【Daily Question: Pump Equipment】Short Answer Question, June 28, 2019

The pressure difference at which cavitation just does not occur in the pump under certain operating conditions, expressed in head.

The excess energy per unit weight of liquid at the pump inlet, above the vaporization pressure, is expressed in meters.

When the 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 on metal surfaces such as those of the impeller, thereby damaging them. This vacuum pressure is known as the vaporization pressure. The net positive suction head is the excess energy per unit weight of liquid at the pump’s inlet that exceeds the vaporization pressure, and it is expressed in meters.

When the 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 on 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 suction 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: that is, the degree of vacuum that the pump can tolerate when sucking in liquid, or in other words, the maximum allowable installation height of the pump. The unit used is 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.

When the 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 on 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 suction 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: that is, the degree of vacuum that the pump can tolerate when sucking in liquid, or in other words, the maximum allowable installation height of the pump. The unit used is 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.

When the 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 on 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 suction 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: that is, the degree of vacuum that the pump can tolerate when sucking in liquid, or in other words, the maximum allowable installation height of the pump. The unit used is 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. 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. Definition of cavitation: When a centrifugal pump is in operation, the liquid pressure decreases as it moves from the pump inlet to the inlet of the impeller; at point K, near the inlet of the blades, the liquid pressure pK is at its lowest value. Thereafter, as the impeller does work on the liquid, the liquid pressure rises rapidly. When the pressure pK near the inlet of the impeller blades is less than the saturated vapor pressure pv at the liquid transport temperature, the liquid vaporizes. At the same time, it allows the gases dissolved in the liquid to escape. They form many bubbles. When the bubble moves with the liquid to a region of higher pressure within the flow channel, the external liquid pressure is greater than the vaporization pressure inside the bubble; as a result, the bubble condenses and collapses, forming a cavity. Instantly, the surrounding liquid rushes toward this cavity at extremely high speeds, causing the liquids to collide with each other and leading to a sudden increase in local pressure (which can reach several hundred atmospheres). In this way, it not only hinders the normal flow of the liquid; more seriously, if these bubbles collapse near the wall of the impeller, the liquid acts like countless small projectiles, continuously striking the metal surface. Its impact frequency is very high (reaching 2000–3000 Hz in some cases), causing the metal surface to crack due to impact fatigue. If the bubbles contain some active gas (such as oxygen), they can act as thermocouples thanks to the heat released during bubble condensation (with local temperatures reaching 200–300°C). This leads to electrolysis, thereby causing electrochemical corrosion and further accelerating the rate of metal degradation. The combined phenomenon of vaporization, condensation, impact, generation of high pressure and temperature, as well as high-frequency impact loads in such liquids, which leads to mechanical spalling and electrochemical corrosion damage of metal materials, is known as cavitation. The area where cavitation is most likely to occur in a centrifugal pump is a. the front cover, where the curvature of the impeller is greatest, on the low-pressure side near the inlet edge of the blades ； b. The low-pressure side near the inlet edge of the volute baffle and guide vanes in the extrusion chamber ； c. 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 ； d. The first-stage impeller in a multi-stage pump. What is net positive suction head? What is suction lift? Their respective units of measurement and designation letters? Answer: When a pump is in operation, a certain amount of liquid vapor is generated at the inlet of the impeller due to a specific vacuum pressure. Under the impact of liquid particles, these vaporized bubbles cause erosion on metal surfaces such as the impeller, thereby damaging them. This vacuum pressure is known as the vaporization pressure. The net positive suction head refers to the excess energy per unit weight of liquid at the pump’s suction inlet, beyond the level required to overcome the vaporization pressure. The unit is meters of liquid column, denoted as (NPSH)r. The suction lift is equivalent to the required net positive suction head Δ/h: it represents the vacuum level at which the pump can draw in liquid, or in other words, the maximum geometric installation height allowed for the pump. The unit is meters. Suction lift = Standard atmospheric pressure (10.33 meters) – NPSH – Pipeline losses – Safety margin (0.5). Standard atmospheric pressure can create a vacuum height of 10.33 meters in a pipeline. 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. Cavitation occurs when, at a certain temperature, the pressure is reduced to the vaporization pressure at that temperature, causing bubbles to form in the liquid. This phenomenon of bubble formation is called cavitation. The bubbles formed during cavitation, when they move to areas of higher pressure, shrink in size and eventually burst. 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 damaged. 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, in addition to causing damage to the flow-through components, it also generates noise and vibration, leading to a decline in the pump’s performance; in severe cases, it can even cause the liquid inside the pump to stop flowing, preventing the pump from functioning properly. Measures to improve cavitation resistance a. Measures to enhance the cavitation resistance of the centrifugal pump itself (1) Improve the structural design from the pump’s suction 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 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 before the pump in order 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 loss in the pipeline before the pump. For example, shorten the pipelines as much as possible within the required range, reduce the flow velocity in the pipelines, minimize the use 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 location where the pump will be used.

It is the pressure drop between the pump inlet and a point slightly downstream of the impeller inlet blades, measured in meters

The net positive suction head required at a given speed and flow rate is known as the required net positive suction head, denoted by NPSHr. Also known as the net positive suction head available, NPSHr is related to the internal flow within the pump and is determined by the pump itself. Its physical meaning is to indicate the degree of pressure drop at the pump’s inlet, and it serves to ensure that cavitation does not occur in the pump.

Height difference between the pump’s suction inlet and the liquid storage tank

NPSH refers to the excess energy per unit weight of liquid at the pump inlet, above its 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 a centrifugal pump can still draw 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. NPSH refers to the difference between the total head of the liquid at the pump inlet and the pressure head at the point where the liquid vaporizes. It is expressed in meters (of water column) and denoted as (NPSH). NPSHr, also known as the required NPSH, is a parameter that specifies the cavitation resistance required of a pump; the lower the NPSHr, the better the pump’s resistance to cavitation.