Specialized in manufacturing compensators, expansion joints, baffle doors

Find out first: What is the fatigue life of expansion joints?

The fatigue life of the expansion joint, to put it bluntly, is how many times the bellows can carry the expansion and contraction. With each expansion and contraction, an alternating stress cycle will occur at the trough and peak of the corrugation-the material is constantly pulled over and pressed over, and when it accumulates to a certain extent, cracks appear.

Fatigue life is never determined unilaterally by material grade. It is common for the same SUS304 to have different corrugation geometries and a difference in life by two or three times. Wave height, wave pitch, wall thickness, number of layers, every parameter changes the stress distribution. The displacement form is also a variable-pure axial tension and compression, which is very different from the transverse shear superimposed on the axial direction, and the calculated stress is very different. What about pressure fluctuations? Every time the pressure in the tube fluctuates, there is an additional layer of circumferential stress cycle on the corrugated wall. So you see, although the general-purpose corrugated expansion joint and the external pressure single axial expansion joint are both called expansion joints, one corrugation is swollen by internal pressure and the other corrugation is compressed by external pressure. The stress state is completely different, and the path of calculating fatigue life is naturally different.

What is the reasonable safety factor?

The recommended practice of GB/T 12777 and EJMA is essentially to push down the theoretical fatigue life with a large multiple of the safety factor. The design fatigue curve given by EJMA itself considers about 10 times the safety factor, and GB/T 12777 also follows this set of logic. Why 10x? Because the fatigue test data of corrugated pipe is very dispersed, the life of the same design and the same batch of materials may jump from 800 to 3,000 times after ten tests. The safety factor is used to cover this spread.

However, the safety factor cannot be one-size-fits-all in different working conditions. After the directly buried (fully buried) expansion joint is buried, it is impossible to repair and replace the pipe at all. The failure is an accident, and it is reasonable for the safety factor to be 15 or even 20. Once the high-temperature axial expansion joint exceeds the creep temperature range of the material-for example, 304 stainless steel is above 425℃-the failure mode changes from pure fatigue to creep-fatigue interaction. At this time, it is gambling on your life to take the safety factor of 10 times of normal temperature.

How to take the number of fatigue life? Conversion from design displacement to actual working condition

Many people step on pits at this step. The "fatigue life 1000 times" written on the sample or drawing is calculated based on a specific cyclic displacement. For example: the design displacement is ±20mm in the axial direction, and the life span is 1000 times, which means that it can run 1000 times under ±20mm pure axial displacement. But in the actual pipeline system, where does pure axial direction come from? Thermal expansion and contraction drive the pipe system to swing, and the axial displacement always comes with the lateral displacement.

The transverse displacement is converted into the equivalent axial displacement according to the geometric characteristics of the bellows, and the equivalent axial displacement is obtained after the two are superimposed, and then substituted into the fatigue life curve to check the number of times. Note that the relationship between fatigue life and displacement is not linear-a 10% magnification of displacement may result in a 30% drop in life. This is why the full sample life cannot be used. The corrugated expansion joint in power station industry has to be discounted by 30% or 20%. Because the transverse expansion joint of compound hinge bears the combination of angular displacement, the conversion is more complicated, and the discount range is often larger.

The most overlooked pits in engineering: temperature correction and corrosion margin

If you take the fatigue curve at room temperature to set the flue gas pipe at 400℃, it is basically a gamble with your life. As soon as the material is at high temperature, the yield strength drops, the creep begins to accelerate, the plastic deformation produced by the bellows every cycle is greater, and the crack initiation is much faster than the room temperature.

A general-purpose corrugated expansion joint used in a cement plant has a design temperature of 350℃. The manufacturer has checked the life of 2000 times according to the normal temperature curve. As a result, the bellows cracked in less than 500 cycles after the actual operation reached 420℃. Some people also blame the manufacturer's poor quality-in fact, the problem lies in the user's negation of the temperature correction coefficient. Corrode the environment harder. The flue gas baffle door and the desulfurization flue gas baffle door are sulfur-containing flue gas, and chloride ions and sulfite make pitting pits on the corrugated surface, which are the natural starting points of fatigue cracks. Corrosion and fatigue are superimposed, not 1+1=2, but 1+1=5. In this environment, the safety factor is at least doubled, and with measures such as the heat insulation of the guide tube and the external heat insulation of the corrugated pipe, it can be played.

