[Test Your Knowledge] Chemical Production Knowledge—Question and Answer (16) is now over! See floor 23 for the answer key

Cavitation: 1. If the installation height of the pump can be reduced, cavitation can be effectively avoided. 2. Install a bypass valve on the centrifugal pump to release air regularly. Flow regulation of centrifugal pumps: 1. By changing the opening degree of the valve, the characteristic curve of the pipeline can be altered. Easy to apply in production. 2. Change the pump’s speed, which in turn alters its characteristic curve. Difficult to apply in production.

Although there are many external reasons for cavitation in liquid pumps – such as factors related to the pump’s structure as well as its installation and operation – the fundamental cause of cavitation is the vaporization of some of the liquid inside the pump; therefore, preventing the vaporization of liquid is the key measure to avoid cavitation in liquid pumps. 1) Reduce the pump’s installation height to increase the pump’s inlet pressure. Increase the liquid subcooling, so vaporization is less likely to occur. 　　2) Strengthen the insulation of the liquid pipelines to prevent the liquid from vaporizing due to temperature rise caused by heat absorption ； 　　3) Do not let the liquid pump run idly for too long. Because when the outlet valve of the liquid pump is closed, the effective power is zero. The power consumed by the motor is used only to stir the liquid inside the pump, which raises the temperature of the liquid and leads to vaporization. As a general rule, it is advisable to open the outlet valve by 1/3 before starting ； 4) If cavitation occurs, ventilation should be carried out immediately until the pump is stopped, in order to ensure the safety of the liquid pump. Centrifugal pump flow regulation: 1. Adjust by changing the valve opening 2. Adjust by changing the pump speed

a. How to prevent cavitation in centrifugal pumps? Answer: To avoid cavitation, the installation height of a centrifugal pump cannot be too high; there are restrictions on the pump’s installation height. The actual installation height of the pump must be lower than or equal to the maximum allowable installation height (allowable net positive suction head). b. What are the methods for adjusting the flow rate of a centrifugal pump? Answer: Based on the operating point of the pump, adjusting the flow rate essentially means changing the characteristic curve of the centrifugal pump or that of the piping system, thereby altering the pump’s operating point. Therefore, the flow regulation of centrifugal pumps should be considered from two aspects: one is to install appropriate control valves in the discharge pipeline in order to modify the pipeline characteristic curve ； The second method is to change the speed of the centrifugal pump in order to alter its performance curve; both approaches can modify the pump’s operating point, thereby regulating the flow rate. 1. Changing the valve opening: Adjusting the flow rate by changing the valve opening essentially means closing or opening the valve more or less in order to increase or decrease the resistance in the pipeline, thereby altering the pipeline’s characteristic curve. This flow regulation method is simple and flexible, and is widely used in industrial production and daily life. 2. Changing the pump speed: Adjusting the flow rate by changing the speed of a centrifugal pump essentially involves keeping the pipeline characteristic curve unchanged, while altering the pump’s own characteristic curve. Another point is that, for a certain piping system, when the flow rate provided by a single centrifugal pump is insufficient to meet the requirements, two centrifugal pumps of the same model can be used in parallel. In this case, the liquid discharged by both pumps flows into the same piping system; both pumps have the same head, and the total flow rate is the sum of the flow rates of each pump. Parallel operation of centrifugal pumps can increase flow rate.

1. Methods to prevent cavitation in centrifugal pumps include: reducing the installation height； Reduce the resistance of the inlet pipe ； Double-suction impeller is used ； Install an inducer wheel ； Use materials resistant to cavitation ； Lower the temperature of the liquid being transported. 2. The methods for flow regulation include: direct control of flow via the outlet valve ； Bypass adjustment ； Use frequency conversion control

