The fire pump cannot pump water:
Some users have measured that although the vertical distance from the water reservoir or water tower to the water surface of the fire pump is slightly less than the head of the fire pump, the water lift is still small or unable to pump water. The reason for fire pumps is often that the pipe is too long, the water pipe has many bends, and the resistance loss of the water flow in the pipe is too large. In general, the resistance of a 90-degree elbow of a fire pump is greater than that of a 120-degree elbow. The head loss of each 90-degree elbow is about 0.5 to 1 meter, and the resistance of every 20 meters of pipe can cause a head loss of about 1 meter. In addition, some users also arbitrarily choose the pipe diameters of the inlet and outlet pipes of the fire pump, which also have a certain impact on the head.

(1) Some users have not filled enough water before the fire pump is started; sometimes it seems that the filled water has overflowed from the vent, but the pump shaft has not been rotated and the air is completely discharged, resulting in a little air remaining in the inlet pipe or pump body in.
(2) The horizontal section of the water inlet pipe in contact with the fire pump should have a downward slope of more than 0.5% against the flow direction
(3) The packing of the fire pump has worn out due to long-term use or the packing is too loose, causing a large amount of water to be sprayed from the gap between the packing and the pump shaft sleeve. As a result, the external air enters the fire pump through these gaps, which affects the lifting of water.
(4) Due to long-term potential underwater, the pipe wall has corroded holes, and the water level continues to drop after the fire pump works. When these holes are exposed to the water surface, air enters the inlet pipe from the holes.
(5) There are cracks in the elbow of the inlet pipe, and there is a tiny gap between the inlet pipe and the fire pump, which may cause air to enter the inlet pipe. 

The suction range of the fire pump is too large, some water sources are deep, and the periphery of some water sources of the fire pump is relatively flat, and the allowable suction range of the fire pump is ignored, which results in little or no water absorption. It is necessary to know that the degree of vacuum that can be established at the suction port of the fire pump is limited. The suction range in absolute vacuum is about 10 meters water column height, and the fire pump cannot establish an absolute vacuum. Moreover, if the vacuum is too large, the fire pump is likely to vaporize the water in the pump, which is not good for the work of the fire pump.
The resistance loss in the water inlet and outlet pipes is too large.