Thread Content
NPSH, also known as net positive suction head, is a parameter that primarily measures a pump’s suction capacity. We all know that 1 standard atmosphere is approximately equal to 10 meters of water column. In other words, if a pump is placed above a very deep pool whose surface is in contact with the atmosphere, and the pump is used to pump water out, it is most likely to lower the water level to a distance of 10 meters perpendicular to the pump’s axis. If the pump continues to operate, the water level cannot drop any further at this point. The pump is also unable to pump water outward; what it discharges is air. This condition is what we call cavitation. But in reality, the pump cannot lower the water level to a distance of 10 meters perpendicular to its axis; some water will always remain. If this remaining amount of water is also expressed in meters, it represents the net positive suction head available for this pump, also known as the required net positive suction head, NPSHr. This value is typically determined by the pump manufacturer using clean water at 20°C and at the pump’s rated flow rate, with the unit being meters. The lower the NPSHr, the better the suction performance of the pump. Definition of net positive suction head: When a pump is in operation, vapor is generated at the inlet of the impeller due to a certain vacuum pressure. These vapor bubbles, driven by the impact of liquid particles, cause erosion of the impeller and metal surfaces, ultimately damaging them. This vacuum pressure is known as the vaporization pressure. Net positive suction head refers to the excess energy per unit weight of liquid at the pump’s inlet that exceeds the vaporization pressure; in simpler terms, it is the head of pressure that corresponds to the excess energy per unit mass of liquid at the pump’s inlet, above the saturated vapor pressure. The unit is indicated in meters, 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). For example, if a pump has an NPSH of 4.0 meters, the suction lift Δh is calculated as follows: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters. The standard atmospheric pressure can create a vacuum in the pipeline up to a height of 10.33 meters. Classification of net positive suction head available. Net positive suction head available 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 can be classified into the following categories: NPSHa – also known as effective net positive suction head available – represents the energy remaining in the liquid before it enters the pump, and this energy can be utilized effectively to prevent cavitation. It is determined solely by the device system of the suction side pipeline of the pump, and has nothing to do with the pump itself. Corrosion will not occur as long as the static head of the liquid flowing toward the center of the pump inlet is high enough to exceed the vaporization pressure of the liquid at that temperature. The larger it is, the less prone to cavitation ； 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 – the experimental net positive suction head, refers to the net positive suction head corresponding to a certain degree of decline in pump performance. It is a value calculated during cavitation tests. There can be multiple experimental net positive suction head values, but there is only one that corresponds to a certain degree of decline in pump performance; this value is known as the critical net positive suction head, denoted as NPSHc ； ——Allowable net positive suction head is the net positive suction head used to determine the operating conditions of the pump (such as the installation height); it should be greater than the critical net positive suction head to ensure that cavitation does not occur during pump operation. Typically, it is taken as = (1.1～1.5) NPSHc. These NPSH values are related as follows: NPSHc ≤ NPSHr ≤ NPSHa. The physical meaning of NPSH: (1) It represents the amount by which the pressure energy of the liquid decreases after it enters a multi-stage pump, before any energy is added by the impeller. This decrease is caused by changes in flow velocity and hydraulic losses. The main factors affecting it are the geometry of the pump’s suction chamber and the flow velocity; it is unrelated to parameters such as the suction pipeline or the properties of the liquid. It depends solely on the structure of the pump, more precisely, on the shape of the impeller and the inlet ； (2) NPSHr and NPSHa are two parameters with different properties. NPSHr is determined by the characteristics of the pump itself; it is a parameter that indicates the pump’s resistance to cavitation. The higher the value of NPSHr, the worse the pump’s cavitation resistance, and vice versa. On the other hand, NPSHa is determined by the characteristics of the external suction system. (3) The limit at which cavitation occurs in the pump is NPSHa = NPSHr; when NPSHa = NPSHr, it corresponds to (pk = pv), and cavitation begins in the pump ； NPSHa NPSHa > NPSHr corresponds to (pk > pv), meaning the pump is free from cavitation. Cavitation phenomenon: At a certain temperature, when the pressure of a liquid is reduced to its vaporization pressure at that temperature, bubbles form in the liquid. This phenomenon of bubble formation is called cavitation. The bubbles formed during cavitation shrink in size as they flow to higher pressure areas, eventually bursting. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse. While the pump is in operation, if the absolute pressure of the liquid being pumped in a certain local area of its flow path – usually somewhere slightly downstream of the inlet to the impeller blades – drops to the vaporization pressure of the liquid at that temperature for some reason, the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly until they burst. As the bubbles condense and burst, liquid particles fill the voids at high speeds; this creates a very strong water hammer effect, which strikes the metal surface at a high frequency. The impact stress can reach several hundred to several thousand atmospheres, while the impact frequency can be in the tens of thousands of times per second. In severe cases, this can cause the wall thickness to be penetrated. The process in which bubbles are formed and burst in a water pump, causing damage to the flow-through components, is known as cavitation in water pumps. After cavitation occurs in a water pump, apart from damaging the flow-through components, it also generates noise and vibration, leading to a decline in the pump’s performance. In severe cases, it can cause an interruption in the flow of liquid, preventing the pump from functioning properly. Measures to improve cavitation resistance: a. Measures to enhance the cavitation resistance of centrifugal pumps themselves: (1) Improve the structural design from the pump’s suction inlet to the vicinity of the impeller. Increase the flow area ； Increase the radius of curvature in the inlet section of the impeller cover plate to reduce the sudden acceleration and pressure drop of the liquid 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 the inlet section of the blades to reduce drag losses ； Extending the blade inlet edge toward the impeller inlet allows the fluid flow to be worked on earlier, thereby increasing pressure. (2) 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. (3) A double-suction impeller is used, allowing the fluid to enter the impeller from both sides; as a result, the inlet cross-section doubles, and the inlet flow velocity can be reduced by half. (4) A slightly larger positive attack angle is adopted in the design conditions 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. (5) Use materials resistant to cavitation. Practice has shown that the higher the strength, hardness, and toughness of a material, as well as its chemical stability, the better its resistance to cavitation. b. Measures to increase the effective NPSH of the liquid inlet device: (1) Increase the pressure of the liquid level in the liquid storage tank ahead of the pump to raise the effective NPSH. (2) Reduce the installation height of the pump in the suction device. (3) Replace the upward suction device with a backflow device. (4) 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. (5) 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.