According to a report by the Voice of China's "News Vertical and Horizontal", recently, the National Bureau of Statistics released data showing that the national birth population last year was 17.23 million, a decrease of 630,000 from 2016, and the birth rate was 12.43‰, both of which decreased compared with 2016. The person in charge of the Guidance Department of the National Health and Family Planning Commission responded that the birth population last year decreased compared with the previous year, but it still maintained a high scale, higher than the annual average level during the "12th Five-Year Plan" period. It has been two years since the official implementation of the "Comprehensive Second Child" policy. In the past two years, the birth population in my country has not exceeded 18 million.
A check valve is a valve used to prevent backflow of media in pipelines or equipment. It opens using the kinetic energy of the fluid. In heating systems, check valves are often installed at the outlet of pumps and other places where fluid is not allowed to flow in the reverse direction. Selecting the appropriate check valve type has a significant impact on water hammer protection. For example, many water hammer accidents in pumping stations are related to the check valves installed at the pump outlet. After an accidental pump stop, the water flow starts to flow backward, and under the action of the backflow, the check valve closes quickly. If the check valve lags behind the start of backflow due to a fault or the influence of support friction and inertia, that is, when the backflow increases to a certain value, or even closes rapidly at the moment of maximum backflow, a very large pressure can be generated at the check valve. The check valve disc is completely closed when the flow rate is zero, but it is unlikely to complete the valve disc closure when the flow rate is zero. It is generally considered normal as long as the closure is completed near zero flow.
In hot water heating systems, in order to reduce water hammer caused by accidental pump stops, a pressure relief bypass pipe with a check valve can be installed between the pressure pipe and the suction pipe of the circulating water pump. When the circulating water pump is running, the check valve is closed because the water pressure on the outlet side of the pump is higher than the water pressure on the suction side. At the instant of a sudden pump stop, the pressure on the pump outlet side drops sharply, while the pressure on the suction side increases significantly. Under this pressure difference, the water in the pipeline on the suction side of the circulating water pump pushes open the check valve to the pipe network system on the pump outlet side, thereby reducing the increase in pressure in the pipe network on the suction side and reducing and preventing the hazards of water hammer. The check valve installed on the pressure relief bypass pipe should be a product with small resistance and flexible opening. The larger the diameter of the pressure relief bypass pipe, the more effective it is in reducing water hammer. The economical pipe diameter can be determined by transient calculations based on the specified pressure limits.
