How to select the inlet diameter of a circulation pump?
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Dear Haihao friends: I really don’t know what to do; I’d appreciate your advice. On-site, the small cooling tower (670 m3/h) needs to be replaced with a larger cooling tower (1000 m3/h). The small cooling tower serves four units, with the return pipes being 4×DN150, which together feed into a DN500 pipe that leads to the circulation pumps (there are currently four such pumps; a certain number of them are activated based on the actual water consumption. The flow rate of these pumps is 340 m3/h, and their suction lift is 3.2 m). The larger cooling tower also serves four units, and for this equipment the return pipes are required to be 4×DN200. However, since the DN500 pipe is already in use as the main collection pipe and it’s not possible to stop operations to make holes in it, it is planned to install DN200 pipes for the drainage from the new cooling tower. These DN200 pipes will be connected via reducers to the DN150 interfaces of the existing butterfly valves, so that the water can flow back into the DN500 main collection pipe. I conservatively estimate that the liquid level is 200 mm above the center of the circulation pump impeller. Assuming a pressure of Pa at the center of the impeller, the maximum head required for a circulation rate of 1000 m3/h is 1.2 m, which is less than the 3.2 m suction head. I talked to the pump manufacturer and they said there’s no problem, but an experienced engineer mentioned that the return pipe has a change in diameter, resulting in too low a flow rate, which might cause overflow from the cooling tower’s water tray I would like to ask how to determine the inlet diameter of a circulation pump so as to ensure it can meet the required circulation water volume Thank you to all the experts
Top, don’t sink. Please give your advice~
I think you should calculate the height difference and the pressure loss in the suction pipe (based on the velocity after the diameter change); the sum of these two values should be within 3.2 meters. Additionally, check the pump’s specifications to see whether it can achieve a flow rate of 1000 m3/h at this head. If all these conditions are met, then there should be no problem
Hello, I have calculated that for a DN150 pipe, the flow velocity is 4.4 m/s. The flow rate exceeds the range specified in the chemical process design manual, which is 1.5–3.5 m/s. We thought of installing pipeline pumps to address the issue of pipe resistance, but the supervisor said that such a high flow rate cannot be achieved. Do you know why?
Now that the flow rate has increased while the pipe diameter remains DN150, the flow velocity must have gone up as well. If I use pipes with dimensions of Φ159x4.5, the flow velocity is approximately 3.9 m/s
(1) Select the pipe diameter appropriately. A larger pipe diameter results in a lower flow velocity at the same flow rate, thereby reducing frictional losses; however, it increases the cost. A smaller pipe diameter leads to a sharp increase in frictional losses, which in turn requires a pump with a higher head pressure, more power to operate the pump, and thus higher costs and operating expenses. Therefore, it should be considered from both technical and economic perspectives. (2) The discharge pipe and its fittings should be designed to withstand the maximum pressure they can handle. (3) The piping layout should be arranged as straight as possible, with as few fittings as feasible and a minimal pipe length. When bends are necessary, the radius of curvature of the elbows should be 3 to 5 times the diameter of the pipe, and the angle should be as large as possible, exceeding 90°. (4) A valve (such as a ball valve or globe valve) and a check valve must be installed on the discharge side of the pump. Valves are used to adjust the operating point of the pump, while check valves prevent the pump from rotating in reverse when the liquid flows backward, thus protecting the pump from water hammer effects. (When the liquid flows back, a huge reverse pressure is generated, which can damage the pump)