Specialized in manufacturing compensators, expansion joints, baffle doors

Many customers always feel that the compensator is installed once and for all. Especially the non-metallic compensator (also called fabric fiber expansion joint), because it doesn't look "tough" like the metal corrugated expansion joint, it feels delicate and easy to break. In fact, on the contrary, the maintenance difficulty of non-metallic compensators is not technical, but that you don't use the right method.

"How to maintain non-metallic compensator?" This question, our after-sales engineers have to be asked hundreds of times a year. Today, I disassembled it and crumpled it to make it clear. Each one is the experience hammered out in the actual project.

Find out the structure of non-metallic compensator first, then talk about maintenance

If you don't even know what layers it consists of, maintenance is busy. The non-metallic compensator is not a whole cloth sleeve, it is a flexible joint made of multiple layers of different materials, and mainly depends on the skin to absorb thermal displacement and vibration. Common structures include:

• Skin (non-metallic band)— — The outermost sealing layer, usually glass fiber cloth coated with fluororubber, silicone rubber or polytetrafluoroethylene, depends on it for temperature resistance and corrosion resistance.
• Thermal insulation-The interior is filled with ceramic fiber blanket or glass wool, which is used to block high temperature and protect the skin.
• guide tube— — Install inside the pipeline to guide the flow of media and avoid high-temperature airflow directly washing the skin. This is the same as the guide tube of the metal expansion joint.
• Flange plates and bolts— — Metal parts that press the skin against the pipe flange, by which standard non-metallic expansion joints are installed.

So what is the nature of maintenance? Is to protect the skin, keep the seal, and prevent the bolts from loosening. You remember the structure in your mind, and you will naturally understand it later.

Where to look for daily inspections? Focus on checking these parts

Inspection is really not difficult, just see with your eyes. It is suggested that in the order of "from the outside to the inside", more than a dozen compensators on a pipeline can be checked once in half an hour.

First Lookskin surface。 Any bulging, delamination, cracking, scratching? The most common injury is abrasion-dust particles, driven by airflow, sand the skin like sandpaper. If the outer fiberglass cloth is found to have broken yarns or exposed yarns, it means that the skin has begun to deteriorate.

Second Lookflange bolt。 This one is the easiest to overlook, but goes wrong the most. Under the temperature change of non-metallic compensator, the thermal expansion and contraction of bolts produce stress relaxation, which has to be tightened once in one or two months. You try the wrench one by one. If you can twist it half a turn, even if the torque is insufficient.

Third Lookguide tube。 Extend a flashlight from the entrance of the compensator and shine it in to see if the guide tube is curled and fallen off. Just two days ago, a cement factory customer reported that the compensator was hot. When he disassembled it, he saw that half of the guide tube was burned through, and the flue gas was sprayed directly on the skin from the gap. It was found that the insulation layer of the whole pipeline would have to be scrapped a few days later.

Remember, the skin is a consumable and can be replaced if it is broken. However, if the deflector and the insulation layer are broken, it is a systemic problem, and the maintenance cost is completely different.

The timing of cleaning and replacing non-metallic compensator

The topic of cleaning is quite controversial in the industry. Here is my view:If you can not wash it, don't wash it. Don't clean the dust.

Why? Once the surface coating of non-metallic skin is corroded by chemical cleaning agent, the sealing performance is degraded directly. Are the solvents you use to remove oil or rust? How much acidity and alkalinity? Don't be smart until the manufacturer gives advice.

If the dust accumulation is too thick and affects the heat dissipation, just purge it with dry compressed air, control the pressure within 0.2MPa, and keep a distance of more than 30cm from the skin. If there is really oily stain, brush lightly with a soft bristle brush dipped in neutral soap, then quickly rinse with clean water and air dry, and the process is controlled within 10 minutes.

So when should I change it?Penetrating cracks or air leakage in skinSwitch it directly without discussion. If only the surface coating is powdered, it can last another month or two, but there must be a replacement plan. There is a rough empirical figure for reference: the skin life of non-metallic compensator is about 3~5 years under normal temperature flue gas conditions; Under the high temperature above 300℃ or the working condition containing sulfur and alkali, the service life is shortened to 1~2 years. This is not said by patting the head, but summarized with reference to JB/T 12235-2015 standard and a large number of engineering cases.

