Factors affecting the efficiency of fluoroplastic centrifugal pumps and solutions

Thread Content

Factors affecting the efficiency of fluoroplastic centrifugal pumps and solutions There is an analysis of the factors that impact the efficiency of fluoroplastic centrifugal pump systems, along with solutions to improve their operational efficiency and reduce energy consumption. The energy losses of a pump mainly include several aspects: 1. Mechanical losses, which are primarily the frictional losses resulting from the liquid, the outer surfaces of the impeller’s front and rear covers, and the pump chamber. The proportion of disk losses is relatively large, even reaching 30% of the effective power. Tests show that the disk loss is proportional to the cube of the rotational speed, and proportional to the fifth power of the impeller’s outer diameter. Therefore, the larger the outer diameter of the impeller, the greater the disk loss. Although disk loss is proportional to the cube of the rotational speed, at a given head, as the rotational speed increases, the outer diameter of the impeller decreases accordingly, causing disk loss to decrease in proportion to the fifth power. Therefore, as the rotational speed rises, disk loss does not increase but rather decreases – this is one of the reasons for the development of high-speed pumps. 2. Volumetric loss: A portion of the liquid in the impeller leaks back to the impeller inlet through the gaps in the impeller sealing ring, failing to be utilized effectively and thus resulting in a loss. Therefore, the gap in the sealing ring should be as small as possible; however, due to factors such as machining and assembly, an excessively small gap may lead to uneven wear or jamming. **Standards specify specific requirements for the gaps in various types of pumps. 3. Hydraulic loss: The fluid flowing through the pump’s flow passage inevitably experiences losses due to changes in its velocity and direction; these two types of losses constitute hydraulic loss. To reduce this type of loss, in addition to improving the smoothness of the flow-through components, it is advisable to use high-quality hydraulic models. Several factors that affect the efficiency of centrifugal pump sets: 1. The efficiency of the pump itself is the most fundamental factor. Pumps under the same operating conditions can have efficiencies that differ by more than 15%. 2. When the operating conditions of fluoroplastic centrifugal pumps are below their rated conditions, the pump efficiency is low and energy consumption is high. 3. The motor efficiency remains essentially constant during operation. Therefore, it is crucial to choose a high-efficiency motor. 4. The impact of mechanical efficiency is mainly related to design and manufacturing quality. Once the pump is selected, subsequent management has little impact. 5. Hydraulic losses include hydraulic friction and local resistance losses. After the pump operates for a certain period of time, wear inevitably occurs on the surfaces of components such as the impeller and guide vanes, resulting in increased hydraulic losses and reduced hydraulic efficiency. 6. The volumetric loss of a pump, also known as leakage loss, includes three types of losses: those due to the impeller seal rings, inter-stage leaks, and the axial force balance mechanism. The level of volumetric efficiency is related not only to design and manufacturing but also to subsequent management. After the pump operates continuously for a certain period of time, friction between its components causes the clearances to increase, resulting in a decrease in volumetric efficiency. 7. The centrifugal pump experiences vacuum conditions and idling due to reasons such as clogging of the filtration tank or air entering the pipelines. 8. Before starting the pump, employees do not pay enough attention to the preparatory work required before starting fluoroplastic centrifugal pumps; basic operating procedures such as warming up the pump, rotating it, and filling it with fluid are not followed properly. This often leads to cavitation in the pump, resulting in high noise levels, excessive vibration, and low pumping efficiency. Measures to reduce energy consumption and improve the efficiency of pump systems: 1. Improve the efficiency of the pump itself ； (1). The blades extend toward the inlet and become thinner, allowing the liquid to come into contact with the blades earlier; this can reduce the outer diameter of the impeller, as well as increase the length of the flow paths within the blades, thereby reducing relative diffusion ； However, the extension must be appropriate; excessive forward extension will result in a too small area for water to enter, reducing the corner angle where the blade inlet meets the blade cover, which in turn increases hydraulic friction losses. It also compresses the inlet flow channel, which is detrimental to both cavitation and efficiency. (2) Keep the ratio of the outlet to inlet area in the flow channel between adjacent blades within the range of 1.0–1.3 to reduce diffusion losses. If this ratio is greater than 1.3, flow channel diffusion is severe and efficiency decreases. (3). The larger the hydraulic radius of the flow channel, the better; the inlet cross-section of the blade should be made as close to square as