Conversion of performance curve and optimal operating point of horizontal centrifugal dg multistage pump
Introduction to the performance curve and best working point of horizontal centrifugal dg multistage pump
Horizontal centrifugal dg multi-stage pump uses the impeller to rotate at a high speed in a fixed casing to transfer energy to the fluid. When the impeller rotates with the shaft, the fluid between the blades also gains centrifugal force with the rotation of the impeller, and the fluid is thrown out from the outlet between the blades. The high-speed fluid enters the diffusion cavity after leaving the top of the blade, and the flow rate begins to decrease. The kinetic energy is converted into higher pressure energy. After the fluid is thrown out, the pressure in the center part of the impeller is reduced, and the external fluid can enter from the suction port of the pump, and the fluid is continuously delivered.
The performance of the horizontal centrifugal dg multi-stage pump can be illustrated by the pump flow-head curve:
Figure 1 Performance curve of horizontal centrifugal dg multistage pump
Figure 1 is a typical 18.5kw horizontal centrifugal dg multi-stage pump performance curve. The flow of the dg multi-stage pump depends on the pump head. When the pump head increases, the flow decreases, and the rate of flow decrease depends on the pump design.
Typical pump performance curve also includes its efficiency curve and shaft dynamic rate (BHP) curve, both of which are drawn according to the corresponding flow rate. The efficiency of the pump is the ratio of the pump fluid power to the pump shaft power.
The intersection of the efficiency curve and the shaft power curve (BHP) is the best working condition point of the boiler feed water pump. At the best working condition point, the boiler feed water pump is the best operating point regardless of energy efficiency or maintenance. Economically, if the operating point of the pump is far away from the optimal operating point, it will accelerate the wear of bearings, mechanical seals and other parts. In actual use, it is very difficult to keep the pump running at the optimal operating point. , Because the load requirements of the system are constantly changing, however, keeping the pump running within a reasonable range near its optimal operating point will greatly reduce the operating cost of the entire system and meet the load requirements of the system.
To
The similarity law of horizontal centrifugal dg multistage pump
The design and manufacture of horizontal centrifugal dg multi-stage pumps are usually carried out in series. In the same series, pumps of different sizes are similar and satisfy the law of similarity. The law of similarity is not only used for pump design, but also for pump design. Operation, adjustment, and even selection are very important.
1. Conversion of performance parameters when the speed changes
The performance parameters of the horizontal centrifugal dg multi-stage pump are all for a specific speed n0. When the actual speed n and n0 are different, the similarity law can be used to find new performance parameters. At this time, the similarity law can be simplified as :
Formula 1)
formula (2)
formula (3)
where: qv——flow rate;
H-head;
P——power;
N——speed.
The above formulas are valid at the same time, that is to say, when the pump speed decreases, the head decreases in a square relationship, and the power decreases in a square ratio.
2. Conversion when the density of the conveying fluid changes
When the density of the conveying fluid changes, the law of similarity is simplified to:
formula (4)
formula (5)
formula (6)
where: qv——flow rate;
H-head;
P——power;
Ρ——density.
It can be seen from the above formulas: when the density of the fluid delivered by the pump changes, the volume flow and head remain unchanged, but the power of the pump will change.
To
Conversion and control of the best operating point of horizontal centrifugal dg multistage pump
Horizontal centrifugal dg multi-stage pump system is composed of frequency converter, pump, pipeline and valve. Because the load demand of the system is constantly changing, it is very difficult to keep the pump running at the best efficiency point.
However, in order to improve the energy efficiency of the horizontal centrifugal dg multi-stage pump system to achieve the purpose of energy saving, we can select the best operating area (point) near the pump's highest efficiency point as the user's adjustment of the operating point of the pump system Guidance is necessary.
The procedure for selecting the best working point of horizontal centrifugal dg multistage pump:
(1) Download the explicit parameters on the pump nameplate, including rated power PN (kw), rated flow Q (m³/n) and rated head H (m);
(2) Ask the manufacturer of the horizontal centrifugal dg multistage pump for performance curves, as shown in Figure 1, including flow-head curve (qv-H), efficiency curve and shaft power curve BHP (p-qv).
Figure 2 Performance curve of horizontal centrifugal dg multistage pump
(3) It can be seen from Figure 2 that the best working point A of the dg multistage pump is 65m³/h, the head is 30m, and the shaft power is 15kw.
Each ton of water (m³) consumes electric energy as
(4) Calculation of the motor speed and frequency at the best operating point A
In operation, the frequency converter is used to control the best working condition point A of the boiler feed water pump. Then the operating speed should be calculated when the flow rate is 65m³/h, and then the frequency of operation at this time should be calculated.
According to formula (3), it can be obtained that the operating speed n of the pump at the BEP point is:
In the formula: P0——The rated power of the motor is 18.5kw
N0——The rated speed of the motor is 1465r/min;
P——The input shaft power of the pump at point A is 15kw.
and so
According to formula (7), we can get:
To
where: f——motor power frequency, that is, the output frequency of the inverter, Hz;
P——The number of pole pairs of the motor, here is 2;
N——Motor operating speed, 1187.8r/min.
and so
In this way, by adjusting the output frequency of the inverter, the motor speed is controlled at 1187.8r/min, that is, the output frequency of the inverter is adjusted to 40Hz.
Of course, in engineering applications, it will not be a point, but a frequency band. Based on this, we can adjust the output frequency of the inverter to the range of 38Hz~42Hz, and think that the dg multistage pump is running in the best area at this time.
