Views: 29 Author: Site Editor Publish Time: 2023-12-27 Origin: Site
A centrifugal pump is a common type of water pump that uses a rotating impeller to generate centrifugal force, which draws liquid into the pump through the inlet and then pushes it out through the outlet. The following is the working principle of a centrifugal pump:
Impeller: A centrifugal pump typically consists of one or more impellers, which are the rotating components of the pump. The impeller has a disk-like shape with slightly tilted edges that can generate centrifugal force when it rotates.
Suction: As the impeller spins, the liquid enters the pump through the suction port, and the rotational motion of the impeller throws the liquid to the outer edge of the impeller, generating centrifugal force that causes the liquid to be thrown in the direction of the pump outlet.
Pressure increase: Due to the centrifugal force, the liquid's velocity increases at the outer edge of the impeller, while according to the law of mass conservation, the pressure decreases. This creates a pressure difference between the suction and discharge ports, causing the liquid to be pushed towards the outlet.
Outlet: Finally, the liquid that has been acted upon by the impeller is pushed out through the outlet and flows into the pipeline or other system.
In summary, centrifugal pumps use the centrifugal force generated by a rotating impeller to increase the pressure and flow velocity of liquid, thereby achieving liquid transportation. This makes centrifugal pumps one of the widely used pump types in industrial and domestic applications.
The performance parameters of a centrifugal pump mainly include flow rate, head, power, efficiency, allowable suction vacuum, and NPSH (Net Positive Suction Head) margin. These parameters reflect the comprehensive performance of the centrifugal pump, and the numerical values of these basic parameters are generally indicated on the nameplate of the pump. Most pump manufacturers use nameplates with content and format as shown in Figure 1.
01 Flow Rate
Flow rate, also known as discharge, refers to the quantity of liquid discharged by the pump per unit of time (measured by an outlet flow meter). There are two types of flow rates: volumetric flow rate and mass flow rate.
Volumetric flow rate: The volume of liquid discharged by the pump per unit of time, usually denoted as Q. Common units include L/s (liters per second), m3/s (cubic meters per second), or m3/h (cubic meters per hour).
Mass flow rate: The mass of liquid discharged by the pump per unit of time, usually denoted as G. Common units include kg/s (kilograms per second), kg/h (kilograms per hour), or t/d (tons per day).
The conversion relationship between mass flow rate and volumetric flow rate is as follows:
G = ρQ
where G is the mass flow rate in kg/s,
Q is the volumetric flow rate in m3/s,
and ρ is the density of the liquid in kg/m3.
The increase in energy from the pump's inlet to its outlet per unit mass of liquid is called the pump's head, which represents the effective energy obtained by the unit mass of liquid through the pump. It is also referred to as the total head of the pump and is commonly represented by the symbol H. In the International System of Units, the unit of head H is J/kg, but it is often expressed in terms of the height of a column of liquid (m), which provides a more visual representation. Although the unit of pump head is the same as that of height, both in meters (m), the pump head should not be simply understood as the height that the liquid can be lifted to, as the total head of the pump is not only used to raise the position height of the liquid but also to overcome the flow resistance and increase the static and kinetic energy of the liquid during transportation.
In this article, to maintain consistency with the unit of pump models and product samples used domestically, the unit of head H is expressed as "meter of liquid column" or "m" (conversion between "J/kg" and "m" can be done using the gravitational acceleration g).
In engineering calculations, the head H provided by the pump in a piping system can be calculated using the Bernoulli's equation, as shown in Figure 2.
Figure 2 Schematic diagram of centrifugal pump device
1 - Pump; 2 - Suction tank; 3 - Bottom valve; 4 - Suction pipeline; 5 - Suction pipeline regulating valve; 6 - Vacuum gauge; 7 - Pressure gauge; 8 - Discharge pipeline regulating valve; 9 - Check valve; 10 - Discharge pipeline; 11 - Flowmeter; 12 - Discharge tank.
Speed refers to the number of times the pump shaft rotates per minute, usually represented by the symbol n, with units of r/min.
Speed is an important performance indicator of centrifugal pumps. When the speed changes, the flow rate, head, and shaft power of the centrifugal pump will all change. The speed specified on the pump product sample generally refers to the maximum allowable speed of the pump, and in actual operation, it should not exceed 4% of the allowable value.
Power is the work done per unit of time, commonly expressed in watts (W) or kilowatts (kW). The power of a pump can be divided into effective power, shaft power, and auxiliary power.
(1) Effective power
Effective power is the output power of the centrifugal pump, that is, the work done by the pump on the liquid delivered per unit of time, represented by the symbol Ne, which can be calculated according to the following formula:
ρ - Density of the liquid, kg/m3;
Q - Volume flow rate, m3/s;
H - Head, m;
G - Acceleration due to gravity, m/s2;
Ne - Effective power, kW.
(2) Shaft Power
Shaft power refers to the input power of the centrifugal pump, which is the power transmitted from the prime mover to the pump shaft, represented by the symbol N. Due to various losses in the pump, it is not possible for the pump to convert all the input power from the prime mover into effective power of the liquid.
(3) Auxiliary Power
The auxiliary power of the centrifugal pump refers to the power of the prime mover that is used in conjunction with the pump, represented by ND. It is related to the shaft power as follows:
ND = (1.1~1.2)N
In general, when N < 4.5 kW, take 1.2; when 4.5 kW < N ≤ 40 kW, take 1.15; when N > 40 kW, take 1.10.
Usually, the power indicated on the pump nameplate is not the effective power, but the shaft power or auxiliary power.
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