Maintenance Myth: These Actions Can Instead Damage the Compensator

The following operations are the "negative teaching materials" seen repeatedly at the after-sales site.

Myth#1: The tighter the bolts are tightened, the better.The skin is sealed by the uniform pressure of the pressure plate. If you tighten the bolts on one side, the pressure plate is warped, and the other side leaks. The correct method is to pre-tighten diagonally several times, and finally tighten it uniformly with a torque wrench. The torque value refers to the requirements of the installation drawings.

Myth 2: If you find small cracks, use glue to mend them.Not only does this fail to restore the mechanical properties of the structure, but the solvent in the glue will further damage the skin coating. Many customers take structural glue on them by themselves, and as a result, the cracks get bigger and bigger.

Myth 3: Ignore maintenance during system downtime.When the machine is shut down, the residual medium in the pipeline (especially sintering flue gas and desulfurization slurry) will crystallize or corrode the skin inside the compensator. If the machine is shut down for more than one week, it is recommended to open the drain outlet to drain the accumulated liquid, and purge it with low-pressure steam if necessary.

To add, a non-metallic compensator cannot be used as a pipe support point. In some sites, in order to save supports, the weight of the pipeline is pressed on the compensator, and the skin is overloaded every day, and its life is more than halved.

Maintenance cycle and record management under different working conditions

There is no uniform standard for maintenance cycle, it depends on your operating environment. We usually divide our customers into three categories:

• Normal operating conditions(The temperature is lower than 200℃, the medium is non-corrosive and dust-free): Inspect once a quarter, and tighten the bolts once every six months.
• bad working conditions(High temperature and high dust, or containing acid, alkali and sulfur components): Inspect once a month, and tighten the bolts every quarter. Under this working condition, the surface temperature of the skin is measured with an infrared thermometer, which is much more reliable than hand touch.
• Extreme operating conditions(For example, near the desulfurization flue gas baffle door and the smoke exhaust system of the power station): See it once a week, focusing on observing whether there are any traces of acid dew point corrosion. By the way, the skin material of the non-metallic compensator in the desulfurization system must be acid-resistant fluororubber coating, otherwise it will be useless no matter how diligent the maintenance is.

Records management, don't bother. Each compensator establishes an independent ledger, which contains three lines: installation date, problems found in previous inspections, and details of replacement parts. With two or three years of data, you can find out the loss law of the compensator on your pipeline and prepare the wearable parts in advance.

To put it bluntly, the answer to the question "how to maintain non-metallic compensators" is twelve words:Understand structure, check frequently, do not move, keep records。

Although this article is about maintenance, if you encounter problems at the selection or installation level, don't shoulder it yourself. Whether the compensator can live long or not is decided from the day of selection and installation. If you are unsure of the specific working conditions, talk to the manufacturer's technical support directly, which is much more efficient than thinking about it on the spot.

Those who do pipeline design and equipment procurement have asked nine times out of ten: Which non-metallic compensator is the best?

This question cannot be answered in one sentence. Non-metallic compensator is not a "universal artifact". It is a general name for a class of products, and there are several kinds below. You pick the wrong type, and it's all trouble behind. Let's break up the key points one by one and explain them clearly. After reading it, you will know in your heart.

First distinguish the types: fabric fiber, rubber, PTFE, it is not the same thing at all

Non-metallic expansion joints (also called fabric fiber expansion joints) are usually covered with glass fiber and ceramic fiber, which have outstanding high temperature resistance and large compensation, so they are especially suitable for working conditions such as flue gas and dust. When you go to the inlet and outlet of the desulfurization tower and the pipelines near the chimney, you basically use this kind of pipeline.

The rubber compensator is different. It is good at vibration reduction and displacement absorption, and can help you isolate the vibration of pump and fan. However, its temperature and pressure resistance are limited. If the temperature exceeds 100℃, you have to weigh it, and it can't stand it even if the pressure is high.

There is also rubber PTFE compensator, which takes into account both anti-corrosion and flexibility, and is more used in acid-base medium pipelines. PTFE compensators also do this, which is more resistant to corrosion.

So you see, marking "non-metallic" doesn't mean omnipotent. Choose the type according to the working conditions first. This step is wrong, and no matter how much money you spend later, it will be in vain.

