I. Matching piping for pipeline centrifugal pumps
Some users of pipeline centrifugal pumps believe this can increase the actual head. In fact, the actual head of a pump = total head − head loss. Once the pump model is determined, the total head is fixed; head loss comes mainly from pipeline resistance. The smaller the diameter, the greater the resistance and therefore the greater the head loss. Reducing the pipe diameter therefore cannot increase the pump’s actual head; on the contrary it will reduce it and lower pump efficiency. Likewise, when a small-diameter pump draws water through a large-diameter pipe, the pump’s actual head does not decrease; rather, because pipeline resistance decreases, head loss decreases and the actual head increases. People also believe that a small-diameter pump drawing water through a large-diameter pipe will greatly increase motor load. They think that after the pipe diameter is increased, water in the discharge pipe will exert great pressure on the pump impeller and thereby greatly increase motor load. As is well known, hydraulic pressure depends only on lift height, not on the cross-sectional area of the water pipe. As long as the head is fixed, the impeller size is constant and the pressure acting on the impeller is constant, independent of pipe diameter. Only when the pipe diameter increases does flow resistance decrease, flow increase, and power consumption increase accordingly. However, as long as operation remains within the rated head range, the pump can operate normally regardless of pipe diameter, while pipeline losses are reduced and pump efficiency is improved.
II. High-head pumps used for low-head pumping
Some users of pipeline centrifugal pumps believe that the lower the pump head, the smaller the motor load. Because of this misconception, a pump with a very high head is often selected at the time of purchase. In fact, for a centrifugal pump, once the pump model is determined, power consumption is proportional to the actual flow rate. Pump flow decreases as head increases, so the higher the head, the smaller the flow and the lower the power consumption. Conversely, the lower the head, the greater the flow and the greater the power consumption. Therefore, to prevent motor overload, the actual pumping head is generally required to be not less than 60% of the rated head. When a high-head pump is used for low-head pumping, the motor is therefore prone to overload and overheating, and in severe cases the motor may burn out. If it must be used in an emergency, a gate valve (or a reduced outlet, for example with a wooden block) shall be installed on the discharge pipe to throttle the flow and prevent motor overload. Monitor the motor temperature rise. If the motor overheats, further throttle the outlet or shut down. This is also easily misunderstood. Some operators believe that blocking the outlet and forcibly reducing flow will increase motor load. In fact the opposite is true. Conventional high-power centrifugal pump irrigation and drainage units are fitted with a gate valve. To reduce motor load at start-up, the gate valve should first be closed and then gradually opened after the motor has started.
III. Elbows on the suction pipe of pipeline centrifugal pumps
If the suction pipe has many elbows, local flow resistance will increase. Elbows should turn in the vertical direction and must not turn in the horizontal direction, to avoid air accumulation.
4. When installing the suction pipe, the horizontal section is level or sloping upward
This will cause air to accumulate in the suction pipe, reducing vacuum in the pipe and pump, lowering the pump's suction head and reducing flow. The correct method is: the horizontal section should slope slightly toward the water source; it should not be level and must not rise.
5. The inlet of the pipeline centrifugal pump is connected directly to an elbow.
When water enters the impeller through an elbow, this causes uneven flow distribution. When the suction-pipe diameter is larger than the pump inlet diameter, an eccentric reducer should be installed, with the flat portion on top and the sloping portion at the bottom. Otherwise air will collect, reducing flow or preventing pumping and causing shock. If the pump inlet and suction pipe have the same diameter, a straight pipe should be added between the suction pipe and the elbow. The length of the straight pipe should be not less than 2 to 3 times the pipe diameter.
6. The discharge pipe of the pipeline centrifugal pump is above the normal water level of the outlet basin.
If the outlet is higher than the normal water level of the outlet, although the pump head is increased, the flow rate is reduced. If due to terrain conditions the outlet must be higher than the water level of the pool, an elbow and a short pipe should be installed at the nozzle so that the outlet pipe is of siphon type and the height of the outlet is reduced.
7. Incorrect location of the suction inlet of the suction pipe.
1. The inlet of the inlet pipe is too close to the bottom and walls of the pool. If there is dirt such as sediment at the pool bottom, and the distance from the pipe inlet to the pool bottom is less than 1.5 times the diameter, this may cause poor suction or inhalation of debris when pumping, blocking the water inlet.
2. When the water inlet of the inlet pipe is not deep enough, a vortex will be caused on the water surface around the inlet pipe, affecting inflow and reducing water volume. The correct installation method is: the water depth for small and medium-sized pumps should not be less than 300–600 mm, and for large pumps should not be less than 600–1000 mm.
8. After the foot valve is installed, the lower section of the suction pipe is not vertical.
If installed in this way, the valve cannot close by itself, resulting in water leakage. The correct installation method is: the inlet pipe is fitted with a foot valve, and the next section is vertical. If it cannot be installed vertically due to terrain conditions, the angle between the water-pipe axis and the horizontal plane should be 60° or greater.
