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Before explaining these two terms, let’s first explain the net positive suction head associated with their meanings. NPSH refers to the *lowest absolute pressure that must be maintained at the suction inlet of a water pump. This absolute pressure is used to compensate for the continuous pressure loss as water enters the pump and moves towards the surface of the impeller, ensuring that even after this pressure loss occurs, the pressure remains above the saturated vapor pressure corresponding to the current water temperature. That sounds a bit abstract; let’s make it more concrete using the example in the figure below: when the atmospheric pressure at the installation location of the water pump (a self-priming magnetic pump) is 10 m (absolute pressure), 7 m is lost due to the height difference in the water level at the pump’s inlet, and 2 m is lost due to local resistances such as elbows and valves. By the time the fluid reaches point P0, the remaining absolute pressure is 1 m. The saturated vapor pressure of water at 20°C can be considered as 0 m. If the pump’s net positive suction head is 2 m, it means that the water pressure will drop below the saturated vapor pressure of 0 m even before it reaches the impeller, causing the water to vaporize; this is what is referred to as cavitation at the impeller location. So, returning to our initial question: what is the relationship between self-priming and suction lift? Self-priming is a description of the pump’s functionality, indicating that the pump has the ability to self-primed; whereas suction lift denotes the height at which water can be drawn up, and it is not entirely related to the self-priming capability. Specifically, self-priming refers to the ability of a water pump, after it is installed, to have its inlet pipe lowered below the liquid level; there is no need to install a check valve at the bottom of the pipe. The pump can be put into use simply by filling it with the amount of water it contains, and it will automatically expel the air from the inlet pipe and draw up water from the low liquid level. This function is what we call self-priming. Thanks to the check valve (foot valve) installed on the inlet pipeline, when filling the pump, not only is the pump itself filled with water but the inlet pipeline as well. Once the water pump is started, it can continuously pump water from the lower water level. The height difference between the pump’s suction inlet and the liquid surface is referred to as the suction head. In principle, the head value is related to factors such as atmospheric pressure, saturated vapor pressure, pipe resistance, and the cavitation head of the water pump. However, for practical purposes, atmospheric pressure is usually taken as 10 m, while saturated vapor pressure and pipe resistance are considered to be 0. Thus, the head = 10 – cavitation head of the pump. For example, if a water pump has a cavitation head of 3 m, then its head is 7 m, which means it can draw water up from a liquid level that is 7 m below. It’s not only self-priming pumps that have a suction lift; generally, centrifugal pumps also possess a suction lift. The amount of this suction lift varies depending on the design of the impeller, as well as the net positive suction head available. Therefore, the operation of drawing water at low water levels is not necessarily limited to self-priming pumps; other products that meet the requirements in terms of flow rate, head, and net positive suction head can also be used.