Look at the working condition parameters: temperature, pressure, medium, none can be missed

The core of choosing a non-metallic compensator is actually how to match the skin material layer. How to match the skin is completely determined by the working conditions.

How much is it resistant to temperature? Is it continuous or intermittent? Is the medium containing sulfur, acid and alkali or dust? Is the pressure negative or positive? With one less of these parameters, manufacturers can only guess according to experience. If you guess right, it is luck, and if you guess wrong, there will be smoke at the scene.

Take an example. The non-metallic expansion joint behind the desulfurization flue gas baffle door is located in a wet and acid environment, and there is a risk of condensation. At this time, the skin has to consider acid corrosion resistance and waterproof penetration. If you move the parameters of the corrugated expansion joint used in the power station boiler or cement kiln, copy it and use it directly, it will probably not last long.

Will the skin be deflated? Some on-site expansion joints are obviously good, but when the negative pressure is large, the skin is directly attached to the internal guide tube, and the wear is aggravated, and it leaks soon.

How to determine the compensation amount and installation size? Don't just look at caliber

The biggest advantage of non-metallic compensator is that it can compensate in multiple dimensions-it can absorb in axial, transverse and angular directions. However, how much it can be compensated is directly related to the structural form and the number of fiber layers.

In the same DN500 circular pipeline, some non-metallic expansion energy saving absorbs 50mm axial displacement, and some can only absorb 20mm. The gap lies in the number of skin layers and the internal structure design.

Rectangular flues generally use rectangular non-metallic expansion joints, while circular pipes use circular structures. This is not difficult to understand, but many people stumble on the installation size.

For example, if the direction of the guide tube is installed backwards, or the flange spacing is not enough according to the drawings, it will be hard-pulled and hard-pulled when installed, and most of the compensation amount will be consumed in the installation stage. Equipment life is directly discounted-it is not that the product is not working, it is that you have installed it incorrectly.

See execution standards and production processes

There is a national standard for non-metallic expansion joints, JB/T 12235-2015, which has clear requirements from materials, welding, inspection to factory test. Products with standard constraints, at least the bottom line is there.

However, the implementation of the standard has to be seen by the eyes. When you get the product, check whether the skin has a cut-proof layer, which protects the skin from scratches during transportation and installation. Then look at whether the flange surface is flat and whether the bolt hole spacing is accurate-these details can be judged with the naked eye.

Skin strength and sealing are not done enough, which is often reflected in these small places. The national standard is not a bondage, but a protection. Products with no standard constraints mostly cut corners out of sight.

Calculate the comprehensive cost: not the cheaper the better, nor the more expensive the more insured

When you buy a non-metallic compensator, you are buying the compensation capacity, not a bunch of metal frames. It is really cost-effective to have no leakage and worry-free maintenance for two years after installation.

I met a customer two days ago, and I bought a non-standard product cheaply. It cracked in less than a maintenance cycle. Not to mention the replacement, it also delayed the production. The loss of a day's downtime is much more than the small purchase fee saved.

Procurement cost + maintenance cost + downtime loss. Put these three items together, and you will not panic when you compare the quotes.

After all, which is the best non-metallic compensator? Suitable for your working conditions, produced according to national standards, and the details can stand to be seen, is the best.

Starting from the name of the industry: What is the relationship between compensator, expansion joint and telescoper?

"Is this expansion joint just a telescoper? Can I use a sleeve type instead of a corrugated one?" In fact, in the field of pipe compensation,Compensator and expansion joint are basically the same thing-You see, article 14 of the question and answer on our site also states that expansion joints and compensators actually refer to the same type of equipment in industrial piping systems. But the word "telescoper" is a little ambiguous. In some old drawings, the sleeve compensator is called a telescoper, and some on-site masters collectively refer to all the telescopic tubes as telescopers. So technically speaking,Compensator (expansion joint) is a collective termWhile scaler is more of a colloquial common name, often referring specifically toSleeve type or packing box type。 Don't underestimate this detail. If you choose the wrong model, the pipe may collapse directly when installed.

Classification by material and structure: metal corrugated, rubber, non-woven, sleeve type, what are the characteristics of each?

