Some questions regarding the circulating water pump; seeking answers

In actual piping processes, it is common to use reducers to increase the pipe diameter in order to achieve the desired flow rate. This phenomenon is quite widespread.

I’m not sure if the original poster is facing problems with design or with on-site operation. If it’s designed this way, it might not be a problem. It might be on-site operation. Are you using the medium flow rate or the volume flow rate?

Question 1: What is the basis on which the manufacturer determines the DN125 outlet size for the pump? In actual piping processes, it is usually necessary to use tees or enlarged pipes to increase the diameter in order to achieve the desired flow rate. Is this phenomenon common? ****It is very common for the inlet and outlet sizes of pumps to be determined by the manufacturer. The sizes of the inlet and outlet piping for the pump are determined by the process system. Question 2: What determines the flow rate of water, and is it related to the pump’s head? In practical use, it always seems that the higher the pump’s head, the higher the water pressure and the faster the flow rate of water. ****The flow rate of water is determined by taking into account both pressure drop and economic feasibility of investment. The flow rate of water is generally around 2 meters per second. Once the pipe diameter and flow rate are determined, the system pressure drop is established, and then the head is determined based on the pressure required by the user. Question 3: Why, when selecting a pump, cannot its rated head be much higher than the actual head required? For example, if a heat exchanger is 10 meters high and requires a water flow rate of 100 m3/h, and a pump with a rated flow rate of 100 m3/h and a rated head of 30 meters is chosen, what problems might arise during use? ****It is not economical to operate. ...... Question 4: Some people say that the diameter of a pipe affects the flow rate of water; the larger the diameter, the less resistance there is and the faster the water flows. Is it true that the flow rate in the main circulation pipe is faster than that in the branch pipes? ****The first half is correct; for the second half, check the principles of chemical engineering and do some calculations. Question 5: If the inlet flow rate of the pipe is definitely 200 m3/h, and flow rate is determined by the product of the cross-sectional area through which the fluid flows and the flow velocity, then the flow velocity should be higher for pipes with a smaller diameter and lower for pipes with a larger diameter, right?

I also try to share my understanding; please point out any differences in perspective. Thank you. 1: What is the basis upon which the manufacturer determines the DN125 outlet size for the pump? In actual piping processes, it is usually necessary to use tees or enlarged pipes to increase the diameter in order to achieve the desired flow rate. Is this phenomenon common? Reply: It’s not clear how the manufacturer calculates it, but in actual operating conditions, it is indeed necessary for the pipe inlets and outlets to have larger diameters, meaning they need to be bigger than the flanges on the equipment itself. Question 2: What determines the flow rate of water, and is it related to the pump’s head? In practical use, it always seems that the higher the pump’s head, the higher the water pressure and the faster the flow rate of water. Reply: I agree with your view. Question 3: Why, when selecting a pump, cannot its rated head be much higher than the actual head required? For example, if a heat exchanger is 10 meters high and requires a water flow rate of 100 m3/h, and a pump with a rated flow rate of 100 m3/h and a rated head of 30 meters is chosen, what problems might arise during use? Reply: The first reason is economic; the second depends on whether the design pressure of the heat exchanger allows for high water pressure. Question 4: Some people say that the diameter of a pipe affects the flow rate of water; the larger the diameter, the less resistance there is and the faster the water flows. Is it true that the flow rate in the main circulating water pipe is faster than that in the branch pipes? Reply: This flow rate is related to both the flow volume and the pipe diameter; a larger pipe diameter results in a higher flow volume. Of course, a larger pipe diameter also means less resistance.

Question 5: When the flow rate is determined, it is based on the pipe diameter, right? There shouldn’t be any ambiguity here, should there?

Circulation water pumps are the most common and fundamental in utility systems; from water pumps, other types of centrifugal transfer pumps can be derived, as they are all related to each other. By clarifying these issues, there will be a clearer approach from the initial selection of the pump to its actual use, which brings convenience. For example, in question three, when the rated head of the pump you choose is much higher than the actual head, the flow rate of water pumped by the pump increases, resulting in an overflow. This increases the load on the pump, raises the current, causes the motor to overheat, and leads to the pump stopping. In such a situation, you can reduce the outlet valve of the pump to lower the flow rate and thus decrease the load on the pump. Therefore, when choosing a pump, it’s not the case that the higher the head, the better.

Thank you for your answer. Design is mostly theoretical, and it differs quite a bit from reality; practical experience may be more effective in solving problems that arise on site than theory!