium forms a step at the entrance of the bellows, the scour is intensified, and a trench is cut out of the root of the bellows. Do you say this deviation value is critical or not?

Are there any deflector diameter deviations specified in ASME and EJMA standards? Where do the reference values come from

Looking through ASME Volume VIII directly, it only specifies the wall thickness, fatigue life calculation and pressure test requirements of bellows, and does not give specific table-format values for the diameter tolerance of deflectors. The EJMA (American Association of Expansion Joint Manufacturers) standard also lacks a direct provision for baffle tolerances. When many buyers get the drawings, they ask: What is the diameter deviation of ASME expansion joint guide tube? It is indeed the norm that a single citable number cannot be found on standard text.

But don't rush to conclusions, although EJMA does not give tolerance values, it gives the recommended clearance range between the deflector and the bellows inner diameter in the design chapter. According to the empirical data of EJMA, the unilateral gap between the outer diameter of the guide tube and the inner diameter of the bellows is usually 6mm to 12mm, depending on the nominal diameter. The larger the diameter, the clearance is appropriately enlarged. This gap is pushed back, which is the lower deviation range of the outer diameter of the guide tube. The domestic supporting GB/T 12777 and HG/T 20205 also follow a similar idea, and the design institute and the manufacturer follow this interval by default.

Therefore, in actual engineering, the diameter deviation of the guide tube is usually determined by the way of "inner diameter as the benchmark and controlling negative deviation". For example: DN1000 universal corrugated expansion joint, the inner diameter of the bellows is 1016mm, the outer diameter of the guide tube is about 1000mm, the deviation is controlled from 0 to-3mm, and the bilateral clearance is about 16mm. For the large diameter thick wall expansion joint of DN2000, the gap must be enlarged to more than 20mm, and the deviation of the outer diameter of the guide tube should be relaxed to 0 to-5mm.

How to control the deflector diameter deviation at the manufacturing end? Measurement location, method, and common deviation ranges

The most common process for rolling guide tubes in manufacturing plants is longitudinal welding of coil plates, and the circumferential direction is inevitably ellipticity. Controlling the diameter deviation does not rely on the accuracy of the plate coiling machine alone-the welding deformation is often greater than the coiling error. It is common that the longitudinal seam shrinks after welding, and the radius of the guide tube shrinks by 2-3mm near the weld.

To control the deviation, the measurement position must be standardized. According to the recommendation of EJMA, the diameter of the guide tube shall be measured at the end and the section not less than 150mm from the end, and the average value shall be taken in two mutually perpendicular directions for each section. This method can effectively avoid the interference of ellipticity. In actual production, inner diameter micrometer and auxiliary support rod are commonly used to measure large diameter guide tubes, and attention should be paid to avoid the weld area when measuring.

The common deviation range is generally classified into three grades in the industry: ordinary grade 0 to-5mm, which is used for general flue ducts and low-pressure pipes; Precision grade 0 to-3mm for high temperature axial type expansion joints and corrugated expansion joints for power station industry; Special Grade (Matching Machining) 0 to-1mm for use in rotary compensators, vacuum-specific and other clearance-sensitive occasions. You will know by comparison, what kind of precision equipment and what inspection methods are used, and the corresponding cost is quite different.

If the deviation is too small, the bellows will be stuck, and if the deviation is too large, it will accelerate the erosion and wear-the field case tells you how to choose the range

The high-temperature axial expansion joint of a power plant, DN1400, with a design temperature of 560℃, has obvious wear on the bellows after more than one month of operation. Disassembly and inspection found that the inner diameter of the guide tube was too large by about 8mm, the medium formed a sudden expansion at the outlet of the guide tube, and the dusty flue gas directly washed the corrugated root. The deflector was later remade with a deviation of 0 to-3mm, and the wear problem disappeared.

