Centrifugal pump net positive suction head

Answer: When the pump is in operation, vapor is generated at the inlet of the impeller due to a certain vacuum pressure. The vaporized bubbles, driven by the impact of liquid particles, cause erosion on metal surfaces such as those of the impeller, thereby damaging them. This vacuum pressure is known as the vaporization pressure. The net positive suction head is the excess energy per unit weight of liquid at the pump’s inlet, above the vaporization pressure. The unit is indicated in meters, as (NPSH)r. The suction lift is equivalent to the required net positive suction head Δh: it represents the vacuum level at which the pump can draw in liquid, or in other words, the maximum installation height allowed for the pump, measured in meters. Suction lift = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters). The standard atmospheric pressure can create a vacuum in the pipeline up to 10.33 meters. For example: If a pump requires a net positive suction head of 4.0 meters, what is the suction lift Δh? Solution: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters. Generally, the net positive suction head available for a pump refers to the required net positive suction head, while the head in the pump’s inlet pipeline is considered the effective net positive suction head. For example: the required net positive suction head for a water pump is 50 meters, which means that for the pump itself, the inlet pressure must be at least 5 kilograms; otherwise cavitation may occur.

“The suction lift is equal to the required net positive suction head Δh”. If the required net positive suction head for a certain pump is 4.0 meters, what is the suction lift Δh? ”Contradictory, right?