A very basic problem with a centrifugal pump

After thinking it over, there’s another question I’d like to add. For a system with a flow rate of 6 m3/h and a head of around 13 meters, would it be possible to use a pipeline pump with a flow rate of 20 m3/h and a head of 30 meters? What problems might arise? ? ? Everyone, help me take a look

Question 1: In fact, when the pump outlet valve is closed, it limits the flow rate both for the pump itself and for the system (the pump’s flow rate is also the system’s flow rate); for the system, this restriction also affects the head pressure. In a system with a pressure drop of 4 meters, a pump with a head of 20 meters can be used; the outlet valve can also be employed for control. By keeping the valve partially closed, the head available for use is 4 meters, with the remaining 16 meters being lost due to throttling at the valve. Of course, this way of using it is quite wasteful. As for the second question, it is recommended not to use it in this way; when the pump operates at a low flow rate for an extended period of time, the likelihood of vibration increases, which can easily lead to damage to the pump.

When reducing the outlet valve on a centrifugal pump, the goal is to lower the pump’s flow rate; however, closing the outlet valve too much is also detrimental to the equipment itself. The most direct way to reduce the head pressure is by cutting the impeller, but cutting the impeller reduces head pressure while also decreasing the flow rate; you need to carry out calculations based on the actual conditions. The flow rate is proportional to the first power of the impeller diameter, Q1:Q = D1:D ； Head is proportional to the square of the impeller diameter, H1:H = (D1:D)2. Note: The final 2 is a square

For a system with a flow rate of 6 m3/h and a head of around 13 meters, using a pipeline pump with a flow rate of 20 m3/h and a head of 30 meters is not suitable, either in terms of flow rate or pressure. The difference in flow rate is too large. It is possible to reduce the flow rate manually by using valves, but for the equipment itself, operating under such conditions for an extended period will cause the pump to overheat, leading to cavitation damage. Additionally, the pump’s head is too high, resulting in excessive outlet pressure. I don’t think a system designed with a head of 13 meters would have such a large design margin.

After reading the post above, I have two questions: 1. If a variable-speed motor is used, can it protect the pump equipment? ? 2. What are the hazards to the entire system if the pump has too high a head and excessive outlet pressure? ? 3. After reducing the flow rate using a valve, according to the characteristic curve, a lower flow rate results in a higher head; in other words, the flow rate is controlled, but the system’s head pressure increases, right? ? ? ? Such a large margin exists because the actual water flow rate in the pump circulation is very low, at 6 m3/h (when six out of the nine pumps are in operation), whereas the water consumption required according to the instructions is quite high, at 20 m3/h (under the ideal condition of all nine pumps being in use). The water inlet valve of the vacuum pump is usually kept very closed as well; if it is opened, the system will trip. It’s very troublesome. This post was last edited by lhxo524 on 2008-3-9 10:08.]

Using variable-frequency motors is feasible to a certain extent, but it increases costs. From what you said above, it seems it should be about reusing existing equipment; in that case, cutting the impeller would be a more direct approach. After the impeller is cut, both the head pressure and flow rate decrease. The system is designed for low pressures; the nominal pressures of all system components such as pipelines, valves, monitoring instruments, and seals are relatively low. When designing it, it is never considered to use pumps with high capacity in this system. The vacuum pump trips when the valve is opened wide, as the pump’s rated load exceeds the motor’s current protection limits; the selection of the pump was not appropriate. When designing any pump, it should not be operated under conditions where the valve is open to a very small degree.

