1. How to calculate the flow rate of centrifugal pump?
1. Flow calculation formula
Flow rate is the volume of liquid passing through the pump per unit time, usually expressed in m³/h, m³/s or L/s.
According to the cross-sectional area of the pipe and the liquid flow rate, the volume flow rate can be calculated:
\[ Q=3600\times A\times v \]
\[ A=\frac{\pi D^2}{4} \]
Among them:
Q: Volume flow rate, m³/h
A: Internal cross-sectional area of the pipe, m²
v: Average liquid flow rate, m/s
D: Pipe inner diameter, m
For example, an industrial water supply pipe has an inner diameter of 100 mm and an average flow rate of 2 m/s, then:
\[ Q=3600\times\frac{3.14\times0.1^2}{4}\times2 \]
The calculated flow rate is approximately 56.5 m³/h.
It should be noted that the actual inner diameter of the pipe must be used for calculation, rather than the DN nominal diameter directly.
2. Determine the design flow rate based on production needs
For chemical production or industrial circulation systems, the design flow rate should usually be determined based on process requirements rather than just calculated from pipe diameters.
For example, a production system needs to transport 80 m³ of liquid per hour, consider a process flow margin of about 10%:
\[ Q_d=80\times1.10=88\ \text{m}^3/\text{h} \]
The preliminary design flow rate is 88 m³/h.
This allowance is for preliminary project estimates only. The actual selection should be based on the maximum, normal and minimum flow conditions to avoid over-amplification of the flow rate causing the pump to deviate from the high-efficiency operating area for a long time.
2. How to calculate the head of a centrifugal pump?
1. The head is not the vertical height of the pipe
The head represents the mechanical energy provided by the pump to the unit weight of liquid, usually expressed in meters (m).
For industrial conveying systems, pumps need to overcome not only the liquid level difference, but also the resistance along the pipeline, the local resistance of valves and elbows, and the pressure difference between the inlet and outlet of the system.
Generally, the total lift required by the system can be expressed as:
\[ H=\Delta z+\frac{\Delta p}{\rho g} +\frac{v_2^2-v_1^2}{2g}+h_f \]
Among them:
Parameters | Meaning | Unit |
|---|---|---|
H | Total head required by the system | m |
Δz | Inlet and outlet reference section height difference | m |
Δp | Inlet and outlet pressure difference | Pa |
ρ | Conveying medium density | kg/m³ |
g | Acceleration of gravity, about 9.81 | m/s² |
v₁, v₂ | Average flow rate at inlet and outlet | m/s |
h_f | Pipeline and local losses along the way | m |
The formula refers to the head calculation method of industrial centrifugal pump systems.
2. Industrial water supply system calculation example
A factory transports normal-temperature clean water from a normal-pressure storage tank to a high-level normal-pressure storage tank. The design conditions are as follows:
Project | Working condition parameters |
|---|---|
Design flow | 80 m³/h |
Tank liquid level difference | 25 m |
Loss along the pipeline | 6 m |
Loss of valves and elbows | 4 m |
The inlet and outlet pressures | are atmospheric pressure |
When the difference in liquid surface flow rates between the two storage tanks is ignored:
\[ H=25+6+4=35\ \text{m} \]
Therefore, when the design flow rate is 80 m³/h, the system requires about 35 m head.
3. Why is it necessary to check NPSH?
When the available NPSH at the pump inlet is insufficient, cavitation may occur, resulting in vibration, noise, performance degradation, or impeller damage.
The basic verification principles are:
\[ NPSH_A>NPSH_R \]
Where NPSH_A is the available cavitation margin of the system, and NPSH_R is the cavitation margin required by the pump. The actual project should also retain a safety margin that meets the requirements of the working conditions and relevant specifications, especially under high-temperature, volatile or low-liquid-level operating conditions.
3. What parameters should be provided when purchasing industrial pumps?
In order to reduce inquiry communication and selection errors, it is recommended that buyers provide complete working condition information to the manufacturer:
Normal, maximum and minimum flow rates, unit m³/h.
Design head or system liquid level, pressure and pipeline resistance data.
Medium name, density, viscosity, temperature and corrosiveness.
Suction liquid level, inlet pressure and installation method.
Working pressure, continuous operation time and explosion-proof requirements.
Power supply parameters, materials, sealing and applicable standards.
When purchasing, it is recommended to ask the supplier to provide Q-H curve, efficiency curve, power curve and NPSHR data, and check the rated operating point and allowable operating range.
Conclusion: When selecting a centrifugal pump, you should first determine the actual flow rate, then calculate the total head of the system, and then check the performance curve, operating efficiency, NPSH and motor power.
