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The flow rate (Q) of a centrifugal pump is a key parameter for measuring its conveying capacity, which directly affects the system design and operation efficiency. This article will deeply analyze the flow rate calculation formulas, influencing factors, and engineering calculation methods to help engineers make accurate selections and optimize operations.

1. Definition and Units of Centrifugal Pump Flow Rate

Flow Rate (Q)

The volume of liquid delivered by the pump per unit time. Common units are as follows:

• International units: m3/h (cubic meters per hour), L/s (liters per second)
• Imperial units: GPM (gallons per minute), ft3/s (cubic feet per second)

Conversion Relations

• 1m3/h≈4.403GPM
• 1L/s=15.85GPM

2. Core Formulas for Centrifugal Pump Flow Rate

2.1 Theoretical Flow Rate Formula (Without Considering Losses)

The theoretical flow rate of a centrifugal pump can be calculated through the geometric parameters of the impeller:

Q=A⋅v=π⋅D⋅b⋅v

• A: The flow - through area at the impeller outlet (m2)
• D: The diameter of the impeller outlet (m)
• b: The width of the impeller outlet (m)
• v: The radial velocity of the liquid at the impeller outlet (m/s)

Application Scenario: It is used to estimate the flow rate in the preliminary design stage, but it does not consider the impact of hydraulic losses and efficiency.

2.2 Actual Flow Rate Formula (Considering Efficiency)

The actual flow rate is affected by the pump efficiency (η) and system resistance, and needs to be calculated in combination with the head (H) and power (P). When the flow rate unit is m3/s:

Q=ρ⋅g⋅HP⋅η

When the flow rate unit is m3/h:

Q=ρ⋅g⋅HP⋅η×3600

P: Shaft power (kW)

• η: Pump efficiency (usually 50% - 85%)
• ρ: Liquid density (kg/m3)
• g: Gravitational acceleration (9.81m/s2)
• H: Head (m)

Key Points:

• The flow rate is directly proportional to the power and inversely proportional to the head.
• High - viscosity liquids will reduce the efficiency (η), and the calculation needs to be corrected.

3. Key Factors Affecting the Flow Rate

3.1 Impeller Parameters

• Impeller Diameter (D): The flow rate is directly proportional to the square of the impeller diameter (Q∝D2).
• Impeller Rotational Speed (n): The flow rate is directly proportional to the rotational speed (Q∝n), following the similarity law: Q1Q2=(n1n2)(D1D2)3.

3.2 System Resistance

Pipe friction, valve openings, and the number of elbows will all increase the system resistance, resulting in the actual flow rate being lower than the theoretical value. The actual flow rate needs to be determined by the intersection of the system characteristic curve and the pump characteristic curve. The system characteristic curve reflects the relationship between the flow rate and resistance in the pipeline system and is usually derived from the pipeline resistance calculation formula. The pump characteristic curve is the curve of the relationship between parameters such as flow rate, head, power, and efficiency of the centrifugal pump under different working conditions, which is determined by the manufacturer through experiments. When the pump is installed in a specific pipeline system, the flow rate corresponding to the intersection of the two curves is the actual operating flow rate of the pump in this system.

3.3 Medium Characteristics

• Viscosity: High - viscosity liquids (such as oils) will increase internal friction and reduce the flow rate.
• Gas Content: When the gas content in the liquid exceeds 5%, cavitation may be induced, and the flow rate will drop sharply.

4. Common Causes and Solutions for Abnormal Flow Rates

5. Summary

The flow rate of a centrifugal pump can be estimated by theoretical formulas, but the actual value needs to be combined with efficiency and system characteristics. The impeller size, rotational speed, and medium characteristics are the core variables affecting the flow rate. In engineering, the flow rate is preferably determined through performance curves and measured data rather than relying solely on calculations. Mastering the flow rate calculation logic can optimize pump selection, reduce energy consumption, and extend the service life of equipment. For complex systems, it is recommended to use CFD simulations or professional software (such as PIPE - FLO) for auxiliary analysis.