When the outlet valve is reduced, the head will increase

The basic situation is as follows: The original vacuum pump system was connected to the main cooling water pipeline. The vacuum return water enters the return water tank, and then is pumped into the cooling tower using level control. When water is discharged from the water tower, it is supplied to the vacuum pump as circulating water through a branch pipe connected to the main circulating water pipeline. Due to the inhalation of ethanol vapor and diphenyl sulfone at work, as well as high temperatures of the medium involved, the water quality is currently very poor and hot (70 degrees); the ethanol content in the return water is 1%, along with many other impurities. When this water is fed into the water tower, it poses a contamination risk to the entire water system. Now, it is planned for the vacuum pump to operate independently, pumping water into the return tank; the water in that tank feeds the pump, and with the addition of a filter and a heat exchanger, an independent circulation system is created. 1. It’s certainly good to make use of existing equipment, but if that’s not possible, it’s better to buy new ones, as the vacuum system is a complex system that can affect vacuum-based production. That’s the current situation: the actual flow rate is 6 m3/h, while the rated flow rate specified in the manual is 15 m3/h. To be safe during design, I had to use a pipeline pump with a capacity of 15 m3/h. However, it seems that for a long time the pump will have to operate at a flow rate of 6 m3/h, so I plan to install a variable-speed motor. 2. Regarding the head of the pipeline pump, as a rough estimate, the pressure drop caused by the filter should be less than 0.1 Mpa, depending on the size of the filter mesh; for the heat exchanger, the pressure drop of 50 m3 is not certain, so it is necessary to contact the manufacturer. Additionally, there are the pipes as well. The pipeline pump, the vacuum tank, and the vacuum pump are all roughly on the same level. All head values should be between 15–20 meters; I’m not sure if that’s appropriate ? ? ? A small question: The pipeline pump sends water to the vacuum pump (they are roughly on the same level), but the outlet pipe of the vacuum pump is 3 meters higher than that of the pipeline pump. So, should the head for this section be considered zero for the pipeline pump (ignoring pipes and other factors for now), or 3 meters? I think since those 3 meters are created by the vacuum pump, it should be accounted for in the vacuum pump’s performance, rather than as part of the pipeline pump’s head – am I correct? ? ? 3. As mentioned on the 7th floor, the vacuum pump trips when the valve is opened wide; this is due to an inappropriate selection of the pump. I need to check this when I go to work tomorrow, but I suspect that the problem lies in the fact that the circulating water in our facility is too dirty. 4. There’s a question I can’t figure out: when a valve restricts the flow rate, the flow becomes smaller but the head increases. For example, a pump with a head of 30 meters now has a head of 40 meters. Yet in reality, the actual head of the system is around 10 meters only. So where does that extra 40–10 = 30 meters of head go? ? ? Could it be that the pipeline pump itself is bearing the load? ?

All the conditions have been listed; let’s discuss them

When the outlet valve of the centrifugal pump is closed, the pump’s head increases while the flow rate decreases. In actual production, reducing the pump head can be achieved by opening the pump’s outlet valve, but the effect is not very significant; it is better to adjust the motor’s output power by using an inverter.

I don’t understand why a pipeline pump was chosen in this way; why wasn’t a pipeline pump that is suitable for the system used instead? Using a pipeline pump with a capacity of 20 m3/h and a head of 30 m should not have a significant impact on the system; however, if not controlled properly, the system could be emptied and the pump might run dry. At the same time, both the flow rate and head of the pump exceed the system’s requirements, which is rather wasteful!

1. The pump’s flow rate decreases while the gauge pressure increases. This has no effect on the pump itself. There is only one way to reduce the head pressure, and that is by cutting the pump’s impeller – making it smaller. Otherwise, the pump must be replaced. 2. It’s possible; simply adjusting the outlet valve will do. This doesn’t have any significant impact on the pump. However, care should be taken to ensure that the valve isn’t opened completely, to avoid excessive flow rates that could cause cavitation. Also, it’s important to ensure that the actual flow rate remains above the pump’s minimum flow rate. Last edited by liuqe001 on 2008-3-10 14:01

But be careful not to open the valves completely, to avoid excessive flow rates that could cause cavitation. I heard for the first time that excessive flow can lead to cavitation – could you explain why? ? ?

