What is the minimum installation slope for HDPE siphon systems

An important feature of the siphon system is that the pipe is in full flow during operation. Siphon drainage is different from traditional gravity flow drainage, which requires a continuous slope along the pipe to maintain water flow velocity and self-purification ability. The siphon drainage system relies on negative pressure suction rather than gravity flow during operation, so it has little dependence on the slope of the pipe and can even achieve "zero slope" laying.

Design basis for the minimum slope of HDPE siphon drainage pipe

In the design, the pipe part of the HDPE siphon system, especially the roof to riser connection part, is generally laid with zero slope or slight slope. Many international and domestic design specifications mention that the horizontal pipe of the siphon drainage system can be designed with a slope of 0‰ to 2‰.

The following are several reference indicators:

GB 50015-2019, "Code for Design of Building Water Supply and Drainage", states that the horizontal main pipe of the siphon drainage can be designed without slope.

EN 12056-3 (European drainage system standard) points out that the pipe slope can be 0‰ under the condition of ensuring the siphon start.

The technical manuals of major manufacturers also clearly state that the recommended slope of the horizontal pipe of the HDPE siphon system is 0‰~1‰, which is conducive to construction leveling and prevents gas accumulation inside the pipe.

Feasibility analysis of the minimum slope of 0‰
When designing the HDPE siphon system, a minimum slope of 0‰ is technically feasible. The basis is as follows:
Full flow operation of the system: When designing the HDPE siphon system, it is ensured that after the rainfall intensity reaches the set value, the system reaches a full flow state, forming a stable negative pressure-driven drainage, without relying on the slope to maintain the water flow.
No gas accumulation inside the pipe: The special siphon rainwater bucket has an anti-gas surge design function, which can prevent air from entering the system during the initial exhaust and operation of the system, fundamentally eliminating the gas blockage problem caused by zero slope.
Structural space saving: The use of a 0‰ slope can significantly reduce the floor height occupancy, especially for large commercial buildings, airports, stadiums and other projects with high clearance requirements.
Convenient construction: HDPE pipes are flexible, the welding connection method is flexible, and the zero-slope layout greatly simplifies the bracket setting and slope finding process, improving construction efficiency and accuracy.

Precautions for zero-slope construction
Although the HDPE siphon system allows zero-slope installation, the following points should still be noted during construction:
The pipeline must have enough support points to prevent local water accumulation or sagging in the pipeline.
Professional calculation software must be used to simulate the system hydraulically to ensure that the siphon state can be quickly established under the designed rainfall intensity.
The welding quality of the pipe joints must be guaranteed to avoid interface leakage caused by negative pressure inside the pipeline.
When installing at zero slope, reserve adjustment space for later maintenance and adjustment.

Differences in slope design of different pipe sections of HDPE siphon drainage system
Although the horizontal main pipe can adopt zero slope, some specific pipe sections can still be set with micro-slope according to needs:
Short pipe connecting the rainwater bucket to the main pipe: a micro-slope (such as 1‰~2‰) can be set to help drain water in the initial stage of system startup.
Start-up section: Some designs recommend using a small slope to guide the flow rate so that full flow can be quickly formed.
End discharge to the rainwater well: If it is not within the siphon control range, consider setting a natural slope to guide the water outflow.