I would like to ask the experts on centrifugal pumps.
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Centrifugal pump characteristic curve: When the pump outlet valve is closed, the head increases while the flow rate decreases. So, what does the head referred to here mean – head before the valve or head after the valve? I’m facing a problem right now: I have two pumps that supply water to other devices. According to the performance curves of these pumps, their flow rate can reach 140 cubic meters per minute, with a head of 80 meters. To maintain the pressure in the pipeline (behind the valve), I use both pumps with the valves fully open. To have a backup pump, I plan to turn on one pump and reduce the opening of its outlet valve in order to maintain the pressure. However, it seems that this doesn’t work – when I reduce the valve opening, the pressure in the pipeline (behind the valve) also decreases, while only the pressure before the valve increases. Could some expert please explain it to me in detail?
The concept of head must be understood accurately; head is the pressure head that a pump needs to provide in order to overcome the resistance and pressure difference in the outlet pipeline. Therefore, it has nothing to do with what’s before or after the valve. Let’s first explain how two pumps are arranged in a process setup, as well as what their parameters are.
The main function of the pump outlet valve is to regulate the flow rate and modify the characteristics of the pump’s outlet pipeline; therefore, to ensure an appropriate pressure, it is necessary to increase the speed in order to generate inlet pressure.
The pipeline pressure is determined by the pump’s head; a higher head results in higher pressure. When the control valves are closed, the pressure before the valves increases while the pressure after them decreases, leading to a reduction in the overall flow rate in the pipeline. This theory originates from Bernoulli’s equation; refer to the fluid mechanics section of \"Principles of Chemical Engineering\" for details
Let’s talk about fake languages to see if we can understand them :) Your pump is designed to meet the needs of users; in other words, you just need to adjust your pump to the parameters specified on its label. If the user’s needs cannot be met, another pump must be started. Things like characteristic curves and head are all nonsense. There are truly just three conditions: first, use the outlet valve to adjust the current to the rated parameters ； Secondly, regardless of the usage at the back end, it is essential to ensure that the pressure (head) remains at or above the specified parameters ； Third, when the outlet valve is opened too slightly, attention should be paid to ensuring that the pump temperature does not rise too high. The above three points are: rated current, the outlet pressure (head) of the pump; if there is a flow meter, this should be used as a reference value, and if not, the first two points are sufficient for normal operation. Mock language, for reference only.
Firstly, the pump head is a characteristic of the pump itself; there is no issue of pressure before or after the valve. According to the performance curve of a centrifugal pump, as the flow rate increases, the head decreases; conversely, as the flow rate decreases, the head increases. With the pump inlet pressure remaining essentially constant, when the outlet valve of the centrifugal pump is closed, the local flow resistance at the valve increases, the flow rate decreases, the pump head increases, and accordingly the pressure before the valve rises while the pressure after the valve falls (as the flow rate decreases, the flow resistance in the pipeline downstream of the valve decreases). https://bbs.hcbbs.com/thread-1764877-1-1.html
Are the 2 pumps started together to ensure flow?
What you are doing now is equivalent to using a hand valve as a control valve. When the manual valve is closed, the pipe resistance increases, the flow rate discharged by the pump decreases, and as a result the pressure at the pump outlet (which is roughly equal to the pressure before the valve) increases. The pressure behind the valve should be calculated by working backwards from the end point. Since the flow rate decreases, the pressure drop in the pipeline downstream of the valve also decreases, and consequently the pressure downstream of the valve drops as well. When the valve is closed, the pressure drop across the valve necessarily increases; the analysis above explains this reason. If the manual valve is closed and the fluid stops flowing, the pressure behind the valve is the static pressure of the downstream system (gas phase pressure + pressure difference).
Increase the pump’s frequency conversion to its maximum level, or switch it to operate at the standard frequency; otherwise, you’ll need to use pipes that are too large, and the pump’s head will not be sufficient, resulting in insufficient pressure in the pipes