The small opening of the pump outlet valve and its impact on the mechanical seal

Is your pump 65-50-160 with a flow rate of 25 and a head of 32? I’m not sure if the actual operating conditions you’re referring to are like this, or if you’re just talking about the data on the pump’s nameplate. Is your valve also DN80? What is the reading on the pressure gauge behind the pump right now? However, as long as the valve is opened to a low setting within the minimum continuous flow rate, some of the mechanical energy is converted into heat, which has little impact on the mechanical seal.

As long as the centrifugal pump maintains a flow rate that is not below the minimum value, there should be no problem. It can’t be that the actual traffic is below the minimum traffic level, right? If so, adding a return line should be better.

1. Add return flow 2. Cut the impeller 3. Add an inverter

Reply to 5# chjzhou: It seems that changing the impeller can only reduce the head; what impact does it have on the mechanical seal?

Reply to 2#: The Jiangtai IHF65-50-160 model has a flow rate of 25 m3/h and a height of 32 m. The outlet valve is an 80-size PTFE ball valve with a very small opening degree; there is no pressure gauge at the suction port. It seems that the flow rate is lower than its minimum value. When it was disassembled today, the mechanical seal was damaged. However, the inlet valve remained open – so shouldn’t that have prevented the mechanical seal from being damaged?

It’s possible that a low opening degree can cause vibration and cavitation; it’s normal for the machine seal to be damaged as a result of cavitation. A pressure gauge should not be installed at the suction port, but one must be installed at the outlet – this way you can know the actual operating conditions and determine whether cavitation is occurring

