【Daily Question】June 11: Methods for adjusting flow rate in centrifugal pumps and reciprocating pumps (First 100 participants + 2 correct answers = additional 3–8 points)

This post was last edited by xlxuiin on 2015-6-11 at 10:29. Variable frequency technology can be used to adjust the flow rate of centrifugal pumps and reciprocating pumps.

Methods for adjusting the flow rate of centrifugal pumps: (1) Reduce the pump speed; (2) Turn the impeller; (3) Change the installation angle of the impeller. Methods for adjusting the flow rate of reciprocating pumps: (1) Reduce the pump speed and thus the number of strokes per unit time; (2) Adjust the stroke length to change the flow rate

This post was last edited by whthday on 2015-6-11 10:44. General methods: 1. Speed adjustment, 2. Bypass adjustment

Let’s talk about the common methods: adjustment of the outlet bypass line, frequency conversion of the motor. For centrifugal pumps: cutting the impeller; slight adjustment of the outlet valve or inlet valve. For reciprocating pumps: unloader

In practical operation, what are the common flow control methods for centrifugal pumps and reciprocating pumps? Centrifugal pumps can be used, and they are a very common method. The flow rate of a centrifugal pump decreases as the head (or pressure) increases; when the valve is closed, the resistance rises, and the pump’s head needs to increase, which results in a decrease in flow rate. Conversely, when the valve is opened, the flow rate increases. A reciprocating pump is a positive-displacement pump; its flow rate is determined by the volume change per unit of time. If the rotation speed remains constant, the flow rate also stays constant. It is not possible to change the flow rate by increasing resistance, as an increase in resistance only leads to greater leakage, resulting in only a slight change in flow rate. Centrifugal pumps can be used, and they are a very common method. The flow rate of a centrifugal pump decreases as the head (or pressure) increases; when the valve is closed, the resistance rises, and the pump’s head needs to increase, which results in a decrease in flow rate. Conversely, when the valve is opened, the flow rate increases. A reciprocating pump is a positive-displacement pump; its flow rate is determined by the volume change per unit of time. If the rotation speed remains constant, the flow rate also stays constant. It is not possible to change the flow rate by increasing resistance, as an increase in resistance only leads to greater leakage, resulting in only a slight change in flow rate. As can be seen from the operating point of the centrifugal pump, changing the opening degree of the pump’s outlet valve alters the local resistance, which in turn changes the pipeline characteristic curve and results in a change in flow rate. Although this method is not economical, it has no other impact on the pump’s operation ； Reciprocating pumps belong to the category of positive-displacement pumps; their head is independent of flow rate. If the outlet valve is closed, the pressure inside the pump rises sharply, which can cause damage to the pump itself, the pipelines, and the motor. Therefore, it is not appropriate to use the outlet valve to regulate the flow rate. For flow rate regulation in reciprocating pumps, common methods used during the operation of steam reciprocating pumps include the bypass return method, changing the piston stroke, and altering the number of piston reciprocations. The bypass return method uses a bypass pipeline to connect the discharge pipeline to the suction pipeline, allowing a portion of the discharged liquid to flow back into the suction pipeline for flow regulation. A bypass control valve is installed on the bypass pipeline. 　　It can be used to simply adjust the amount of reflux in order to regulate the flow rate, as shown in Figure 4-4. This regulation method incurs power consumption losses, so it is not cost-effective. A common method for changing the piston stroke is to adjust the position of the crank pin, modify the clearance at the connection between the plunger and the crosshead, or use a mechanism for adjusting the piston stroke length. The piston stroke adjustment mechanism allows for stepless adjustment; the stroke can be set to zero, enabling the pump’s flow rate to be adjusted arbitrarily between maximum and zero. It is currently widely used for stepless flow regulation and accurate metering in piston pumps. 　　The method of changing the number of piston reciprocations involves using a tower wheel or gearbox to adjust the speed of the pump shaft, thereby changing the number of piston reciprocations. However, it should be noted that as the rotational speed increases, the power of the prime mover, the strength of the pump’s components, and its maximum allowable speed must meet the required standards. For reciprocating pumps driven directly by steam, as long as there are fluctuations in the instantaneous flow rate and the average flow rate remains constant, this is because the suction and discharge processes of the liquid medium in such pumps occur alternately. Moreover, the speed of the piston (or plunger) changes continuously during its movement. In a pump with only one cylinder, the instantaneous flow rate not only varies over time but is also discontinuous. The use of a double-acting structure (as shown in Figure 4-2), a differential structure (as shown in Figure 43), and a multi-cylinder pump structure can reduce the amplitude of fluctuations in the instantaneous flow rate in the discharge pipeline, but such fluctuations are inevitable. Theoretically, it can be considered that the flow rate is independent of the discharge pressure as well as of the properties of the liquid. The flow rate from a single-acting pump is discharged intermittently, and it is not as uniform as that of a centrifugal pump; however, precisely because the liquid is discharged cylinder by cylinder, it can be used for metered transfer. 　　The discharge pressure of a reciprocating pump is independent of its structural dimensions and speed; the maximum discharge pressure depends solely on the pump’s own power, strength, and sealing performance. The flow rate of an electric reciprocating pump is almost independent of the discharge pressure; only at higher pressures does the pump’s flow rate change slightly due to factors such as gas dissolution in the liquid, leakage in valves and seals, etc. Therefore, reciprocating pumps cannot be used to regulate flow by closing the outlet valve; closing the discharge valve will cause an abrupt increase in discharge pressure, leading to motor overload or pump damage. Reciprocating pumps have self-priming capability; they can draw in liquid on their own without the need for priming before starting. But in actual use, it is still desirable to have liquid inside the pump cylinder, as this allows for immediate suction and discharge of liquid ； On the other hand, dry friction between the piston and the cylinder, or between the plunger (piston rod) and the packing, is avoided to reduce wear. The suction capacity of a reciprocating pump is related to its speed; as the speed increases, not only do flow losses rise, but inertial losses also increase, resulting in a decrease in the suction pressure inside the pump cylinder. When the pressure inside the pump cylinder is lower than the vaporization pressure of the liquid, part of the liquid begins to vaporize within the cylinder, which reduces the suction fill level of the pump and can even lead to cavitation. Severe cavitation will lead to water hammer, damaging the pump’s components and shortening its service life.

Adjustment of centrifugal pumps: outlet adjustment by changing the speed. Reciprocating pump: Adjust the speed. The general method is to adjust the speed. However, a reciprocating pump can have a tee at the outlet for backflow, which is regulated by a backflow valve.

1. Both can be adjusted through frequency conversion. 2. They can also be adjusted via the return flow rate

In practical operation, what are the common flow control methods for centrifugal pumps and reciprocating pumps? Bypass adjustment, speed adjustment