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NPSH is primarily a parameter used to measure a pump’s suction capacity. The NPSH of a water pump refers to the head of energy per unit mass of liquid at the pump’s inlet that exceeds the saturated vapor pressure; it is expressed in meters and denoted as (NPSH)r. Definition of NPSH: NPSH, also known as net positive suction head, is mainly a parameter used to assess a pump’s suction capacity. We all know that one standard atmosphere is equivalent to about 10 meters of water column. In other words, if a pump is placed above a very deep pool of water, the water surface remains in contact with the atmosphere. At this point, if the pump is used to discharge water outward, it is most likely to lower the water level to a distance of 10 meters perpendicular to the pump’s axis. If the pump continues to run, the water level cannot drop any further at this point. The pump is also unable to pump water outward; what it discharges is air. This condition is what we call cavitation. But in reality, the pump cannot lower the water surface to a distance of 10 meters perpendicular to its axis; some water will always remain. If this remaining amount of water is also expressed in meters, it represents the net positive suction head available for this pump, also known as the required net positive suction head, NPSHr. This value is typically determined by the pump manufacturer using clean water at 20°C and at the pump’s rated flow rate, with the unit being meters. The lower the NPSHr, the better the suction performance of the pump. The formula for net positive suction head: When a water pump is in operation, under certain vacuum pressures at the impeller’s inlet, gas is generated from the liquid. These gas bubbles, due to their collision with liquid particles, cause erosion on the impeller and metal surfaces, ultimately damaging the impeller and other metal components. This vacuum pressure is known as the vaporization pressure. Net positive suction head refers to the excess energy per unit weight of liquid at the pump’s suction inlet, beyond the vaporization pressure. In simpler terms, it is the excess energy per unit mass of liquid at the pump inlet, relative to the saturation vapor pressure; this excess energy is expressed in terms of head. 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 head = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters). For example, if a pump has an NPSH of 4.0 meters, the suction head Δh is calculated as follows: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters. The standard atmospheric pressure can create a vacuum in the pipeline up to a height of 10.33 meters. Classification of net positive suction head: Net positive suction head refers to the difference between the total head of the liquid at the pump inlet and the pressure head required for the liquid to vaporize. It can be classified into the following categories: NPSHa – System net positive suction head, also known as effective net positive suction head, represents the energy remaining in the liquid before it enters the pump; this energy can be utilized to prevent cavitation. It is determined solely by the device system of the suction side pipeline of the pump, and has nothing to do with the pump itself. Corrosion will not occur as long as the static head of the liquid flowing toward the center of the pump inlet is high enough to exceed the vapor pressure of the liquid at that temperature. The larger it is, the less prone to cavitation ； NPSHr —— pump net positive suction head, also known as the required net positive suction head or the dynamic pressure drop at the pump inlet; the lower this value, the better the pump’s resistance to cavitation ； NPSHc – the experimental net positive suction head, refers to the net positive suction head corresponding to a certain degree of decline in pump performance. It is a value calculated during cavitation tests. There can be multiple experimental net positive suction head values, but there is only one that corresponds to a certain degree of decline in pump performance; this value is known as the critical net positive suction head, denoted as NPSHc ； ——Allowable net positive suction head is the net positive suction head used to determine the operating conditions of the pump (such as the installation height); it should be greater than the critical net positive suction head to ensure that cavitation does not occur during pump operation. Typically, it is taken as = (1.1～1.5) NPSHc. These net positive suction heads have the following relationship: NPSHc ≤ NPSHr ≤ NPSHa. The physical meaning of net positive suction head: (1) After a liquid enters a multi-stage pump, before its energy is increased by the impeller, there is a reduction in its pressure head. This reduction is caused by changes in flow velocity and hydraulic losses. The main factors affecting it are the geometry of the pump’s suction chamber and the flow velocity; it has nothing to do with parameters such as the suction piping or the properties of the liquid. It depends solely on the pump’s structure; more precisely, it depends only on the shape of the impeller and the inlet configuration ； (2) NPSHr and NPSHa are two parameters with different properties. NPSHr is determined by the characteristics of the pump itself; it is a parameter that indicates the pump’s resistance to cavitation. The higher the value of NPSHr, the worse the pump’s cavitation resistance, and vice versa. On the other hand, NPSHa is determined by the characteristics of the external suction system. (3) The boundary at which cavitation occurs in the pump is NPSHa = NPSHr; when NPSHa = NPSHr, that is, when pk = pv, cavitation begins in the pump ； NPSHa NPSHa > NPSHr corresponds to (pk > pv), meaning the pump is free from cavitation. Method for calculating the net positive suction head of a device (1) Formula for calculating net positive suction head Image Where ps/ρg —— inlet pressure head converted to the reference level (m) ； v2s/2g —— Measured average velocity head (m) of the liquid at the ps cross-section ； pv/ρg — Vapor pressure head at the temperature of the pumped liquid (m) ； NPSH — Net Positive Suction Head (m); its value is expressed as a value converted to the reference plane (i.e., NPSH is calculated using the pressure head ps/ρg converted to the reference plane).