This post was last edited by Refinery Operator on 2011-11-12 at 19:46. What are the common methods for adjusting the flow rate of centrifugal pumps? Clicks: 3313. Publication date: 2010-4-21. Analysis of common methods for adjusting centrifugal pumps. Centrifugal pumps are widely used in industries such as water management and chemicals, and there is an increasing emphasis on selecting the appropriate operating conditions for them as well as analyzing their energy consumption. The so-called operating point refers to the actual flow rate, head, shaft power, efficiency, and suction vacuum level of the water pump at a given moment; it reflects the performance of the water pump. Typically, the flow rate and head of a centrifugal pump may not match those of the piping system, or changes may occur due to production requirements or process needs, which necessitates adjusting the pump’s flow rate; in essence, this involves changing the operating point of the centrifugal pump. In addition to the accuracy of centrifugal pump selection during the engineering design phase, the choice of operating conditions in the actual use of centrifugal pumps also directly affects the user’s energy consumption and cost expenses. Therefore, it is particularly important to determine how to fairly shift the operating point of the centrifugal pump. The working principle of a centrifugal pump is to convert the mechanical energy generated by the high-speed rotation of the electric motor into the kinetic and potential energy of the liquid being pumped; it is a process of energy transfer and conversion. Based on this characteristic, it can be seen that the operating point of a centrifugal pump is determined by the balance between the energy supply and demand in the pump and pipeline system; whenever there is a change in either of these factors, the operating point will shift. The change in operating conditions is caused by two factors: first, the change in the characteristic curve of the piping system, such as valve throttling ； II. Changes in the characteristic curves of the water pump itself, such as variable frequency speed control, cutting of the impeller, and series or parallel connection of water pumps. The following methods are analyzed and compared: 1. Throttling the valve to regulate the flow rate of the centrifugal pump. The simplest way to change the flow rate of a centrifugal pump is to adjust the opening degree of the valve at the pump’s outlet, while keeping the pump’s speed constant (usually at the rated speed); in essence, this involves changing the position of the pipeline characteristic curve in order to alter the pump’s operating point. As shown in Figure 1, point A, the intersection of the pump characteristic curve Q-H and the pipeline characteristic curve Q-∑h, represents the extreme operating condition of the pump when the valve is fully open. When the valve is closed, the local resistance in the pipeline increases, causing the operating point of the centrifugal pump to shift to the left to point B, with a corresponding decrease in flow rate. When the valve is fully closed, the resistance is infinitely large and the flow rate is zero; at this point, the pipeline characteristic curve coincides with the vertical axis. As can be seen from Figure 1, when the flow rate is controlled by closing the valve, only the water supply capacity of the centrifugal pump itself remains unchanged, and its head characteristic stays the same; whereas the pipe resistance characteristic changes as the valve opening degree changes. This method is easy to operate, provides a continuous flow rate that can be adjusted freely between a maximum value and zero, requires no additional investment, and is therefore highly practical. However, throttling regulation relies on consuming the excess energy of the centrifugal pump (the shaded area in the diagram) to maintain a certain supply volume; as a result, the efficiency of the centrifugal pump decreases, which is not economically favorable. II. Adjusting the flow rate of centrifugal pumps using variable frequency speed control: The fact that the operating point is outside the high-efficiency range is the fundamental condition that necessitates speed control for centrifugal pumps. When the speed of the centrifugal pump changes, the valve opening remains constant (usually at its maximum value); the characteristics of the piping system stay unchanged, and it is only then that the water supply and head characteristics change. As shown in Figure 2, A represents the equilibrium operating point (also known as the operating point) of the centrifugal pump, corresponding to the efficiency ηa. To reduce the flow rate, the speed can be decreased; at this point the operating condition is B, with an efficiency of ηb, and the water pump remains within the high-efficiency range. If valve throttling is used for regulation, the operating point will be C, with an efficiency of ηc; the efficiency of the centrifugal pump decreases as a result. It can be seen that when the required flow rate is less than the rated flow rate, the head generated by variable frequency speed control is lower than that achieved through valve throttling; therefore, the power required for water supply under variable frequency speed control is also lower. The shaded area in Figure 2 represents the amount of water supply power saved by using variable frequency speed control. It is evident that, compared to valve throttling, variable frequency speed control yields significant energy-saving benefits, and centrifugal pumps operate with higher efficiency. Furthermore, the use of variable frequency speed control not only