Definition of pump speed and flow rate:
The pump speed refers to the number of revolutions per minute of the water pump shaft. It is represented by the symbol n. Rotation speed refers to the number of revolutions per minute of the water pump shaft when the power machine drives the water pump to pump water. It is represented by the symbol n and the unit is r/min.
The flow rate of the water pump is also called the water delivery volume. The volume or mass of the liquid discharged by the water pump per unit time is the water pump flow rate, or the water pump flow rate refers to the volume of water pumped by the water pump per unit time. The flow of water pump is usually expressed by the English letter Q. The units of pump flow are m3/s, m3/h, L/S, etc.
The relationship between pump speed and flow rate and calculation formula:
1. The working point of the pump is determined by the characteristic curve of the pump and the characteristic curve of the pipeline:
The characteristic curve of the water pump H = Ho-SoQ^2 is a downwardly concave decreasing curve
The characteristic curve of the pipeline H = Z2-Z1 + SQ^2 is a concave upward curve
In the formula: H——pump head,
Ho ——the head when the flow is zero,
So——The internal friction of the pump,
Q——Water pump flow rate,
Z1——The water level of the pump suction pool,
Z2——The water level of the outlet pool,
S-pipeline friction.
The change in the opening of the pump outlet valve means that the characteristic curve of the pipeline changes. When the opening of the valve becomes smaller, the resistance of the pipeline increases (S increases), and the characteristic curve of the pipeline becomes steep, moving from the intersection of the characteristic curve of the water pump to the direction where the flow becomes smaller and the head becomes larger. When the opening of the valve becomes larger, the opposite is true.
As for the change of shaft power and efficiency, it should be determined on the characteristic curve of the pump and the characteristic curve of the pipeline. For centrifugal pumps, the shaft power becomes smaller as the opening of the valve becomes smaller.
2. In the water supply equipment driven by frequency conversion, the frequency determines the speed of the motor, that is, the speed of the water pump. For the same pump, the proportional law of the pump can be used to calculate the head, flow, and power at different speeds. The definition of the law of proportionality: the same pump, when the impeller diameter is unchanged, but the speed is changed, its performance changes.
Q1/Q2=N1/N2, H1/H2=(N1/N2) square,
P1/P2="(N1/N2) cube.
Q, H, P are the flow, head and shaft power at the corresponding speed N respectively.
3. The flow rate and the speed have a linear relationship: Q1/Q2 = n1/n2;
The relationship between head and speed is quadratic: H1/H2 = (n1/n2) 2
The motor shaft power has a cubic relationship with the speed: P1/P2 = (n1/n2) 3
From the above derivation, we can know that the motor speed formula:
n=60f/p,
Among them, n is the synchronous speed of the motor,
f is the power supply frequency,
p is the number of pole pairs of the motor, and it can be seen that the power supply frequency f of the motor is proportional to the speed. In this way, frequency and flow, head and motor shaft power also have the aforementioned n-th power (n=123) proportional relationship.
3. The relationship between pump displacement, flow and speed is as follows:
The relationship between pump displacement, theoretical flow and speed qt=nV
Where n-the speed of the hydraulic pump; V-the displacement of the hydraulic pump.
The actual flow rate refers to the hydraulic pump in unit time when considering the leakage loss of the hydraulic pump
The actual volume of oil output. Due to the leakage loss of the water pump during operation, the actual flow rate of the water pump is less than the theoretical flow rate. That is, q=qt-Δq, where Δq-a leakage amount.
Rated flow refers to the actual output flow when the pump is working at rated speed and rated pressure. The flow rate marked on the product catalog or nameplate of the pump is the rated flow rate of the pump.
The displacement of a water pump refers to the volume of oil discharged by one revolution of the pump shaft, represented by V, and the unit is mL/r. The displacement of the water pump depends on the geometric size of the sealing cavity of the hydraulic pump. Different pumps have different displacements due to different structural parameters.
The flow rate of a water pump refers to the volume of oil output by the hydraulic pump in a unit time. It is often expressed by q, and the unit is L/min.
The theoretical flow rate refers to the volume of oil output by the water pump per unit time without considering the leakage loss of the hydraulic pump. It is often expressed in qt and the unit is L/min.
