Key points of compensator in pipe network layout
In recent years, with the continuous development of urban central heating in my country, compensators, as key components, have been increasingly used in thermal pipe networks. However, if the compensator is improperly used and arranged in the pipe network, it will cause damage to the entire pipe system. damage or even cause serious accidents.
Introduction: In recent years, with the continuous development of urban centralized heating in my country, compensators, as key components, have become more and more widely used in thermal pipe networks. However, if the compensator is improperly used and arranged in the pipe network, it will cause the entire pipe to malfunction. The damage to the system may even lead to a vicious accident. This article is based on the design and layout of compensators in thermal pipe networks and summarizes several experiences based on years of practical experience for the reference of relevant personnel. Keywords: Compensator application issues and reasonable arrangement
Foreword:
Compensators are increasingly used in heating networks due to their compact structure, large compensation capacity, small flow resistance, zero leakage, and no need for maintenance. But it also has shortcomings that are not easy to solve: for example, the axial compensator generates pressure thrust on the fixed bracket, causing the fixed bracket to have a large thrust and thus high cost; in addition, the compensator tube wall is thin and cannot withstand torsion and vibration, and is not safe; equipment investment is high , strict design requirements, high construction and installation accuracy, and a series of shortcomings such as often not reaching the expected life span. In view of these shortcomings of the compensator, and because many design and construction personnel are not fully familiar with the compensator, accidents easily occur during construction and operation. Analyzing the causes of accidents, some accidents are related to the manufacturing quality of the compensator itself or improper material selection, some are construction problems, and a considerable part are design and layout problems. When problems occur in the design, most of them are due to ignorance of the design characteristics of the corrugated compensator pipeline, resulting in calculation errors and unreasonable selection of the compensation piping system.
The main performance of the compensator includes: compensation amount, elastic stiffness, compressive strength, stability, fatigue strength, etc. The general design requirement of the thermal pipe network is to satisfy the strength, stability, and fatigue life. The larger the compensation amount, the better the stiffness value. The smaller the better. The compensator can be combined with additional tie rods, hinges and other accessories with bellows components to form a compensator with various functions. Through different combinations of compensators, various forms of compensation piping systems can be formed to complete the compensation needs of the thermal pipe network. . The compensator combination is divided into axial compensator, angular compensator, and compound tie rod compensator piping system. The angular and compound tie rod compensators are closer to the force form of the natural compensation piping system. There is no need to consider the internal pressure and thrust. Axial compensation is used. Because the device bears large internal pressure, the compensation amount is large. The requirements for concentricity accuracy are high and problems often occur.
The following focuses on some experiences and improvement opinions on the use of axial compensator piping systems.
Basic principles of stress on the compensator bracket:
The axial compensator stress bracket is divided into main fixed bracket, secondary fixed bracket and guide bracket.
Fixed bracket thrust calculation:
The horizontal thrust of the main fixed bracket is composed of the resultant force of three forces:
Internal pressure thrust due to working pressure F=PA:
Where P is the working pressure and A is the effective cross-sectional area. The internal pressure thrust is determined by the effective cross-sectional area and working pressure. The internal pressure thrust is proportional to the working pressure and effective cross-sectional area. Generally speaking, the internal pressure thrust of the compensator is larger.
The elastic force generated by the compensator stiffness PA=KfL
Among them, K is the compensator stiffness, L is the actual elongation of the pipe, f is the coefficient, which is 0.5 when pre-stretched, otherwise it is 1.
Sliding friction reaction force qμl between fixed brackets
Where q is the weight of the pipe, μ is the friction coefficient, and l is the distance from the free end of the pipe to the fixed end.
Main fixed bracket horizontal thrust = internal pressure thrust + friction reaction force + elastic force
If it is not concentric, the bending moment and lateral thrust on the fixed bracket caused by eccentricity will also be included. The horizontal thrust of the main fixed bracket is huge, and the large pipe diameter can reach hundreds of tons. The civil engineering layout is difficult and requires comprehensive structural accounting. It is a heavy-duty bracket.
