Jiangyin Chengchen Pipe Industry Co., Ltd.

The principle of the siphon roof rainwater system relies on the design of special rainwater heads to separate air from water, thereby keeping the rainwater riser full. As the water in the riser reaches a certain capacity, the siphon effect is initiated. During rainfall, the continuous siphon action enables the entire system to astonishingly quickly drain the roofwater.

1Introduction

1.1 Features of the Siphonic Roof Drainage System

The siphon drainage system operates similarly to a gravity drainage system during the early stages of rainfall when the height of the roof rainwater does not exceed the height of the rain gutters.

As the rainfall continues, when the height of roof rainwater exceeds the height of the rainwater gutters, the scientifically designed vortex prevention rainwater gutters significantly reduce the amount of air carried into the drainage system by controlling the flow rate of rainwater entering the gutters and adjusting the flow pattern to minimize vortices. This results in the drainage pipes being fully flowing. Utilizing the height of the building's roof and the potential energy of the rainwater, as the continuous flow of rainwater passes through the suspended rainwater pipes into the vertical rainwater pipes and falls, an siphon effect is formed, creating the maximum negative pressure within the pipe at that location. Under the suction action of the negative pressure in the pipes, the roof rainwater is discharged to the outside at a higher velocity.

1.2 Differences Between Siphon and Gravity-Based Surface Rainwater Discharge Systems

The siphon roof rainwater drainage system is designed with drainage pipes in a full-flow and pressurized state, allowing the rainwater suspended pipes in the siphon drainage system to be laid without any slope. Moreover, when the siphon effect is generated, the flow velocity within the pipes is high, which results in an excellent self-cleaning function for the system. In contrast, the gravity drainage design does not calculate based on full flow, and the laying slope of the rainwater suspended pipes must not be less than 0.005.

In an siphon drainage system, the discharge flow rate of the drainage pipe is significantly greater than that of the same-diameter pipe in a gravity drainage system. This means that to discharge the same volume of rainwater, the diameter of the drainage pipe in an siphon drainage system is smaller than that in a gravity drainage system.

The siphon drainage system is essentially a multi-pit pressure flow rainwater drainage system. Consequently, the buried pipes required are significantly less compared to gravity drainage systems.

The system is just beginning its domestic application, but internationally, it has nearly two decades of usage history, involving buildings such as terminal buildings (Charles de Gaulle Airport Terminal in France, Hong Kong International Airport Terminal, Zurich Airport Terminal in Switzerland), exhibition centers (Hong Kong Convention and Exhibition Center), stadiums (Copenhagen Football Stadium in Denmark, Sydney Stadium in Australia), industrial factories (Chrysler Factory in Austria, Citroën Factory in France), commercial centers, parking lots, cargo warehouses, office buildings, and more. According to incomplete statistics, there are nearly 40,000 engineering projects using Geberit's siphon drainage system, covering approximately 30 million square meters of roof drainage area.

2System Composition and Operation Status

2.1 Overview

The roof rainwater drainage system typically consists of an siphon rainhead, level suspended pipe, vertical pipe, and rainwater outflow pipe (discharge pipe).

The prerequisite for creating an siphon roof rainwater drainage system is the presence of a rainhead equipped with an effective air-water separator. Under the designed rainfall intensity, the rainhead prevents air from entering, utilizing the pressure difference formed by the height difference between the rainhead and the outflow pipe during the rainfall process. This pressure difference carries the rainwater through the roof's internal drainage system and discharges it through the outdoor outflow pipe. Throughout this process, the drainage pipes are in a full-filled pipe pressure flow state, and the roof rainwater discharge is a result of the siphon effect. Hence, such a system is termed an siphon roof rainwater drainage system.

The siphon rainwater discharge system is characterized by a continuous change in the internal pressure and flow state of water.

Initially, during the early stages of rainfall, the amount of rain is typically light, creating a wave flow with a free liquid surface inside the suspended pipe. Depending on the intensity of the rainfall, in some cases, the siphon effect cannot be formed in the early stages, and the flow is primarily gravity-driven. As the rainfall increases, the pipe gradually exhibits pulsating and pull-push flows, which eventually lead to full-pipe bubble flow and full-pipe steam-liquid mixture flow, until a unidirectional water flow is achieved.

As the end of the rainfall period approaches, the volume of rainfall decreases, causing the water level in the inlet of the rain grate to drop to a specific level (which varies depending on the design of the rain grate), allowing air to enter the grate. This disrupts the siphon action within the drainage pipe, shifting the drainage system from siphon flow back to gravity flow.

