1 Yinguan polyurethane prefabricated heat insulation pipes are widely used in central heating systems.
The most widely used production technology is still a discontinuous, on-site pouring technology, which fills the reactive foam material between the preset working tube and the casing. When particularly good fluidity is required to ensure a balanced distribution of the entire pipe, special foam materials should be used. Another key requirement is that in the case where the operating temperature of the central heating network is as high as 140X: the foam material can still maintain long-term high insulation performance.
More advanced pipeline production technology has been developed. This technology can reduce the cost of prefabricated insulation pipes by reducing the density of the foam material and thinning the sleeve. However, these continuous production technologies require changes in the reactivity of the foaming system, viscosity growth curve, and curing performance.
2 The performance of rigid polyurethane foam products for the production of prefabricated insulation pipes for central heating systems requires good adhesion of the foam to ensure the long-term performance of the pipeline composite. Product performance requirements of rigid polyurethane foam for pipes. Low foam thermal conductivity, long-term high thermal resistance and high-strength compression performance will ensure that the foam material can withstand heavy pressure under the highway. For example, when the pipeline material component is in transit or buried in the ground, the foam is required to be able to bear a heavy load. For prefabricated thermal insulation pipe systems used in underground hot water networks, the minimum performance of polyurethane foam must meet the requirements of the European quality standard EN253. China is now developing similar quality standards.
At present, there are basically: â‘ full water foaming system; â‘¡HCFO141b / water foaming system; â‘¢cyclopentane / water foaming system. These three alternative rigid polyurethane foam systems are used in the manufacture of prefabricated insulated pipes.
The foam system manufactured in place of HFCCM41b is currently under development.
3 The recommended pipe filling technology and processing technology have some different technologies that can be used to produce prefabricated insulated pipes.
Of course, each technology has its advantages and disadvantages. We can divide two main types of production technologies, continuous production and discontinuous production. The suitability of a production technique depends on the specific needs of each pipeline producer. The most widely used production techniques will be discussed below. 3.1 Non-continuous production technology In the non-continuous production method, the steel pipe is placed in the center of the HDPE tube shell, which is shorter than it. .
The gap between the steel pipe and the shell at both ends is sealed with an end cap, which tightly seals the steel pipe and HDPE pipe. There are some small holes in the end cap to inject foam material and let air out. In principle, pipes with a length of less than 6m can be used, but the standard length of steel pipe is the key parameter of the 6m high-quality pipe infusion process, including the following aspects: a foam system with excellent flow properties to ensure the average density in the pipe, the temperature of the raw material and the pipe Control, tube surface treatment, and perfusion density and time. The temperature of polyol and isocyanate should reach 2023t: as good. When assembling the tubes, especially in winter, preheat them to 20 ~ 30C. If the temperature of the assembled tubes is too low, the heat of the foam mixture will be lost too much, resulting in poor reaction between the foam material and the surface of the tube. This will cause the foam to be brittle and reduce the adhesion of the foam to the tube. At this time, more foam material may be needed to completely fill the space in the tube.
The steel pipe should be degreased and derusted to achieve proper adhesion. It is recommended to pre-heat the HDPE tube shell with fire or corona to ensure that the foam material is firmly bonded to the tube shell. Adjust the output of the foaming machine so that the calculated amount of the mixture can be poured into the mannequin in the milky time. The infusion density depends on the actual size of the tube. The infusion density is usually high enough so that the foam fills the entire tube well within the gel time. If the foam does not reach the end of the tube within the gel time, then the foam will be stretched and its cell structure will become slender. This will result in poor mechanical strength of the foam material at the end of the pipe.
3.1.1 Bottom-up pouring technique In the bottom-up pouring method, the preset tube is usually at an angle of r to 15 from the horizontal plane (see). The size of this angle depends on the length of each tube and the fluidity of the materials used. A specified amount of foam mixture is poured into the gap between the steel pipe and the HDPE pipe through the hole in the end cap at the bottom of the pipe assembly. The foam starts to expand inside. Immediately after the fixed amount of foam material was injected, a plug was used to plug the feed hole. The foam material is discharged along the tube under the pressure. Once the expanded foam rises to the top air hole, the air hole is well sealed. Wait for the component to cure for a period of time before removing the end cap.
The main advantage of this technology is easy production. There is no strict regulation on the tilt angle of the preset tube. The skill requirements for operators are also low. With this technology, pipes of various lengths can be produced with only minor changes. The technology is quite flexible in this regard. The main disadvantage is that the distribution of the foam material throughout the tube is not uniform, and therefore its mechanical properties are not uniform. The bottom foam has the highest density and the top has the lowest density. Since the foam material has to travel a long path in a narrow tube gap, more material needs to be poured.