There is also a case in a chemical plant. The medium is a slurry with particles, and the expansion joint model is a general-purpose corrugated expansion joint. The outer diameter of the guide tube is enlarged by 1.5mm, and it can't rotate when installed at room temperature. When the corrugated tube is compressed during pressure test, the end face of the guide tube directly bends the corrugated valley. After rework and removal, the ellipticity of the guide tube exceeded the standard, and the long axis direction was nearly 5mm larger. The manufacturer re-rolls it and strictly controls the deviation before it is solved. When the two cases are put together, the truth is clear: the deviation should leave enough gaps, but it should not be large enough to form a step scouring. What about that? Select the deviation gear according to the medium properties and working conditions, take the intermediate value for dusty medium, take the small gap for pure medium, and make the inlet chamfer of the expansion joint of high-pressure and high-speed fluid.

When signing the technical agreement with the supplier, the deviation of the guide tube diameter must be written in to avoid discord in acceptance

The following four items are clearly written in the technical agreement, which can save ten quarrels during acceptance.

First, give the benchmark size. Make clear whether the inner diameter or outer diameter of the guide tube is the reference, and indicate the design nominal size and deviation range. Don't just write "conforming to EJMA". EJMA doesn't give a value, and everyone will hold their own opinion when the time comes. Write directly: the outer diameter of the guide tube is D =1000mm, the deviation is 0/ -4mm, and the ellipticity is not more than 1/2 of the diameter tolerance.

Second, specify the measurement method. Note that the measurement position is 150mm from the end face, and each section is measured in two vertical directions, except for the weld area. If it is not written clearly, the supplier will take a caliper on the end face and hand it in for inspection, and the measured data can't reflect the true ellipticity at all.

Third, the gap range is determined. According to the one-sided clearance of 6-12mm recommended by EJMA as the acceptance basis, and specify the maximum clearance limit. This is particularly important-the design time gap is 8mm, and as a result, the guide tube is made 13mm smaller, and the scene looks fine. In fact, the risk of scour has doubled.

Fourth, clarify the end treatment requirements. The inlet end of the guide pipe must be chamfered or rounded, and the outlet end should be burred smoothly. The chamfer radius R shall not be less than 2mm. If conditions permit, the hard surface treatment of surfacing welding shall be done. The reason for writing this one is simple, the outlet end burrs and steps are the starting point of bellows wear.

These items are written into the agreement, so suppliers dare not fool them, and inspectors are worried. If you have signed the contract but have not written these, the factory inspection report requires that the measured data be filled in, and then the gap mark on the final assembly drawing is checked. After this set of actions is completed, you will have a bottom in your heart.

How to arrange the bracket of the expansion joint? This question is asked almost every few days. Most of the people who asked were that the pipeline stress calculation had just been completed, and the result was stuck in the bracket step-the bellows model was selected and the compensation amount was calculated. As a result, the bracket layout was wrong, and the expansion joint was selected for nothing.

Fixed brackets and guide brackets. The fixed bracket is responsible for dividing the pipe system into several independent pipe sections, each section can digest its own thermal displacement, and prevent force from being transmitted randomly to equipment, valves or weak points. The transverse displacement of the guide bracket tube ensures that the tube goes in a straight line when it expands and contracts, and the bellows is not screwed as a universal joint. With the cooperation of the two, the bellows can honestly do the expansion and contraction it should do. Mixed? The bellows cracked when twisted, the fixed bracket was crooked, and the thrust pushed to the place where it shouldn't have been pushed. There were many accidents.

How to fix the distance of the fixed bracket? Don't slap your head

The fixing bracket is the anchoring point of the pipe system. When the spacing is large, the thermal elongation of the pipe between the two anchor points exceeds the absorption capacity of the expansion joint; The spacing is small, the number of brackets is too large, and the cost can't be suppressed. How does that count? Looking at the type of expansion joint, compensation amount and pipeline stress calculation results, all three are indispensable.

The spacing between the fixed brackets of the universal corrugated expansion joint (axial type) is usually divided according to the natural compensation ability of the pipe system. The natural compensation is not enough, so it is considered to absorb with expansion joints. The hinge-type and compound-hinge transverse expansion joints with tie rods are different-their compensation ability comes from angular displacement and transverse displacement, and the spacing of fixed brackets should be checked according to the length of reverse thrust arm. To put it bluntly, the bellows is subjected to lateral thrust and bending moment in addition to axial force. When the bracket distance changes, the force arm changes.