Throughout the rainfall process, the pressure and flow state within the suspended pipeline will undergo repeated changes with the increase or decrease of rainfall.

Similar to suspended pipes, the water flow within the vertical pipes transitions from wall adherence to bubble flow, then to air-water emulsion flow, ultimately reaching a near-unidirectional state when the siphon effect is formed.

2.2 Rain Gutter

Generally, the design of the rainhead is a critical factor in the functioning of the entire siphon system. The better its flow stability, the lower the roof water collection height required for the siphon, resulting in superior overall performance.

Standard rain heads consist of a base (PE material), a disc (ASA), and a top grate (PE). Additionally, universal accessories such as insulated bases, fasteners, flanges, welding plates, fire-retardant caps, and miniature heating coils are available upon request.

The pressure flow (siphon) rainwater gullies are made of HDPE, cast iron, or stainless steel. Each part serves a distinct structural function. The gully is positioned within the roof layer, topped with an inlet grille. During rainfall, water enters the gully through the grille cover's side. As the roof collects water to a certain height, the anti-turbulent device within the gully blocks air from entering from the outside while eliminating turbulent conditions, allowing the water to steadily submerge and flow into the drainage pipe. The siphon rainwater gully minimizes the depth of water accumulation in the gutter, reducing the roof's water load to the minimum while enhancing the gully's rated flow rate.

Our leading siphon drainage system products offer partial universality. Their greatest advantage lies in their broad applicability to various roof systems with different functions and materials. In other words, a single rainhead, when combined with the appropriate accessories, can be adapted to different roofs, such as concrete, metal, wooden roofs, roofs designed for pedestrian traffic or greenery, and roofs with trapezoidal shapes. The rainhead is a critical component of the siphon system. For the overall siphon roof drainage system, the key is to prevent air from entering the system through the rainhead. If air directly enters the rainhead, it can form air pockets in the pipes, significantly reducing the system's drainage efficiency, ultimately resembling traditional gravity-based drainage systems.

Therefore, the rain heads used in the siphon roof rainwater drainage system must have a hood with an optimized anti-turbulence function to prevent air from being carried into the entire system through the water flow at the entrance of the rain head, and it aids in forming a water seal to completely block air entry when the water level in front of the head rises to a certain extent.

The design and installation of rain heads also have certain stringent requirements:

The rain head should be at least 1 meter away from the wall.

The distance between rain heads should generally not exceed 20 meters.

(3) If the flat roof is covered with gravel, the thickness of the gravel around the rain gutter grate cover cannot exceed 60mm, and the minimum particle size must be 15mm.

(4) If the rain gutter is installed within the downspout and焊接件is used, the minimum width of the rain gutter should be 350mm. The installation opening for the rain gutter within the downspout should be between 70mm x 270mm and 290mm x 290mm.

(5) If the rainwater pipes are installed within the concrete roof surface, the roof must be at least 160mm thick.

(6) The roof rainwater gutter openings, with a continuous trapezoidal cross-section, must be 280mm × 280mm in size for the installation of fastening components. If the opening exceeds 300mm × 300mm, the roof will require reinforcement.

(7) If the roof is made of concrete, the diameter of the rainwater pipe connected to the rainhead should be at least 35mm (connected with electric welding pipe clamps), corresponding to a roof thickness of 180mm to 190mm.

(8) The roof insulation layer with an isolation layer must be at least 40mm thick. If the isolation layer exceeds 180mm, the base of the rain leader must extend to an appropriate length to connect with a pipe of 56mm diameter.

2.3 System Pipeline

Pipes, as the most critical component of the siphon roof rainwater drainage system, must ensure the system's safety, reliability, and efficient, continuous operation. As a specialized drainage system, the siphon pipes must guarantee complete sealing and comprehensive fire protection measures, while also minimizing noise, absorbing vibrations, withstanding external impacts, and accommodating as much as possible the deformation caused by temperature changes.

Completely impermeable pipes do not necessarily guarantee system密封 integrity. Generally, leakage allowances are permitted within a small range, as long as there are remedial measures in place. However, leaks in siphon systems are often difficult to detect. In the event of a sudden heavy rainfall, it may lead to an immediate collapse of the entire system. Furthermore, if the roof rainwater cannot be discharged in time, it may exceed the roof's load-bearing capacity, causing the roof to collapse.