3.1.2 Top pouring technology In the top pouring technology, the preset pipe is usually at an angle of r to 15 from the horizontal, but at this time the specified amount of foam material is poured into the steel pipe and the hole through the hole in the top end cap of the pipe assembly The gap between the HDPE pipes. Gravity causes the less viscous mixture to flow down the tube. The expansion of the fluid depends on the pouring angle of the pipe assembly. The greater the angle of inclination, the more material flows below. Before the foams actually start to expand, they already have an initial distribution along the tube. The foam material then flows from the middle of the tube to both ends to fill all the voids.
Experience has shown that when the foam material reaches the bottom vent hole, about 20s faster than the time to reach the top, the material distribution obtained is the best.
Of course, once the foam material overflows from the air hole, the vent hole is sealed.
This initial distribution before the foam expands reduces the path through which it must flow to fill the entire gap. In this way, the amount of overfilling can be reduced and a lower perfusion density can be used.
Longer tubes are also easier to infuse. If the installation angle is correct, the density distribution of the bubble material in the entire pipeline can be within a small range, and the distribution is good. This technology requires stricter tube placement angles than when the bottom is poured upwards, so the operator's technical level must be higher. It is recommended to use the existing pipe pouring table and adjust the angle accordingly.
3.1.3 Intermediate filling technique In the intermediate filling method, the tube assembly is placed horizontally (see). The foam material of the child is required to be poured through the hole in the middle of the HDPE tube shell. With this technique, the full length of the tube that the foam had to pass through was reduced to half of the tube length. In this case, less overfilling can be used, and it is easier to manufacture long tubes with a satisfactory density distribution. Air is discharged from the air holes on the two end caps. Compared with other methods, using this technology is more or less likely to entrap air.
After the foam material is poured, the feed hole in the center of the tube shell is immediately plugged.
After the foam material is cured, you must ensure that the hole is welded. And that place has become a potential weak point for this tube.
Intermediate infusion technology 3.1.4 Boom pull-back technology The boom pull-back technology is a discontinuous production technology, but during its processing, the distribution of foam material is carried out continuously. In this production method, the preset tube assembly is placed horizontally. Adjust the foaming machine and place the small mixing head at the end of the boom (see). The spray rod is inserted into the gap of the tube so that the mixing head is located at the distal end of the tube.
Obviously, the design size of the mixing head must be suitable for the narrow space between the inner working tube and the outer tube shell. The combination with a high-pressure foaming machine is the most successful aspect of the technology. The size of the mixing head limits the use of this technique in very small diameter pipes. Put the mixing head in the correct position 3.1.5 Penetration technology Penetration technology is also a discontinuous production technology, but its foam material supply is continuous. At this point, it is similar to the boom pull-back technique. The preset tube assembly is placed horizontally. There is a thin semi-permeable paper band under the inner working tube. The foam mixture is injected onto the thin strip and passes from one end of the tube to the other (see). After the foam material only needs to flow for a short distance, the foam material begins to be poured. During priming, the boom is pulled back little by little. This method ensures that the foam distribution of the entire pipe is uniform and good, regardless of the length of the pipe. At this time, the path through which the foam flows is greatly reduced. It only needs to run around the pipe. The amount of overfilling perfusion is also very small. In operation, as the mixing head moves, the foam material is evenly laid along the tube. This eliminates the limitation of the injection time imposed by the top or bottom up technique. Therefore, very small tubes can be used to fill very large tubes.
Diameter, so the amount of overfilling is also very small. And the foam material of the whole pipe is evenly distributed. The advantage of uniform distribution of the foam mixture makes this method used to pour long and thin tubes. For example, pipes up to 30m in length can also be easily manufactured. The disadvantage of this technique is that the paper tape is left in the foam material, which affects the adhesion between the foam material and the shell. The injection rate of foam material and the loss of paper tape 3.2 Continuous production technology The continuous production technology of the pipeline consists of two stages. In the first stage, the foam material is poured on the surface of the inner working pipe by a mold method or a spray method. In the second stage, the tube shell is wound or extruded onto the preformed foam material. Continuous production technology requires changes in the reactivity, viscosity growth curve and curing characteristics of the foam system. This requires the use of a modified polyurethane system, usually up to several hundred meters, which is bent after use. For such special requirements, continuous production technology is very suitable. Because the most outstanding feature of this technology is that it is not affected, in the continuous production of the spray method, the reactive foam mixture is sprayed onto the surface of the rotating central working tube (see). Obviously, these foams must react quickly to adhere well to the surface of the central tube without being turned off. This method can spray multiple layers of foam materials to meet the thickness requirements of thermal insulation. The foam material forms a very uniform foam layer in a very short flow path. In fact, the spray coating method can be used to produce any thickness of insulation.
