Regarding the allowable suction vacuum level of water pumps

Allowable suction vacuum: When the vacuum level at the inlet of the pump reaches a certain level during operation, the liquid may vaporize inside the pump, preventing it from functioning. The maximum suction vacuum level permitted during pump operation is referred to as the “allowable suction vacuum,” denoted by Hs, with the unit being MPa. 　　The allowable suction vacuum degree of a pump is an important indicator of its suction performance and cavitation resistance. The higher the vacuum level Hs that can be achieved by suction, the better the pump’s suction performance and its resistance to cavitation. It is mainly related to the form and structure of the pump; simultaneously, as atmospheric pressure decreases, liquid temperature rises, and pump flow rate increases, the allowable suction vacuum level also decreases. The Hs value indicated on the pump nameplate is obtained by the manufacturer through testing under standard atmospheric pressure (760 mmHg), with clean water at room temperature (20°C), under rated operating conditions. According to national standards, during testing, the suction vacuum degree of the pump is increased gradually. For positive displacement pumps, the calibration value of Hs is defined as the suction vacuum degree corresponding to a flow rate that is 3% lower than that under normal operating conditions. In contrast, impeller pumps use a specified value for the decrease in head or efficiency as the critical condition, with a certain margin added, which is indicated in the form of the required net positive suction head Δhr. 　　The allowable suction vacuum degree of a water pump is often expressed in terms of the height of a water column in meters; this is known as the allowable suction vacuum level, denoted by [Hs]. ［Hs］＝Hs／ρg. [Hs] can be used to calculate the maximum allowable suction lift of a water pump (allowable suction height). 　　Based on the suction head of the pump: The allowable suction vacuum can be determined. When the pressure at the liquid surface being sucked in, psr, is equal to the atmospheric pressure pa, then the allowable suction vacuum is as follows. The permissible suction height of the pump is therefore given by the allowable suction vacuum value. For water pumps, this can be expressed as follows: The permissible suction height of the pump can be calculated by subtracting the head loss due to the suction velocity and the resistance in the suction pipeline from the allowable suction vacuum height [Hs], under conditions of normal temperature and when the pressure at the liquid surface is approximately equal to standard atmospheric pressure.

The explanation was very detailed and easy to understand; thank you

The allowable suction vacuum level indicates how deep the pump can draw water from below the inlet, and this relates to the operating conditions during on-site installation. The net positive suction head provided by the manufacturer is closely related to the pump design and manufacturing process. Estimation formula: NPSH = 10.33 meters – allowable suction vacuum – 0.5 meters (safety margin). 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 of 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 that exceeds the vaporization pressure, and it is expressed in meters. 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.

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