Someone asked, are there any experience points? Yes, but don't use experience points as a master key. For small-diameter steam pipes below DN300, the distance between fixed brackets is 10 meters and 20 meters. Large-diameter high-temperature pipeline, five or six meters have to be set up. Why? The expansion amount is large, the wave number of the bellows is large, the stiffness is low, and the spacing is slightly enlarged, and the middle pipe section sinks. Calculate the pipe stress, and there are any answers.

The spacing and position of the guide brackets is more specific than you think

The guide bracket is too close to the expansion joint, and the bellows cannot absorb the lateral displacement; Too far, the pipe is unstable. How to handle this sense of proportion? The distance between the first guide bracket and the expansion joint is generally calculated according to 4D (D is the nominal diameter of the pipe), and the second guide is controlled according to 14D. For example: In the pipeline of DN200, the first guide bracket is about 800mm away from the expansion joint, and one is set every 2.8 meters behind it. However, note that this is only the recommended value of the general axial expansion joint-specific to the products of a certain manufacturer, the rigidity, pressure thrust and fatigue life of the bellows are different, so it is best to check the limit value in the product sample.

The role of the guide bracket is to give the tube a sliding constraint so that the thermal displacement goes in the intended direction. The pipe cannot swing laterally at the guide bracket, but it should be free to expand and contract in the axial direction. The most common mistake at the site is that the guide bracket is used as a fixed bracket-the angle steel is welded to death, the axial direction can't move, the thermal expansion and contraction forces are all suppressed on the bellows, and the pipe wall is pushed to bulge. On the other hand, the distance between the guide brackets is enlarged, the pipe shakes laterally, one side of the bellows is stretched and the other side is compressed, and the fatigue life drops directly from a cliff.

The bracket arrangement logic is completely different for different expansion joints

The single axial expansion joint is guided by the pipeline itself, and the bracket arrangement is the most conventional; But what about replacing it with a double straight pipe bypass pressure balanced expansion joint? It comes with its own balance structure, the thrust on the fixed bracket is small, and the bracket can be made lighter. The pressure balance expansion joint of curved tube is suitable for the elbow, and it can balance the internal pressure thrust itself, so the load of the fixed bracket is mainly friction.

It is an iron rule that hinged expansion joints must be used in pairs. Single hinges can only absorb angular displacement, and pairs can form lateral displacement compensation. The bracket must withstand the internal pressure thrust-the hinge structure can balance the internal pressure blind plate force, but the external load must be carried by the bracket. It would be a joke for someone to take the bracket scheme of the universal expansion joint to set the rotary compensator. The rotary compensator absorbs the displacement by the packing seal and the rotation of the rotating cylinder. The bracket layout pays attention to making the pipe rotate around the rotation center, which is completely different from the force logic of the bellows expansion joint.

The Three Most Common Mistakes in Real Cases

First, the fixing bracket is not welded firmly. Two days ago, I met a customer. During the pressure test of the pipeline, the whole bracket was pushed away, the bellows was pulled in a straight line, and the compensation amount was all gone. Check the reason, the welding length between the bracket and the embedded parts is not enough, and the height of the weld seam also shrinks. The internal pressure thrust is hundreds or thousands of kilograms, and the welding joint is not solid, so it is strange that the glue is not opened.

Second, the spacing between the guide brackets is too large. The waste heat power generation pipeline of a cement plant, DN500, and the guide brackets are set at an empirical spacing of 8 meters, nearly double that of 14D. After three months of operation, the bellows was twisted and deformed, and the lateral displacement pushed the bellows out of the pit. When I remove it, I see that the trough is full of fatigue cracks.

The third is to ignore cold tightness or pre-displacement. After the pipe system is cold tight, the bellows will have an initial displacement. The bracket was welded according to the position after cold tightening. As a result, as soon as the operating temperature rose, the displacement direction was opposite to the design, and the initial force of the bracket exceeded the standard. The squeak is considered light, and it is not uncommon for the bellows to be squeezed out of the tie rod limit.

So how to arrange the bracket of the expansion joint? There is no unified formula, but there is a main line: first distinguish the responsibilities of the fixed bracket and the guide bracket, and then determine the spacing and position according to the type of expansion joint, compensation mode and pipeline stress calculation result.