helps to reduce the likelihood of cavitation in centrifugal pumps, but it also allows the start-up and shutdown processes to be extended by presetting the time taken for acceleration/deceleration. This results in a significant reduction in dynamic torque, thereby substantially minimizing the destructive water hammer effect and **extending the lifespan of the pump and pipeline systems**. In fact, variable-frequency speed control also has its limitations; aside from the high initial investment and maintenance costs, excessive changes in the pump’s speed can lead to a decrease in efficiency. Once beyond the range dictated by the pump’s proportional laws, it becomes impossible to adjust the speed indefinitely. III. Adjusting the flow rate of a centrifugal pump by using a cutting impeller: When the rotational speed remains constant, both the head and flow rate of the pump are related to the diameter of the impeller. For pumps of the same model, the cutting method can be used to modify their characteristic curves. Let the original impeller diameter of the centrifugal pump be D, its flow rate be Q, its head be H, and its power be P; the diameter of the impeller after cutting be D’, its flow rate be Q’, its head be H’, and its power be P’. Then the relationships between these values are as follows: The above three equations are collectively referred to as the cutting laws for centrifugal pumps. The cutting law is based on a large amount of empirical experimental data; it states that if the amount of cutting on the impeller is kept within certain limits (and this cutting limit is related to the specific speed of the water pump), then the efficiency of the centrifugal pump remains essentially unchanged before and after cutting. Cutting the impeller is a simple and effective method for altering the performance of water pumps; it is what is known as diameter adjustment. To a certain extent, it resolves the conflict between the limitations in pump types and specifications and the diversity of requirements from different water supply applications, thereby expanding the range of applications for water pumps. Of course, cutting the impeller is an irreversible process, and users must conduct accurate calculations and assess economic feasibility before proceeding. IV. Series and parallel connection of centrifugal pumps to regulate flow rate The series connection of centrifugal pumps refers to the situation where the outlet of one pump feeds fluid into the inlet of another pump. Taking the simplest case of two centrifugal pumps of identical model and performance connected in series as an example: as shown in Figure 3, the performance curve for the series connection is equivalent to the sum of the head values from the individual pump’s performance curves, at the same flow rate. The flow rate and head at the series operation point A are both higher than those at the single-pump operation point B, but neither reaches twice the values of the single pump. This is because, when pumps are connected in series, the increase in head is greater than the increase in pipeline resistance; thus, the increased head facilitates an increase in flow rate. On the other hand, the increased flow rate leads to an increase in resistance, which limits further increases in the total head. When centrifugal pumps are operated in series, it is essential to pay attention to whether the subsequent pump can withstand the increased pressure. Before starting, the outlet valves of each pump must be closed, after which the pumps and valves are turned on in sequence to supply water outward. Parallel operation of centrifugal pumps refers to the use of two or more pumps to deliver fluid into the same pressure pipeline, with the goal of increasing flow rate while maintaining the same head pressure. Still taking the simplest case of two centrifugal pumps of identical model and performance connected in parallel as an example: as shown in Figure 4, the performance curve for the parallel operation is equivalent to the sum of the flow rates corresponding to the single-pump performance curve at the same head. Both the flow rate and head at the parallel operating point A are greater than those at the single-pump operating point B, but due to pipe resistance, they still do not reach twice the values of the single pump. If the sole goal is to increase flow rate, then ultimately whether to use a parallel or series configuration should depend on the flatness of the pipeline’s characteristic curve. The flatter this curve, the closer the flow rate in a parallel configuration will be to twice that of a single pump in operation; thus, it yields a higher flow rate compared to a series configuration, which is more advantageous for operation. V. Regulating the flow rate of centrifugal pumps Conclusion: Although valve throttling results in energy loss and waste, it remains a fast and straightforward method for flow rate regulation in some simple applications ； Variable frequency speed control is increasingly favored by users due to its excellent energy-saving effects and high degree of automation ； Cutting impellers are generally used in water purification pumps; due to the alteration in the pump’s structure, their versatility is limited ； Pump series and parallel connections are only useful when a single pump is not sufficient to meet the transportation requirements, and having too many pumps in series or parallel is actually uneconomical. In practical application, various factors should be taken into consideration, and the best plan should be devised by combining different flow regulation methods to ensure the efficient operation of the centrifugal pump.

