The difference between siphon drainage system and traditional gravity drainage system
Siphon drainage function:
1. As long as the calculation requirements are met, there is no limit to the number of rain buckets in a single system.
2. The rainwater bucket has large flow, gas-water diversion, fast drainage speed and good effect.
3. The suspension pipe is in a horizontal state without any slope, which is convenient for construction.
4. The riser is small in diameter and quantity, which is convenient for decoration.
5, less buried pipes, less ground excavation workload, effectively shortening the construction period.
Characteristics of traditional gravity drainage:
1. The number of rain buckets in a single system is limited.
2. The rainwater bucket has small flow, gas and water confluence, and the drainage effect is poor.
3. Risers are large in diameter and large in quantity, and cross construction is easy to play.
4. There are many buried pipes, the workload of civil engineering is heavy, and the gutter is easy to return to water.
Design principle of siphon drainage system
When the designed rainfall reaches, all the pipes in the real siphon system are full of flow, and the pressure distribution in the pipes is also different from that in the traditional drainage system. Because the full height difference between the roof rainwater hopper and the ground drainage well is fully utilized, compared with the gravity drainage system, when the diameters of the siphon drainage pipeline and the gravity drainage pipeline are equal, the water flow in the siphon pipeline is much greater than gravity.
Siphon system uses the height of the building as the water head, fully considers the hydraulic loss in the pipeline, reduces the diameter of the pipeline as much as possible, and discharges the rainwater from the roof into the rainwater well. Hydraulic balance is achieved by adjusting the structure and length of vertical tailpipes and horizontal headers.
The pressure in the pipeline is quite different from atmospheric pressure. There is no limit to the positive pressure in the pipeline, but the problem of evaporation pressure, that is, cavitation, must be considered for the negative pressure. Therefore, the pressure in the pipeline must be kept higher than the evaporation pressure in the area where the system is located.
1, design of siphon rainwater hopper
The main difference between gravity drainage rainwater hopper and siphon system rainwater hopper is that there is an air baffle in the rainwater hopper. The wind deflector is laminated and has no through holes. The design of roof bonding parts, waterproof layer and leaf cover all follow the conventional ideas. The diameter combination of rainwater bucket body, wind deflector and water outlet is the guarantee of stable function under any water flow condition. It is difficult to optimize the design of this combination in theory, and a conclusion can only be drawn through a lot of experiments and summary.
2. System piping design
Piping systems that must conform to local national codes and can withstand positive and negative pressures can be used for siphon system drainage pipes. Such as ABS, PVC, HDPE, PP, copper pipes, steel pipes and cast iron pipes, have been successfully applied to a large number of UV systems. Different plastic pipes have different negative pressure resistance, which is determined by different materials, wall thickness and pipe diameter. Pipeline system is generally selected according to the actual situation of the project, and factors such as applicability, drainage capacity, durability, rigidity, fire resistance, noise, insulation, construction cost and installation cost need to be comprehensively considered.
3. Relevant design points:
(1) Selection of rainwater hopper
The flat-bottomed metal rain bucket is adopted to ensure the anti-aging performance and anti-leakage performance of the rain bucket under long-term sunlight irradiation. Flat-bottomed rain bucket ensures simple and safe construction. Because the gutter has an outer gutter and an inner gutter, the patented flat-bottomed rainwater bucket is installed on the concrete roof, which is simple and quick, and can effectively prevent water leakage.
According to the roof structure of the building, the siphon rainwater hopper used in this project is:? The displacement of siphon rainwater hopper is generally 20? L/s to 40? L/s, when choosing the rain bucket, the design displacement should be 70~80% of the maximum displacement. Compared with gravity flow system, it reduces the size and number of gutter openings, reduces possible leakage points and saves engineering construction time. Siphon systems are mostly series systems, and roof drainage passes through the building slope to the gutter or water collection point. When dividing rain buckets on the same roof, the calculation system is based on the same elevation (there is an error in construction, and the elevation of each rain bucket in the same system on the same roof must be within 50mm, otherwise the system calculation is invalid and the system is destroyed). On the other hand, when calculating the system, it is necessary to control the water depth in front of the rain bucket. If the displacement and water depth in front of each rainwater bucket cannot be controlled well, the system cannot drain water. Only after careful calculation can the depth of roof water be reduced and the drainage effect be better. Some systems are connected by roof rainwater systems with different elevations to realize rapid and safe drainage. On the premise of sufficient drainage capacity of rainwater hopper, the drainage capacity and the rapid formation of siphon are ensured through accurate calculation.
(2) Pipeline
The selected pipes and interfaces must meet the needs of positive pressure and negative pressure. ? The selected pipelines can be divided into the following categories:
HDPE pipe: plastic pipe with low cost, smooth inner wall, good hydraulic conditions and simple construction. It is welded by hot melt. ? Plastic-coated steel pipe: steel-plastic composite pipe with high cost, good rigidity, good fire resistance, smooth inner wall, good hydraulic condition and long service life, connected by flat clamp or flange.
Galvanized steel pipe: common steel pipe, with high cost, good rigidity, good fire resistance and long service life, connected by groove clamp. ? Stainless steel pipe: advanced drainage steel pipe, with the highest cost, good rigidity, good fire resistance and long service life, connected by sub-arc welding or electric welding.
4, siphon roof drainage system of hydraulic calculation points:
(1) The design velocity of the pipeline shall not be less than 1? Meter/second.
(2) The total head loss of the drainage pipe should be less than or equal to the geometric height difference between the top surface of the rainwater hopper and the critical point, and the pressure margin should be △Pr≤ 100mbar.
