Real case: A room is located on the ground floor, three buildings with 12 floors and two buildings with eight floors, the floor height is 2.8 meters, the life pump and fire pump are all DN80, the life pump head is 53 meters, the fire pump head is 60 meters, and the flow rate is 10L/S. The water supply is five-way, and the DN80 pipe enters the roof water tank. The DN80 water pump supplies five DN80 water pipes at the same time, so the water pressure is not high, and it can barely enter the water tank. The emergency shut-off valve sets the pressure at 0.5MPa, and supplies five channels of water at the same time, the pressure will not exceed 0.5MPa, and it can only replenish water to the water tank. However, if four water tanks are full, the liquid level control valves on the water tanks are closed, and only one water tank is still full of water and still needs to be replenished. The next water tank can’t replenish water. In order for each water tank to replenish water smoothly, it is necessary to stabilize the pressure at 0.45MPa, so that it is less than 0.5MPa, so as not to cause shutdown. This requires adding a pressure relief valve to the life pump. In most units now, there is no pressure relief valve on the living pump. Some units have used electric contact pressure gauges, which trip automatically when the water pressure exceeds 0.5MPa, and the water pump stops running, which still cannot solve the problem of replenishing water for a single water tank.
The second problem is that when the fire pump is started, all the emergency shut-off valves are open. The DN80 water pump is divided into five DN80 water pipes. At this time, if the water in the water tank is not full, the water pressure will not go up, and it is impossible to exceed 0.5MPa. The pressure will not increase until the pool is full and the liquid level control valve is closed. Here the designer should consider increasing the capacity of the water pump or reducing the diameter of the water pipe. The difference between the two should be at least two levels. For example, the water pump is changed to DN100, and the five-way water pipe is changed to DN65. function, in order to send fire water to the building. For another example, there is a DN100 fire pump in a residential area on Kongjiang Road in Shanghai, and the six water pipes are DN80, but they still cannot be closed, so the difference between the water pump and the multiple water supply pipes should be two gears apart.
The third problem is that in the drainage system of the pump house, the water discharged from the pressure relief valve should be discharged into the fire pool or sewer, and should not be discharged into a narrow open ditch. Still taking the above example, during debugging, the five water pipes were gradually closed one by one. As a result, all the water in the water pump was pumped out and released by the pressure relief valve in the basement. The drainage pump was much smaller than the fire pump, and the underground pump room was flooded. . A better solution is to use variable frequency pumps for both domestic pumps and firefighting, and the water pressure is constant during commissioning without being affected by the flow. Or it is also possible to add a pressure relief valve with a quantitative pump, but the pressure relief valve must be connected to the pool or the sewer with a water pipe, and the caliber of the sewer is one to two grades larger than the water inlet pipe. It is better to have a direct connection to the user after closing the valve in an emergency. An anti-pollution isolation valve should be added between the fire-fighting pipe network and the domestic pipe network to prevent backflow of fire-fighting water from polluting domestic water.
How emergency shut-off valves work
The emergency shut-off valve is one of the hydraulic control valves. It is composed of main valve, throttle valve, pilot valve, pressure gauge and other pipe fittings. When the inlet pressure of the main valve is lower than the set pressure of the pilot valve, the pilot valve is in the open state at this time, the pressure in the upper cover cavity of the main valve is equal to the outlet pressure of the main valve, there is a pressure difference between the inlet and the outlet, and the water flows through the valve port and flows out from the outlet . When the fire pump starts, or the pressure of the variable frequency pump rises and exceeds the set pressure of the pilot valve, the pressure of the control pipe from the inlet of the main valve to the pilot valve rises, and the pressure acting on the spring increases, pressing down to drive the valve core Move down to close the pilot valve. At this time, the inlet pressure of the main valve is continuously replenished to the upper cavity of the main valve bonnet through the throttle valve. Because the downward pressure on the disc is greater than the upward thrust, the main valve is closed.
Special attention should be paid here, if the inlet pressure is lower than the set pressure, the valve is always open, and if the inlet pressure is greater than the set pressure, the valve is closed. Nowadays, many designers mistakenly think that as long as the emergency shut-off valve is installed, it will be closed, no need to mention any conditions, then it is wrong.