This post was last edited and replied to by Refinery Operator on 2011-11-12 at 19:46. Reply 6# lm1986: The Impact of Impeller Cutting Methods on Pump Performance. Authors: Fan Chaopu, Yan Xuelan, Ma Hongzhen. Abstract: This paper describes the differences in pump performance that occur when the outer diameter of the impeller is machined using two different methods; it explains the reasons for these differences through analysis, and proposes ways to improve the machining of the impeller’s outer diameter. 1. Introduction: The characteristic curves of water pumps under different operating conditions can be obtained by changing the rotational speed. However, this method is limited in practical applications because most water pumps are driven by AC three-phase asynchronous motors. The speed of this type of motor cannot be changed arbitrarily; using variable-frequency speed control increases the cost of the equipment. Turning the outer diameter of the impeller can also change the characteristics of the pump, and it is a simple and easy method; however, this approach can only be used in situations where it is necessary to reduce the flow rate, head, and power of the water pump. The method of cutting the outer diameter of the impeller expands the application range of a pump, which is why it is commonly used in single-stage centrifugal pumps. Cutting the impeller is based on the similarity principle in pump theory; when changing the performance of a pump by adjusting the impeller diameter through turning, calculations are usually carried out using the cutting law formula. Tests have shown that impellers cut to a diameter calculated using similarity laws often fail to achieve the desired performance. The more the impeller is cut, the greater the gap between its actual performance and the desired performance, resulting in losses due to the impeller becoming unusable. For new products, it is often necessary to leave enough margin based on personal experience; the impeller should be cut in several stages, and after each cut, testing must be carried out on a test bench. This increases the workload associated with testing and hinders the improvement of testing efficiency. II. Tests on Two Impeller Turning Methods Performance tests were conducted on a 50–32–315 type single-stage, single-suction magnetic pump using two different methods for cutting the impellers; notable differences were observed, which are presented here. The diameter D2 of the pump impeller before cutting is 324 mm, and the base circle diameter D3 at the partition of the pump casing is 334 mm. The sequence of impeller cutting tests is shown in Figure 1. 1) The impeller shown in Figure 1a is in its original size. The performance conditions for testing are as follows: Figure 1 shows cross-sections of the impeller – (a) the impeller in its original size; (b) the outer diameter of the blades turned to D2=324 mm; (c) the outer diameter of the blades turned to D2=285 mm; (d) the outer diameter of the blades turned to D2=276 mm; (e) the diameters of the front and rear cover plates of the impeller turned from p2=305 mm to 276 mm. 1. Blades; 2. Front cover plate; 3. Rear cover plate. The parameters are: Qa=6.8 m3/h, Ha=152 m, n=2950 r/min, η=12%. Since the user requires a head of H=135 m at this flow rate, it is necessary to machine the outer diameter of the impeller shown in Figure 1a in order to meet the user’s requirements. 2) The impeller shown in Figure 1b was designed to meet the requirement of Hb=135m; after calculation using the cutting law formula, the outer diameter of the impeller was adjusted, and in accordance with standard practices, the outer diameters of both the front and rear cover plates as well as the blades were turned to D2=305mm. The test results were: Qa = 6.8 m3/h, H = 130 m, n = 2950 rpm; clearly, the actual head did not reach the desired value. This is because the head cannot be controlled independently when cutting the impeller. The actual operating point of the pump will automatically move downward from point a along the cutting parabola ab to point c (as shown in Figure 2). At this point, the head meets the requirements but the flow rate is low, with Qb < Qa. To achieve the desired flow rate, the outlet valve is opened to adjust the flow rate to Qa; at this time, the operating point of the pump moves downward to the right along the Q-H curve at D2=305mm, from point b to point F. The head of the pump at this point is definitely lower than that at point b, with Ht=130m < Hb=135m. This is because the characteristic curve of a centrifugal pump is determined by the inherent property that the head decreases as the flow rate increases. The shape of the characteristic curve of a centrifugal pump depends on its specific speed. The range of specific speeds for centrifugal pumps is quite wide, which makes it difficult to carry out precise calculations using the cut-off law; therefore, it is necessary to include a sufficient margin in the calculations, and the size of this margin often depends on individual experience. 3) The impeller shown in Figure 1c is obtained by turning the outer diameter of the blades of the impeller in Figure 1b to D2=285 mm, while the outer diameters of the front and rear cover plates remain unchanged. Using the formulas for the cutting law, Qc=QbDc/Db and Hc=Hb(Dc/Db)2, it can be determined that the operating point of the impeller in Figure 1c should be Qc=6.35 m3/h and Hc=113.5 m. If the flow rate of the impeller shown in Figure 1c is adjusted to Qc=Qa=6.8 m3/h, then the corresponding head will be Hc’< Hc=113.5 m, for the reasons shown in Figure 1b. However, the test results for the impeller shown in Figure 1c show that at Qc=6.8 m3/h, the head Hc’ equals 19 m, which is higher than Hc=113.5 m. In other words, the head obtained without using a cutting cover plate is nearly 5% higher than the value calculated using the cutting law. 4) To further verify this effect, the outer diameter of the impeller blades shown in Figure 1cN was turned again to D2=276 mm, while the outer diameters of the front and rear cover plates remained unchanged; this is the impeller shown in Figure 1dN. Using the cutting law formula, it is calculated that the operating point of the impeller shown in diagram ld should be Qd=6.15 m3/h and Hd=106.5 m. If the flow rate of the impeller shown in Figure 1dN is also adjusted to Qd=Qa=6.8 m3/h, then the corresponding head will also be Hd’< Hd= 106.5 m, for the same reason as above. However, the test results for the impeller shown in Figure 1d show that at Qd=6.8 m3/h, the head Hd’=114.7 m, which is greater than Hd=106.5 m; it is also nearly 8% higher than the value calculated using the cutting law. The test results also show that, at the same flow rate, the pump efficiency of the three impellers shown in Figures 1b, 1c–1d remains essentially unchanged. 