Introduction of siphon system
1. 1 characteristics of siphon roof drainage system
At the beginning of rainfall, the siphon drainage system works in the same way as the gravity drainage system under the condition that the height of rainwater on the roof does not exceed the height of rainwater bucket.
With the continuous rainfall, when the height of rainwater on the roof exceeds the height of rainwater hopper, the scientifically designed anti-vortex rainwater hopper is adopted. By controlling the flow rate of rainwater entering the rainwater hopper, the flow pattern is adjusted and the vortex is reduced, thus greatly reducing the amount of air entrained by rainwater entering the drainage system and making the drainage pipeline in the system in a full circulation state. Using the height of the building roof and the potential energy of rainwater, the siphon effect is formed when rainwater continuously flows into the rainwater riser and falls, and the maximum negative pressure appears in the pipeline. Roof rainwater is discharged to the outdoor at high speed under the suction of negative pressure in the pipeline.
1.2 differences between siphon and gravity and ground rainwater drainage system
The drainage pipe of siphon roof rainwater drainage system is designed according to the full flow and pressure state, so the rainwater suspension pipe in siphon drainage system can be laid without slope. At the same time, when the siphon effect occurs, the water velocity in the pipeline is very high, so the system has good self-cleaning effect. However, the design calculation of gravity drainage is not based on full flow, and the laying slope of rainwater suspension pipe should not be less than 0.005.
The drainage capacity of siphon drainage system is much larger than that of gravity drainage system with the same diameter, that is to say, excluding the same rainwater flow, the diameter of siphon drainage system is smaller than that of gravity drainage system.
Siphon drainage system is essentially a multi-bucket pressure flow rainwater drainage system. Therefore, compared with the gravity drainage system, the buried pipes are obviously reduced.
At present, the application of the system in China has just begun, while the system has been used internationally for nearly 20 years, involving buildings such as terminals (Charles de Gaulle Airport Terminal, Hong Kong New Airport Terminal and Zurich Airport Terminal), exhibition halls (Hong Kong Convention and Exhibition Center), stadiums (Copenhagen Football Stadium in Denmark and Sydney Stadium in Australia), industrial plants (Chrysler Automobile Factory in Austria and Citroen Automobile Factory in France), commercial centers, parking lots, freight warehouses and office buildings. According to incomplete statistics, there are nearly 40,000 projects using Geberit siphon drainage system, and the roof drainage area is about 30 million m2.
2. System composition and working conditions
2. 1 Summary
Roof rainwater drainage system is generally composed of siphon rainwater hopper, non-gradient suspension pipe, riser and rainwater outlet pipe (drainage pipe).
The precondition of siphon roof rainwater drainage is that there must be a rainwater hopper with a good gas-water separation device. Under the design rainfall intensity, the rainwater hopper is not mixed with air, and the pressure difference formed by the height difference between the rainwater hopper and the outdoor pipeline is used to be discharged from the outdoor exhaust pipe through the drainage system in the roof during the rainfall. In this process, the drainage pipeline is full of pressure flow, and the drainage process of roof rainwater is the result of siphon action. Therefore, this system is called siphon roof rainwater drainage system.
The pressure and water flow state in siphon rainwater drainage system are constantly changing processes.
At the beginning of rainfall, the rainfall is generally small, and there are waves with free surfaces in the suspension pipe. According to the different rainfall, in some cases, siphon cannot be formed in the initial stage, and it is a flow pattern dominated by gravity flow. With the increase of rainfall, pulsating flow and pulling flow gradually appeared in the pipe, and then the mixed flow of bubble flow and soda water appeared in the pipe until the unidirectional flow of water appeared.
At the end of rainfall, the amount of rainwater decreases, and the water level in front of the rainwater hopper drops to a certain value (depending on the design of different rainwater hopper products). The rainwater bucket gradually began to mix with air, and the siphon effect in the drainage pipe was destroyed, and the drainage system changed from siphon flow state to gravity flow state.
During the whole rainfall process, with the increase or decrease of rainfall, the pressure and water flow state in the suspended pipe will change repeatedly.
Similar to the suspended pipe, the water flow in the riser will gradually transition from wall-attached flow to bubble flow and gas-water floating flow, and finally it will be close to one-way flow when siphon is formed.
2.2 rain bucket
Generally speaking, the design of rainwater hopper is one of the keys to whether the whole siphon system can work according to the design requirements. The better its flow stability, the lower the height of roof water collection required to produce siphon, and the better its overall performance.
The standard rain bucket consists of a rain bucket base (PE material), a disc (ASA) and a grille top cover (PE). In addition, we can provide general insulating base, fixing parts, flange parts, welding parts, fire protection caps, miniature heating coils and other accessories as required.
