The pump is a mechanical device used to transport liquids or increase their pressure. It typically operates by using an electric motor or other power equipment to drive the impeller to rotate, drawing liquid from a lower position and, through the impeller's action, converting its kinetic energy into pressure, thereby transporting the liquid to a higher position or over a long distance. The main performance parameters of a water pump include: flow rate, head, power, and efficiency, etc.
1. Pump performance parameter - Flow rate
The flow rate of a pump refers to the volume or weight of liquid that the pump transports per unit time, represented by the symbol Q. Common units include m³/h, m³/s, L/s, or t/h.
The flow rate indicated on the pump's nameplate is the design flow rate of this pump, also known as the rated flow rate. The pump operates at the highest efficiency when it is running at the rated flow rate.
The formula for calculating the flow rate of a pump is:
Q = P × η / 2.73H
Where: Q is the flow rate, in m³/h; P is the shaft power, in KW; η is the pump efficiency, in%; 2.73 is a constant; H is the head, in m.
2. Pump performance parameter - Head
The head of a pump refers to the energy increase of a unit gravitational liquid from the pump's inlet to the outlet, that is, the energy gained by a unit gravitational water after passing through the pump. In simple terms, it refers to the height that the pump can lift water, represented by the symbol H, with the unit mH2O. Generally, it is simplified to m.
Centrifugal pump head
The head of a centrifugal pump is based on the center line of the impeller and is composed of two parts. The vertical height from the center line of the pump impeller to the water surface of the water source, that is, the height that the pump can lift water, is called the suction head, abbreviated as suction head; the vertical height from the center line of the pump impeller to the water surface of the discharge pool, that is, the height that the pump can pressurize water, is called the discharge head, abbreviated as discharge head. The head of the centrifugal pump is the sum of the suction head and the discharge head.
The marked head on the pump nameplate is the design head of this pump, also known as the rated head, corresponding to the head when the designed flow rate is passed.
The formula for calculating the pump head is:
H = (p2 - p1) / ρg + (v2² - v1²) / 2g + z2 - z1
Where: H is the head, unit m; p1, p2 are the pressure of the liquid at the pump inlet and outlet, unit Pa; v1, v2 are the flow velocity of the liquid at the pump inlet and outlet, unit m/s; z1, z2 are the inlet and outlet heights, unit m; ρ is the liquid density, unit kg/m³; g is the gravitational acceleration, unit m/s².
3. Pump performance parameters - Power
Pump power refers to the work done by the pump within a unit of time, usually represented by the symbol N, and is commonly measured in KW.
Pump power can be further divided into shaft power, effective power and matching power, etc.
The shaft power indicated on the pump nameplate refers to the shaft power corresponding to the designed flow rate, also known as the rated power.
Shaft power is the power transmitted from the power machine (motor) to the pump shaft, which can be understood as the input power of the pump. Generally, the pump power refers to the shaft power, represented by the symbol P.
Effective power refers to the energy that the liquid flowing through the pump obtains from the pump within a unit of time, also known as the output power of the pump, and is usually represented by Pu.
Matching power refers to the power of the power machine (motor) that matches the pump, and the value of matching power is usually indicated on the pump nameplate or sample.
The formula for calculating pump power is:
P = ρ × g × Q × H / 1000 η
Where: P is the rated power, measured in KW; ρ is the density of the liquid, measured in kg/m³; g is the gravitational acceleration, measured in m/s²; Q is the flow rate, measured in m³/s; H is the head, measured in m; η is the pump efficiency, measured in %.
4. Performance parameters of water pumps - Efficiency
The efficiency of a water pump refers to the percentage of the effective power of the pump compared to the shaft power. It is an important technical and economic indicator of the pump and is represented by the symbol η.
The efficiency marked on the pump's nameplate corresponds to the efficiency when the designed flow rate is passed through. This efficiency is the highest efficiency of the pump.
The shaft power of the pump cannot be fully transmitted to the output liquid. A part of the energy is lost, and the energy loss within the pump can be divided into three parts: hydraulic loss, volumetric loss, and mechanical loss. Correspondingly, there are hydraulic efficiency ηh, volumetric efficiency ηV, and mechanical efficiency ηm.
The efficiency η of the pump is the product of the hydraulic efficiency ηh, volumetric efficiency ηV, and mechanical efficiency ηm. The higher the efficiency of the pump, the smaller the energy loss during its operation.
The formula for calculating the efficiency of the pump is:
η = (H × Q) / P × 100%
Where, η is the efficiency of the pump, in percentage; H is the head, in meters; Q is the flow rate, in m³/s; P is the power, in KW.
5. Hydraulic losses and hydraulic efficiency:
When the liquid flows through the suction chamber, impeller, and discharge chamber of the pump, friction losses, local losses, and impact losses occur.
Friction loss is the loss caused by the friction between the liquid and the walls of the flow components.
Local loss is the loss caused by the change in speed magnitude and direction of the liquid within the pump.
Impact loss is the loss caused by the liquid at the inlet, outlet, and discharge chamber of the pump when it operates under non-design conditions.
The greater the hydraulic loss, the smaller the pump's head.
The head without considering the hydraulic loss in the pump is the theoretical head HT. Then, the ratio of the pump's head H to the theoretical head HT and the theoretical head HT is called the hydraulic efficiency ηh.
6. Volume loss and volume efficiency:
After the liquid flows through the impeller, a small portion of the high-pressure liquid leaks through the gaps within the pump body (such as the sealing ring) and the axial force balancing device (such as the balance hole) to the inlet of the impeller. Another portion leaks from the shaft seal device to the outside of the pump, consuming part of the energy, which is the volume loss.
The larger the leakage q, the smaller the water output Q of the pump.
The ratio of the flow Q through the pump outlet to the sum of the flow Q and q through the pump inlet is called the volume efficiency ηV.
7. Mechanical loss and mechanical efficiency:
When the impeller rotates in the liquid, the outer surfaces of the front and rear covers generate friction losses (wheel disc losses) with the liquid. During the rotation of the pump shaft, the shaft and the shaft seal, as well as the bearings, also generate friction losses. The energy consumed to overcome these friction losses is called mechanical loss, and the mechanical loss power is represented by Pm.
After deducting the mechanical loss from the pump's input power, the power transmitted by the impeller to the liquid is called hydraulic power, denoted by Pw.
The ratio of the hydraulic power of the pump to the shaft power is called mechanical efficiency ηm.
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