Find out first: What exactly does the "delivery status" of the expansion joint mean?

It's not simply the four words "do a good job of packing" and it's done. The delivery status of the expansion joint is a complete set of technical definitions: whether the bellows is pre-deformed, where the tie rod nut is locked, whether the direction of the guide tube is correct, how to protect the flange sealing surface, and whether the transportation tool is assembled or disassembled. These details are bundled together, which determines whether you can hoist and weld directly after receiving the goods, or you have to squat on the site to do a bunch of pretreatments first.

Two days ago, I met a customer and received a batch of general-purpose corrugated expansion joints. I opened the wooden box and welded it. Halfway through the welding, it was found that the bellows was crushed to death, and the tie rod was still locked tightly-the manufacturer issued the transportation protection state, which should have loosened the nut and adjusted the pre-deformation before installation. And the result? As soon as the welding heat comes up, the bellows bulges directly. Who takes the blame? The delivery status was not clearly written in the contract, so the manufacturer sent it in the safest way, and no one understood it on the spot. The final construction period was delayed for one week.

What are the common delivery states?

Free Condition, Pre-Stretched/Pre-Compressed Condition, With Limit Tie Rod Condition, Overall Delivery and Part Delivery。

• Free state: The bellows has no artificially applied displacement and the length is equal to the design length. Most general-purpose corrugated expansion joints are sent by default, because they are safest and are not afraid of accidental collisions during transportation.
• Pre-stretched/pre-compressed state: According to the cold tightness of the pipe, the bellows is pre-elongated or compressed for a section, and then locked with tooling. It is commonly used in corrugated expansion joints in power station industry, or straight pipe pressure balance expansion joints that need to absorb large displacements.
• State with limit tie rod: The tie rod nut is locked to limit the bellows from shaking during transportation. But there is a pit here-some tie rods are for transportation protection, and they must be disassembled after installation; Some tie rods are working components, such as the tie rods of external pressure single axial expansion joint and double hinge transverse expansion joint, which are used to restrain axial force or distribute transverse displacement. If you dismantle them, you will have a big problem.
• Block Delivery vs Part Delivery: Small calibers are usually delivered as a whole; Large diameter thick wall expansion joints or non-metallic expansion joints may be shipped separately and reassembled on site. When the parts are delivered, flange bolts, gaskets and fabrics have to be counted separately, and one less is troublesome.

Why does the manufacturer send free state by default? Because the free state has the widest adaptability surface, the site can be adjusted according to the actual cold tightness value. However, some working conditions must be pre-deformed: for example, the design temperature of the pipeline is 500°C, the installation temperature is 20°C, and the thermal expansion is very large. If the bellows is not pre-stretched during delivery, the bellows may be pushed to the limit after operation, and the life will be greatly reduced.

How is pre-stretching and pre-compression achieved? Is the tie rod disassembled or not?

By tie rod nuts, or special tooling. The manufacturer uses a jack or screw to pull the bellows to the specified displacement before leaving the factory, and then locks it with a tie rod nut. On delivery, the tie rod has a lead seal or a yellow warning label that says "loosen nut before installation" or "strictly prohibited removal". But you must not just look at the label, you must look at the structure.

How to identify? Remember one thing:There are usually only one or two transportation protection rods, which are slender and directly connected to the flanges or connecting pipes at both ends, and the bellows cannot move at all after the nut is tightened; Working tie rods tend to be multiple uniformly distributed rods with spherical washers or spherical bearings that allow the bellows to generate lateral displacement.Take a practical example: the tie rod of the transverse expansion joint of a compound hinge. If you remove it, the hinge structure will lose its constraint, and the transverse displacement will twist the bellows. However, the short tie rod that comes with the general corrugated expansion joint from the factory, if it is not disassembled after installation, the expansion joint cannot be deformed at all, which is equal to a rigid short tube.

How to check the delivery status during acceptance?