The pressure flow (siphon) rain bucket is made of HDPE, cast iron or stainless steel. Its various parts have different structural functions. The rain bucket is placed on the roof layer, and the upper part is covered with a water intake grille. When it rains, the rainwater enters the rainwater bucket through the side of the grille cover. When the roof water collection reaches a certain height, the vortex prevention device in the rainwater hopper will prevent the air from entering from the outside, and at the same time eliminate the vortex state, so that the rainwater can overflow smoothly and be discharged into the drainage pipe. Siphon rain bucket minimizes the depth of water accumulation in the gutter, minimizes the rain load on the roof, and improves the rated flow of the rain bucket.
At present, the leading products can be partially universal. Its greatest advantage lies in its wide applicability to roof systems with different functions and materials. In other words, a rain bucket can be applied to different roofs through the combination of corresponding accessories, such as concrete roofs, metal roofs, wooden roofs, roofs considering people walking or greening, and uneven trapezoidal roofs. Rainwater bucket is the key part of the whole siphon system. For the whole siphon roof rainwater drainage system, the most important thing is to prevent air from entering the whole system through the rainwater hopper. If the air directly enters the rain bucket, it will form an air mass in the pipeline, which will greatly reduce the drainage efficiency of the system, and finally it will be the same as the traditional gravity drainage system.
Therefore, the rainwater hopper used in the siphon roof rainwater drainage system must have an optimized cover with vortex prevention function to prevent air from being brought into the whole system through the water flow at the entrance of the rainwater hopper, and help to form a water seal when the water level in front of the hopper rises to a certain extent, completely blocking the air from entering.
There are also some strict requirements for the design and installation of rain buckets:
(1) The rain bucket shall be at least 1 m away from the wall.
(2) The spacing between rain buckets is generally not more than 20m.
(3) If it is a gravel layer on a flat roof, the thickness of the gravel around the top cover of the rainwater hopper grille should not be greater than 60mm, and the minimum particle size should be15 mm. ..
(4) If the rain bucket is installed in the gutter and welded parts are used, the width of the gutter is at least 350 mm, and the installation opening of the rain bucket in the gutter is 70 mm× 270 mm to 290 mm× 290 mm. ..
(5) If the storm sewer is installed on the surface of a concrete roof, the thickness of the roof should be at least160mm.
(6) The opening of the roof rainwater hopper with continuous trapezoidal section must be 280mm × 280mm, so as to install the fasteners. If the opening is larger than 300mm ×300mm, the roof needs to be reinforced.
(7) If the roof is made of concrete, the diameter of the rainwater pipe connected under the rainwater bucket shall be at least 35mm (connected by welded pipe hoop joints), and the corresponding roof thickness shall be 180mm to 190mm.
(8) The insulation layer thickness of the roof with insulation layer shall be at least 40mm. If the thickness of the isolation layer is greater than 180mm, the base of the rain bucket must be extended by an appropriate length to connect with the connecting pipe with a diameter of 56 mm
2.3 system piping
As the most important part of siphon roof rainwater drainage system, pipeline must ensure the safe, reliable, efficient and continuous operation of the system. Siphon system, as a special drainage system, must ensure complete sealing and complete fire prevention measures, and reduce noise, absorb vibration, resist impact external force and meet the deformation caused by temperature change to the greatest extent.
The fact that the pipeline is completely leak-proof does not mean that the system tightness is satisfied. Generally speaking, the requirement of leakage prevention is to allow a small range of leakage, as long as there are remedial measures. But once the siphon system leaks, it is not easy to find it. When there is a sudden rainstorm, the whole system may collapse immediately. Furthermore, because the rainwater on the roof can't be discharged in time, it exceeds the tolerable strength of the roof, which leads to the collapse of the roof.
Of course, a small unsealing may not necessarily cause air leakage, but it is enough to cause air leakage. Once the air mass appears in the drainage pipeline, the efficiency of siphon drainage will be greatly reduced, and even the siphon effect will be destroyed seriously.
Because the siphon system uses negative pressure to drain water, the wall of the pipeline must have considerable bearing capacity. But it is not a completely rigid object. Because the negative pressure of siphon system is generally not greater than -0.08Mpa, excessive negative pressure will lead to excessive water flow velocity in the pipe and cavitation, which will cause great damage to metal pipes or metal joints (-0.09Mpa is close to the critical value of cavitation). At the same time, too high negative pressure will also bring great vibration to the system and reduce the service life of the system.
Pipes and fittings must have flame retardant conditions. When a fire breaks out in one part of a building, the system can prevent the fire from spreading quickly to other parts of the building. Therefore, the flame retardancy of the material itself is not the most important, and the fire diffusion of the whole pipeline system is the key to minimize disaster losses.