The secondary fixed bracket has the same force as the main fixed bracket, but the internal pressure and thrust are balanced and offset, and the total thrust is smaller. It is not of the same order of magnitude as the main fixed bracket, and it is an intermediate load-reducing bracket.
When calculating the fixed point thrust, the forces on each side of the fixed point should be calculated separately and then combined. When the directions on both sides of the fixed point are the same, the vector sum of the two forces is used as the fixed point thrust. When the two forces have opposite directions, the force with the larger absolute value minus 0.7 times the force with the smaller absolute value is used as the thrust force at the fixed point.
The guide bracket controls the movement along the movement direction of the pipeline or compensator to ensure that the expansion of the pipe section acts on the compensator and ensures that the pipeline does not become unstable.
The general compensator manufacturer's catalog not only provides detailed descriptions of product specifications, "structural" parameters, but also has application examples, thrust calculations and "general installation requirements," which are relatively comprehensive and can be used as a design basis.
The influence on the compensator during the small displacement of the fixed bracket:
Many piping systems and even directly buried piping systems are arranged in a movable design with fixed brackets and slight thermal displacement. In natural compensation piping systems, the entire piping system participates in compensating deformation, and the deformation of the pipes is relatively uniform. This arrangement makes the piping system It has good integrity, high reliability, and can reduce stress concentration. The situation is quite different in the compensator piping system. Improper handling will have a great impact on the safety of the compensator. A movable design form with small thermal displacement is that the connection between the pipe and the bracket is not welded but is welded and fixed at the root close to the limit baffle. National Standard Atlas 403.022-02 baffle-type fixed bracket There is a big debate about whether the natural compensation piping system should be welded. In addition, directly buried steam pipelines now mostly use steel sleeves and steel internal fixation. This structural method is to reduce the transmission of thermal bridges. Heat, the fixed ring adds insulation materials such as rubber plates between the inner and outer ring plates. The inner and outer ring plates are usually not welded and can move freely. When the fixed bracket is subjected to large force or water shock vibration, a certain amount of displacement will occur, and sometimes longitudinal traces will occur. The displacement produces a torque effect on the compensator, and this displacement has a certain impact on the compensator.
Discussion on the setting position of compensator
According to common practice, axial compensators are arranged close to the fixed bracket, and then followed by two guide brackets, with distances of 4Dg and 14Dg respectively. The main purpose is to prevent axial instability. Directly buried steam pipes rely on insulation materials and The outer steel pipe is used for support or guidance, and the hot water directly buried pipe is mainly formed by the insulation material and is controlled by the soil and sand layer. However, the author believes that this layout method has a good starting point, but in actual application, it is limited by terrain. If there are too many overhead pipe system supports, the layout will be difficult; there are too many underground obstacles for directly buried pipe systems, which may cause excessive bending. , it is required that the compensator can only be arranged in the straight pipe section. This form of locating the compensator on the side of the fixed bracket may cause uneven transmission of the displacement-absorbing capacity of each node of the compensator due to pipeline displacement, and the compensation capacity will not be sufficient. . We believe that solving the problem of axial instability of the compensator is not only related to its layout and setting location, but also mainly depends on the performance and quality of the compensator itself. The performance and quality requirements of the compensator only arranged on the fixed bracket side should be higher. , the pipeline segment distance should generally be smaller. When selecting, be sure to choose a compensator with good guidance and strong anti-instability ability. The design and layout should be based on basic principles and according to the actual situation of the project, and should be handled flexibly. Practice has confirmed , whether it is overhead or directly buried in the trench, as long as the guide structure is well controlled, the compensator can be set at any position of the two fixed brackets.
