Primary issues with the circulating water pump

For a circulating water pump (usually a centrifugal pump), the greater the opening degree of its outlet valve, the higher the power consumption of the motor; the lowest power level is achieved during safe shutdown, when it is close to operating without load, but at this point the motor is doing no useful work.

The smaller the opening degree of the outlet valve of the circulating water pump, the lower the load on the pump, and consequently the lower the motor power. Conversely, the larger the opening degree, the higher the power.

Look at the flow-power curve in the performance curve.

For circulation water pumps with high flow rates and low head, the power curve is relatively flat; therefore, when the valve opening is increased, the rise in power is not very significant, and in some cases there may even be a decrease – this is all normal. The power of circulation water pumps with low flow rates generally increases as the valve is opened wider.

The person on floor 5 in bag #5 is probably referring to another type of pump； It is called a mixed-flow pump; in terms of appearance and structure, the mixed-flow pump lies between centrifugal pumps and axial flow pumps ； The pumping principle of a mixed-flow pump involves the high-speed rotation of the impeller, which generates both the centrifugal force of a centrifugal pump and the thrust force of an axial-flow pump; the mixed-flow pump relies on the combined action of these two forces to pump water. The performance of mixed-flow pumps also lies between that of centrifugal pumps and axial flow pumps. Compared to centrifugal pumps, they have a lower head but a higher flow rate ； Compared to axial flow pumps, it has a higher head but a lower flow rate. 　　The performance of mixed-flow pumps is expressed in terms of operating parameters. Such as flow rate, head, power, efficiency, speed, etc. At a certain speed, with flow rate as the variable, in other words, if the flow rate of the water pump is changed, then the pump’s head, power, and efficiency also change accordingly. By graphically representing these patterns of change in the interrelationships through several curves, we obtain the performance curves of the water pump. The performance curve of a mixed-flow pump also lies between that of a centrifugal pump and an axial-flow pump. For mixed-flow pumps with high head, the relationships between flow rate and head, as well as between flow rate and power, follow patterns similar to those of centrifugal pumps. In operation, it is possible to start the pump with the valve closed; this results in the lowest power consumption and ensures safety for the driving motor. For low-head mixed-flow pumps, the variation patterns among their performance parameters are similar to those of axial flow pumps. In terms of operation, it is not advisable to start such pumps with the valve closed; instead, they should be started with the valve open. This results in lower power consumption, reducing the risk of the motor being damaged.

The performance curve of a circulating water pump (centrifugal pump) is such that the flow rate is at its minimum, the head is at its maximum, and the power is at its minimum. When the outlet valve is closed, the flow rate decreases, the head increases, and the power decreases.

Look at the flow-power curve in the performance curve.

Generally, for centrifugal water pumps, the power increases as the flow rate increases; that is, the greater the opening of the outlet valve, the higher the motor current. However, this is the opposite case for axial flow centrifugal pumps.