Certainly, minor non-sealing may not necessarily cause leakage, but it is enough to cause air leakage. Once an air mass forms within the drainage pipes, the efficiency of siphon drainage is immediately greatly reduced, and in severe cases, it can even disrupt the siphon action.

Due to the vacuum-assisted drainage of the siphon system, the pipeline walls must possess adequate pressure-bearing capacity. However, they are not completely rigid. Generally, the vacuum of the siphon system does not exceed -0.08 MPa. Excessive vacuum can lead to rapid water flow inside the pipe, causing cavitation, which can cause significant damage to metal pipelines or metal connections (-0.09 MPa is close to the cavitation threshold). Moreover, overly high vacuum can also result in considerable vibration for the system, reducing its service life.

Pipes and fittings must meet flame-retardant requirements. In the event of a fire in one part of the building, the system is designed to prevent the rapid spread of the fire to other sections. Therefore, the inherent flame-retardant properties of the material are not the most critical; the overall fire spread resistance of the piping system is the key to minimizing disaster losses.

Advantages of HDPE Pipes

Excellent pressure resistance; the pipe wall will not burst under external loads. It can withstand impact pressure, reduce the damage caused by water hammer, ensure the safe operation of the system, and maintain the vacuum required for the siphon action.

Pipe connection methods are convenient and flexible. Pipes can be connected using various methods as needed, such as: butt welding, electrically welded coupling, flange connection, threaded connection, and expansion joints. HDPE pipes can also be connected with other pipe materials like steel pipes, cast iron pipes, and ceramic pipes. Operation can be performed simply by using a specialized heating electric welder.

HDPE pipes are produced under thermal conditions, with the material's tension already reduced during the manufacturing process. Therefore, any minor dimensional changes in the finished product will not pose any harm, minimizing the risks associated with thermal expansion and contraction.

HDPE pipes exhibit exceptional corrosion resistance, unaffected by various acidic, alkaline, and saline-induced electrochemical reactions. They are more wear-resistant than metal pipes and can withstand extreme temperatures ranging from -400°C to 1000°C. The lightweight nature of the pipes facilitates easy construction, allows for pre-fabrication, and significantly boosts installation efficiency.

HDPE pipes, as a new type of energy-saving material, hold significant development potential in China's construction industry, given the current trends towards residential industrialization, standardized design, material intensification, factory-based construction, and scientific management.

2.4 Auxiliary Fixed System

The primary function of the installation fixing system is to assist in the installation and fixation of pipelines.

The fixed devices of the siphon rainwater pipe system include parallel square steel guide rails, pipe clips connecting the pipes to the square steel guide rails (installed at intervals of 0.8 to 1.6 meters depending on pipe diameter), hangers for securing the steel guide rails, and galvanized angles. The installation system also includes pipe clip accessories, which can fix the pipe's axial direction and utilize anchoring pipe clips to secure points on the pipe.

In the drainage process of the carbonated beverage mix flow, a crucial requirement is the limitation of vacuum pressure within various parts of the system, stipulating that the vacuum pressure must not fall below -0.8 kg. The reason is that when the vacuum pressure is around -0.92 kg, the bubbles within the system will burst under pressure, causing the entire pipeline and system to experience severe vibrations.

Therefore, to ensure the smooth operation of the system, the hazards of pipeline vibration are an issue that cannot be overlooked. If vibrations are not prevented, they may affect the lifespan of the building structure and could even lead to the destruction of the entire system. One of the main functions of the installation of the fixing system is to absorb these vibrations, thereby avoiding their impact on the building structure.

Due to temperature changes, pipelines will inevitably experience thermal expansion and contraction. This creates tension or pressure within the system, affecting the pipeline joints.

The installation of a fixed system can prevent damage to the building structure caused by forces resulting from thermal expansion and contraction in rigidly installed exhaust systems. It absorbs pipe displacement due to thermal expansion and contraction. Additionally, it can also prevent deformation of pipes caused by suspended forces.

Whether it's external forces from system vibrations, internal forces from thermal expansion and contraction, or even the gravity on suspended pipes, all are transmitted to the square guide rail through the connecting components, thus avoiding system changes and minimizing the impact on the building structure.

The fixed system not only serves to secure pipelines and transfer pipeline forces but also aids in increasing the distance between the roof and horizontal pipelines without affecting the horizontal force on the pipelines.

In summary, while the fixed system is an auxiliary part of the siphon rainwater drainage system, it plays a crucial protective role.