Problems existing in a design method of directly buried steam pipelines
In order to reduce the number of fixed brackets, the directly buried steam pipeline system is sometimes arranged in a "stationary point" form: there is no fixed point at the midpoint of the pipeline between two adjacent compensators of the same specifications and models in the directly buried pipeline. When the pipeline is heated evenly During expansion, a relative balance point of force, that is, a stagnation point, must be formed between the two compensators. Theoretically, with this point as the boundary, the pipeline expands uniformly in the left and right directions. It is generally believed that the balance point of the force may be slightly offset due to uneven stress on the pipeline. Generally, a 20% margin is used to consider the compensator setting. The author believes that this arrangement is worthy of discussion. Our company has a business unit that built a φ630 directly buried steam pipeline in 1992 and adopted this arrangement. The distance between the fixed brackets is 80 meters. Two compensators have the same specifications and models, both are 120mm. This was carried out in 2000. A section of pipeline was replaced. After disassembly, it was found that one compensator had been flattened and compressed by 200mm. The other compensator not only failed to compensate for the compression, but was stretched by 50mm. The elongation of one compensator caused excessive damage to the other compensator. The compression causes both compensators to fail. The reasons for this situation are more complicated: First, the compensator itself has a large mass deviation. Although the model and specifications are the same, the stiffness value gap is large and cannot be freely compressed; second, due to the quality of pipe processing and installation, it cannot be freely expanded and contracted, and the "stationary" The uneven stress on the pipes on both sides of the "point" fixed bracket causes the stagnation point to deviate greatly and the "stagnation point" is not fixed, making the corrugated compensator unable to withstand it and eventually causing damage. Unless major improvements are made to the compensator itself to ensure that the compensator is uniformly distributed and the compensator stiffness is balanced and consistent, under the condition of using ordinary compensator, only one fixed bracket should be installed between every two fixed brackets in accordance with the US EJMA regulations. Compensator Principle.
Requirements for compensator layout due to pipeline water hammer
Water hammer has a great impact on the compensator. Whether steam pipelines are overhead, underground trenches or directly buried pipelines, there is a water hammer problem. The energy generated by water hammer cannot be released, and ultimately acts on the pipeline insulation structure, brackets, compensators and valves. Water hammer often occurs at elbows or where pipes exit. However, because the pipeline is rigid and has strong resistance to water hammer, the compensator corrugation is a flexible body and cannot withstand the sudden increase in pressure wave impact vibration caused by water hammer, causing damage. Judging from the damaged parts, one is the corrugation, and the other is the diversion sleeve. The weakest link is the corrugation of the compensator. The result of water hammer is that the compensator is deformed or even broken, and the diversion sleeve is toppled or torn, causing serious harm. Pipe network security. Measures to prevent water hammer: In addition to reasonably determining the corresponding pipe diameter based on the heat load, setting up drainage points in a targeted manner, and conducting effective and timely drainage, the design and layout of the compensator should also be improved. It is recommended to move the compensator away from the bend and the upward fixing bracket, and to fix the bracket close to the other side. In this way, even if there is a small amount of water in the pipe, the action position is far away from the compensator, which can greatly reduce the damage caused by water hammer to the compensator. destroy. In addition, using an external pressure compensator and improving the form of the diversion sleeve can also play a certain role in preventing water hammer.
The impact of on-site changes on the compensator:
Sometimes, although the original design of the thermal pipe network is very good, due to obstacles often encountered after construction, the actual site conditions are often very different from the design, and a large number of actual design changes have to be made. As long as the natural compensation pipes are handled properly, there will be no major consequences. However, it has a great impact on the axial compensator pipeline. Many construction units are not fully familiar with this. After the pipeline changes direction, some fixed brackets do not bear the pressure thrust but instead bear the pressure thrust or produce large bends. distance, the stress-bearing structural form of the bracket undergoes major changes. Improper handling can easily push the fixed bracket apart, leading to accidents. Since the degree of specialization of construction units is generally low, the design unit mainly relies on the design unit to control the integrity of the construction's heating network layout. In the case of major pipeline changes, special attention should be paid to whether the stress form of the pipeline complies with the basic principles of compensator layout. Reasonable segmentation ensures that the pipeline is in a straight line, controls the occurrence of inflection points, and reduces the bending moment and lateral thrust acting on the fixed bracket and guide bracket, thereby ensuring the safety and reasonableness of the pipeline system. This is the most important thing for designers. In addition to constantly accumulating experience, they must form clear design ideas in order to improve the level of designing compensator piping systems.
