Misuse of centrifugal pumps
1. High-lift Pump for Low-lift Pumping Many people believe that pumping at a lower lift means less motor load, which is a common misconception. This misunderstanding often leads to the selection of pumps with higher head ratings than necessary. However, for centrifugal pumps, power consumption is directly proportional to the actual flow rate, not the head. As the head decreases, the flow increases, and so does the power consumption. Therefore, if you use a high-head pump for low-lift applications, the motor may become overloaded or overheat, potentially causing damage. To prevent this, ensure that the actual pumping head is at least 60% of the rated head. In emergencies, install a gate valve or block the outlet slightly to reduce flow and avoid motor overload. Be cautious about motor temperature, and if it becomes too hot, shut it down immediately. Some mistakenly think that blocking the outlet increases motor load, but in reality, many high-power pumps are equipped with gate valves to reduce initial load during startup—close the valve before starting and gradually open it once running.
2. Large Pump with Small Pipe Some believe that using a larger pump with a smaller pipe can increase the actual lift, but this is incorrect. The actual head of a pump equals total head minus loss head. When the pump model is fixed, the total head remains constant, but the loss head depends on pipeline resistance. A smaller pipe increases resistance, thus reducing the effective lift. Conversely, a larger pipe reduces resistance and improves efficiency. Some worry that using a large pipe with a small pump increases motor load, but pressure depends only on head, not pipe diameter. As long as the head is within the rated range, increasing the pipe size won’t harm the pump—it just reduces losses and improves performance.
3. Inlet Pipe Installation When installing the inlet pipe, avoid making the horizontal section level or upward. This can trap air, reducing vacuum and lowering the suction head. The correct approach is to have the horizontal section slightly sloped toward the water source, not level or upward.
4. Too Many Elbows on Inlet Pipe Adding too many elbows increases local resistance. If bends are made horizontally, they can trap air. Always make bends vertically to prevent air accumulation.
5. Direct Connection Between Pump Inlet and Elbow Connecting an elbow directly to the pump inlet can cause uneven water flow. If the inlet pipe is larger than the pump’s inlet, use an eccentric reducer, placing the flat side up and the beveled side down. Otherwise, air may accumulate, reducing flow and causing noise. If the inlet pipe and pump have the same diameter, add a straight section of at least 2–3 times the pipe diameter between the elbow and pump.
6. Foot Valve Not Installed Vertically If the foot valve isn’t vertical, it may not close properly, leading to leaks. The correct method is to install the valve at the end of the inlet pipe, ideally vertically. If terrain prevents vertical installation, the pipe should be at least 60° from the horizontal.
Incorrect Inlet Position (1) If the inlet pipe is too close to the bottom of the pool or wall (less than one diameter), it may suck in sediment or debris. It should be at least 1.5 times the diameter away from the bottom. (2) Insufficient submersion can cause vortices around the inlet, reducing flow. For small and medium pumps, submerge at least 300–600 mm; for large pumps, 600–1000 mm.
7. Outlet Pipe Above Water Level If the outlet is above the normal water level, even though the head increases, the flow decreases. If unavoidable, install an elbow and short tube to create a siphon effect, lowering the outlet height and improving efficiency.
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