First, the surface moving in the liquid should be streamlined to prevent vortices from forming in certain areas, as low pressure in vortex zones can easily lead to the formation of bubbles. Furthermore, reducing the gas content in the liquid and the disturbances in the liquid flow will also limit bubble formation. 　　Choosing the appropriate material can improve cavitation resistance. Generally, metal materials with high strength and toughness exhibit better cavitation resistance, and improving the corrosion resistance of the material also reduces cavitation damage. The methods for adjusting the flow rate of a centrifugal pump include: (1) installing a control valve in the outlet pipeline of the centrifugal pump; by changing the opening degree of this valve, the value of B in the pipeline characteristic curve He=A+BQe2 is altered. When the valve is opened wider, the operating point moves further away from the vertical axis ； The valve is closed, and the operating point is close to the vertical axis. The advantage of this adjustment method is its simplicity and flexibility in operation. Its disadvantage is that when the valve is closed, the resistance in the pipeline increases, leading to greater energy loss; as a result, the pump cannot operate within its most efficient range, which is uneconomical. Using the method of adjusting the valve opening to regulate flow is commonly applied in situations where the required flow adjustment range is small but frequent adjustments are needed. (2) Change the pump speed, thereby changing the pump’s characteristic curve. (3) Turning the outer diameter of the impeller also changes the pump’s characteristic curve. Using either of the above two methods can change the pump’s I-curves. Using these methods to adjust the flow rate within a certain range ensures that the pump operates in its high-efficiency zone, resulting in more economical energy use; however, it is inconvenient and the range of flow rate adjustment is limited, which is why it is not widely used.

a. How to prevent cavitation in centrifugal pumps? Answer: 1. First, drain the residual gas from the pump ； 2. When installing the pump, try to minimize the suction vacuum level as much as possible ； 3. In terms of operation, the temperature at which the liquid is heated should be kept as low as possible ； 4. Reduce resistance losses in the pipelines ； 5. Minimize hydraulic losses in the suction pipe so that the pressure at the pump’s inlet is much higher than the saturated vapor pressure of the liquid. 6. Reducing the speed at which liquid enters the impeller, that is, lowering the pump’s rotational speed and flow rate, can also prevent cavitation. 7. Using an inducer wheel can also improve the cavitation of the pump. b. What are the methods for adjusting the flow rate of a centrifugal pump? Answer: 1. Changing the pipeline characteristic curve. The simplest way to adjust the flow rate of a centrifugal pump is by controlling the opening degree of the pump’s outlet valve; essentially, this involves changing the position of the pipeline characteristic curve in order to alter the pump’s operating point. 2 Changing the characteristic curve of centrifugal pumps: According to the law of proportionality and the law of modification, two methods can be used to alter the characteristic curve of a pump – by changing its rotational speed or by modifying its structure (such as by reducing the outer diameter of the impeller). These methods enable the regulation of flow rate (while also changing the head pressure). However, for pumps that are already in use, it is not convenient to change their structure, and such structural changes reduce the versatility of the pumps. Although it can be economical and convenient to adjust the flow rate in some cases, this method is rarely used in production. Here, only the method of adjusting flow rate by changing the speed of the centrifugal pump is analyzed. When the flow rate is reduced from Q1 to Q2 by changing the pump speed, the pump’s speed (or the motor speed) decreases from n1 to n2. At this speed of n2, the pump’s performance curve Q-H intersects the pipeline performance curve He=H0+G1Qe2 (which remains unchanged) at point A3(Q2, H3); point A3 represents the new operating point after adjusting the flow rate via speed control. This adjustment method offers significant, rapid, and safe results; it can extend the pump’s service life and save electrical energy. Additionally, operating at a lower speed helps to reduce the NPSHr of the centrifugal pump, keeping it away from the cavitation zone and thereby decreasing the likelihood of cavitation occurring in the pump. The disadvantage is that to change the pump’s speed, it is necessary to use frequency conversion technology to adjust the speed of the prime mover (usually a motor); the principle behind this is complex, the investment required is high, and the range of flow rate adjustment is limited. 3 Parallel and series connection methods for pumps: When a single centrifugal pump is not sufficient to meet the transportation requirements, these pumps can be operated in parallel or in series. By connecting two centrifugal pumps of the same model in parallel, the head change is minimal, but the total flow rate of delivery increases; the overall efficiency of the pumps in parallel is the same as that of a single pump ； When centrifugal pumps are connected in series, the total head increases while the flow rate remains roughly the same; the overall efficiency of the series-connected pumps is equal to that of a single pump.