possible in order to reduce friction losses. As is known from hydraulics, the ratio of the cross-sectional area through which water flows to the wet perimeter is called the hydraulic radius, that is, hydraulic radius = cross-sectional area through which water flows / wet perimeter. A larger wet perimeter means, in effect, a larger contact area between the liquid and the wall surface. When the cross-section of the flow channel is changed from approximately square to an elongated rectangle, it essentially means that the liquid has to flow through the narrow gaps in that elongated cross-section, so the resistance is bound to be high. (4) Due to the high hydraulic losses in curved diffusion tubes, most systems now use a diffusion section that is slightly curved but still nearly straight. For the deflector vanes, their inlet angle and circumferential position should be determined in consideration of the flow pattern as it exits the diffusion section; the principle is to create a continuous flow channel, avoiding overly narrow inlet cross-sections for these vanes, as this can lead to vortices and shock losses at the inlet. (5) The pre-rotation caused by the outlet angle of the deflector vanes has a significant impact on the characteristics of the next stage of impellers. During design, in order to make the term 1Vul in the theoretical head formula Ht = U²/Vu² – 1Vul equal to zero, the outlet angle of the deflector vanes should be set at 90°; this helps to eliminate the rotational component in the case of the final stage vanes. However, experiments have shown that this is detrimental to both efficiency and the achievement of a stable performance curve, especially for some pumps with low specific speed. In order to obtain a descending characteristic curve, the exit angle of the counter-rotating vanes should be set to less than 90°, typically between 60° and 80°. The ends of the blade should be thinner to avoid impact and vortex losses. (6). Increasing the outlet width of the impeller reduces the absolute velocity at the impeller outlet, thereby reducing the hydraulic losses in the water chamber. (7) Tilting the impeller outlet, reducing the length difference between the upstream and downstream flow paths, or selecting different blade outlet angles for the various flow paths, in order to decrease the pressure difference along the flow paths at the upstream and downstream covers, thereby reducing the secondary recirculation at the outlet. (8). Increasing the throat area of the pressurized water chamber helps to prevent flow obstruction when the original design area is small. 2. Reduce mechanical and friction losses: (1) Mechanical friction losses caused by bearings and packing are generally very small and have little impact on efficiency. The mechanical friction loss of a packing seal is greater than that of a mechanical seal; it is better to use a mechanical seal. (2). Improve the surface finish of the impeller and guide vane flow channels. If possible, it is best to grind the surface of the flow channel using tools such as a hand-held grinder, as this will significantly reduce hydraulic friction losses. (3). The disk friction loss generated between the surfaces of the front and rear cover plates of the impeller and the liquid is proportional to the fifth power of the impeller’s outer diameter. Choosing a larger blade exit angle can reduce the outer diameter of the impeller, thereby reducing disk friction losses. The disk friction loss is highly dependent on surface roughness; the outer wall of the impeller cover should be as smooth as possible. Appropriately reducing the gap between the impeller shroud and the vanes can also reduce disk friction losses. 3. Reduce leakage: Reduce the gaps between various components appropriately, increase the sealing gap, or use labyrinth seals, etc., to increase the resistance to leakage and thereby reduce volume losses. Leakage points inside the pump occur at the impeller and seal ring, between the stages of multi-stage pumps, and in the axial force balance devices. Improve the efficiency of the pump by optimizing the piping system to reduce resistance. The pipeline length should be kept as short as possible and in a straight line; the flow velocity should be reduced to minimize frictional head losses ； Reduce the number of components such as gate valves, foot valves, elbows, and orifice plates to minimize local head losses. The excess amount that reduces the outlet pressure of the fluoroplastic centrifugal pump meets the requirements of the piping system for the outlet pressure in an appropriate manner. If the pressure margin of the fluoroplastic centrifugal pump is too large, and its outlet pressure exceeds the pressure required by the system, it becomes necessary to use throttling methods such as closing valves to reduce the pressure, which results in a waste of power. At this point, modifications must be made to the fluoroplastic centrifugal pump. The first and second impellers can be removed depending on the pressure required by the system; if the excess pressure is not too high, the impellers can be turned down in size to reduce the pressure, so that the fluoroplastic centrifugal pump in the system operates as close as possible to its optimal efficiency point, thereby avoiding operation at high or low flow rates.