In the product list on our site, there are dozens of kinds of light expansion joints. Such asUniversal corrugated expansion joint、High temperature axial expansion joint、directly buried expansion joint、External pressure single type axial type…These are metal bellows, which absorb displacement by the elastic deformation of the corrugations. rubber compensator andRubber PTFE compensatorRelying on the flexibility of rubber, it is suitable for scenes with low pressure, large displacement and vibration reduction, such as fan import and export.Non-metallic expansion joint(also called fabric fiber expansion joint) is resistant to high temperatures and corrosion, which is common in flue gas pipelines.Sleeve type pipe expansion joint(that is, the telescoper in some people's mouths) is compensated by the sliding of the inner and outer sleeves. The pressure is high and the axial displacement is large, but the sealing ring is easy to age. AndRotary compensator、Curved tube pressure balance typeThese are specific to operating conditions. At the end of the day, there is no universal compensator, only matching selection.

What to look at when choosing a model? Temperature, pressure, displacement, and medium must not be missing

You use a rubber compensator on the steam pipe? As soon as the high temperature of several hundred degrees goes up, the rubber is directly carbonized. So the high temperature pipeline has to beHigh temperature axial expansion jointOrExternal pressure single type axial type。 If it is desulfurization flue gas, which is highly corrosive, it must bePTFE-lined hoseOrPTFE compensator。 The direction of displacement is also very critical-axial displacement menu, transverse displacement selectionCompound hinge transverse typeBoth axial and transverse,Straight pipe pressure balance typeOrDouble straight pipe bypass pressure balance typeMore appropriate. Don't forget that there arebaffle doorAndIsolation doorSuch ancillary equipment, such asFlue gas baffle door、Round flapper door、Electric plug-in insulation doorThey work with a compensator, but have completely different functions. When selecting the model, fill in the working condition parameters clearly, and don't pat your head.

Don't step on the pits of installation and maintenance

Many people think that the compensator can be installed, but it leaks within two months. There are three common problems: First, the transportation screw was not disassembled before installation-the expansion joint will be fixed by a screw in order to prevent transportation deformation when it leaves the factory, and it must be disassembled or adjusted according to the instructions after installation. Article 13 Q&A specifically talks about this matter. Second, cold-tight pre-stretching was not done, especially high-temperature pipelines, which were not pre-stretched during installation and cracked during operation. Third, the direction of the guide tube is reversed. It is mentioned in the seventh question and answer that the function of the guide tube is to prevent the medium from washing the ripples, and the direction of the arrow must point to the flow direction of the medium. And the packing gland of the sleeve-type telescoper can't be screwed too dead, otherwise the sliding resistance is large and can't be compensated. In terms of maintenance, the rubber compensator should be checked for aging cracks in about three years, and the fatigue life of the metal bellows should be paid attention to. For details, please refer to Article 18 Q&A.

Summary: Different names, the same core functions, but the specific products should be carefully looked at

Back to the original question —Is a compensator the same as a telescoper?Functionally speaking, they are all devices that absorb the thermal displacement of pipelines and reduce vibration and noise, which can be equated. However, when it comes to the procurement link, you report the "telescoper" to the manufacturer, and the other party may give you the sleeve type; You quote "compensator", and the other party may give you bellows. Therefore, it is recommended to clarify the product type directly: for example, "I need an axial corrugated expansion joint with DN200, PN16 and a compensation amount of ±50mm", or "Give me a non-metallic fabric compensator with a temperature of 350℃ and a smoke environment". In this way, you can't go wrong, and you can quickly match the most suitable one from the 29 products on our site. If you are not sure, just throw the working condition parameters over, and we will help you choose-we do this every day, which is more reliable than you patting your head.

Do you really understand the role of corrugation compensator?

What is the corrugated compensator (also called expansion joint and compensator in the industry) used for? Simply put, it is a flexible joint that absorbs all kinds of displacements and stresses in the pipeline system. Don't underestimate this function. The pipeline can run for tens of meters. When the temperature changes, it expands and contracts. If it is hard, the weld will collapse and the flange will leak, which is a loss of real money. So figuring out its specific role is more important than anything else.