3.3 Auxiliary infusion technology 3.3.1 Infusion technology at the joint When constructing a district heating network, 616m long pipelines are auxiliary-installed, and then connect them. This includes the steel tube layer. The HDPE tube shell is then wound or extruded out of the insulation layer.
This technology can use lower density foam materials and thinner HDPE shells. This saves material. In practical applications, spraying technology is particularly suitable for large diameter pipes. The use of this technology to produce small-diameter pipes will cause greater waste and is not economical. Like the continuous production technology of the mold method, the spray method takes a long time to set when changing the pipe diameter.
Point: the temperature of the raw material and the tube, the surface condition of the tube and the density of the perfusion. Pay attention to these points when pouring at the joint of the construction site.
Although a small foam machine can be used on the job site, the foam material used at the joint is usually stirred by hand. Special attention should be paid to the welding during manual mixing, the welding of HDPE shells and the infusion with PU foam material. The key factor of the reactive tube infusion effect of the mixed foam material for the joint is still the gap mentioned above. Go (see). These junctions should be regarded as the weak links of the entire network. Therefore, it is necessary to select high-level and experienced employees to perform this work to ensure the best quality of on-site work. This is extremely important. Of course, it is necessary to obtain a lower quality than the usual reactivity used in the production of pipes. This is because the manual configuration and careful pouring of the mixture into the gap through the small holes in the top of the housing of the joint take longer than usual production time.
Medium, or used to repair insulation on existing pipelines. Pipe insulation is produced with half-pipe models, or cut from large blocks of material. In order to avoid the foam material sticking to the mold, some release agent should be added to the mold. In the process of forming, the mold is preferably heated to the system to produce the pipe insulation. However, in order to improve the production speed, the reaction activity of the system can be increased by adding a catalyst. This can shorten the demoulding time, that is, the foam insulation layer can be taken out of the mold without deformation. If less overfilling is used, a system with a higher free foam density must be selected, which will also shorten the polyoxyethylene foam system used for various pipe infusions. In the laboratory, we developed a variety of foam systems to meet the market The different needs and requirements of pipeline manufacturing technology. These systems have been successfully used in industrial production.
4.1 Full water foaming system If the formula of full water foaming system is formulated through correct calculation, a very fine cell structure can be formed. It can provide good mechanical and thermal insulation properties. The appraisal report of the appraisal committee proves that the material can theoretically work continuously for 30 years (CCOT) at a temperature of 140t :. The all-water foaming system is an environmentally friendly product and can be processed into a regular all-water foaming system. The expansion of the polyurethane foam material is a product of Fan. In the production process, the temperature of the pipe and the raw material reacted with water and isocyanic acid to produce carbon dioxide should be carefully controlled.
The thermal conductivity of the all-water foaming system is slightly higher. This is because carbon dioxide has a slightly lower thermal insulation ability than other blowing agents such as HCFC441b or cyclopentane. In addition to the generation of carbon dioxide, the reaction of water with isocyanate also generates polyurea in the foam. These polyurea components improve the heat resistance of the material. However, this large ft polyurea increases the brittleness of the foam, especially in systems with low reactivity.
Operate to avoid brittle foam.
Huntsman Polyurethanes has developed a series of all-water foaming systems. The contents of polyols, surfactants, catalysts and water have been rigorously calculated. Table 1 lists the main performance of Daltofoam series foaming system TE44204 / Suprasec5005. These materials have good fluidity and have a broad processing field. Due to the longer emulsification time of this system, the operation time of the perfusion can be correspondingly longer. Therefore, this system is more suitable for small-capacity foaming machines. It is also easier to fill large diameter tubes with this system.
Table 1 Performance of series of all water-based foam materials (recommended for non-continuous pipe filling) Performance test method Typical value EN253 European standard polyurethane rigid foam material Average cell size // Im closed cell rate /% foam density kgTn Compressive strength / MPa Water absorption /% Shear strength before aging of pipe components / MPa Pressurized at 23t EN at 140X: Pressurized at 231: Add T-shaped shear compression COOT Humidity) (30 years) / T: The foam system dedicated for production and infusion has been generally adopted.
The foam system for pouring joints has been improved to operate at about Ot :. Foam systems using spray bar pull-back technology, through-the-head technology and die-pipe continuous production technology have all been successfully developed and become commercial products.
The generated carbon dioxide and physical blowing agent HFCCM41b expand the polyurethane foam.
