Working principle of Low Resistance Backflow Preventer

1. Normal Flow Condition

The supply pressure at the inlet is always higher than the pressure at the outlet, allowing water to flow smoothly through the backflow preventer in the forward direction. At this time, the pressure at the inlet is greater than the pressure inside the valve chamber, creating a pressure difference of ΔP1-2, where ΔP1-2 = P1-P2 ≥ 8kPa. This pressure difference keeps the bypass drain valve closed, preventing water from leaking out.

2. Zero-Flow Condition (1)

No water flows out of the outlet. If the supply pressure at the inlet remains constant and is higher than the pressure inside the valve chamber, with a pressure difference of ΔP1-2 = P1-P2 ≥ 8kPa, the drain valve remains closed.

3. Zero-Flow Condition (2)

No water flows out of the outlet. If the supply pressure at the inlet decreases, although the pressure at the inlet is still higher than the pressure inside the valve chamber, when ΔP1-2 = 5kPa, the bypass drain valve opens to release water, and some of the water inside the valve chamber is discharged. If the pressure in the outlet pipeline increases for some reason and the outlet check valve leaks, causing the pressure inside the valve chamber to rise, the drain valve will open again when ΔP1-2 = 5kPa to discharge the reverse-flowing water. Once the drain valve opens, air immediately enters the valve chamber through the air vent connected to the atmosphere, creating an air barrier.

4. Backsiphoning Condition

When the supply pressure at the inlet of the backflow preventer drops below 6kPa, regardless of the pressure P2 inside the valve chamber, the drain valve opens to release water. At the same time, air immediately enters the valve chamber through the air vent, creating an air barrier. Even if the pressure at the inlet continues to drop and becomes negative, backsiphoning will not occur.

5. When ΔP1-2 ≥ 5kPa, the drain valve automatically closes and returns to its original position.

The forward pressure difference ΔP2-3 at the outlet check valve is ≥ 7kPa, ensuring that it can close quickly in an instant at the onset of backpressure (when the downstream pressure increases and the water flow tends to reverse). In current municipal water supply networks, where the downstream flow rate changes frequently, water hammer in the supply system can cause significant pressure fluctuations at the outlet of the backflow preventer. The instant closure of the outlet check valve prevents the possibility of backflow contamination caused by backpressure, and it also eliminates the pressure fluctuations in the intermediate chamber P2, preventing the drain valve from leaking frequently. This not only prevents backflow contamination but also avoids excessive waste of water resources.

The dual-channel structure of the drain valve ensures that a large amount of air is introduced into the valve chamber while water is being discharged, creating an air barrier. This barrier separates the water in the valve chamber that is at the critical point of backflow or is flowing towards the atmosphere from the water at the inlet. Especially when backsiphoning occurs in the upstream pipeline, a large amount of air enters the valve chamber, breaking the siphonic vacuum and effectively preventing the reverse flow of the medium.

Low-resistance backflow preventers (based on the pressure-reducing principle) are mainly used in supply systems, such as those that supply water to general users, to strictly regulate the one-way flow of pressurized water in the pipelines. Their function is to prevent the reverse flow or siphonic flow of the medium in the pipeline, thereby avoiding backflow contamination.

1. Utilizes the pressure-reducing principle to achieve low resistance; under normal operating conditions at the economic flow rate, the actual water loss is only 1.8–2.5 meters.

2. Compact structure with shorter dimensions. The two-stage independent check valves have low flow resistance and open and close quickly, effectively preventing damage to the valves and pipelines caused by sudden increases in backpressure. They also have a silent operation feature.

3. The drain pressure of the drain valve is adjustable, with minimal interference from pipeline fluctuations, and its opening and closing are safe and reliable.

4. The dual-diaphragm design prevents backflow contamination caused by diaphragm damage.

5 The air barrier structure with dual inlet and outlet channels ensures that the air intake and drainage in the barrier chamber do not affect each other, preventing problems such as blocked drainage poor drainage. It completely eliminates the possibility of pressurized water backflow or siphonic backflow.

6. Built-in safety control system, which prevents the exposed bypass pipe from being stolen or damaged and also prevents it from being used for malicious activities as poisoning. This ensures use and an attractive appearance.

7. Can be tested and maintained online.

- Medium: 0–100°C

- Applicable medium: Clean water and substances with physical and chemical properties similar to clean water

- Nominal pressure: 1.0–1.6MPa

- Sealing pressure: 1.1 times the working pressure

- Strength test pressure: 1.5 times the working pressure

1. Before formal installation, all pipelines should be thoroughly flushed.

2. The backflow preventer should be installed in a horizontal position (for vertical installation, this should be specified in the technical agreement in advance). The installation location should be convenient for commissioning and maintenance, and it should allow for prompt detection of water leakage or malfunctions. After installation, the valve body of the backflow preventer should not bear the weight of the pipelines, and care should be taken to prevent it from being damaged by freezing or human action.

3. Maintenance gate (butterfly) valves should be installed at both ends of the backflow preventer. A filter should be installed before the inlet, and at least one end should be equipped with a flexible joint. However, for pipelines that are only used in emergency situations (such as fire protection systems), the possibility of the filter screen being blocked by impurities, leading to a disruption in water supply during emergencies, should be considered.

4. The drain outlet of the drain valve should not be directly connected to the drainage pipeline but should be discharged into the ground drainage ditch through an air vent. The lower end of the air vent should be at least 300mm above the highest liquid level of the drainage facility.

5. In principle, it is not allowed to install the backflow preventer outdoors or in a pipe shaft. If it is absolutely necessary, it must comply with national, industry, and local regulations; it should be installed by certified technicians in a suitable space with a protective structure that allows for proper installation and maintenance; and the correct installation and maintenance of the air vent must be ensured to guarantee the safe use of the backflow preventer.

6. When the backflow preventer needs to be maintained without shutting down the water supply, two backflow preventers of the same model and diameter should be installed in parallel on the pipeline.

7. After installation, when starting up for the first time, to prevent the O-ring from shifting or the internal components from being damaged during sudden movements, the following steps should be followed: Close the outlet gate valve, slowly open the inlet gate valve, allow water to slowly fill the backflow preventer, open the test ball valves to release the air inside the valve chamber, and after the valve chamber is filled with water, slowly open the outlet gate valve to allow water to fill the entire pipeline.

8. If a pressure test is to be conducted, all the air inside the backflow preventer must be released, and the device must be filled with water. The test should be carried out in accordance with the GB/T13927 standard for general valve pressure tests.

9. If the pressure of the inlet medium fluctuates due to the water supply conditions, it is normal for the drain valve to leak a small amount of water.