5) The impeller shown in Figure 1eN has its front and rear cover diameters turned down from D2=305mm to D2=276mm, just as is done for ordinary impellers. At this point, the performance of the impeller shown in Figure le is also calculated using the operating condition point corresponding to the impeller in Figure 1b; it is predicted that its operating condition point will be Qe=6.15 m3/h and He=106.5 m. If the flow rate is adjusted to Qe=Qa=6.8 m3/h, the test results show that He’=102.7 m < He=106.5 m at this point. This outcome has been analyzed earlier; it is expected. It is worth noting that the only difference between the impeller shown in Figure 1d and that shown in Figure 1e lies in whether the front and rear cover plates are present or not; yet at the same flow rate, the head difference between them is Hc’ – He’ = 114.7 – 102.7 = 12 m, resulting in a relative error of 10%, which shows just how significant the difference is. 3. Reasons for the performance differences between the two turning methods: The focus of research on centrifugal pumps is the motion of liquid particles within the space between two adjacent blades inside the impeller; these liquid droplets undergo rotational motion in a flow channel confined by the two blades and the front and rear cover plates. The cutting law is derived under the assumption that the proportion of reduction in all velocities in the exit velocity triangle of the impeller before and after cutting is the same as the diameter ratio D2’/D2. In a centrifugal pump, the head is generated by the impellers; the role of the pump casing is to convert the kinetic energy imparted to the liquid by the impellers into pressure energy. It does not generate head itself, but rather it should minimize the hydraulic losses of the liquid entering the pump. Tests have shown that the hydraulic losses of a pump occur mainly within the pump body; therefore, the impeller and the pump body are designed to work together. It is not possible to obtain a pump with excellent performance simply by using a good impeller along with any pump body that can be assembled with it. The relationship between the impeller diameter D2 of the pump and the base circle diameter D3 of the pump casing is also determined based on the optimal rotational clearance. The ideal approach is that if the size of the impeller changes, the size of the pump casing should also be adjusted accordingly to match it. For example, in Figure 1, both the base circle diameter D3 and the cross-sectional area of the volute should decrease as the outer diameter of the impeller decreases. However, such a requirement is impossible to meet in actual production. The head generated by the impeller is primarily reflected in the ratio of the circumferential component Vu2 of the absolute velocity of the liquid at the impeller outlet to the circumferential velocity u2 of the blades at that outlet. Once the liquid enters the pump body, a well-designed pump body will not destroy or alter this ratio. However, cutting the impeller while simultaneously cutting the front and rear cover plates increases the gap between the base circle diameter D3 of the pump body and the outer diameter D2 of the impeller, creating an annular space. The liquid in the annular space is the fluid that has gained energy from the blades; if not affected by any external forces, this fluid will maintain its velocity moment as it flows into the diffuser cone, where its kinetic energy is converted into pressure energy. At this point, the hydraulic losses within the pump are at their minimum. However, the cutting of the diameters of the front and rear cover plates of the impeller causes the fluid flowing out of the blade outlets to be mixed with and disturbed by the fluid inside the pump chamber. The front and rear cover plates of the impeller, together with the inner wall of the pump body, form the pump’s front and rear chambers. These chambers are created due to structural requirements; they are not channels for the liquid flowing within the pump. The pump chamber functions like a still water area; under the action of the impeller cover, the liquid particles within the pump chamber move in a limited space, performing both circular motion as well as radial and axial motion. The flow pattern of the liquid in the pump chamber is completely different from that at the blade exits; when the cover plate does not cut through, it can be considered to separate the liquid in the pump chamber from the liquid at the blade exits, preventing interference between them. When the cover plate cuts the impeller, the greater the cutting amount, the greater the disturbance caused by the fluid in the pump chamber to the flow of fluid at the blade outlets; as a result, the hydraulic losses increase and the head decreases more. The results of this experiment confirm this conclusion. IV. Conclusions 1) The tests were conducted on a centrifugal pump with a low specific speed, and the results showed that at the same flow rate, the head produced by the two cutting methods could differ by 5%–10%. Since the specific speed range of centrifugal pumps is very wide, this percentage difference in head is not highly representative. What is truly meaningful is the method of not turning the cover plate, which meets and improves the application conditions and accuracy of the cutting law. 2) The cutting method that involves cutting only the blades and not the cover plate is a beneficial and harmless approach, as it minimizes the influence of the liquid in the pump chamber on the fluid flow at the impeller outlet. Tests show that a decrease in the cover plate diameter does not lead to an increase in the disk friction loss of the pump. This is because the frictional losses in the disk are not solely determined by the diameter of the impeller cover; they are also related to the energy consumed by the liquid within the pump chamber, which constitutes the main portion of the losses. (end)

The above is an article from another company’s website. The original poster should take a look at the governing law for centrifugal pumps: (H1:H2)^2 = D1:D2, Q1:Q2 = D1:D2. Related articles