Knowledge about pumps

This formula applies to clear water.

Error 1: The statement that “the suction lift is equal to the required net positive suction head Δh” is incorrect; the suction lift is approximately equal to standard atmospheric pressure minus the pump’s required net positive suction head. Error 2: “Suction lift = standard atmospheric pressure (10.33 meters) – net positive suction head – safety margin (0.5 meters); based on this formula, the maximum height at which a pump can be installed is 9.83 meters.” This is incorrect (or rather, it’s a misconception), because for certain types of pumps, the net positive suction head can be negative (for example, in vertical tubular pumps), and in such cases the pump can be installed at a height higher than 9.83 meters.

Error 1: The statement that “the suction lift is equal to the required net positive suction head Δh” is incorrect; the suction lift is approximately equal to standard atmospheric pressure minus the pump’s required net positive suction head. Error 2: “Suction lift = standard atmospheric pressure (10.33 meters) – net positive suction head – safety margin (0.5 meters); based on this formula, the maximum height at which a pump can be installed is 9.83 meters.” This is incorrect (or rather, it’s a misconception), because for certain types of pumps, the net positive suction head can be negative (for example, in vertical tubular pumps), and in such cases the pump can be installed at a height higher than 9.83 meters. What was said on the third floor is right, exactly

May I ask, is the NPSH required for a pump the same concept as NPSH? Confused:(

1. The view that “the suction lift is equal to the required net positive suction head Δh” is correct. 2. The claim that “based on this formula, the maximum height at which a pump can be installed is 9.83 meters” is incorrect, as some pumps can be installed at heights of over 10 meters

Actual NPSH = Required NPSH + 0.5

It should also be related to the density of the medium sampled

NPSH refers to the excess capacity of the liquid pressure at the pump inlet over the vaporization pressure of the liquid. The required NPSH is determined by the structure of the pump itself, and it is established by the pump manufacturer through experiments.