To prevent cavitation before starting the pump, the following steps should be taken: 1. First, drain any residual gas from within the pump； 2. When installing the pump, try to minimize the suction vacuum level as much as possible ； 3. The temperature at which the liquid is heated should be kept as low as possible during operation ； 4. Reduce resistance losses in the pipelines ； 5. Minimize hydraulic losses in the suction pipe so that the pressure of the liquid at the pump’s inlet is much higher than the saturated vapor pressure of the liquid. 6. Reducing the speed at which liquid enters the impeller, that is, lowering the pump’s rotational speed and flow rate, can also prevent cavitation. 7. Using an inducer wheel can also improve the cavitation of the pump. 8. Use materials with strong cavitation resistance to manufacture the impeller. Methods for flow regulation include: (1) Installing a control valve on the outlet pipeline of the centrifugal pump; by changing the opening degree of this valve, the value of B in the pipeline characteristic curve He=A+BQe2 is altered. When the valve is opened wider, the operating point moves away from the vertical axis ； The valve is closed, and the operating point is close to the vertical axis. The advantage of this adjustment method is its simplicity and flexibility in operation. Its disadvantage is that when the valve is closed, the resistance in the pipeline increases, leading to greater energy loss; as a result, the pump cannot operate within its most efficient range, which is uneconomical. Using the method of adjusting the valve opening to regulate flow is commonly applied in situations where the required flow adjustment range is small but frequent adjustments are needed. (2) Change the pump speed, thereby changing the pump’s characteristic curve. (3) Turning the outer diameter of the impeller also changes the pump’s characteristic curve. Using either of the above two methods can change the pump’s I-curves. Using these methods to adjust the flow rate within a certain range ensures that the pump operates in its high-efficiency zone, resulting in more economical energy use; however, it is inconvenient and the range of flow rate adjustment is limited, which is why it is not widely used.

