Technical AdvantageEditor
The technical advantages of the pressure flow roof rainwater drainage system are evident. During the rain process, the rainwater collected from the roof slopes towards the gutters. Traditional rainwater drainage involves channeling the collected water from the gutters through rainwater hoppers, vertical pipes, and discharge pipes to rainwater inspection wells, or through rainwater hoppers, suspended pipes, vertical pipes, and discharge pipes to rainwater inspection wells. Generally, roof drainage systems are categorized into external and internal drainage systems based on the location and direction of their discharge pipes. From a hydrodynamic perspective, they can be divided into gravity flow roof drainage and pressure flow roof drainage systems. The latter emphasizes the pressurized state within the roof drainage system under the design rainfall intensity. Different roof rainwater drainage systems adopt various design calculation methods based on their flow state analysis. Traditional roof rainwater drainage systems are designed based on gravity flow, using gravity-type rainwater hoppers, which result in free weir flow, with rainwater seeping into the air to form a water-air mixture. The design flow rate of the rainwater hoppers is usually small. Suspended pipes calculated for gravity flow require less than an 80% fill rate and a slope greater than 5‰, necessitating larger pipe diameters and gradients. To ensure the proper functioning of rainwater hoppers connected to the same suspended pipe, it is limited to no more than four, thereby increasing the number of vertical pipes. The gravity flow roof rainwater drainage system, due to its hydraulic characteristics, results in many vertical pipes, large diameters, and limited drainage capacity, which is more pronounced for large industrial and public building roofs. The pressure flow roof rainwater drainage system, using pressurized rainwater hoppers, significantly improves drainage capacity. Under hydrodynamic calculations, the number of rainwater hoppers connected to suspended pipes is not limited, thereby reducing the number of vertical and buried pipes. Suspended pipes do not require slopes, making installation convenient and aesthetically pleasing. The system, calculated based on pressure flow, can reduce the diameter of the chosen pipes. Since the length of a single suspended pipe can reach 150 meters, the main vertical pipe can be close to the exterior wall, allowing for the avoidance of pipeline shafts inside the building, and the non-burial of pipes. This is particularly suitable for places with many underground pipes or where it is not advisable to set up wells. It is clear that the pressure flow roof rainwater drainage system has a significant technical advantage over the gravity flow system.
Working PrincipleEditor
The working principle of the pressure flow roof rainwater drainage system consists of a pressure flow rainwater inlet, rainwater suspended pipe, rainwater riser, buried pipe, and rainwater outflow pipe. The pressure flow rainwater inlet has excellent straightening functions, preventing air from seeping in during the design rainfall intensity. During the rainfall, it acts like a small pool of stable water surface on the roof draining downwards, passing through the roof's internal drainage system and exiting via the discharge pipe. The pipeline is in a fully filled pressure flow state, making the roof rainwater drainage process an siphon drainage process. Therefore, it is quite appropriate to call the roof rainwater drainage system with siphon drainage capability a pressure flow rainwater drainage system. The pressure and flow state of the pipeline within the pressure flow roof rainwater drainage system change. Initially, with less rainfall, the suspended pipe contains a wave flow with a free surface. As the rainfall increases, the pipe exhibits pulsating and pulling flows, eventually leading to full pipe bubble flow and full pipe air-water emulsion flow, until reaching a single-phase flow state. As the rainfall decreases at the end, the water level in the rainwater inlet drops to a certain value, allowing air to seep in, disrupting the vacuum in the drainage pipe, and shifting the drainage system from siphon flow to gravity flow. Throughout the rainfall, the pressure and flow state within the drainage pipe change repeatedly with the variation in rainfall. The hydraulic analysis of the pressure flow roof rainwater drainage system is shown in Figure 1.
Equation (1) lists Bernoulli's equation for the sections 2-2 and x-x: H + P2/γ + v2^2/(2g) = Hx + Px/γ + vX^2/(2g) + h2-x (1). The right side of the equation consists of four terms: the first is the position head, the second is the pressure head, the third is the velocity head, and the fourth is the head loss between the two sections. Since P2 = 0, v2 = 0, γ = 1, and hx = H - hx, substituting these into equation (1) yields Px = hx - vX^2/(2g) - h2-x (2), where Px is the pressure head at the pipe section x. Equation (2) is the basic formula for calculating the pressure head at any section within the pipe, with its physical meaning being that the pressure head at any point in the pipe equals the difference in height between that point and the rain gully, minus the velocity head at that point and the total head loss from that point to the rain gully. By changing the section x to different positions within the pipe system, the full system's pressure can be clearly identified; the connecting pipe below the rain gully exhibits slight negative or positive pressure within the pipe. On the suspended pipe, as the section x moves from the farthest end of the suspended pipe towards the vertical pipe, the head loss within the pipe increases, while the available head loss remains constant, resulting in a continuously increasing negative pressure within the pipe, with the maximum negative pressure occurring at the intersection with the vertical pipe. Subsequently, from the intersection point between the vertical and suspended pipes downwards, the available head increases rapidly, far exceeding the head loss due to the increased pipe length, and the negative pressure within the vertical pipe also decreases quickly to zero. This is followed by a gradual increase in positive pressure, reaching a maximum at the bottom of the vertical pipe before gradually decreasing. At the point of connection with the drainage well, the pressure within the pipe returns to atmospheric pressure, and the flow state transitions to gravity flow. From the above analysis, it can be concluded that the effective operating height is from the water level of the rain gully to the critical total height, which should be fully utilized in design calculations. On the other hand, the total head loss from the rain gully to the end of the suspended pipe should be limited to control the maximum negative pressure at the end of the suspended pipe.
