Dear seniors, I would like to ask how to understand the net positive suction head; what is the effective net positive suction head and the required net positive suction head?
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How can the net positive suction head be understood more easily? What is the relationship between the net positive suction head, the effective net positive suction head, and the required net positive suction head? Also, do self-priming and backflow affect the pump’s net positive suction head? Thank you to all the seniors~~~
The effective NPSH is determined by the system, while the required NPSH is determined by the pump characteristics; the former must be greater than the latter.
The net positive suction head is divided into the effective net positive suction head NPSHa and the required net positive suction head NPSHr. A stands for available, meaning it is feasible to provide; this is determined by the system and pipelines, and strict calculations are required ； r stands for required, meaning it is determined by the pump itself, and is related to factors such as speed and impeller design ； To prevent cavitation in the pump, NPSHa must be greater than NPSHr. Exactly how much more, there are empirical values for various types of pumps ； 1. The basic conditions for cavitation to occur in a pump are: (1) the lowest liquid flow pressure Pk at the inlet of the blades ≤ the saturated vapor pressure Pt of the liquid at that temperature. 2. Effective NPSH and required NPSH (1) Effective NPSH: The excess head above the saturated vapor pressure of the liquid that exists when the liquid flows from the suction tank, through the suction pipeline, to the pump’s inlet. Denoted by Δha. (2) Required net positive suction head of the pump: The total energy loss of the fluid flow from the pump inlet to the lowest pressure point K inside the impeller, denoted as Δhr. (3) Differences and relationships between Δhr and Δha: When the pump’s effective net positive suction head is greater than its required net positive suction head, the pump does not experience cavitation; when the effective net positive suction head is equal to the required net positive suction head, the pump begins to experience cavitation; when the effective net positive suction head is less than the required net positive suction head, the pump suffers from severe cavitation. (4) Generally, an excess head of 0.5–1 m is added to the pump’s required net positive suction head to determine the allowable net positive suction head. 3. The required net positive suction head of a pump is a characteristic of that pump, determined by its design, while the effective net positive suction head of the pump is determined by the process piping.