I met a customer two days ago. The steam pipe has been used for two years, and the flange interface began to leak. When I removed it, I saw that the bellows were cracked. After asking, I found out that the displacement was not counted at all in the original selection, so I bought a general-purpose corrugated expansion joint to deal with it. Alas, I have seen many cases of this kind, and I have one feeling: if I don't understand the function of the corrugated compensator, I will have to pay tuition sooner or later.

First role: to compensate for thermal displacement-this is the skill of eating

Steam pipes, hot water pipes and flue gas pipes, the temperature goes up and down, and the pipes stretch and contract. For example, corrugated expansion joints are used in the power station industry, and hundreds of degrees of steam can easily be expanded and contracted by several millimeters to tens of millimeters, which are all absorbed by bellows. Another example is the metal corrugated expansion joint in the cement industry, which is transported by high-temperature clinker and has a larger displacement. Calculate, a 30-meter steam pipe, the temperature difference is 200℃, the linear expansion coefficient of steel is 0.012mm/m·℃, and the optical axial elongation is 72mm. Hard solder dead? The weld stress has already exceeded the yield limit.

When selecting the model, it also depends on axial displacement, lateral displacement or angular displacement. Like a straight tube pressure balance expansion joint, which is specially used to deal with axial expansion and contraction; Curved tube pressure balance type expansion joint, used at the bend of pipe system, absorbs angular displacement and transverse displacement. A few days ago, there was a desulfurization project, and the pipe system turned around. The design institute used three double hinge transverse expansion joints, and all the displacements were eaten, and the site passed at once. So don't use the general-purpose model to deal with all working conditions, that's called laying mines.

The second function: vibration reduction, noise reduction and impact resistance-bellows can also be used as shock absorbers

As soon as pumps, fans and compressors are turned on, vibration is transmitted along the pipeline, which not only annoys the noise, but also loosens the pipe frame and cracks the weld. Corrugated compensator can eat a part of vibration energy, especially rubber compensator and rubber PTFE compensator, which have good flexibility and obvious vibration isolation effect. Metal hoses also do this job, such as the double hinge expansion joint of air-cooled island vacuum pipeline, which has to bear the vacuum negative pressure and absorb the vibration of the pipeline. And guess what? The pipeline on the air-cooled island of the power plant vibrated at a ridiculously high frequency. Without this thing, the welds would crack in fatigue within three months.

I have seen a cement factory where the fan outlet pipe used a non-metallic expansion joint (fabric fiber expansion joint). The original noise was 90 decibels, but it dropped directly below 75 decibels after replacing it. Why? Fabric fiber expansion energy-saving absorbs multi-directional displacement, and at the same time converts vibration energy into heat energy and dissipates it. Tsk, that's a lot easier than adding a soundproof cover.

Third function: solve installation deviation and facilitate maintenance-less quarrel with field workers

It is impossible to install the pipeline on the spot-if it is slightly crooked by a few millimeters, the stress will be great if it is hard connected. Corrugated compensators absorb these installation errors and allow the pipes to connect smoothly. Especially for large-diameter pipes, it is common for flanges to mismatch. I have seen a chemical plant, DN1000 pipeline, which was misaligned by 8mm during on-site welding. It was adjusted by a universal corrugated expansion joint + tie rod, but it was not reworked. Otherwise, cutting and re-welding, the construction period is at least three days.

In addition, during equipment maintenance, such as manual plug-in insulation doors and electric plug-in insulation doors, a section of pipe needs to be removed. The corrugated compensator can provide additional telescopic space, so there is no need to move the pipe. Think about it. As soon as the expansion joint is compressed during overhaul, the space will come out. After replacing it, it will be stretched back, which will save trouble. Without this function, the entire pipe gallery would have to be demolished.

The fourth role: material adaptation under special working conditions-not all pipes eat the same set

Chemical industry runs acid and alkali, with PTFE-lined hose or PTFE compensator, corrosion resistance. Why? PTFE can carry almost all strong acids and alkalis, but ordinary rubber is scrapped in one month. What about high temperature flue gas pipes? It is necessary to use non-metallic expansion joints (fabric fiber expansion joints), which can withstand the temperature up to 1200℃ and can absorb multi-dimensional displacement. In wet desulfurization environment such as desulfurization flue gas baffle door, acid and alkali resistance and leakage prevention have to be considered. Two days ago, in a power plant desulfurization project, a rectangular non-metallic expansion joint was equipped next to the flue gas baffle door, which was both acid-and alkali-resistant and anti-condensation. After two years of use, nothing happened.

