【A Long Journey Begins with a Single Step – Centrifugal Pumps】1.1 Understanding the Concepts: Cavitation and NPSH

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This post was last edited by 3983596_FPPZ on 2020-4-7 15:06. 1.1.7 Cavitation and NPSH: We often hear the term “cavitation evacuation”; in fact, cavitation is the cause, while evacuation is the macroscopic result of severe cavitation. Cavitation is a process in which, at a certain point on a liquid impeller (slightly downstream of the impeller inlet), the pressure drops to the vaporization pressure, resulting in microscopic vaporization followed by re-liquidification. As this process progresses to a certain extent, we observe an increase in the vibration noise of the pump, severe fluctuations in the outlet pressure, and cavitation damage occurring on the metal surface at the inlet of the impeller. In the later stages of cavitation, the pump stops pumping and becomes underloaded, while the motor operates with reduced load. Due to the expulsion and flow-around effect at the inlet blade of the impeller, cavitation often occurs on the back side of the blade, slightly downstream of the inlet edge. The location of damage on the metal surface of the impeller is an important indicator to distinguish whether corrosion, erosion, or cavitation is responsible for the impeller’s damage. Since cavitation is entirely caused by a decrease in pressure, it is crucial to properly design the pressure levels in the areas near the pump inlet in order to control the occurrence of cavitation. The net positive suction head, just like head, is an energy difference; it represents the difference between the pressure energy of a liquid at a certain location and the vaporization pressure energy of that liquid, and is measured in meters. The cavitation head of the unit: The unit refers to an entire set of equipment systems connected to the pump in order to ensure its proper operation, including components such as the inlet liquid supply tank, pipelines, valves, and elbows. The net positive suction head available at a device is a pressure value provided at the inlet of the pump; this value, which is higher than the vaporization pressure of the liquid (or can be considered as a safety margin), is what is referred to as the net positive suction head available at that device. In conditions where vaporization pressure is low and temperatures are low, making vaporization difficult, the net positive suction head of the device is generally high ； In conditions where vaporization pressure is high and temperature is high, due to the costs associated with increasing the system pressure and lowering the temperature, the net positive suction head available in such systems is generally low. The cavitation head of a device is expressed as NPSHa, also known as the effective net positive suction head. The greater the flow rate of a pump and the faster the flow velocity, the greater the losses and the lower the pressure; as a result, the safety margin decreases. Therefore, the relationship between NPSHa and flow rate is a curve that declines as the flow rate increases. Pump net positive suction head: As the fluid enters the pump inlet, its velocity increases gradually, especially when it first reaches the impeller blades. At this point, the blades have not yet had time to do any work, but axial pressure energy must be converted into kinetic energy; as a result, the pressure drops to its lowest level. The back side of the impeller blades is the area most prone to cavitation. Since there is always a pressure drop once the fluid enters the impeller, in order to prevent cavitation at the inlet of these blades, the liquid at the pump inlet must possess a certain amount of energy – an energy level that is higher than the vaporization pressure of the fluid, by a sufficient margin. This energy difference is known as the required net positive suction head, or pump NPSHr. The pump’s NPSHr represents a capability inherent to the pump itself. As the flow rate increases, the pump’s net positive suction head requirement NPSHr follows an upward curve. In pump design, the magnitude of the pump’s net positive suction head and pump efficiency are two conflicting parameters; the smaller the pump’s net positive suction head (the better), the lower the pump efficiency. The pump NPSHr represents an energy requirement with respect to the system’s NPSHa; the lower the pump NPSHr, the better, as it indicates that the medium entering the pump does not require much energy and can cope with various conditions close to the vaporization point ； The higher the NPSHr of the device, the better, as it indicates that the energy of the fluid at the pump inlet is much higher than its vaporization pressure, thereby providing a favorable suction condition for the pump. However, an excessively high NPSHr can sometimes lead to uneconomical design of the device’s operational parameters. Therefore, it is necessary to ensure that NPSHa > NPSHr, with an additional safety margin, i.e., NPSHa - NPSHr > 0.3–0.5 m (API61: 0.5 m). Suction height: The suction height is a concept commonly used in self-priming pumps or in situations where liquid is drawn in from above. Under ideal conditions, one standard atmosphere can lift water to a height of 10.33 meters; thus, the ideal maximum suction height is 10.33 meters. However, in reality, (1) the pump has requirements regarding its net positive suction head (NPSHr), and (2) there are local and overall losses along the suction path. (3) A certain flow velocity must be generated during pumping; the medium needs to have its kinetic energy increased as it goes from a stationary state to a flowing state. (4) A safety margin is also reserved, usually around 0.5. All of the above factors must be deducted from the potential energy of 10.33 meters generated by atmospheric pressure; therefore, the final lifting height h is