(3)? Siphon roof drainage system has a large negative pressure at the intersection of suspension pipe and main riser. The negative pressure value at this point should have different limits according to different pipes. The drainage system made of cast iron and steel pipes should be less than 900mbar plastic pipe, the pipe diameter De50-De 160 should be less than 800mbar, and the pipe diameter De200-De300 should be less than 450? Millibar.
(4) The calculated pressure difference of different branches at each node of siphon roof drainage system shall not be greater than 150? Millibar.
(5) The siphon roof drainage system adopts plastic-coated flexible drainage cast iron pipe or steel pipe and high-density polyethylene pipe, and the corresponding calculation chart should be used to calculate the head loss along the pipeline.
Practical steps of siphon drainage pipe design
1.？ First, according to the requirements of design drawings, determine the roof area with siphon drainage.
2.？ Measure the area of the selected area on the cad drawing, that is, the design catchment area f of the roof.
3.？ Find the rainstorm intensity value q(l/s? 10000m？ )
4.？ According to the formula, roof rainwater design flow: Q=k? qFk & lt 1,? φ& gt； 1? By communicating with the staff of the local design institute, determine which formula to use for calculation. Generally, we use the formula Q=qF for calculation. In order to change the unit to l/s, we change the formula to Q=qF/ 10000? The resulting roof rainwater design flow Q (unit: L/s)
5.？ A. Determine the number of rain buckets according to the design flow of roof rainwater and the given rated flow parameter value of rain buckets. Under normal circumstances, the calculated rain bucket sharing value should not exceed 20l/s, and the number of buckets should be increased if it exceeds too much.
B. According to the determined number of rain buckets, find the position of the gutter in the cad drawing, measure the length of the gutter, and then calculate the distance between the rain buckets. In general, the distance between two rain buckets should not be more than 20m, and then determine the specific position of the rain buckets in the drawing.
6. Technical specification for siphon roof rainwater drainage system:
( 1)? Yes, the catchment area is more than 5000? m？ For large roofs, at least two independent siphon roof rainwater drainage systems should be set up.
(2)? When the diameter of the riser is not greater than DN75, the height difference from the top surface of the rain bucket to the transition section should be greater than 3m; When the riser diameter is not less than DN90, it should be more than 5m;
(3)? The design velocity of the suspension pipe should not be less than1.0m/s; The design velocity of the riser should not be less than 2.2m/s and not more than10m/s;
(4)? The flow velocity downstream of the transition section of siphon roof rainwater drainage pipeline system should not be greater than 2.5m/s; When the velocity is greater than 2.5m/s, energy dissipation measures should be taken;
(5)? The diameter of the riser should be determined by calculation, which can be smaller than the diameter of the upstream suspension pipe.
(6)? Siphon rain buckets and roofs should be set at the lowest point of the roof or gutter in each water collection area, and the number of rain buckets in each water collection area should not be less than 2. The spacing between two rain buckets should not be greater than 20m. The distance between the siphon rainwater hopper set on the roof of the podium and the junction of the podium and the tower should not be less than 1m and not more than 10m.
(7)? The shape of the gap between the shutters of the siphon rainwater bucket can be hole-like or groove-like. The diameter of the pore should not be less than 6mm and not more than 15mm.
(8)? The siphon rainwater hopper shall be symmetrically arranged with rainwater risers.
(9)? When connecting multiple siphon rain buckets, the drainage connecting pipe of siphon rain buckets shall be connected to the suspension pipe, and shall not be directly connected to the top of the rainwater riser.
(10) The starting depth of the gutter should be determined according to the catchment area and slope of the roof and the water depth before the siphon rainwater bucket, and the slope of the gutter should not be less than 0.003.
(1 1) The section of the drainage ditch shall be calculated and determined according to the design flow of the catchment area. The width of the drainage ditch should consider the uniform water inflow around the rainwater.
(12) Pipes and fittings shall be made of high density polyethylene (HDPE) with a grade not lower than PE80.
(13) The longitudinal shrinkage of the pipe should not be greater than 3%;
(14) Rainwater risers shall be provided with inspection ports according to the design requirements, and the center of inspection ports shall be 1.0m from the ground. When using high density polyethylene (HDPE) pipes, the maximum spacing between inspection ports should not be greater than 30m.
(15) The rainwater pipeline is installed at the designated position in the design;
(16) The connection between the connecting pipe and the suspension pipe should adopt a 45-degree tee;
(17) Two 45-degree elbows or 90-degree elbows shall be used for the connection between the suspension pipe and the riser, and between the riser and the discharge pipe, with r not less than 4D.
(18) When the high-density polyethylene pipeline passes through the wall, floor or parts with fire protection requirements, fire-retardant rings, fire-retardant belts or fire-retardant sleeves shall be set according to the design requirements;
(19) When rainwater pipes pass through walls and floors, metal or plastic sleeves shall be provided. The top of the sleeve in the floor should be 20mm higher than the decorative floor, and the bottom should be flush with the bottom of the floor. Both ends of the sleeve in the wall should be flush with the finish. The gap between casing and pipeline shall be filled with flame retardant dense material;
(20) When installing, temporary sealing measures should be taken for the pipe opening and rainwater hopper.
From the design principle of siphon drainage system to the practical steps of its design, we understand what siphon drainage system is about from the conceptual level, and then try to simulate the design of siphon drainage system from the practical steps. A deep understanding of siphon drainage system can remove stagnant water so skillfully and quickly, and people have to admire the magic and power of this siphon drainage system.