• nameplate: Model, nominal diameter, pressure, displacement, factory number. Focus on whether there is a label "pre-stretch xx mm" or "cold tightness xx mm".
• Certificate of Conformity: Material, welding process, inspection date. Compare the contract to see whether the bellows material is 304 or 316L. Don't find the wrong delivery after a few days of rust.
• appearance: Whether there are bruises, scratches and pits in the bellows, whether the flange sealing surface is bumped, and whether the direction of the guide tube is consistent with the medium flow direction (the direction of the arrow must point to the medium flow direction). Flexible products such as non-metallic expansion joints and rubber compensators also depend on whether the fabric or rubber has wrinkles and cracks. They are easier to deform during transportation, so they must be laid flat or hung, and cannot be stacked.

Whether the tie rod nut is loose is also a key check item. If the nut is loose during transportation and the pre-deformation amount runs away, you have to measure the actual length of the bellows and compare it with the installation length marked on the nameplate. If the difference exceeds 5mm, don't hard install it, contact the manufacturer to readjust it.

Relationship between delivery status and installation status

Don't think that you can weld it directly when you receive the goods. Especially for corrugated expansion joints used in power station industry, pipelines often have strict cold tightness requirements. For example, the main steam pipeline has a designed cold tightness value of 30mm. When the manufacturer delivers the goods, it is pre-stretched by 20mm. When the ambient temperature changes during on-site installation, you may have to pull another 10mm. If there is no adjustment allowance reserved at the time of delivery, or the tie rod has been locked to the limit, then the scene is blind.

Look at the cold tightness table of the design drawing first, then measure the actual length of the arriving expansion joint, and then correct the cold tightness value according to the installation temperature. This process is called "secondary cold tightening". Some expansion joints have adjustment marks when they leave the factory, such as engraving lines on the tie rod to tell you how much displacement each turn of the nut corresponds, which is for the field micro-tuning.

Status of delivery clause written into the purchase contract

To avoid scrambling, these five articles must be clearly written in the contract:

• Delivery status: free state or pre-stretched/pre-compressed state, what is the amount of pre-deformation in mm.
• Nature of tie rod: whether it is a transportation protection tie rod (removed during installation) or a working tie rod (strictly forbidden to remove), and whether there is any mark.
• Shipping protection: with or without additional support, anti-collision wooden frame, bellows sheath.
• Packing Method: Wooden Case, Naked Package, Pallet. If the large-diameter thick-walled expansion joint is naked, the outside of the bellows must be wrapped with a protective pad, and the bolts on the flange surface should be coated with rust-proof oil.
• Storage period: Non-metallic expansion joints and rubber compensators are stored for more than 6 months, the rubber may age and must be turned over or deflated; If metal expansion joints are stored outdoors for more than one year, check the bellows for rust.

Don't bother, these terms would rather be written in detail than slapped on the head on the spot. And guess what? Many procurement disputes, finally check the contract, it says "delivery according to the manufacturer's standard"-what is the manufacturer's standard? Of course, people come by the most hassle-free.

Attached: Common delivery status comparison table

• Free stateApplicable to universal corrugated expansion joint and rotary compensator; Delivery length = design length; The site needs to be adjusted by itself.
• Pre-stretched state: Applicable to corrugated expansion joints and straight pipe pressure balance expansion joints for power station industry; Delivery Length> Design Length; Retest according to the requirements of cold tightness on site.
• Pre-compressed stateApplicable to working conditions with large axial compensation requirements; Delivery length
• With limit tie rod (transport protection): Suitable for long distance transportation; Must be loosened and removed before installation.
• With working tie rodApplicable to external pressure single axial type expansion joint and double hinge transverse type expansion joint; It is strictly prohibited to remove, but the nut can be adjusted to adjust the displacement distribution.
• Overall deliverySmall-diameter expansion joint, metal hose; Install as soon as it arrives.
• Parts deliveryLarge diameter non-metallic expansion joint, rectangular expansion joint; It needs to be assembled on site, and the parts list should be checked.

In what state the expansion joint is delivered, this question should not be remembered on the day of arrival. Before signing the contract, throw the real needs of your site to the manufacturer-free state or pre-stretch, how to deal with the tie rod, and the packaging is not in place. It is much more reliable to say it once than to call for help when you arrive at the scene.

It is the most flexible and the most vulnerable. The wall thickness of bellows is often only a few tenths of millimeters to a few millimeters, but it has to bear the displacement compensation of the whole pipe system. When the scene is bad, it is bad to scold the material, but the real murderer is nine times out of ten scouring. Want to Solve Metal Expansion Joint Scour? First, understand the following five questions, which works better than changing the model ten times.