In the design, consider extending the life of the compensator and preventing corrosion:
There are many factors that affect the life of the compensator. One is damage and instability, and the other is corrosion. The theoretical calculation life of the compensator used in urban heating networks is about 6,000-10,000 times, and its safety factor is 15 times. The actual allowable life should be greater than 400 times. If a continuously operating heating network is started about 20 times a year, , its allowable normal life should be more than 20 years. This is not the case in actual applications. They will be replaced within three to five years. There are almost no compensators installed more than ten years ago. There is a famous saying in design that "corrosion begins with the drawings", which requires us to not only The position of the fixed bracket must be reasonable, the distance between the guide brackets must be appropriate, and the guide brackets must have measures to prevent the compensator from becoming unstable. In addition, the design and layout should also consider preventing corrosion, which is often ignored. Through actual inspection, it was found that the compensators arranged in inspection wells or trenches corrode quickly. In particular, the compensators on the water supply pipes in the inspection wells of the hot water pipe network are the most serious, while the return pipes are basically free of corrosion. After analysis, the main reason is that the water supply pipes are corroded very quickly. The return water compensator and pipe sections form a galvanic cell effect and cause electrochemical corrosion. Such problems can be solved through design optimization methods. When arranging compensators, it is particularly important to avoid placing them side by side. If possible, the spacing between compensators should be increased. When laying, it is best to use a fully buried method without inspection wells. Make good marks. If it must be located in an inspection well, it must be waterproof and insulated to prevent sewage and rainwater from entering, reduce corrosive conditions, and block the formation of galvanic cell effect loops.
Consider the hydraulic test plan in advance during the design:
A certain thermal pipeline uses an axial compensator. During the construction, the construction team used segmented compression tests and set up temporary blind plates at selected segment points. The blind plate force did not act on the main fixed bracket, but on the secondary fixed bracket. When increasing the pressure by 1.5 times the test pressure, the fixed bracket will be damaged. When axial type compensator pipeline is pressed in sections, the main fixed bracket that can withstand the hydraulic test pressure should be selected at the segmented point. When this cannot be achieved, the secondary fixed bracket that can withstand the blind plate force at the segmented point should be temporarily reinforced. , enabling it to withstand the blind plate force. Since the thrust force of the primary and secondary fixed brackets is too different, it is difficult to implement segmentation through temporary reinforcement. Therefore, the best way is to consider the hydraulic test or purge plan in advance in the design. The location of the segmented points for suppression is best determined by the owner. , the designer, and the construction unit jointly determine, the design unit is responsible for the technical briefing, and the owner organizes implementation based on the design unit’s opinions.
The impact of construction and installation on axial compensator:
Sometimes the layout of the compensator is unreasonable or the design measures are not taken properly. Deviations can easily occur during construction and installation, causing the force direction to be mainly not the axial force, but the deflection force. The deflection force produces a certain torque on the compensator, and for the axis For compensators, thinner tube walls have poor torque resistance and are easily unstable. Therefore, in order to ensure that the coaxiality tolerance of the piping system at the location where the compensator is installed is at a minimum during construction, it is recommended to lay the pipe section before installing the compensator, then cut a section of the pipe at the location where the compensator is to be installed, and then install the compensator. Weld it up and install it by cutting the pipe. Although a small amount of pipe waste is caused, the concentricity of the pipe can be ensured.