When a centrifugal pump is in operation, the pressure inside the impeller varies; the pressure is lower at the inlet and higher at the outlet. And the vaporization temperature of a liquid is related to pressure ； The lower (or higher) the pressure, the lower (or higher) the corresponding vaporization temperature. If the temperature of the liquid entering the pump is higher than the vaporization temperature corresponding to the inlet pressure, part of the liquid will vaporize, forming bubbles. And when the bubble is carried by the liquid to an area with higher pressure, the corresponding vaporization temperature increases, causing the vapor to condense back into a liquid once again, and the bubble ruptures rapidly. At this point, since the densities of gas and liquid differ by hundreds of times, at the moment the bubble condenses and its volume shrinks suddenly, the surrounding liquid rushes into the space previously occupied by the bubble at high speed, resulting in intense shock waves within the liquid. If this phenomenon occurs on the surface of a blade, the metal material is eroded due to repeated exposure to high impact stresses; therefore, it is called cavitation. 　　 There are two measures to improve the cavitation resistance of centrifugal pumps: a. Measures to enhance the cavitation resistance of the centrifugal pump itself (1) Improve the structural design from the pump’s suction inlet to the vicinity of the impeller. Increase the overcurrent area ； Increase the radius of curvature at the inlet section of the impeller cover to reduce the sudden acceleration and pressure drop of the fluid flow ； Appropriately reducing the thickness at the blade inlet and rounding it to make it more streamlined can also reduce the acceleration and pressure drop around the blade tip ； Improve the surface finish of the impeller and blade inlet sections to reduce drag losses ； Extending the blade inlet edge toward the impeller inlet allows the fluid flow to be worked on earlier, thereby increasing pressure. (2) A pre-induction wheel is used to enable the liquid flow to do work in advance within the pre-induction wheel, thereby increasing the pressure of the liquid flow. (3) A double-suction impeller is used, allowing the fluid to enter the impeller from both sides; as a result, the inlet area doubles, and the inlet flow velocity can be reduced by half. (4) A slightly larger positive attack angle is adopted in the design conditions to increase the blade inlet angle, reduce bending at the blade inlet, minimize blade blockage, and thereby increase the inlet area ； Improve working conditions under high flow rates to reduce flow losses. However, the impact angle should not be too large, otherwise it will affect efficiency. (5) Use materials resistant to cavitation. Practice has shown that the higher the strength, hardness, and toughness of a material, as well as its chemical stability, the better its resistance to cavitation. b. Measures to increase the effective net positive suction head of the liquid inlet device: (1) Increase the pressure of the liquid level in the liquid storage tank ahead of the pump to raise the effective net positive suction head. (2) Reduce the installation height of the pump in the suction device. (3) Replace the upward suction device with a backflow device. (4) Reduce the flow loss in the pipeline before the pump. For example, shorten the pipelines as much as possible within the required range, reduce the flow velocity in the pipelines, minimize the use of bends and valves, and maximize the opening degree of the valves. Flow regulation method: (1) Install a control valve on the outlet pipeline of the centrifugal pump; by adjusting the opening degree of the valve, the value of B in the pipeline characteristic curve He=A+BQe2 is changed. When the valve is opened wider, the operating point moves away from the vertical axis ； The valve is closed, and the operating point is close to the vertical axis. 　　The advantage of this adjustment method is its simplicity and flexibility in operation. Its disadvantage is that when the valve is closed, the resistance in the pipeline increases, leading to greater energy loss; as a result, the pump cannot operate within its most efficient range, which is uneconomical. Using the method of adjusting the valve opening to regulate flow is commonly applied in situations where the required flow adjustment range is small but frequent adjustments are needed. 　　(2) Change the pump speed, thereby altering the pump’s characteristic curve. 　　(3) Turning the outer diameter of the impeller also changes the pump’s characteristic curve. Using either of the above two methods can change the pump’s I-curves. Using these methods to adjust the flow rate within a certain range ensures that the pump operates in its high-efficiency zone, resulting in more economical energy use; however, it is inconvenient and the range of flow rate adjustment is limited, which is why it is not widely used.

Preventing cavitation: 1. Avoid the formation of internal bubbles. There are various factors that can cause bubbles to form: such as air being drawn in at the inlet, high temperature of the medium, low inlet pressure, and so on. 2. You can try increasing the inlet pressure. Such as increasing the backflow height, etc. 3. The impeller is made of a material with good cavitation resistance. Methods for adjusting the flow rate of centrifugal pumps: (1) Install a control valve on the outlet pipeline of the centrifugal pump; by changing the degree of opening of this valve, the value of B in the pipeline characteristic curve He=A+BQe2 is altered. When the valve is opened more widely, the operating point moves further away from the vertical axis ； The valve is closed, and the operating point is close to the vertical axis. 　　The advantage of this adjustment method is its simplicity and flexibility in operation. Its disadvantage is that when the valve is closed, the resistance in the pipeline increases, leading to greater energy loss; as a result, the pump cannot operate within its most efficient range, which is uneconomical. Using the method of adjusting the valve opening to regulate flow is commonly applied in situations where the required flow adjustment range is small but frequent adjustments are needed. 　　(2) Change the pump speed, thereby altering the pump’s characteristic curve. 　　(3) Turning the outer diameter of the impeller also changes the pump’s characteristic curve. Using either of the above two methods can change the pump’s I-curves. Using these methods to adjust the flow rate within a certain range ensures that the pump operates in its high-efficiency zone, resulting in more economical energy use; however, it is inconvenient and the range of flow rate adjustment is limited, which is why it is not widely used.

a Controlling the temperature and concentration of the medium; b is generally achieved by adjusting the opening degree of the reflux line valve of the pump itself