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There are 6 types of pump protection lines. Their function is to ensure that the pump body is not damaged and operates normally. The pump protection lines are set according to the usage conditions. 1. Warm pump line - when transporting high-temperature oil products with a medium temperature greater than 200°C, a DN20~25 warm pump line should be set up if there is a backup pump. ； 2. Small flow line - when the working flow of the pump is lower than 30% of the rated flow of the pump, a small flow line should be set for the pump to operate normally at the lowest flow rate. ； 3. Balance line - For transporting liquids whose saturated vapor pressure is higher than atmospheric pressure at normal temperature or liquids at bubble point, a balance line should be added to prevent the liquid entering the pump from generating steam or bubbles from entering the pump and causing cavitation. ； 4. Bypass line - used for trial operation of the pump or when the outlet main valve is closed under abnormal operating conditions, the pump can still be running. Generally, a bypass valve with a flow-limiting orifice is installed in situations where the pressure difference between the front and rear of the valve is very high. ； 5. Anti-condensation line - When transporting liquids with high pour points or high freezing points that solidify at room temperature, the backup pumps and pipelines should be equipped with anti-condensation lines to avoid blockage of the backup pumps and pipelines (pump stewards deserve attention) ； 6. Safety valve line - For volumetric pumps such as electric reciprocating pumps, gear pumps and screw pumps, a safety valve line is set on the outlet side. When the outlet pressure exceeds the constant pressure value, the safety valve will trip and the fluid will return to the pump inlet pipe. 02 The minimum flow pipeline of the centrifugal pump has the following functions: 1. When the centrifugal pump operates at low flow rate, fluid detachment will occur in the pump, which can easily cause vibration and noise in the pump. Setting the minimum flow pipeline can ensure that a certain amount of fluid always flows through the pump after it is started to prevent cavitation. ； 2. When a high-power centrifugal pump is running with the outlet valve closed, the fluid in the pump will get hotter and hotter, and in severe cases, the bearings will be burned. After setting the minimum flow pipeline, it can be ensured that a certain flow of liquid will always flow through the pump during the operation of the pump to prevent this phenomenon. ； 3. When working at a small flow rate, the pump shaft will be subject to additional radial force. Setting the minimum flow pipeline can ensure that a certain amount of fluid always flows through the pump when the pump is running, preventing the pump shaft from being subject to excessive radial force. ； 4. Sometimes a flow-limiting orifice plate is used on the minimum flow line of the pump, so that even if a misoperation occurs and the pump outlet valve is closed, the centrifugal pump can operate safely without causing damage. The advantage of using a restrictor orifice plate is that it does not require manual adjustment. If only the stop valve is used for adjustment, it is difficult to control the flow rate near the minimum flow rate of the pump, either large or small. The difference between each flow line : 1. Distinguish between small flow lines and circulation lines. The former has a flow rate less than 30% of the rated flow during normal production, while the latter is used in places where switching is frequent or to prevent condensation and blockage. Similarities: The flow rate of the pump is much larger than the flow rate required by the actual process. 2. Distinguish between minimum continuous stable flow and minimum continuous heat flow. The minimum continuous stable flow rate refers to the minimum flow rate at which the pump can operate normally without exceeding the noise and vibration limits specified in the standard. It should generally be determined by the pump factory through tests and provided to the user. Minimum continuous thermal control flow refers to the minimum flow rate at which the pump can operate continuously without being lost by the temperature rise of the pumped liquid.