HDPE pipe has good pressure-bearing performance, and the pipe wall will not break under external load. It can resist the impact pressure, reduce the impact damage of water hammer, ensure the safe operation of the system and maintain the negative pressure of rainbow action.
The pipeline connection mode is convenient and flexible. Different methods can be used to connect pipes according to needs, such as butt welding, electric welding pipe hoop connection, flange connection, threaded connection, telescopic pipe joint, etc. HDPE can also be connected with steel pipes, cast iron pipes, ceramic pipes and other pipes. It can only be operated by a special heating welder.
HDPE pipeline is produced under hot conditions, and the tension of the material itself has been reduced in the manufacturing process, so the possible slight size changes after the finished product will not cause any harm, and the harm caused by thermal expansion and cold contraction will be minimized.
In terms of physical and chemical properties, HDPE pipeline has strong anti-corrosion ability and is not affected by electrochemical reactions caused by various acids, alkalis and salts. Pipes are more wear-resistant than metal pipes. The extreme temperature resistance is–400 C ~1000 C. The pipe is light in weight, convenient to construct and can be prefabricated in advance, which greatly improves the installation efficiency.
As a new type of energy-saving pipe, HDPE pipe has great development potential from the development trend of housing industrialization, design standardization, material intensification, factory construction and scientific management in China's construction industry.
2.4 Auxiliary Fixation System
The main function of the installation and fixing system is to assist the installation and fixing of pipelines.
The fixing device of siphon rainwater pipeline system includes a square steel guide rail parallel to the pipeline, and connecting pipe clamps between the pipeline and the square steel guide rail (pipe clamps are set every 0.8-1.6m according to different pipe diameters) for fixing the hanger of the steel guide rail and galvanized angle steel. The installation and fixing system also includes pipe clamp fittings, which can fix the axial direction of the pipeline and be installed at the fixed point of the pipeline by anchoring the pipe clamp.
In the process of hydrophobic steam-water mixed flow, there is a very important requirement, that is, the negative pressure of each part of the system should not be less than -0.8 kg. The reason is that when the negative pressure is about -0.92 kg, the bubbles in the system will burst under the pressure, which will cause the whole pipeline system to vibrate violently.
Therefore, in order to ensure the normal operation of the system, the harm of pipeline vibration is a problem that can not be ignored. If vibration is not prevented, it may affect the service life of the building structure and may also lead to the destruction of the whole system. One of the main functions of the installation and fixing system is to absorb these vibrations, so as to avoid the influence of vibration on the building structure.
Due to the change of temperature, the pipeline will inevitably expand with heat and contract with cold. Tension or pressure is formed inside the system and acts on the joint of the pipeline.
Installing a fixed system can prevent the damage to the building structure caused by the blocking force of thermal expansion and cold contraction in the rigidly installed drainage system, and absorb the pipeline displacement caused by thermal expansion and cold contraction. At the same time, it can also avoid pipeline deformation due to suspension stress.
No matter the external force caused by the vibration of the system, the internal force caused by thermal expansion and contraction, or even the gravity borne by the suspension pipeline, it is transmitted to the square guide rail through the connector, so as to avoid the change of the system and reduce the impact on the building structure.
The fixing system can not only fix the pipeline and transmit the force of the pipeline, but also help to increase the distance between the roof and the horizontal pipeline without affecting the horizontal force of the pipeline.
In a word, although the fixed system is an auxiliary part of the siphon rainwater drainage system, it plays a vital role in protection.
3. Technical conditions of siphon roof rainwater drainage system
3. 1 continuous fluidity of water
Under the condition that the flow velocity is greater than or equal to 0.7m/s, it is the key to maintain the siphon effect to ensure the continuous fluidity in the direction of water flow. Especially when the turning angle of the pipeline is relatively large, even 90, it is very likely that the siphon effect will be destroyed because of the sudden drop of the flow velocity in the pipeline.
Therefore, when the water flow changes in the direction of 90, it is necessary to design a connecting pipe section in the elbow connection mode here to ensure that the flow will not suddenly drop sharply, but keep rising, so that the whole siphon roof rainwater drainage system can operate normally.
When the 90 T branch pipe appears in the system, when the water in the horizontal pipe rushes to the pipe wall at a faster speed, it suddenly encounters obstacles and the speed drops to zero in a very short time. On the one hand, the impact on the pipe wall is great, on the other hand, the water flow quickly forms a backflow in the pipe with a speed opposite to the initial direction after hitting the pipe wall. In this way, two water flows in opposite directions collide in the pipeline, which is easy to form a water plug, hinder the drainage of the drainage pipe and destroy the siphon effect.
Therefore, a relatively large pipe diameter must be adopted, and the specific situation can be selected according to the space and environmental conditions of the pipeline. The best choice of hydraulic situation is to design a joint pipe section to avoid 90 changes.
