Technical and Economic Analysis of Constant Speed ​​Pump and Speed ​​Control Pump

Understanding the working principles and characteristics of constant-speed pumps and variable-speed pumps is essential for optimizing pump performance in fluid systems. When a pump is installed in a pipeline, its actual operating condition is determined not only by its own performance curve but also by the system's resistance characteristics. The intersection point of these two curves defines the pump’s operating point within the piping system. In the same system, different pumps will have different operating points, leading to variations in flow rate, head, and efficiency. A constant-speed feedwater pump typically operates as a centrifugal pump, which uses centrifugal force generated by the rotation of the fluid to transfer energy. It is driven by a motor at a fixed speed, imparting both pressure and kinetic energy to the fluid. On the other hand, a variable-speed pump, such as the YT62 type fluid coupling, includes a hydraulic coupling between the prime mover (like an electric motor) and the pump. This coupling transmits power through the kinetic energy of the working fluid. The working principle involves a pump impeller on the drive shaft and a turbine on the driven shaft, with a cavity formed between them. As the prime mover rotates the drive shaft, the fluid gains energy in the pump wheel and is directed toward the turbine, transferring power and driving the turbine to rotate. The circulation of the working oil within this cavity depends on the difference in centrifugal force between the impeller and the turbine, known as slip. Under normal operating conditions, this slip is less than 3%. By adjusting the amount of oil in the cavity, the speed and power transmission can be controlled, allowing for continuous speed regulation. In terms of technical and economic comparison, Hebei Thermal Power Co., Ltd. implemented an upgrade project involving two 200 MW heat units and four 410 t/h circulating fluidized bed boilers. Each unit was equipped with two turbo pumps and one fixed-speed pump, capable of delivering 440 t/h at a head of 1,400 m. Various configurations were tested, including running a single constant-speed or variable-speed pump, running two constant-speed or variable-speed pumps in parallel, and using a combination of fixed-speed and variable-speed pumps. When a single constant-speed or variable-speed pump is used, the pump performance curve can be adjusted through methods like speed control, bucket adjustment, or cavitation control. Pipeline characteristic curves are modified using outlet or inlet throttling. Outlet throttling is the most common method, where valve opening is adjusted to regulate flow. Fixed-speed pumps usually use outlet throttling, while variable-speed pumps rely on speed control. Steam turbines have specific requirements for cold, warm, and hot starts, depending on the main steam parameters and the time needed to reach 410 t/h load. Similarly, boiler startups have defined parameters and timeframes to reach full load. As shown in the data, boiler startup takes longer, and the pressure must be gradually increased according to machine-side requirements. A variable-speed pump can adjust its speed to meet these demands without altering the feedwater control valve, unlike a constant-speed pump, which requires significant throttling, resulting in higher energy consumption. From the pump characteristic curve, it is clear that at a flow rate of 120–450 t/h, the head can increase from 800 m (or lower) to 1,400 m as the furnace pressure rises. The excess energy is dissipated through the water supply control valve, as shown in the formula: N = (g * Q * H) / (η * 100) Where: - N: Motor power consumption (kW) - g: Acceleration due to gravity (m/s²) - Q: Mass flow rate (kg/s) - H: Head (m) - η: Pump efficiency (%) Under constant conditions, power consumption is directly proportional to the head. By adjusting the pump speed to match the furnace demand, significant energy savings can be achieved. At 120 t/h, a variable-speed pump can save up to half the power consumed by a constant-speed pump.

Polyquaternium-39 PQ-39 CAS 25136-75-8

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