Puzzles regarding single-stage double-suction centrifugal pumps
Thread Content
Motor: 1000KW, rated current: 68.8A, speed: 1480 r/min. Water pump: flow rate: 3000 m3/h, head: 85m, efficiency: 82%. Inlet diameter: 505mm, inlet pipe diameter: 800mm; outlet diameter: 405mm, outlet pipe diameter: 600mm. The pipe leading to the main pipeline has a diameter of 900mm (the pipeline network is already filled with water). The height of the water tank is 5 meters. (1) At an outlet opening of 10%, the current is 43A, the pressure in the pipe is 1.2 MPa, and the flow rate is around 1500 (for reference only). (2) At an outlet opening of 28%, the current is 63A, the pressure in the pipe is 0.9 MPa, and the flow rate is 3523. (3) At an outlet opening of 35%, the pressure in the pipe is 0.5 MPa, the current is 59A, and the flow rate is 4524. (4) At an outlet opening of 45%, the current is 54A, the pressure in the pipe is 0.4 MPA, and the flow rate remains at 4521 (with little change). When the opening is increased further, the flow rate doesn’t change much, but the pressure keeps dropping as does the current. What’s going on here?
Each pump has its own performance curve, with 3000 being the point of maximum efficiency. When this flow rate is exceeded, the pump begins to operate outside its optimal conditions, its efficiency starts to decline, and the pressure also drops. As the flow rate changes, the vapor feed at the inlet of the pump impeller also increases gradually, until it reaches a certain value. At a balance point. The pump flow will no longer increase, and the power will decrease due to steam feedback. It is recommended to keep the flow rate below 3500; otherwise, the impeller will be damaged and the motor’s energy efficiency will be low.
Pump characteristic curve and pipeline characteristic curve
If you can’t generate pressure, it’s because your traffic volume is too high. Do you want flow or pressure?
When the flow rate is high, the pressure is low; if you want to increase the pressure, you need to reduce the valve opening.
As the flow rate increases, the head decreases, and the current does not increase but rather decreases, indicating that it is not in an overloaded condition. As the flow rate of the water pump increases, the required NPSH must also increase. The pressure at the pump inlet should be lower than the required NPSH at high-flow conditions; as a result, cavitation occurs and the flow rate of water output decreases.