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There are 6 types of protection lines for pumps, whose purpose is to prevent damage to the pump and ensure its proper operation; the protection lines for pumps are selected based on the operating conditions. (1) Warm pump line – When transporting high-temperature oils with a temperature exceeding 200°C, a DN20–25 warm pump line should be installed if a backup pump is available ； (2) Low-flow line – When the pump’s operating flow rate is 30% below its rated flow rate, a low-flow line should be provided to allow the pump to operate properly at the lowest flow rate ； (3) Balance line – For liquids with a saturated vapor pressure at room temperature that is higher than atmospheric pressure, or liquids in the bubble point state, a balance line must be installed to prevent vapor from forming in the liquid entering the pump or bubbles from entering the pump and causing cavitation ； (4) Bypass line – used for the trial operation of the pump, or to keep the pump running when the main outlet valve is closed under abnormal operating conditions. Bypass valves equipped with flow-limiting orifices are generally installed in situations where the pressure difference before and after the valve is very high ； (5) Anti-freezing measures – When transporting liquids with a high pour point or high freezing point that solidify at normal temperatures, anti-freezing measures should be implemented in the standby pumps and pipelines to prevent them from getting clogged ； (6) Safety valve line – For positive displacement pumps such as electric reciprocating pumps, gear pumps, and screw pumps, a safety valve line is provided on the outlet side; when the outlet pressure exceeds a set value, the safety valve opens and the fluid returns to the pump inlet pipe.