What is the appropriate number of fatigue life times?

Landing recommendations are in two gears. For conventional pipeline systems, such as thermal pipelines and general chemical pipelines, 70% ~80% of the design life is taken as the allowable cycle number. The sample is marked 1000 times, and you will set the maintenance and replacement cycle according to 700~800 times, leaving a safety margin to cope with the fluctuation of working conditions.

In critical situations or parts where maintenance is difficult, the control line should be pressed lower. The double hinge expansion joint of air-cooled island vacuum pipeline is on a pipe frame tens of meters high, and it is necessary to build a full hall of scaffolding to change it; Once the straight pipe pressure balance expansion joint fails, the blind plate force of the whole pipe system loses balance, and the consequence is chain. Such products are controlled at 50 percent or less of the design life-and replacement plans are required for less than half the life. The reason is simple: the fatigue life itself is a statistical value. Some people are still running when they use 120%, while others leak 60%. You lower the line of control, not conservatively, but to leave uncertainty alive.

How to communicate fatigue life requirements with manufacturers during model selection?

Don't just drop "Give me something that lives longer" and be done. The design engineer of the manufacturer is not a fortune teller. You have to give him at least these things: medium temperature, cyclic displacement (how many millimeters in the axial direction and how many millimeters in the transverse direction, if the measurement is not accurate, give the stress analysis report of the pipe system), cycle frequency (how many times a day to start and stop, how many thermal cycles a year), whether there is pressure fluctuation, and the type and concentration of corrosive medium. In this corrosive environment, sulfide or chloride content should be declared separately.

Only when the parameters are in place can the manufacturer select the safety factor to a reasonable gear, and then deduce the fatigue life times. If you don't say anything, the other party can only match you with a universal corrugated expansion joint according to the general working conditions of "normal temperature, pure axial direction and no corrosion". It looks quite cheap when you get it, and it will be revealed after running two laps on the scene. How to take the safety factor of fatigue life of expansion joint and the number of times of fatigue life of expansion joint? The answer is never in the sample, but in your own working condition data.

Friends who purchase or engage in equipment maintenance have asked this question nine times out of ten: Which non-metallic compensator is good? If you report the model directly, nine times out of ten are unreliable. There are several types of non-metallic compensators. The working conditions are different, and the things you choose can be a hundred and eighty thousand miles away.

First distinguish one thing: what kind of non-metallic compensator are you talking about?

Don't rush to compare the good with the bad, first see which type to install on your pipeline. There are roughly three types of non-metallic compensators common in industrial scenes: fabric fiber expansion joints, rectangular non-metallic expansion joints and rubber compensators. The name all sounds "non-metal", but the structure, material and scope of application are completely different things.

Fabric fiber expansion joint, also called non-metallic expansion joint, is mainly made of multi-layer fiber fabric, sealing material and heat insulation layer. It has good flexibility, can absorb multi-directional displacement, and is especially resistant to high temperatures. Rectangular non-metallic expansion joint, to put it bluntly, is a rectangular cross-section fabric fiber compensator, which is specially used on rectangular smoke duct. As for the rubber compensator, the main body is rubber and fiber reinforced layer, which relies on elastic deformation to reduce vibration and noise, and its temperature and pressure resistance are not as good as the first two.

Therefore, the premise of the question "which non-metallic compensator is good" is which category your working conditions are suitable for. If you choose the wrong type, no matter how good the product is, it will be useless. The rubber compensator is used on the flue gas pipe, and it burns in two months; Fabric fiber expansion joints are used on the water pump connection, and the vibration still cracks the pipe.