Basic Knowledge of Pumps I. What is a pump? A pump is a machine used to transport liquids or to increase their pressure. It transfers the mechanical energy of the prime mover or other external energy to the liquid, thereby increasing the energy of the liquid. Pumps are mainly used to transport liquids such as water, oil, acids, alkalis, emulsions, suspensions, and liquid metals; they can also handle mixtures of liquids and gases, as well as liquids containing suspended solids. Pumps can generally be classified into three categories based on their working principle: positive displacement pumps, dynamic pumps, and other types of pumps. In addition to classification by working principle, it can also be classified and named using other methods. For example, based on the driving method, they can be divided into electric pumps and hydraulic pumps, etc ； Based on structure, they can be divided into single-stage pumps and multi-stage pumps ； Based on their purpose, they can be divided into boiler feed pumps and metering pumps, etc ； Based on the properties of the liquid being transported, they can be classified into water pumps, oil pumps, slurry pumps, etc. There is a certain interdependent relationship among the various performance parameters of a pump, which can be represented by curves; these are known as the pump’s characteristic curves. Each pump has its own specific characteristic curve. II. Classification criteria for pumps There are many types of pumps, which can be classified according to their working principle as follows: ① Dynamic pumps, also known as impeller pumps or vane pumps, rely on the rotating impeller to exert force on the liquid, thereby continuously transferring energy to it and increasing its kinetic energy (the main form) and pressure energy. Subsequently, the kinetic energy is converted into pressure energy in the discharge chamber. These pumps can be further divided into centrifugal pumps, axial flow pumps, partial flow pumps, and vortex pumps, among others. ②Positive displacement pumps transfer energy to a liquid periodically by means of periodic changes in the volume of the sealed working space that contains the liquid, thereby increasing the pressure of the liquid enough to force it out. Depending on the movement pattern of their working elements, they can be divided into reciprocating pumps and rotary pumps. ③Other types of pumps transfer energy in other forms. For example, a jet pump relies on the high-speed jet of the working fluid to draw in the fluid to be transported, mix it within the pump, and exchange momentum to transfer energy ； A water hammer pump uses part of the water in the flow during braking to be lifted to a certain height in order to transfer energy ； An electromagnetic pump achieves transportation by causing the electrically charged liquid metal to flow under the action of electromagnetic force. Additionally, pumps can also be classified according to the properties of the liquid they transport, the driving method, structure, and application. III. Basic parameters of pumps: The basic parameters that characterize the main performance of a pump are as follows: 1. Flow rate Q – The flow rate is the amount of liquid (by volume or mass) that the pump can deliver per unit of time. Volumetric flow rate is denoted by Q, with units such as m3/s, m3/h, l/s, etc. Mass flow rate is denoted by Qm, with units such as t/h, kg/s, etc. The relationship between mass flow rate and volume flow rate is: Qm = ρQ, where ρ is the density of the liquid (kg/m3 or t/m3); for plain water at room temperature, ρ = 1000 kg/m3. 2. Head H: The head is the increase in energy per unit weight of liquid pumped by the pump, from the pump inlet (pump inlet flange) to the pump outlet (pump outlet flange). That is, the effective energy obtained by a Newtonian fluid through a pump. Its unit is N•m/N = m, that is, the height of the liquid column pumped by the pump, commonly abbreviated as meters. 3. Rotational speed n: The rotational speed is the number of revolutions per unit time of the pump shaft, denoted by the symbol n, with the unit being r/min. 4. NPSH – NPSH, also known as net positive suction head, is the main parameter that indicates cavitation resistance. The net positive suction head was previously denoted as Δh in China. 5. Power and efficiency: The power of a pump usually refers to the input power, that is, the power transmitted from the prime mover to the pump shaft; it is therefore also known as shaft power, denoted by P ； The effective power of a pump, also known as the output power, is denoted by Pe. It is the effective energy acquired by the liquid pumped out from the pump per unit of time. Since head refers to the effective energy obtained by a unit weight of liquid pumped out of the pump, the product of head, mass flow rate, and gravitational acceleration represents the effective energy obtained by the liquid pumped out of the pump per unit time – that is, the effective power of the pump: Pe=ρgQH(W)=γQH(W). Here, ρ is the density of the liquid pumped by the pump (kg/m3) ； γ — specific weight of the liquid pumped by the pump (N/m3) ； Q —— Pump flow rate (m3/s) ； H —— Pump head (m) ； g — gravitational acceleration (m/s2). The difference between the shaft power P and the useful power Pe is the power lost within the pump, and its magnitude is measured by the pump’s efficiency. The efficiency of a pump is the ratio of useful power to shaft power, denoted by η. IV. What is head? Which letter is used to represent it? What unit of measurement should be used? The conversion and formulas related to pressure? The energy gained per unit weight of liquid by a pump is called head. The head of a pump, including the suction head, is approximately equal to the pressure difference between the pump outlet and inlet. Head is denoted by H, with the unit being meters (m). The pressure of a pump is denoted by P, with the unit being Mpa (megapascals). H = P/ρ. If P is 1 kg/cm2, then H = (1 kg/cm2) / (1000 kg/m3) = (1 kg/cm2) / (1000 kg/m3) = 10,000 kg/m2 / 1000 kg/m3 = 10 m. 1 Mpa = 10 kg/cm2. H = (P2 – P1) / ρ, where P2 is the outlet pressure and P1 is the inlet pressure. V. What is cavitation in pumps? 1. Cavitation phenomenon: When the pressure of a liquid is reduced to its vaporization pressure at a certain temperature, bubbles form within the liquid. This phenomenon of bubble formation is called cavitation. The bubbles formed during cavitation shrink in volume as they flow to higher-pressure areas, resulting in their collapse. 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 drops to the vaporization pressure of that liquid at the current temperature in a certain local area of the flow path – usually somewhere slightly downstream of the inlet to the impeller blades – 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 breached. The process in which bubbles are formed and burst in a water pump, causing damage to the flowing 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 interrupt the flow of liquid within the pump, preventing it from functioning properly. 2 Basic equations for pump cavitation: The conditions under which cavitation occurs in a pump are determined by both the pump itself and the suction system. Therefore, to study the conditions under which cavitation occurs, it is necessary to consider both the pump itself and the suction system. The basic relationship for pump cavitation is NPSHc ≤ NPSHr ≤ NPSHa. NPSHa = NPSHr / (NPSHc) – this is the point at which cavitation begins in the pump; when NPSHa > NPSHr / (NPSHc), there is no cavitation in the pump. Here, NPSHa represents the net positive suction head available, also known as the effective net positive suction head, and 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. 3. Calculation of the device’s net positive suction head available NPSHa = Ps/ρg + Vs/2g – Pc/ρg = Pc/ρg ± hg – hc – Ps/ρg 4. Measures to prevent cavitation To avoid cavitation, it is necessary to increase NPSHa so that NPSHa > NPSHr. The measures to prevent cavitation are as follows: (1) Reduce the geometric suction height hg (or increase the geometric backflow height) ； (2) Reduce the suction loss hc; to this end, it is possible to increase the pipe diameter, minimize the length of the pipelines, as well as reduce the use of elbows and other fittings ； (3) Prevent operation at high flow rates for extended periods ； (4) At the same speed and flow rate, a double-suction pump is used, as it reduces the inlet flow velocity and thus makes cavitation less likely to occur ； (5) When cavitation occurs in the pump, the flow rate should be reduced or the pump should operate at a lower speed ； (6) The conditions of the pump suction tank have a significant impact on pump cavitation ； (7) For pumps operating under harsh conditions, to avoid cavitation damage, cavitation-resistant materials can be used. VI. What is the efficiency of a pump? How about the formula? It refers to the ratio of the effective power of the pump to its shaft power. η=Pe/P. The power of a pump usually refers to the input power, that is, the power transmitted from the prime mover to the pump shaft; hence it is also called shaft power, denoted by P. Effective power is equal to the product of the pump’s head, mass flow rate, and gravitational acceleration. Pe=ρg QH (W) or Pe=γQH/1000 (KW) ρ: Density of the liquid pumped by the pump (kg/m3) γ: Specific weight of the liquid pumped by the pump; γ=ρg (N/m3) g: Acceleration due to gravity (m/s) Mass flow rate Qm=ρQ (t/h or kg/s)