The directly buried (fully buried) type expansion joint is specially used for buried pipelines, which takes into account both resistance to soil pressure and compensation for thermal displacement. Some customers use ordinary expansion joints to bury the ground cheaply, but they are crushed by the soil in half a year-that is called the gain is not worth the loss.

Finally, talk about the selection idea: don't let the corrugated compensator take the pot

It is not just buying a corrugated compensator to solve the problem, but it has to be matched according to the medium, temperature, pressure, displacement direction and installation space. For example, large-diameter thick-walled expansion joints are used in high-pressure pipelines, external pressure single axial type is suitable for long straight pipes, and compound hinge transverse type is suitable for pipe systems with many turns. The attachments of the guide tube and the tie rod are not decorations-the guide tube protects the inner wall of the bellows from being washed by high-speed media, and the tie rod limits excessive displacement. Understand these, and the selection will not overturn.

What does a corrugation compensator do?To put it bluntly, it was four words: Keep the life of the dao. If you choose the right one, the pipeline will run as steady as an old dog for more than ten years; The wrong choice will make you doubt your life every three days. Therefore, the next time you choose a model, don't just look at the price. First, calculate the displacement, check the medium and ask about the working conditions-this is a serious business.

Two days ago, a buddy who designed flue gas pipelines called and asked, "How to calculate the resistance of the expansion joint? I calculate the pressure drop according to the straight pipe section. If the pump is selected small, the test run will trip." Alas, this sounds familiar. Too many people have stumbled in the selection of expansion joints-the system pressure drop is quite accurate, but they have slapped their heads in the expansion joints. Today, let's talk about this matter clearly. After you understand it, you can pat your chest and say that you can count.

First, don't rush to set the formula-what problem does the resistance calculation solve?

I calculate this resistance, what is it for?
To put it bluntly, there are two purposes: first, ensure that the head of the pump or fan is sufficient, so that the system will not be pressurized or the flow rate is insufficient; Second, avoid the pipeline stress exceeding the standard due to excessive local resistance, which will tear the expansion joint.

To put it bluntly, the expansion joint is installed in a pipe, and the fluid passing through it will inevitably cause pressure loss. If this loss is ignored, the chosen pump will either trip like the guy above, or the big horse-pulled cart will waste the electricity bill. Calculate it accurately, and you can match an economical and reliable system.

Second, the physical nature of resistance: the expansion joint is not a straight pipe, and the fluid turns a corner here

What is the structure inside the expansion joint? Take the most commonUniversal corrugated expansion jointFor example, the ripples raise one by one, and when the fluid flows through it, it is like driving a car on a washboard road-every wave peak has to change direction, accelerate, and slow down, and energy is lost in the process. This loss belongs to local resistance, which is completely different from the friction resistance along the pipeline.

If you think about it, a straight pipe of the same caliber and a section of corrugated expansion joint, at the same flow rate, the pressure drop of the latter may be 5 to 10 times that of the former. Why? Because the fluid repeatedly "bends" in the ripples, a large number of vortices are created. This kind of local resistance cannot be applied hard by Darcy's formula, but has to be calculated by a special resistance coefficient method.

Then how to quantify? The empirical formula is as follows: Δ P = ξ · (ρ · v²/2), where ξ is the drag coefficient, ρ is the medium density, and v is the flow rate. The difficulty is all about this.

Third, different types of expansion joints, the resistance difference is greater than you think (metal corrugation vs non-metal vs sleeve)

Different types of expansion joints have very different internal geometries, and the drag coefficients can be different by an order of magnitude.

• Metal corrugated expansion joint(For example, those used in the power station industryHigh temperature axial expansion joint、Large diameter thick wall expansion joint): The ripple bulge is obvious, and the fluid turbulence is violent. The resistance can be two or three times different from the one with a guide tube and the one without one. More on that later.
• Non-metallic expansion joints (fabric fiber expansion joints): The interior is generally a smooth rectangular or circular channel, without obvious corrugated bumps, and the resistance is much less than that of metal corrugated expansion joints. However, it depends on the fabric to bear pressure, and the flow rate cannot be too high, usually controlled within 15m/s.
• Sleeve type pipe expansion joint: The displacement is compensated by the sliding of the inner and outer sleeves, and the inner channel is basically straight through, with the smallest resistance, which can be calculated almost according to the straight pipe section. But seals are prone to wear and tear, which is another matter.