1. How exactly does scouring happen?

Not all wear is called scouring. Particles in the medium, excessive flow rate, gas-liquid two-phase flow, and sudden change in flow direction-at least two of these four factors account for at least two, and the bellows wall surface will be repeatedly cut. To put it bluntly, this is the high-speed friction between the medium and the metal surface, which is the same as polishing with sandpaper. If you touch the flushed bellows with your hand, the surface is shiny, and you can even see the metal texture, then it doesn't run.

In some working conditions, the flow rate is not high, but the medium is steam with water, and the water droplets hit the trough at high speed, which still cuts iron like mud over time. What is more hidden is the sudden change of flow direction, such as the elbow immediately behind the expansion joint, and the medium directly turns against the bellows, which is equivalent to opening a sandblasting machine locally.

Second, how long does it take to scouring to destroy an expansion joint?

Three months, or even less. The wall thickness of the bellows is inherently thin. Once it is washed and thinned, the pinhole leaks first, and then it tears apart along the trough. Many scenes thought that the material was not good, so they changed 316L and 254SMO, but it still leaked. The roots are washing away, not corroding.

So the deflector is not an optional accessory, it is a shield for the bellows. The expansion joint without the deflector is equivalent to going to the battlefield without body armor. When selecting, if the medium has particles, high flow rate and gas-liquid two phases-any of these three words appear, the guide tube must be matched.

Third, how to prevent scour in design?

Add the guide tube, change the material and change the structure, all three tricks are indispensable. The guide tube guides the high-speed medium into the inside of the tube, so that the bellows avoid direct flushing, which is the first line of defense. When the dust content of the medium is large, you can consider surfacing cemented carbide on the inner wall of the pipe. Don't be reluctant to give up this cost. The replacement cost of an expansion joint is enough for surfacing for several years.

If the working temperature is high, choose high-temperature axial expansion joint or external pressure single axial expansion joint, and move the bellows to a position that is not directly impacted by the medium. The external pressure structure is particularly easy to use. The bellows is outside, the medium goes through the internal channel, and the trough can't be touched on the scour path at all. Two days ago, I met a cement factory customer. The temperature of the smoke duct was 800℃. It used to be an ordinary axial type, and it wore out in three months. Later, the external pressure single axial expansion joint was changed, and it took more than a year to see that the bellows was intact when disassembled.

4. What can be done by on-site operation?

Don't think everything will be fine if you put it on. Controlling flow rates, avoiding running at critical flows for long periods of time, checking bellows troughs for bright spots or pits during shutdowns — these details are more important than anything else. And guess what? Many scour accidents are caused by incomplete pipeline purging in the early stage of driving and welding slag stuck in the gap of the guide tube. The welding slag tumbles inside at a high speed, which is equivalent to filing the bellows every day.

When the expansion joint is installed, the direction of the guide tube must be consistent with the flow direction of the medium. The arrow is clearly marked, but someone is acting backwards. The consequence of reverse installation is that the medium is directly poured into the interlayer between the bellows and the guide tube, so it is not called washing, but cutting.

5. How to avoid stepping on pits when selecting models?

Calculate the medium first, then check the flow rate, and finally determine the structure. Flue gas pipeline uses non-metallic expansion joints or rectangular non-metallic expansion joints. The erosion resistance depends on the skin material. Don't just look at the thickness of the frame. The skin is the layer that directly contacts the medium. For general-purpose corrugated expansion joints or metal hoses for liquid pipelines, focus on the length and wall thickness of the guide tube-the front end of the guide tube should extend beyond the turbulent zone of the medium, and the wall thickness should be at least the same as that of the connection tube.

For power plant desulfurization system, corrosion-resistant expansion joints matched with desulfurization flue gas baffle doors have to be used. The concentration of chloride ions in desulfurization slurry is high and there are many particles, which ordinary stainless steel can't bear at all. Non-metallic or fluorine-lined structure is the correct solution. All in all, don't take the general structure to harden the bad working conditions. The first principle of type selection is to choose the structure under any working condition.