Select structure according to working conditions: temperature, pressure and medium determine life and death

Temperature and pressure are hard indicators. Fabric fiber expansion joints can usually hold hundreds or even thousands of degrees of smoke, but their pressure bearing capacity is general, which is suitable for low-pressure and large-diameter pipelines. The temperature resistance of rubber compensator generally does not exceed 100℃, and the pressure can be higher. However, there are oil, acid and alkali, ozone and so on in the medium, and the rubber will age soon.

Therefore, why do we recognize fabric fiber expansion joints in places such as flue gas pipelines, desulfurization systems and dust removal fan outlets? The flue gas temperature is high, the sulfur contains corrosion, and it is accompanied by vibration and displacement. Rubber can't do this work. On the other hand, the rubber compensator is the natural counterpart in places with low temperature but vibration, such as water pump inlet and outlet, chiller and diesel engine exhaust port. Its damping properties are there, and the vibration damping effect is far better than the fabric fiber layer.

Don't ignore installation position and displacement direction

The same equipment, installed at the outlet of the fan and installed at the inlet of the chimney, requires completely different compensators. The air flow at the outlet of the fan is unstable, with radial displacement and certain pressure pulsation; In the section of the chimney entrance, it is mainly axial thermal expansion, and may be accompanied by lateral wind load displacement. If you take a model that only does axial compensation and hard-mount it where you need lateral displacement, it won't take long to tear.

Rectangular non-metallic expansion joints are common in rectangular smoke ducts. At this time, it is not only necessary to look at the cross-sectional size, but also to understand the thermal expansion direction of the equipment. Wherever the pipe goes, the compensator has to be able to absorb displacement in which direction. Circular pipeline, axial or transverse compensation, when selecting a type, your mind should be clear. Is there enough space for installation? Flange connection or plug-in welding? These details must be clearly marked on the drawings.

Look closely at the skin material and the number of layers

The core of the fabric fiber expansion joint is not the metal frame on the outside, but the flexible skin in the middle. The skin is generally divided into several layers: the outermost layer is corrosion-resistant and aging-resistant fiber cloth, the middle layer is heat insulation layer, and the inner layer is sealing layer. How to match the number of layers and how to choose the material all depends on the working conditions.

For example, if the flue gas temperature is 600℃, it is necessary to choose silicon-titanium glass fiber cloth with high silicon fiber, and then reinforce it with stainless steel wire mesh. If the medium contains sulfur dioxide, polytetrafluoroethylene film should be considered as an anti-corrosion layer. Do you want to add the deflector? When there is high-speed dusty airflow inside, the guide tube can prevent the dust from directly washing the skin, but adding the guide tube will occupy a certain circulation area and may cause vibration. Only manufacturers who have done similar cases can get this balance accurately.

When selecting materials, don't just look at the temperature resistance meter. Some skins are marked with a temperature resistance of 800℃. In actual working conditions, the temperature fluctuates greatly, the material expands and contracts repeatedly, and the life span still drops from a cliff. So we have to ask clearly: What are the creep characteristics of this material at long operating temperatures? Have you ever had a fatigue test?

The last thing is to look at the manufacturer

After the first few steps, you already know which structure and material of non-metallic compensator you need. At this time, go to the manufacturer. It is not universal by marking "non-metallic expansion joint". It is the bottom line to make inspection according to JB/T 12235-2015 standard. This standard clearly writes the technical requirements, test methods and inspection rules of non-metallic expansion joints.

In addition to standards, it also depends on whether there are actual cases in the same industry. Those who have done the smoke duct of power station boilers and those who have done the tail of cement kilns have a completely different understanding of the working conditions. You ask him to take out two cases, the acceptance record, the running time and the replacement cycle, and you will know it when you ask.

The price of this thing, put it to the last comparison. The front structure is wrong, the material is low, and it is a hidden danger to buy it back and install it. Which non-metallic compensator is good, you ask? The answer is not the model, but the matching degree-working condition matching, structure matching, material matching, plus a manufacturer that does things according to standards. With all these four things together, the one in your hand is the best.

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.