Understanding pump head and flow rate
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This post was last edited by atkqn0030 on 2010-4-16 16:57. Let’s discuss my personal understanding of pump operation and head； 1. If the pump remains unchanged but its actual head is lower than the rated head, then its actual flow rate will increase ； 2. Its flow rate and head are calibrated under the condition that the medium is water ； 3. If the medium changes while the total flow rate remains constant, the actual head will be: H/(density of the medium/density of water). 4. If the size of the outlet pipeline changes, its head should also be affected. Please discuss whether this is correct
Regarding point 4, based on my experience, increasing the diameter of the pump’s connection pipe reduced the outlet pressure, that is, the head decreased
[color=DarkRed]1. If the pump remains unchanged but its actual head is lower than the rated head, then its actual flow rate will increase； Correct. 2. Its flow rate and head are calibrated under the condition that the medium is water ； Correct. 3. If the medium changes, the total flow rate remains unchanged, but the actual head is given by H/(density of the medium/density of water). The head stays the same, while power changes and the inlet and outlet pressures change, yet the head remains unchanged. 4. If the size of the outlet pipeline changes, its head should also be affected; correct
But if the motor remains unchanged, the total flow rate should remain constant; however, the actual head is: H/(medium density/water density)
Upstairs: Just because the motor used remains unchanged does not mean that its power stays constant; for example, if a water-pumping motor is used to pump concentrated sulfuric acid, it will experience overcurrent, which can severely damage the motor.
The head is generally controlled by the opening degree of the valve for discharge
1. If the pump remains unchanged but its actual head is lower than the rated head, then its actual flow rate will increase； Correct. 2. Its flow rate and head are calibrated under the condition that the medium is water ； It’s not entirely correct; when transporting media with low viscosity, both the flow rate and head remain constant. 3. If the medium changes, the total flow rate stays the same, but the actual head is given by H/(density of the medium/density of water). This is incorrect – when the medium changes, the pressure changes accordingly, but the head remains unchanged. 4. If the size of the outlet pipeline changes, the head should also be affected. This is incorrect; the head of a pump has no direct relationship with the size of the outlet pipeline
Regarding point 4 mentioned by the brother upstairs, head is related to the height difference and pipeline losses, and pipeline losses are in turn related to the diameter of the pipes. If the flow rate remains constant, an increase in pipe diameter results in lower pipeline losses, thereby reducing the required head. In the case of a centrifugal pump, the flow rate increases automatically to compensate for the reduced head.
The head is independent of the medium; it remains constant.
Everyone is working quite hard. It’s already good enough to have such an understanding of pumps; many people still think that for transporting materials with a high specific gravity, a pump with a high head is necessary, which is utterly ridiculous. Head and flow rate are characteristic values of the pump itself, and they remain basically unchanged after the pump is manufactured – only one changes while the other changes as well. The flow rate and head can be adjusted within certain limits.
This post was last edited by happyhefei on 2010-5-4 09:42. There are basically no objections to the other questions except for the third one. Third question: If the medium changes while the total flow rate remains constant, the actual head should be H/(density of the medium/density of water). Incorrect! ! ! When the density of the medium changes, the pump’s head, flow rate, and efficiency remain unchanged. The most significant change is in the shaft power p, which becomes equal to the previous value of p multiplied by (density of the medium before the change / density of the medium after the change)