3Technical Conditions

3.1 Continuous Fluidity of Water

Maintaining the continuous flow direction is crucial for the siphon effect under conditions where the flow rate is greater than or equal to 0.7 m/s. Particularly when the pipe bend angle is relatively large, even up to 90 degrees, the siphon effect may be disrupted due to a sudden decrease in the flow rate within the pipe.

Therefore, when the water flow changes direction by 90 degrees, the connection method of the elbow here must ensure the design of a transition section to maintain the flow rate from dropping suddenly and sharply, instead keeping it ascending, thereby ensuring the smooth operation of the entire siphon roof rainwater drainage system.

When a 90oT branch pipe appears in the system, if the water flow in the horizontal pipe suddenly encounters an obstacle at a high speed, its velocity drops to zero in an extremely short time. This results in a significant impact on the pipe wall, and on the other hand, the water, after colliding with the wall, rapidly reverses direction and forms a backflow within the pipe. As a result, the two opposing water streams collide within the pipe, which can easily lead to a water plug, obstructing the drainage pipe's discharge and disrupting the siphon effect.

Therefore, it is necessary to use relatively large pipe diameters, with specific choices depending on the space and environmental conditions of the pipeline. The best hydraulic option is still to design a transition section that avoids a 90-degree change.

The presence of a gas-water mixed flow

When siphon action forms within the system pipeline, due to the pipe diameter available for use not necessarily being the exact calculated size, there are often many small bubbles dissolved in the water within the pipeline, which is not a completely idealized single-phase fluid flow. These tiny bubbles gradually release during the flow process; however, this gas-water mixture flow, rather than a two-phase gas-water flow, can still be considered a permissible state for siphon action, as it does not affect the formation of siphon action nor the drainage capability of the system.

However, bubbles dissolved in water do not equate to gas pockets within the pipes. If within a drainage pipe, the middle section contains gas pockets while the wall section has flowing water, this represents the flow state in a traditional gravity-based rainwater drainage system. The presence of gas pockets within the pipe severely affects the formation of full flow within the pipe during the siphon action, resulting in a low degree of water filling within the pipe, significantly reducing the system's drainage capacity.

3.3 System Integrity and Sealing

To ensure the generation and continuous effect of siphon drainage, the entire drainage system from the rain head to the pipeline must be integrated, with all parts tightly connected.

If the rain gully has an entirely open entrance, the air will be drawn into the entire drainage system by the rotational force of the water flow, rendering it impossible to form a full-flow siphon state. Consequently, the system is no longer an efficient siphon drainage system; instead, it is actually functioning as a traditional gravity drainage system.

However, to achieve better emission effects, the gravity-based discharge system requires a minimum slope of 2% for the suspended pipeline during installation. In contrast, the suspended pipeline of the siphon system has a slope of zero, lacking the effect of gravitational potential energy, rendering the entire system ineffective for drainage.

Thus, air can only be effectively prevented from entering the system at any time when the inlet of the storm drain is semi-open. When the water depth in front of the pit meets certain requirements, a water seal can form, completely blocking air, and quickly create a siphon effect.

In addition to effectively blocking air from entering at the entrance, it is also crucial to ensure that no air enters the system pipes. Consequently, another requirement is the system's complete密封性, ensuring there are no leaks in the pipes.

Due to this, rubber sealing rings cannot be used for component connections; instead, a socket-and-spigot method should be employed (see Figure 9-1). This makes it difficult to effectively ensure the system's airtightness, which can easily lead to pipeline leaks. Because during the siphon action, the fluid inside the pipeline is in a pressure flow state, on one hand, the pipe walls are under pressure, and the socket and spigot joints are also subjected to pressure, making leaks more likely; on the other hand, once a leak occurs, the pressure state inside the pipe changes, affecting the normal siphon action.

3.4 Roof Water Level

The entire system only functions as a siphonic rainwater drainage system when the roof water level reaches a certain point (which varies depending on the specific rainhead product).

During a continuous rainfall event, the water level initially remained below the height required to initiate the siphon effect. As the water level rose gradually, once it reached this specific threshold, the system began to form a siphon. The water continued to rise until the amount of rainfall on the roof was less than the drainage capacity of the siphon system.

However, water levels must be strictly controlled and limited to a certain height; otherwise, accumulated rainwater on the roof can exert an unforeseen load on the roof, potentially leading to structural deformation or damage, or even leakage.

