1. An overview of the

In the water supply pipeline, backflow preventers are installed in some important water pipelines to prevent backflow from polluting the water source.For some connection between construction water and municipal water supply, backflow preventer device can effectively cut off backflow between downstream end water and upstream end water, prevent backflow of construction water from polluting municipal water supply, so as to ensure the quality and safety of municipal water supply.

2. Structural performance

The key function of backflow preventer is to prevent backflow of water at the downstream end or infiltration of water at the upstream end when there is no water forward flow or abnormal water supply in the pipeline, so as to effectively eliminate the possible danger of backflow pollution.The reverse flow preventer shell adopts a diversion structure to drain the backflow and critical backflow water out of the pipe, completely preventing the backflow of the downstream end water or infiltration of the upstream end, so as to avoid backflow pollution and ensure the safety of water supply.The backflow preventer consists of two separate check valves and a cavity and hydraulically controlled drain valve (figure 1).There is no water medium flow in the water supply pipeline system, and when the downstream pressure may be higher than the upstream pressure and there is backflow of water at the downstream end, it will be discharged through the intermediate.If one of the check valves fails to turn upside down and fails to close, the downstream end water will also be discharged through the intermediate.

According to the role of the backflow preventer, the key to its product performance is to ensure that the drainer is properly opened in backflow and closed when the forward flow is closed.The backflow preventer can not be opened in the middle of the drainer can not guarantee the backflow preventer partition effect, at the same time, the displacement of the drainer is too small, high pressure backflow water at the downstream end of the pipe may infiltrate into the upstream end.Moreover, the overall strength of the product, the strength of the disc and the outlet disc, and the positive differential pressure performance of the sealed inlet/outlet check valve when it is closed are the basic indicators to ensure the long-term normal use of the valve.In order to save energy, reduce emission and improve efficiency, the valve as a whole should also achieve as small as possible head loss.

3. Performance test method and verification

In the literature [1] ~ [5], clear regulations and requirements are given on the service performance, manufacturing, installation and test testing of the backflow preventer, among which, there are differences in the setting requirements, resistance performance test and performance indexes of the backflow preventer.The use and performance test items and methods of backflow preventer mainly include opening and closing performance test of drainer, sealing performance test of drainer, backflow pollution function test, durability test, pressure loss test, shell strength test and disc strength test.

3.1 opening and closing performance test of the drainer

The opening and closing performance test device of the drainer (FIG. 2) has a pressure less than 0 3 MPa.The inlet pressure of the tested valve is reduced. When the pressure is not greater than the outlet pressure, the drainer should be opened to drain water, but no continuous drainage.In the test valve water, the drainer should be quickly closed, no water leakage.Repeat the above steps for more than 3 times continuously, the opening and closing action of the drainer should be sensitive and reliable.

3.2 sealing performance test of drainer

The test device for sealing performance of the drainer is consistent with the test device for opening and closing performance of the drainer.During the test, there is flow out of the outlet, adjust the tested valve to make the outlet flow from micro flow to rated flow (the average flow rate in the pipe is 2m/s), and then reduce the rated flow to micro flow.When there is flow through the whole machine, the drainer should be sealed and closed in the whole test process without any leakage.

3.3 functional test for preventing backflow pollution

When the inlet pressure is less than the outlet pressure, the backflow preventer should be able to effectively prevent backflow from polluting the water source.When the inlet pressure is less than the outlet pressure, the drainer should be able to open effectively so that the intermediate chamber is communicated with the atmosphere.The internal moving seal structure of the check valve and the whole machine (including the drainer) should ensure that when the seal fails, the leaking media should flow directly to the intermediate cavity or outside the valve.When the sealing port of the outlet check valve is hung with a metal wire with a diameter of 3.0mm, the maximum leakage flow should meet the requirements.When the flow rate of the leakage medium through the drainer reaches the specified level and the pressure in the intermediate chamber is no more than 105kpa, the outflow amount of the drainer is measured.

3.4 durability test

The endurance test device is shown in figure 3.The whole machine should maintain its original performance after 20,000 continuous open and close operation tests.

3.5 pressure loss test

Test method for pressure loss of backflow preventer according to standard GB/T20832, the smaller the head loss, the better.

3.6 shell strength test

Plug the outlet of backflow preventer and the outlet of drainer, close all detection valves, pressurize from the inlet, the test pressure is 1.5 times of the nominal pressure, and the pressure holding time is more than 5min. The test results show that the shell shall be free of visible leakage, structural damage and permanent deformation.
3.7 strength test of valve disc

The strength test of disc and outlet piston includes the strength test of inlet check valve disc, outlet check valve disc and outlet piston. During the test, connect the inlet and outlet of the valve to be tested, pressurize at the same time, open the test valve of the intermediate chamber, make P2 = 0, the test pressure is 1.5 times of the nominal pressure, and the pressure holding time is more than 5min. Check the intermediate chamber. The test result shows that the intermediate chamber shall be free of visible leakage, structural damage and permanent deformation.

3.8 sealing performance test
(1) overall sealing performance test
The sealing performance test of the whole machine includes the sealing performance of the valve body and bonnet of the drainer, the valve body and bonnet of the backflow preventer, the drainer, the additional device, the inspection valve and the pipeline connection. During the test, pressurize from the inlet of the valve to be tested, close or block all the outlets. The test pressure is 1.1 times of the nominal pressure, and the pressure holding time is more than 3 minutes. Check all sealing parts. No visible leakage is found at all sealing parts according to the test results.
(2) sealing performance test of check valve and induction valve disc
Open the test ball valve of the middle chamber of the valve to be tested, so that there is no pressure at this place, and pressurize the inlet and outlet at the same time. The test pressure is 1.1 times of the nominal pressure, and the pressure holding time is more than 3min. The test results show that there is no visible leakage in the middle chamber.
(3) measurement test of positive pressure difference △ PJ when the inlet check valve is closed
Connect the lower port of the glass tube with the inlet detection valve of the valve to be tested with a hose, and close the water inlet valve when there is water flowing out of the ball valve in the middle chamber. Observe that the liquid level of glass tube drops and stops finally, keep for 3min, and measure the vertical distance between the stop liquid level and the outlet of ball valve in the middle chamber.
(4) test for determination of positive pressure difference △ PC when outlet check valve is sealed and closed
Connect the hose at the lower port of the glass tube with the ball valve in the middle chamber, observe that the liquid level in the glass tube will drop and finally stop, maintain for 3min, and measure the vertical distance between the static liquid level in the glass tube and the outlet of the ball valve.

4. conclusion
The backflow preventer is composed of two separate check valves and a middle cavity and hydraulically controlled drain valve. The series connection of common check valves does not have the required backflow discharge function. The correct opening and closing and sealing of the drainer in the flowing state are the key performance to ensure the anti pollution partition and normal use. The effectiveness of the function of the backflow preventer can be confirmed only after the performance test.