What is the difference and connection between the allowable suction vacuum level and the net positive suction head?
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The vacuum level at the suction inlet of a water pump is referred to as the suction vacuum head. To prevent cavitation in the pump, it is necessary to keep this suction vacuum head below a certain limit value; this limit value is the allowable suction vacuum head specified by the manufacturers of centrifugal pumps； The net positive suction head is the total head at the pump’s inlet, minus the head remaining to prevent vaporization at the point of lowest pressure inside the pump (i.e., the actual net positive suction head). To avoid cavitation, this remaining head must be greater than the required net positive suction head, which is equal to the pressure loss from the inlet to the point of lowest pressure plus a certain safety margin. It can be seen that the allowable suction vacuum level and the required net positive suction head are conditions for preventing cavitation in water pumps, viewed from different perspectives.
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. NPSH refers to the excess energy per unit weight of liquid at the pump’s suction inlet that exceeds the vaporization pressure; it is expressed in meters and denoted as (NPSH)r. The suction lift is the required net positive suction head △h: it represents the vacuum level at which the pump can draw in liquid, or in other words, the installation height of 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.
:) NPSH refers to the excess energy per unit weight of liquid at the pump inlet, above its vaporization pressure
Please provide a detailed explanation for this image; I don’t understand it