【A journey of a thousand miles begins with a single step – Centrifugal Pumps】1.1 Understanding the concept: Head

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This post was last edited by 3983596_FPPZ on 2018-11-14 at 15:33. Head pressure: The term \"head pressure\" is often heard, but few people truly understand its meaning; many consider it to be the height to which water can be lifted, and intuitively it can be viewed in that way. Head: The increase in energy per unit weight of the fluid after it passes through the pump. Keywords: specific weight, energy, increase value. Specific weight: refers to /mg. Energy: There are three forms of energy in liquids: pressure energy (commonly known as head), kinetic energy, and potential energy. Pressure energy is the energy resulting from a pump applying pressure to a liquid, thereby creating stress on the walls of containers and pipes. Kinetic energy is what enables the pump to give the liquid a certain flow rate. Potential energy is the result of the pump raising the liquid to a certain height. The symbolic expressions corresponding to these three forms of energy are mP/ρ, 1/2mv2, and mgh respectively. In many cases, the liquid pumped by a pump possesses all three of these forms of energy: pressure, flow velocity, and an increase in height. Therefore, the energy possessed by the liquid flowing through the pump is the sum of these three forms of energy: mP/ρ, 1/2mv2, and mgh. By dividing each of these by the unit weight /mg, the energy of the liquid can be expressed as Total liquid energy = P/γ + v2/2g + h = C; this is the famous Bernoulli equation. If the total energy of a liquid remains unchanged, with no external energy added to it or lost through consumption, then its total amount stays constant. The three forms of energy can undergo internal changes in form while keeping the total energy constant. For example, the kinetic energy of the liquid is highest at the outer diameter of the impeller, while the pressure energy is usually highest at the pump outlet. Value added: The value added in terms of energy is the mechanical energy of the drive mechanism, which is used by the impeller to do work on the liquid, thereby increasing the liquid’s energy. This value added is determined by the flanges at the pump’s inlet and outlet – that is, the outlet pressure minus the inlet pressure; the pressure difference represents the value added in terms of pressure energy ； Similarly, for the exit flow velocity minus the inlet flow velocity, the difference in velocity represents the increase in kinetic energy ； Exit height – inlet height; the difference in height represents the increase in potential energy. The sum of these three added values is the head value, measured in meters. In performance tests, the pump head value is determined based on this principle: during such tests, pressure is measured at the pump’s inlet and outlet, the flow velocity at the outlet is calculated, and the height difference between the inlet and outlet is measured; the sum of these three values gives the instantaneous head value. The relationship between outlet pressure and head: Generally, when the inlet pressure is at atmospheric pressure (slightly higher than atmospheric pressure), it can be assumed that the inlet pressure Ps is zero. Without considering the effect of viscosity, the outlet pressure is given by: Po = H*γ, where γ is the specific gravity. For example, if the head is 120 meters, the inlet pressure is at atmospheric pressure, and the fluid is plain water with a specific gravity of 1.0, then the pump’s outlet pressure can be approximated as: Po = 120*1 ≈ 1200 KPa = 12 kgf/cm2 = 1.2 MPa. Homework question (multiple choice): Given that the pump’s head is 87.5 meters and it transports an alkaline solution with a specific gravity of 1.35, with the inlet pressure at atmospheric pressure, what is the approximate value of the pump’s outlet pressure? A. 8.75 kilograms B. 11.8 kilograms C. 0.875 MPa D. 0.118 MPa E. 1.18 MPa BE