Effective NPSHA (NPSHA) and Required NPSHB (NPSHB) for centrifugal pumps

Category: Water pump knowledge

2024-06-07

Centrifugal pump uses the impeller to rotate at high speed in a fixed shell to transfer energy to the fluid. When the impeller rotates with the shaft, the fluid between the blades also obtains centrifugal force with the rotation of the impeller, and the fluid is thrown out from the outlet between the blades. The high-speed fluid enters the diffusion chamber after leaving the top of the blade, the flow rate begins to decrease, the kinetic energy of the fluid is converted into higher pressure energy, and the fluid is thrown out, the pressure of the central part of the impeller is reduced, and the external fluid can enter from the suction port of the pump, which can continuously transport the fluid. Compared with other types of pumps, centrifugal pumps are currently the most widely used.

The cavitation margin of the centrifugal system refers to the ability of the pump to avoid cavitation when the flow rate is large under rated conditions. Cavitation refers to the presence of gas or steam in the liquid in the centrifugal pump, due to the decrease of the static pressure of the liquid, the gas or steam begins to evaporate to form bubbles, and then collapses in the high pressure area to produce liquid impact force. Pump NPSH refers to the unit weight of the fluid from the pump suction inlet to the impeller inlet pressure at the lowest pressure drop, known as net positive suction head (NPSH). It is an important parameter to characterize the pump cavitation resistance.

For NPSH, there are two main reference indexes: effective NPSH (NPSHA) and required NPSH (NPSHB).

1. Required NPSH (NPSHB)

Required NPSHB (NPSHB) refers to the pressure drop per unit weight of fluid from the pump suction port to the lowest pressure at the impeller blade inlet, which is a function of the pump and flow rate.

2. Effective NPSHA (NPSHA)

Effective NPSHA (NPSHA) refers to the excess energy per unit weight of the fluid at the west inlet of the pump that exceeds the saturation vapor pressure, which is a function of the system and flow rate;

If the effective NPSHA (NPSHA) is greater than the required NPSHB (NPSHB), the pump should be free of cavitation. Excessive cavitation will not only affect the efficiency of the pump, but may also cause damage to the pump.

The size of the NPSH is closely related to the design and selection of the centrifugal pump. For different types and specifications of centrifugal pumps, the NPSH requirements will be different. Typically, the design and selection process will refer to relevant standards and empirical data to determine the appropriate NPSH. Considering safety and performance stability, the cavitation margin of the centrifugal pump should be as large as possible to prevent potential cavitation problems.

This liquid impact force negatively affects the operational stability and performance of the pump. The cavitation margin of the centrifugal system is actually a safety margin in the design and operation of the centrifugal pump, which ensures that cavitation does not occur during the operation of the pump. The NPSH of a centrifugal pump requires consideration of a number of factors, including the nature of the liquid, the geometry of the pump, the design of the impeller and the operating conditions. In general, the NPSH should be large enough to ensure stable performance of the pump under rated conditions and to avoid cavitation under high flow conditions.

The cavitation margin of centrifugal pump is an important parameter in the design and operation of centrifugal pump, which is to ensure that the centrifugal pump can avoid cavitation under rated conditions, maintain stable operation and good performance, through reasonable design and selection, to ensure that the centrifugal pump has sufficient cavitation margin, can improve the reliability and efficiency of the pump, and extend the service life of the equipment.

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