So don't take all models with one formula. You're choosingCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointWhen the structure is more complicated, the drag coefficient has to be corrected according to the specific wavenumber and waveform.

4. Calculate it hand-in-hand: empirical formula + key parameter values

Okay, straight to the case. Assume a steam pipeline with nominal diameter DN300, design flow rate of 25m/s and medium density of 0.6kg/m³. Select a steam pipeline with guide tubeUniversal corrugated expansion joint, Single Wave.

Check the drag coefficient ξ. According to industry experience, ξ of single-wave metal corrugated expansion joint with guide tube is generally between 0.5 and 1.2. Take 1.0 for conservative points. Without the deflector, ξ can soar above 3.0.

Calculate the pressure head. ρ ·v²/2 =0.6×25²/2=187.5 Pa.

Δ P = ξ ×187.5=1.0×187.5 ≈ 188 Pa. This is just the loss of a single wave. If you choose a multi-wave expansion joint (such asCompound straight pipe bypass pressure balanced expansion joint), also multiply by the wavenumber, but pay attention to the interwave interference, not a simple linear superposition-usually corrected by the square root of the wavenumber.

The contrast system allows pressure drop. If the outlet pressure of the fan is only 500Pa, an expansion joint will dry out 188Pa, plus the loss of the pipe valve, it is definitely not enough. At this time, either change to a low-resistance type with a guide tube, or increase the diameter to reduce the flow rate.

5. Guide tube, flow rate, medium-those "invisible" factors that affect resistance

Just now, we mentioned the guide tube. What is this thing for?Specific Function of Expansion Joint Guide TubeIt is to let the fluid take a relatively straight path inside the corrugation, so as to avoid direct impact on the root of the corrugation to generate large vortex. With it, the drag coefficient can be reduced to one-third or even lower than without it. So don't be reluctant to give up that cost, especially for high-speed airflow pipelines, the guide tube is a power-saving artifact.

Medium viscosity. Gas and water are very different. The gas has low density and low viscosity, and the resistance mainly comes from eddy loss; Liquids such as water or oil have high density and high viscosity. In addition to local resistance, the friction loss of corrugated wall should not be underestimated. When calculating liquid pipeline, it is recommended to refer toMetal hose pressure standardEmpirical data in, or do CFD simulation directly.

In addition, the higher the flow rate, the better. The flow rate doubles and the resistance becomes quadrupled (because v²). Some people choose small diameter expansion joints to save money, but as a result, the pressure drop is too large, and the pump consumes much more energy than the equipment money saved. Tsk, the gain outweighs the loss.

6. Completion of calculation is not equal to completion-common cases of resistance rollover in engineering

Let's tell me a few real rollover scenes.

Case A: The desulfurization flue of a power plant was usedNon-metallic expansion jointIt would have been fine, but the design flow rate was mentioned above 20m/s, and as a result, the fabric layer was torn by the airflow after half a year of operation. Turning back and calculating, the local pressure drop exceeds the standard, resulting in negative pressure fluctuation and fabric fatigue damage. Later replaced with a deflectorMetal rectangular expansion jointJust steady.

Case B: Steam pipeline of a chemical plant, selectedExternal pressure single axial expansion jointThe pressure drop of the deflector is not counted, and as a result, the system safety valve jumps frequently. Finally, it was found that the corrugated root inside the expansion joint accumulated scale, the actual circulation area shrank by 30%, and the resistance tripled. So regular inspections and cleaning are also important.

And guess what? These rollovers all have one thing in common: the expansion joint is regarded as "part of the pipe" in the design stage, and the resistance is not counted separately at all. In fact, the expansion joint is a local resistance member in the pipeline, which must be listed separately in the pipeline hydraulic calculation table.

How is the resistance of the expansion joint calculated?Remember this formula: Δ P = ξ · (ρ V²/2), then honestly check ξ, set the flow rate, and check the system. Don't slap your head, don't be lazy. Understand these steps, and selection is no longer metaphysics.