Cavitation in centrifugal pumps

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What is cavitation? When the local pressure of the liquid in the pump drops to the critical pressure, bubbles form within the liquid. Cavitation is the entire process of bubble formation, movement, splitting, and disappearance. The critical pressure is generally close to the vaporization pressure. What are the hazards of cavitation? 01 Corrosion of components subject to overcurrent. There are two reasons for this corrosion: one is the high-frequency impacts (600–25,000 Hz) generated when bubbles burst, with pressures as high as 49 Mpa, which cause mechanical erosion of the metal surface ； Secondly, heat is released during vaporization, and the temperature difference battery effect leads to hydrolysis; the oxygen generated causes the metal to oxidize, resulting in chemical corrosion. 02 Decline in pump performance: When cavitation occurs in the pump, the energy exchange within the impeller is disrupted and impaired. This is reflected in the external characteristics as a decline in the Q-H curve, as well as in the Q-P and Q-η curves. In severe cases, the fluid flow within the pump is interrupted, preventing it from functioning. For low specific speed, due to the narrow and long flow channels between the blades, once cavitation occurs, bubbles fill the entire flow channel, causing a sharp drop in the performance curve. For medium to high specific speed values, the flow channel is short and wide; as a result, it takes a transition period for bubbles to develop from their formation point to filling the entire flow channel. Consequently, the performance curve starts to decline gradually before dropping sharply once a certain flow rate is reached. The part of a centrifugal pump where cavitation is most likely to occur is the front cover, where the curvature of the impeller is greatest, on the low-pressure side near the inlet edge of the blades ； The low-pressure side near the inlet edge of the volute baffle and guide vanes in the extrusion chamber ； The sealing gap between the outer circumference of the blade tip of the high specific speed impeller without a front cover and the housing, as well as the low-pressure side of the blade tip ； The first-stage impeller in a multi-stage pump. Measures to improve cavitation resistance 01 Measures to enhance the cavitation resistance of the centrifugal pump itself: Improve the structural design from the pump’s inlet to the vicinity of the impeller. Increase the overcurrent area ； Increase the radius of curvature at the inlet section of the impeller cover to reduce the sudden acceleration and pressure drop of the fluid flow ； Appropriately reducing the thickness at the blade inlet and rounding it to make it more streamlined can also reduce the acceleration and pressure drop around the blade tip ； Improve the surface finish of the impeller and blade inlet sections to reduce resistance losses ； Extend the inlet edge of the vane toward the impeller inlet so that the liquid flow is made to do work earlier, thereby increasing the pressure. A pre-induction wheel is used to enable the liquid flow to do work in advance within the pre-induction wheel, thereby increasing the pressure of the liquid flow. By using a double-suction impeller, the fluid flow enters the impeller from both sides, doubling the inlet area and allowing the inlet flow velocity to be reduced by half. The design condition uses a slightly larger positive attack angle in order to increase the blade inlet angle, reduce bending at the blade inlet, minimize blade blockage, and thereby increase the inlet area ； Improve working conditions under high flow rates to reduce flow losses. However, the impact angle should not be too large, otherwise it will affect efficiency. Use materials resistant to cavitation. Practice has shown that the higher the strength, hardness, and toughness of a material, the better its chemical stability and its resistance to cavitation. 02 Measures to increase the effective NPSH of the liquid feed device: Increase the pressure of the liquid level in the reservoir before the pump in order to raise the effective NPSH. Reduce the installation height of the suction unit pump. Replace the upward suction device with a backflow device. Reduce the flow losses in the pipeline before the pump. For example, minimize the length of the pipelines within the required range, reduce the flow velocity in the pipelines, decrease the number of bends and valves, and maximize the opening degree of the valves. Reduce the temperature of the fluid at the pump inlet (when the fluid being transported is close to its saturation temperature). The above measures can be appropriately applied through comprehensive analysis based on factors such as pump selection, material choice, and the operating environment of the pump. NPSH and suction head: When a 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. NPSH refers to the excess energy per unit weight of liquid at the pump’s inlet that exceeds the vaporization pressure; it is expressed in meters and denoted 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 lift = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters). The standard atmospheric pressure can create a vacuum in the pipeline up to 10.33 meters. For example: If a pump has a net positive suction head of 4.0 meters, what is the suction lift Δh? Solution: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters. What are the units and symbols used for each value? NPSH 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 is expressed in meters (of water column) and denoted as (NPSH). It can be classified into the following types: NPSHa – also known as available NPSH; the higher this value, the less likely cavitation will occur ； 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 —— Critical net positive suction head, refers to the net positive suction head at which the pump’s performance declines by a certain amount ； ——The allowable NPSH is the NPSH used to determine the operating conditions of a pump, and it is usually taken as = (1.1–1.5) NPSHc. What is the difference between required NPSH and available NPSH? NPSH is divided into available NPSH, NPSHa, and required NPSH, NPSHr. The required net positive suction head of a pump is a characteristic of the pump determined by its design, while the effective net positive suction head of the pump is determined by the process piping. For a given pump, the net positive suction head required at a specified speed and flow rate is known as the required net positive suction head, commonly denoted as NPSHr. Also known as the net positive suction head of a pump, it is a cavitation performance parameter that must be achieved by the pump. NPSHr is related to the internal flow within the pump and is determined by the pump itself. Its physical meaning is the degree of pressure drop in the liquid at the pump inlet; in other words, to prevent cavitation, it is necessary for the liquid at the pump inlet to have an excess of energy in the form of head over the vaporization pressure. The net positive suction head must be independent of the device parameters, and depend only on the dynamic parameters of the pump inlet section (vo, wo, wk, etc.), which are determined by geometric parameters at a certain speed and flow rate. In other words, NPSHr is determined by the pump itself (the geometric parameters of the suction chamber and the impeller inlet). For a given pump, regardless of the medium (with the exception of highly viscous media, which affect the velocity distribution), when fluid flows into the pump inlet at a certain speed and flow rate, the same pressure drop occurs due to the identical velocity levels; in other words, the NPSHr is the same. Therefore, NPSHr is independent of the properties of the liquid (excluding thermodynamic factors). The smaller the NPSHr, the lower the pressure drop and the lesser the NPSHa that the system must provide; consequently, the better the pump’s cavitation resistance. Therefore, r stands for required, meaning it is determined by the pump itself, and is related to factors such as speed and impeller design ； The available net positive suction head is the net positive suction head determined by the pump’s installation conditions, and it is commonly denoted as NPSHa. Also known as the vaporization margin of the unit, it is the excess energy per unit weight of liquid at the pump inlet, provided by the suction unit, that exceeds the vaporization pressure head. The greater the NPSHa, the less likely the pump is to experience cavitation. The magnitude of the effective net positive suction head is related to the device parameters and the properties of the liquid (p, pv, etc.). Since the hydraulic loss of the inhalation device is proportional to the square of the flow rate, NPSHa decreases as the flow rate increases. Therefore: A stands for available, meaning it is usable and can be provided; this is determined by the system and pipelines, and strict calculations are required ； To prevent cavitation in the pump, NPSHa must be greater than NPSHr. As for exactly how much more, there are empirical values for various types of pumps; generally, an additional head of 0.5–1 m is added to the pump’s required net positive suction head to serve as the allowable net positive suction head.