VI. Handling sequence of scour accidents

First stop the machine for inspection, then judge the degree of damage, and then decide whether to repair the weld or replace it as a whole. If the bellows is not perforated but thinned obviously, replace it directly-the strength and fatigue life of the thinned bellows are discounted, and the welding repair will only leave the stress concentration point. If only the guide tube is worn out, you can only change the guide tube, and the cost can be saved by half.

Remember, the expansion joint is the weak link in the pipe system, but it is because of its weakness that it protects the whole pipe system for you. When it is broken, I think of replacing it. It's better to ask more questions when selecting a model: Will this medium be washed? What is the flow rate? How big are the particles? — — This sentence can save hundreds of thousands of parking losses.

It is also a corrugated expansion joint. Why is the height difference of the wave sidewall so much? Find out exactly where this parameter refers first

What is the sidewall height of expansion joint wave? To be honest, there is a problem with this question itself-the height of the wave sidewall is not a fixed value, it follows the working conditions. Don't rush to ask "how much" first, find out which paragraph it refers to.

The waveform of the corrugated expansion joint, you can understand it as a wave. The hypotenuse between the peak and the trough, that is, the vertical distance from the bottom of the trough to the top of the peak, is called wave height in the industry, but when the engineering drawings are actually processed and inspected, it is actually the "wave sidewall height"-it refers to the vertical projection size of the inclined wall on the side of the bellows. Many people confuse it with wave height, which is not exactly the same thing. When you measure with a caliper, you measure the height from the trough to the peak, but the height of the wave sidewall should also consider the transition radius between the peak arc and the trough arc. The measured value will be slightly different from the wave height.

This size directly determines the stiffness, pressure resistance and compensation amount of the expansion joint. The same nominal diameter, high wave side wall height, good flexibility, large compensation, but the pressure resistance will decrease; On the other hand, if the wave side wall height is low, the pressure bearing capacity is strong, but the compensation capacity is limited. So do you say that this parameter is heavy or not? When you can't figure out the selection, most of the problems will occur when you install the pressure test later.

Is there a fixed standard for wave sidewall height? From GB/T 12777 to the actual product, see how it is determined

Some people think that the national standard will specify an exact wave sidewall height for each path, which is overthinking. GB/T 12777 General Technical Conditions for Expansion Joints of Metal Corrugated Pipe mainly stipulates the design, manufacture, inspection and acceptance requirements of corrugated pipe, and gives framework things such as design calculation method, material selection principle and fatigue life test method, which will not determine the height of wave sidewall alone.

There is a parameter table of bellows section dimensions in the standard, which lists the recommended values such as wave height, wave pitch, wave number and wall thickness, but these are design references, not mandatory standards. The same corrugated expansion joint of DN200 is used in low-pressure and large-compensation applications and high-pressure and small-compensation applications, and the wave side wall height is likely to be more than double. For this reason, you can't find a "standard answer" in the national standard. What about that? It has to be pushed from the design source.

There are four key factors affecting the height of wave sidewall: pressure, caliber, material and compensation amount

Let's talk about how the height of the wave sidewall was "forced" out in actual design. Four factors are indispensable:

Stress.This is the first deciding factor. The higher the internal pressure, the greater the circumferential stress on the bellows wall. If the height of the sidewall of the wave is too high, the stress is easy to concentrate at the trough, and the pressure resistance is immediately discounted. Therefore, under high-pressure working conditions, the height of the wave side wall is usually pressed relatively low, the wave shape is flat, and multi-layer thin walls are used to make up the flexibility. On the other hand, in the working conditions of low pressure and large displacement, such as non-metallic expansion joints or large-diameter metallic expansion joints for smoke ducts, the height of wave sidewall can be made higher, and the waveform can be stretched, so that the compensation amount can go up.

Caliber.The larger the diameter, the larger the pressure bearing area, and the greater the total thrust under the same pressure. The design requirements of the expansion joints of DN100 and DN2000 are completely different from the same order of magnitude. Large diameter expansion joint should control the ratio of wave side wall height to avoid excessive plastic deformation of wave trough. For example, the rectangular expansion joint on the smoke duct of power station, which has a super large diameter, is often iterated by finite element analysis, instead of copying the empirical value of small diameter.

