The issue with the pump speed

The rotation speed is related to the pump’s head; a higher rotation speed results in a higher head, while a lower rotation speed leads to a lower head. In addition to considering the head, it is also necessary to take into account whether the medium being transported might experience centrifugal blockage at high speeds; for example, when transporting starch slurry, high-speed pumps with 2900 rpm cannot be used, and instead medium-speed pumps with 1400 rpm must be chosen.

The higher the rotational speed, the more prone the pump is to cavitation. When the pump speed is low, the velocity of the material entering the pump at the inlet is reduced, which gives more time for compensation; as a result, the net positive suction head of the pump decreases. That’s how I understand it.

There are various methods to prevent cavitation in centrifugal pumps. For the user (or buyer), the best approach is to reduce the pump’s installation height (by using reverse flow if necessary). The advantage of this method is that it allows the use of pumps with higher efficiency and easier maintenance; the downside is that it requires additional installation costs. Reducing the speed of the pump is also a common method used by manufacturers (or sellers). Its advantage is that it is simple to implement, but the downside is that the pump’s efficiency decreases; moreover, at the same flow rate and head, pumps operating at lower speeds are more expensive. As for which of the two methods is appropriate, a comprehensive evaluation based on specific circumstances is required ； Overall, the former might be more appropriate. The cost of reducing the pump’s installation height is a one-time expense, whereas reducing the pump’s speed leads to a decrease in its efficiency, which is a long-term issue. For reference!

When the speed of the pump changes, its flow rate and head also change, which in turn leads to corresponding changes in the pump’s efficiency and power. The area near the pump’s suction inlet is a low-pressure zone. When the lowest pressure near the inlet of the blade is equal to or less than the saturated vapor pressure of the liquid at the conveying temperature, the liquid will vaporize at this point, or the gas dissolved in the liquid will come out and form bubbles. When the bubble-containing liquid enters the high-pressure area of the impeller, the bubbles shrink rapidly and burst under the high pressure. The disappearance of these bubbles creates a local vacuum, causing the surrounding liquid to rush into the space formerly occupied by the bubbles at extremely high speeds, resulting in shock waves and vibrations. Under the repeated action of severe impact forces, the material on the surface of the blade becomes fatigued; this leads to pitting at first and then the formation of cracks, ultimately resulting in damage to the impeller or pump casing. This phenomenon is called cavitation. When the pump operates at high speed, the pressure at the pump’s suction inlet is low, which makes cavitation more likely to occur

Changes in the pump speed can alter flow rate and head

A low rotation speed can indeed reduce the impact of momentum; with all other conditions remaining unchanged, lowering the pump’s rotation speed results in a decrease in flow rate, head, and efficiency, as well as in power consumption. It is important to take into account the relationship between efficiency and power of the pump, and to make use of the pump’s characteristics in order to use the minimum amount of power necessary to meet the water demand for production, thereby achieving optimal operating conditions.

To align the views above: the higher the rotational speed, the more likely the pump is to experience cavitation; therefore, the rotational speed should be reduced as much as possible while still meeting the operational requirements

Pumps with low rotational speeds are costly because they also have the potential for lifting capacity. The best way is to reduce the installation height.

“Our factory is planning to purchase a circulating water pump; we initially considered ones with 1400 revolutions per minute, but then changed our mind to those with 900 revolutions per minute. Which option is more cost-effective? ” Let me add a few more points: motors with 1400 revolutions per minute cost less than those with 900 revolutions per minute. The difference in motor speed has a direct impact on the pump’s flow rate and head pressure. Technical support

The pump speed is only related to the head, not to cavitation

In my opinion, the pump should be chosen based on your needs; as for the motor that matches it, it is determined by the requirements of the pump itself, and has nothing to do with the speed. As for the cavitation issue mentioned by everyone, it is something that needs to be considered in pumps handling saturated liquids; however, normal temperature water does not experience cavitation under normal conditions.

The speed of the pump is related to flow rate and head, but not to the net positive suction head. At the same flow rate and head, a pump with a lower rotation speed requires a larger pump body, as well as larger pipe diameters at the inlet and outlet. The price of such a pump is also higher. The motor used with it has more poles at lower rotation speeds, which makes it more expensive as well. Pumps with low rotational speeds are mainly used in applications that require a large flow rate but low head; sometimes, high-speed pumps are not suitable for such applications. Additionally, when the material contains particles, low-speed pumps are chosen to reduce the abrasive wear on the pump casing and impeller caused by high rotational speeds.

Additionally, to meet the requirements for fluid transportation, the goal is to select equipment that minimizes investment and energy consumption, regardless of other factors such as rotational speed.

I don’t think there is any necessary connection between the two

The lower the rotational speed, the lower the net positive suction head of the pump; moreover, a pump with a lower rotational speed has a larger volume. The flow rate and head are selected based on the spectra of the units provided by various suppliers; of course, the rotation speed is also a factor to consider, and generally suppliers will take the user’s opinions into account.

The areas of a centrifugal pump where cavitation is most likely to occur are: a. the front cover, where the curvature of the impeller is greatest, on the low-pressure side near the inlet edge of the blades； b. The low-pressure side near the inlet edge of the volute baffle and guide vanes in the extrusion chamber ； c. The sealing gap between the outer circumference of the blade tip of the high specific speed impeller without a front cover and the housing, as well as the low-pressure side of the blade tip ； d. The first-stage impeller in a multi-stage pump. 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) Improving 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 before the pump in order 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 losses in the pipeline before the pump. For example, minimize the length of the pipelines within the required range, reduce the flow velocity in the pipelines, decrease the number of bends and valves, and maximize the opening degree of the valves. The above measures can be appropriately applied through comprehensive analysis based on factors such as pump selection, material choice, and the location where the pump will be used. From the above, it can be seen that rotational speed has no relation to cavitation. Why? To achieve the same head and flow rate, it is necessary to have the same rotational speed, which in turn determines the centrifugal force. If the rotational speed is reduced, then for a pump with the same diameter, the head will also be lower. Therefore, to achieve the same flow rate and head rate, it is possible to increase the diameter of the impeller. The problem is that when the impeller diameter increases, the rotational speed (i.e., angular velocity) decreases. However, since the diameter has increased, the linear velocity increases. According to the formula for centrifugal force, f = mω²r, if the head remains unchanged, then the centrifugal force remains unchanged as well, and thus the pressure generated also remains unchanged. In this case, cavitation has nothing to do with the rotational speed when maintaining the same head and flow rate. Last edited by mashang123 on 2009-4-19 18:23