According to European standards, the water level height of roof rainwater must be limited to 55 millimeters. This figure is the result of long-term experiments and practical engineering experience.

Convert millimeter water volume to the weight of rain per square meter:

It is evident that the load on the roof is related to the depth of water in millimeters. Clearly, when the water level exceeds 55 millimeters, it imposes a substantial weight load on the roof structure. This aspect must be taken into account during the design of the roof or gutter.

Especially for gutters, the water level must absolutely not exceed 55 millimeters; otherwise, over time, the gutters will gradually deform, which can have a significant impact on the drainage system and the entire building.

4Tech Development

4.1 Gravity Flow Technology

Currently, the majority of roofs in China still use gravity drainage technology. Its advantages include ease of design and construction, and low cost. However, with the continuous development of construction technology, this method is increasingly difficult to meet the requirements for drainage on complex structures or large-area roofs.

In this context, the pressure flow technology has emerged.

4.2 Pressure Flow (Siphon) Technology

4.2.1 Gravitational-Pressure Flow

This technology utilizes an immersed rainfall collector with a deeper water depth before the collector; the flow pattern is calculated as a single-phase flow, disregarding aeration factors. The suspended pipe is horizontally installed, and pressure balancing calculations are conducted at the junction points where the pipes converge. However, the head loss calculations primarily focus on the along-the-line head loss. Due to the pressure zero point at the rainwater riser, the upper section of this riser is also under negative pressure. The actual flow in the piping system is a gravity-pressure flow. The entire system imposes higher requirements solely on the rainfall collectors.

Due to the fact that the calculation does not fall under the category of precise calculation, the efficiency of the siphon is relatively low, the system requires a high load on the roof, the stability of operation is low, and the lifespan of the system is difficult to guarantee. It belongs to the early siphon technology.

4.2.2 Siphon - Pressure Flow

This is the most advanced siphon technology currently available internationally.

This technology employs a forced siphon rainhead with a deeper water level before the head. The computed flow regime is a mixture of vapor and water, taking into account aeration factors, making it highly accurate in real-world scenarios. The suspension is horizontally installed, with full system pressure balance calculations typically performed using computer software. The material, roughness, and equivalent length of pipe fittings are key focus points in the calculations. Siphoning is triggered at certain instants. This technology demands a high level of integrity and calculation accuracy from the system, which is directly related to a vast amount of experimental and engineering experience data.

The system demonstrates high efficiency in siphon formation, with minimal roof load requirements. It boasts high operational stability and ensures a sufficiently long lifespan. It represents mature siphon technology.

Our company offers a total of ten specifications of rainheads, including models I, II, and III, all of which meet the requirements for concrete and metal structural roofs. After comprehensive evaluation by our independently designed and installed large-scale siphon rainwater demonstration and testing system, their performance has reached the advanced level of similar foreign products. Additionally, the establishment of this system has filled a technical gap in the domestic market.

The unique mechanical fixing method (similar to a flange structure) between the rain head and the roofing waterproofing layer fills a technological gap both domestically and internationally, comprehensively solving the waterproofing issue and achieving a perfect integration of rainwater drainage and roofing waterproofing.

A unique rectifier design allows the system to form a siphon with extremely low forefoot water depth.

The Douti body boasts an aesthetically pleasing and durable design, high mechanical strength, and easy construction.

Our company's rain head has been granted a national patent, and all models of the rain heads have passed the national testing authority's inspections.

5Design Standards

"Code for Design of Water Supply and Drainage in Buildings, GB50015-2003"

"Rain Harvesting Diverter" CJ/T245-2007

"Technical Code for Rainwater Utilization in Construction and Community Engineering - GB50400-2006"

"Code for Rainwater Drainage of Siphon Roof Technology: CECS183:2005"

6Engineering Standards

"High-Density Polyethylene (HDPE) Pipes and Fittings for Building Drainage - CJ/T250-2007"

"Unwelded Steel Tubes for Fluid Conveyance - GB/T12771-2000"

Welded Steel Pipe for Low-Pressure Fluid Transportation, GB/T3091

"Plastic Coated Steel Pipe for Water Supply" GB/T 120-2008

"Technical Code for Construction Water Supply Steel-Plastic Composite Pipe Pipeline Engineering - CECS125-2001"

7Acceptance Criteria

"Code for Acceptance of Construction Quality of Building Water Supply, Drainage, and Heating Engineering - GB50242-2002"