Materials.The yield strength and fatigue characteristics of austenitic stainless steels (e.g. 304, 316L) determine how much bending strain the bellows can withstand. The material strength is high, which can make the height of the wave sidewall larger, and the wave shape can hold it anyway; If the material is low in strength or thin in thickness, the situation is the other way around. There is also the problem of material creep under high temperature working conditions. As soon as the temperature rises, the allowable stress of the material drops, and the height of the wave side wall has to be lowered accordingly, otherwise the bellows is easy to become unstable under high temperature.

The amount of compensation.The larger the displacement to be absorbed, the larger the deformation of each wave, and the height of the wave sidewall should naturally be made higher. However, the fatigue life of bellows is directly linked to the displacement of single wave, and it is not because you can increase the compensation infinitely by pulling the height of the wave sidewall hard. Axial type, transverse type and hinge type, each type has different requirements for the height of wave sidewall. Under the working condition of transverse displacement, the height of wave sidewall has a great influence on the shear deformation of corrugated pipe, so it should be specially checked during design.

The height of the wave side wall is not determined by patting the head: what data are mainly looked at in design calculation?

Then how exactly do the designers set this size? The core path is as follows: first, according to the pipeline stress analysis, the displacement to be absorbed-axial displacement, lateral displacement and angular displacement is obtained, and then the materials and layers are selected in combination with the design pressure, design temperature and medium corrosion margin, and then the geometric parameters of the bellows are iteratively calculated by the calculation method recommended by EJMA standard or GB/T 12777. The wave sidewall height is not calculated separately, it is a related dimension that follows the wave height, wave distance and wave number.

One is the compressive strength check, to ensure that the bellows will not yield or instability under the most severe working conditions; The other is the fatigue life check, to ensure that penetrating cracks will not occur within the expected number of cycles. If you find that the sidewall height of the expansion knuckle wave reported by the two manufacturers is much different under the same working conditions, don't rush to compare the price. Let them take out the design calculation book first. If the wave sidewall height deviates too far from the conventional value, either the stiffness and fatigue life are not up to standard, or there is another mystery in the material or wall thickness.

How to confirm the wave sidewall height with the manufacturer when selecting the model? What to pay attention to in field measurement and drawing verification

If your project has entered the procurement stage, when confirming the height of the corrugated sidewall with the manufacturer, the safest way is to ask the other party to mark the expansion drawing of the corrugated pipe on the drawing, instead of simply quoting a number. The wave height, wave distance, wave side wall height, number of layers and wall thickness of a single layer should be marked on the drawing. These data are meaningful when put together. If you ask the height of a wave sidewall separately, and the manufacturer gives a number, you can't see whether it is reasonable or unreasonable.

For on-site measurement, just use a vernier caliper or depth gauge to directly measure the distance from the trough to the peak of the bellows. But one thing to remind: Many bellows have protective sleeves or insulation layers on the surface, so you have to remove the outer sheath and measure it again. The measured value is allowed to deviate from the value marked on the drawing. GB/T 12777 has requirements for the dimensional tolerance of corrugated pipe. Generally, the tolerance of wave height is about ±3%, and the wall thickness tolerance is according to the material standard. If the measured deviation exceeds 5%, we should be alert to manufacturing quality problems.

In addition, attention should be paid to distinguish whether it is a single layer or a multilayer structure during measurement. The wave sidewall height of multi-layer bellows is the total height. What you measure from the outside is the outermost contour, but there may be gaps or misalignments between each layer. This should be confirmed with the manufacturer. Two years ago, there was a customer, and the height of the wave sidewall measured by a caliper was 3mm smaller than the drawing. It was not said that the manufacturer cut corners. As a result, it was found that the outer protective ring blocked part of the measurement position. This kind of low-level misunderstanding, it is good to talk about it, but I am afraid that it will not talk about it.

At the end of the day, what is the expansion knuckle wave sidewall height? This question should not be answered by a "standard answer", but by your working condition data. Report the parameters of pressure, temperature, medium, displacement and number of cycles to the manufacturer, and ask them to take out the design calculation book. You will naturally know what range the reasonable wave sidewall height should fall in.