What are the protective lines for pumps?

1. Warm-up Line  
When the medium temperature exceeds 200°C and a standby pump is available, a warm-up line is required to prevent high-temperature liquid from rushing abruptly into the standby pump during switching. This could cause uneven heating of the pump body and impeller, leading to damage, deformation, and potential jamming between rotating and stationary components. The warm-up line keeps the standby pump in a ready-to-start condition by preheating it, enabling smooth and immediate switching. After switching, the warm-up line should be closed. The inlet must remain open, allowing oil to flow through the pump body via the warm-up line to prevent condensation. Typically, a bypass pipe with minimal recirculation is installed before or after the pump outlet valve (or group) as the warm-up line. Alternatively, a small hole can be drilled in the check valve disc to serve as a warm-up path.  
It should be noted that such pumps often handle high-temperature media; therefore, regardless of whether the pump is in operation or on standby, cooling medium must remain active to prevent motor overheating.

2. Minimum Flow Line  
When the operating flow rate of a pump falls below 30% of its rated capacity, radial thrust increases significantly. Additionally, low-efficiency operation raises the liquid temperature at the suction side, increasing vapor pressure and making cavitation more likely. To prevent cavitation, a minimum flow line should be installed to ensure normal pump operation under low-flow conditions. There are several reasons for installing a minimum flow line:  
1) When centrifugal pumps operate at low flow rates, fluid separation may occur inside the pump, causing vibration and noise. A minimum flow line ensures a continuous flow of fluid through the pump after startup, preventing cavitation.  
2) For high-power centrifugal pumps, running with the discharge valve fully closed causes internal fluid temperatures to rise sharply, potentially damaging bearings. Installing a minimum flow line maintains a constant flow through the pump during operation, avoiding this risk.  
3) At low flow rates, the pump shaft experiences additional radial forces. A minimum flow line ensures sufficient fluid passes through the pump continuously, reducing excessive radial load on the shaft.  
4) Sometimes, an orifice plate is installed in the minimum flow line. This allows safe pump operation even if the operator mistakenly closes the discharge valve, preventing damage. Using an orifice plate eliminates the need for manual adjustment—unlike control valves, which make it difficult to precisely maintain flow near the pump’s minimum requirement, often resulting in either too much or too little flow.

3. Bypass Line  
When starting a high-head pump, the outlet check valve may be difficult to open due to excessive backpressure. Forcing it open risks damaging the valve stem. Installing a bypass line equipped with an orifice plate before and after the outlet check valve facilitates easy opening. The bypass line also helps reduce pipeline vibration and noise. It further enables pump operation during trial runs or abnormal conditions when the main outlet valve is closed. Note that the installation requirements for the bypass line are similar to those of the warm-up line, but the direction of medium flow differs.

4. Balance Line  
For liquids whose saturated vapor pressure at ambient temperature exceeds atmospheric pressure or are at their boiling point, a balance line is generally recommended to prevent vapor formation or bubble ingress into the pump, thus avoiding cavitation. The balance line connects from the pump inlet to the gas phase section of the suction tower (or tank).

5. Anti-Condensation Line  
When pumping high-pour-point or high-freezing-point liquids that solidify at ambient temperature, anti-condensation lines should be installed for both the standby pump and associated piping to prevent blockage. Typically, two DN20 anti-condensation lines are provided: one connecting from the pump outlet isolation valve to the upstream side of the check valve, and the other from the outlet isolation valve to the upstream side of the inlet isolation valve. Installation should minimize dead spots in the pump inlet and outlet piping. Anti-freezing lines are typically heated using steam or trace heating. In winter, when lubrication performance is poor, a steam line can be installed inside the lubrication tank to provide heating by introducing steam. Trace heating involves wrapping both the steam pipe and the heated pipe with insulation material to achieve the desired heating effect. It's important to prevent condensate formation when steam cools down; any condensate should be promptly drained from the system. A steam valve should be installed on the return line to maintain proper temperature control. The anti-freezing line, also known as a connecting line, is opened during pump oil withdrawal and maintenance to prevent oil from solidifying in the inlet and outlet pipelines. When draining oil for pump maintenance, the inlet and outlet valves are generally closed while the connecting line is opened. Additionally, during plant line sweeping operations, steam must not pass through the pump body for extended periods; therefore, the connecting line should remain open during sweeping. Anti-freezing lines and warm-up lines are similar—both are bypass lines that go around the pump discharge check valve and hand valve (possibly for different media, such as preventing water from freezing and damaging the pump in winter). For standby pumps, the inlet valve should be fully open, and the warm-up or anti-freezing line slightly opened so that flow does not cause reverse rotation of the pump. This ensures continuous flow of high-temperature medium through the pump and piping, preventing solidification and enabling immediate switching. After switching, the warm-up line is usually closed.

6. Pump Discharge Safety Valve Line  
For positive displacement pumps such as electric reciprocating pumps, gear pumps, and screw pumps, a safety valve line is installed on the discharge side. When the discharge pressure exceeds the set value, the safety valve opens, allowing fluid to return to the pump suction line. Piping should be arranged as compactly as possible without interfering with operation.