3.2 The existence of gas-water mixed flow
When the siphon phenomenon occurs in the pipeline of the system, because the diameter of the available pipeline is not completely the calculated diameter, there will be many small bubbles dissolved in water inside the pipeline, which is not a completely idealized liquid single-phase flow. These tiny bubbles will be gradually released during the flow. However, the flow pattern of gas-water mixed flow rather than gas-water two-phase flow can still be regarded as a state of allowing siphon, which does not affect the formation of siphon and the drainage capacity of the system.
However, bubbles dissolved in water do not mean that there are air masses in the pipeline. If the middle part of the drainage pipeline is air mass and the part along the wall is water flow, this is the flow pattern in the traditional gravity rainwater drainage system. The existence of air mass in the pipeline seriously affects the formation of full flow state in the pipeline under the action of siphon, and the water fullness in the pipeline is quite low, which greatly reduces the drainage capacity of the system.
3.3 System integrity and sealing
In order to ensure the generation and continuous function of siphon drainage, it is required that the whole drainage system from the rainwater hopper to the pipeline system must be integrated and the parts are closely connected.
If the rainwater hopper has a completely open entrance, air will flow out of the entrance and enter the whole rainwater drainage system under the rotation of the water flow, so that the siphon state of the whole flow can not be formed at all, and the whole system is no longer an efficient siphon drainage system, but actually has been working as a traditional gravity drainage system.
However, in order to achieve good drainage effect, the gravity drainage system requires that the minimum slope of the suspended pipe should be 2% when installing the pipeline. However, the installation slope of suspension pipe in siphon system is zero, and there is no gravitational potential energy, so the whole system can not effectively drain water.
Therefore, only when the inlet of the drain is half open can air be effectively prevented from entering the system at any time. When the water depth in front of the bucket reaches a certain requirement, a water seal can be formed, which completely isolates the air and quickly forms a siphon effect.
In addition to effectively preventing air from entering the inlet, it is also necessary to ensure that no air enters the system pipeline. Therefore, another requirement is the complete sealing of the system to ensure that the pipeline has no leakage.
For this reason, rubber sealing rings cannot be used when connecting accessories, they are connected through sleeves. In this way, it is difficult to effectively ensure the air tightness of the system and it is easy to cause pipeline leakage.
Because the pipe flow in the pipeline is in the state of pressure flow when siphoning, on the one hand, the pipe wall is compressed, and the socket is also compressed, which is easy to leak; On the other hand, once leakage occurs, the pressure state in the pipe changes, which affects the normal siphon effect.
3.4 Roof water level
Only when the roof water level reaches a certain level (with different fixed values according to different rainwater hopper products) can the whole system really play the role of siphon rainwater drainage system.
In a certain continuous rainfall process, the initial water level is lower than the height of siphon. As the water level gradually rises, the system begins to form siphon after reaching this specific value. The amount of rainwater that the water level continues to the roof is less than the drainage capacity of the siphon system.
However, the water level must be strictly controlled and limited to a certain height, otherwise the rainwater accumulation on the roof will form a great unpredictable load on the roof, which may lead to deformation or damage of the roof structure and even leakage.
According to European standards, the level of rainwater on the roof must be limited to 55 mm, which is the result of long-term experiments and practical engineering experience. Millimeters of water can be converted into the weight of rain per square meter.
Therefore, the relationship between the load borne by the roof and the millimeter water depth. Obviously, when the water level is greater than 55 mm, it will produce considerable weight load on the roof structure. This situation must be taken into account when designing roofs or gutters.
Especially in the gutter, the water level must not exceed 55 mm, otherwise, with the passage of time, the gutter will slowly deform. It has a great impact on the drainage system and the whole building.
4. Development of roof drainage technology
4. 1 gravity flow technology
At present, most roofs in China still use gravity flow technology to drain water. Its advantages are convenient design and construction and low cost. However, with the continuous development of building technology, it is increasingly difficult for this technology to meet the drainage requirements of complex structures or large-area roofs.
In this context, pressure flow technology came into being.
4.2 Pressure Flow (Siphon) Technology
Gravity pressure flow
This technology adopts a sunken rainwater bucket with water depth before the bucket; The calculated flow pattern is single-phase flow, regardless of gas permeability. The suspension pipe is installed horizontally, and the pressure balance is calculated at the joint of the pipe, that is, the confluence point, but the calculation of head loss is mainly based on the head loss along the way. Because the pressure in the rainwater riser is zero, the upper part of this riser is also in a negative pressure state. The actual flow pattern in the pipeline system belongs to gravity-pressure flow. The whole system only requires more rain buckets.
Because the calculation does not belong to the category of accurate calculation, the siphon power generation efficiency is low, the system requires a large load on the roof, the working stability is low, and the system life is difficult to guarantee. Belonging to the early siphon technology.