The HCFC441b / water foaming system has good fluidity, so it is easy to process. Compared with the full-water foaming system, its thermal resistance performance is generally slightly lower, the mechanical properties are almost the same, and the initial thermal conductivity is lower. HCFCM41b series foaming system can be processed into standardized products.
Because it has the potential to destroy the ozone layer (ODP) and increase the global warming effect (GWP), it will eventually be banned from use, so local governments should exercise caution in this legislation. Products that can replace HCFO141b include HRM34a, HF0245 or HF0365. These products do not destroy ODP, and GWP is low. Because a plan has been made to stop the production of HFCCM41b, we are currently developing HFC434a, HF0245, HFC365 or their combination to replace HFCCM41b to manufacture a series of foaming systems.
The HFCCM41b foaming system produced by correct calculation has good mechanical and thermal insulation properties. Life appraisal proves that its physical activator, catalyst, water and HCF (the content of M41b has been rigorously calculated. Table 2 lists the special foam system used for pouring and joint use of Daltofoam series foaming system TE production. Has been widely adopted.
Table 2 Properties of HCF (M41b series foam materials (recommended for non-continuous pipe infusion> performance test method typical value EN253 European standard polyurethane rigid foam material average cell size / fim closed cell rate /% foam density / (kg, m3) Compressive strength / MPa water absorption /% shear strength before aging of pipe components / MPa at 23X: under pressure EN at 140X: under pressure 231: under T-cut pressure 50t: thermal conductivity / VHm-K) ccor (calculated continuous working temperature) (30 years) / C 4.3 Expansion of materials such as cyclopentane / water foaming system is produced by carbon dioxide and physical blowing agent cyclopentane The thermal stability is similar to the full-water foaming system. Cyclopentane has no destructive effect on the ozone layer, and the potential impact on the global temperature effect is very small, less than 0.01. Therefore, this type of foam is considered an environmentally friendly product.
However, strict precautions must be taken when handling cyclopentane.
Cyclopentane is a flammable liquid with a flash point much lower than room temperature. It will burn when mixed with air and may cause simmering. Therefore, fire must not be allowed to enter. From the viewpoint of safety prevention, the equipment should be adjusted accordingly before processing materials containing pentane.
Huntsman Polyurethanes has developed a series of cyclopentane / water foaming systems and applied them to the production of prefabricated insulated pipes. Cyclopentane is always added to the polyol before being poured into the tube on site, and our cyclopentane foaming system has good mechanical and thermal properties. The life appraisal proves that CCOT is up to 140t :, up to 30 years. Table 3 lists the main performance of Daltofoam series foaming system TE 34201 / SUprasec5005. The material has improved fluidity and broad processing fields. The improved fluidity allows the density of tube filling to be reduced, especially in the top filling technique.
The typical performance of the experimental method is such that the pipe manufacturer can save costs by reducing the consumption of foam. In addition, compared with the HCFC> 141b foaming system and the full-water foaming system, the pentane foaming system has the lowest initial thermal conductivity. Compared with the full-water foaming system, the pentane foaming system has a lower thermal conductivity aging rate; that is, its rate of thermal insulation deterioration with time is relatively small.
The general improved foam system has reached the European standard EN 253 and can work continuously for 30 years at 1491C. The foam system developed for continuous process technology can work continuously for 30 years under 146t :. The foam system dedicated for infusion has been widely adopted in production.
Table 3 Performance of cyclopentane series foam materials (recommended for non-Lianbin pipe infusion) European standard polyurethane rigid foam material average cell size / pun closed cell rate /% foam density / (kgTn compression strength / MPa Water absorption /% Shear strength before aging of pipe components / MR at 23X: under pressure at 140X: under pressure at 231: under T-cutting and 50C thermal conductivity / W.On.Kr1 Pipeline perfusion technology has been adopted. Discontinuous production technology requires relatively little capital investment. When producing a large range of pipes of different sizes, it is generally more flexible and only requires mid- to low-level skilled operators. We prefer the top pouring technique, because if the correct inclination is set, the density distribution of the entire pipe it produces is more uniform and good. Continuous production technology is more suitable for manufacturing a large number of pipes of the same size, which can be afforded by the machine Investment in equipment. Continuous production technology can reduce the production cost by reducing the filling density of the pipe, of course, the minimum core density required must be reached. In addition, HDPE can also be thinned The shell thickness to reduce costs.
Huntsman Polyurethanes offers a variety of polyurethane foam systems, which can meet the requirements of special pipeline manufacturing technology. Existing water, HCFC441b and cyclopentane series foaming systems are available, and successfully achieved the scale of industrial production. These products all meet the European standard EN 253. Prefabricated thermal insulation pipes made of these materials are used in district heating systems and as industrial thermal insulation pipes, for example in oil pipelines.
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