Siphon drainage system
The siphon drainage system, a negative pressure or pressure drainage system, is widely used in large-span buildings with complex structures, such as stadiums and factories. It is an effective solution for roof drainage.
The Drainage Principle: Utilizing the height of the building's roof to give rainwater potential energy, thus creating a siphon effect during full pipe flow. This results in the maximum negative pressure at the point where the rainwater suspended pipe transitions into the rainwater vertical pipe. Under the suction action of the negative pressure within the pipe, the roof rainwater can be drained to the outside at a high velocity.
The System Composition: The siphon roof rainwater drainage system consists of anti-turbulent rainwater gutters, suspended rainwater pipes, rainwater vertical pipes, buried pipes, and rainwater outflow pipes.
The formation process of the siphon rainwater drainage system includes five stages: wave flow, pulse flow (pulsating flow), piston flow (drawing flow), foam flow (emulsifying flow), and full-tube flow. Initially during rainfall, the rainwater in the suspended pipe of the drainage system is in a non-full-tube flow, primarily wave and pulse flow, with the system in a gravity flow state. As the rainfall increases, the water level in the sump gradually rises, and the flow gradually transitions to piston and foam flow, intermittently producing full-tube flow siphon states, resulting in a significant negative pressure within the suspended pipe. The formation of the siphon suddenly increases the system's drainage capacity, and the water level in the sump falls again, returning the system to a gravity flow mode. This transformation continues back and forth for a period until the rainfall further increases, the water level in the sump stabilizes, the system's aeration reduces, and finally, a stable full-tube flow siphon is formed.
Siphon Drainage vs. Gravity-Driven Traditional Drainage Advantages
According to regulations, the suspended pipes of the gravity flow roof rainwater drainage system should be designed under non-full flow conditions, with a fill level not exceeding 0.8. The flow velocity within the pipes should not be less than 0.75 m/s, and the slope should not be less than 0.5%. This requires larger suspended pipe diameters and a greater gradient. Additionally, to ensure normal discharge of rainwater from each rainhead on the same rainwater pipe, the number of rainheads on a single suspended rainwater pipe is limited to no more than 4. This limitation leads to an increase in the number of suspended rainwater pipes and rainwater stacks, thereby increasing the roof load and the cost of the project.
The siphon-type roof rainwater drainage system enhances drainage capacity, quickly removing roof rainwater to alleviate roof load and reduce the harm caused by roof surface water accumulation to the building's roof structure. The number of suspended pipes connected to the rainwater buckets is unrestricted, saving rainwater vertical pipes. Suspended pipes do not require slope adjustments, making installation easy and aesthetically pleasing, and maintaining a single installation height within the mechanical and electrical installation space for convenient coordination. There is no need for inspection or cleaning ports (since the siphon-type rainwater buckets are designed to intercept garbage and large granular materials from entering the system, and the system has an auto-cleaning function); the system's calculation based on siphon pressure flow can reduce the diameter of the selected pipes, and engineering practice has proven that this can significantly reduce construction costs.
Engineering Case Study
The office building project of Changsha-based Industrial Co., Ltd. is a comprehensive facility integrating conference, reception, and office spaces. The project covers an area of 14,422 square meters. The main building is 20 floors, with a局部 of 7 floors, the podium is 4 floors with a局部 of 5 floors. Rooftop area: the main building is 1,754 square meters, the podium is 12,154 square meters.
Technical data for the rainwater system design of the annex building: Design return period P = 5 years; 5-minute storm intensity in Changsha q5 = 0.0626 L/(s/m2); Total catchment area of the annex building Fw = 12154 m2; Flat roof (slope < 2.5%), K1 is taken as 1. Based on Qy = K1·Fw·q5/100 = 12154 × 6.26 = 760.84 L/s, 61 rainwater heads are required, with each head covering 200 m2.
If using a standard gravity-based rainwater drainage system, with a drainage area of 250m2 allocated per De10 vertical pipe, 49 vertical pipes are required. Using De10 and De160 pipes, 41 De10 vertical pipes with 61 rainwater heads or 19 De160 vertical pipes with 61 rainwater heads would be needed.
Based on the aforementioned, the siphon rainwater drainage system boasts numerous advantages such as high flow rate, rapid velocity, high intensity, excellent durability, low leakage, ease of flat layout, convenient construction, material savings, short construction period, and lower cost. Therefore, the siphon rainwater drainage system is adopted for this project (annex part). After further design refinement, it was determined that there will be 1 De90 system, 5 De10 systems, 2 De125 systems, and 5 De160 systems, totaling 13 systems with 62 rainwater inlets.
The project has been completed for two years and is currently operating well. Since its launch, the drainage has been smooth, yielding positive results. It has also withstood the natural conditions during Hunan Province's rainy season (April to June), achieving the anticipated outcomes and delivering both economic and social benefits.