I didn’t quite understand the basic information you provided, but regarding the few issues you mentioned, I’d like to share some of my thoughts: the relationship between head and flow rate is such that as the flow rate increases, the head generally tends to decrease. However, it is absolutely impossible for a pump designed with a head of 30 meters to achieve a head of 40 meters simply by closing the outlet valve. Regarding the issue of reducing the outlet valve as you mentioned, when the valve is closed, the pump will operate at a flow rate lower than its rated flow. If the actual flow rate is close to the rated value, there will only be a slight loss in efficiency and some electricity waste; it should not cause any serious problems for the equipment. However, once it falls below a certain limit, it will cause noise and vibration, and the performance of the pump will also become unstable; API610 in the United States has specific regulations regarding this situation. I came across an article in some literature discussing equipment temperature rise: generally, the power provided by the drive mechanism is used primarily to convey the medium, after accounting for mechanical losses such as those incurred by bearings and shaft seals. Once the pump operates at a low flow rate with it in a semi-closed state, a large portion of the remaining power of the drive mechanism is converted into heat, raising the temperature of the medium. When the temperature of the medium rises to the saturation point, it vaporizes, causing cavitation and eventual mechanical failure. As for your specific situation, the appropriate degree to which the valve should be closed can only be determined based on the actual conditions on site; the criteria for making this decision should also take into account the equipment’s noise, vibration, and temperature levels. By the way, high flow rates are unlikely to cause cavitation; on the contrary, low flow rates may lead to it.

What is the relationship between cavitation and flow rate? Could you provide a detailed explanation of the energy conversion process? ? ? ? What does that have to do with the same head? ? ?

Under normal conditions, flow rate has little to do with cavitation, which is usually taken into account during pump design. What truly affects cavitation are the cavitation margin, suction specific speed, working medium, operating temperature, etc. However, under abnormal operating conditions, there is a certain relationship between flow rate and cavitation. The suction specific speed of the pump is proportional to the square root of the flow rate; when the speed and the net positive suction head (the required margin against cavitation) remain constant, the greater the flow rate, the higher the suction specific speed, and the better the pump’s resistance to cavitation. But this increase is relative rather than infinite. The relationship between flow rate and head is such that, within a certain range, as the flow rate increases, the head tends to decrease. This is because the energy output by the prime mover remains constant; assuming no energy losses, the energy possessed by the fluid being pumped should also remain constant. Mass is inversely proportional to head – this is a purely theoretical concept. When designing a process, the selection of a pump should take into account the condition in which the pump operates most efficiently under normal use, where both the flow rate and head are within relatively ideal ranges. The flow rate should not be used to adjust the head.

When a centrifugal pump operates for an extended period at a flow rate below 30% of its rated flow, the frictional heat generated within the pump cannot be dissipated promptly by the fluid, which makes cavitation more likely to occur.

My understanding of this issue is as follows: with a flow rate of 6 cubic units, using a pump with a capacity of 20 cubic units will have a significant impact on the pump if it operates for long periods of time; it can cause surging, and moreover, the efficiency will be very low. You said there are 9 pumps, so the method of cutting the impellers from some of those pumps can be used; using variable-frequency motors would be too wasteful. Our company has encountered a similar situation, and we resolved it by cutting the impellers from some of the pumps.

For a system with a flow rate of 6 m3/h and a head of around 13 meters, using a pipeline pump with a flow rate of 20 m3/h and a head of 30 meters is somewhat wasteful. This holds true both in terms of flow rate and pressure. It is possible to reduce the flow rate by closing the valves manually, but from an equipment perspective, operating the pump at such a low flow rate for extended periods can cause the pump to overheat, leading to cavitation damage. Additionally, the high head and high outlet pressure resulting from this pump represent a waste as well! Additionally, the poster mentioned using variable frequency drives; theoretically, this is possible, but variable frequency drive systems are quite expensive, and they are generally used only in devices with high load requirements!

Now, variable frequency speed control technology is quite mature, and its costs have also decreased. Pumps with a flow rate of 6 m3/h are very small, so they shouldn’t cost much