The significance of the pump’s head value
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This post was last edited by 3983596_FPPZ on 2019-2-25 07:55. For example, a multi-stage centrifugal pump is used to deliver a certain fluid into a system, where the system pressure remains constant. The flow rate of the pump can be adjusted by controlling the size of the outlet valve; the greater the flow rate, the lower the pressure at the pump’s outlet. Does it mean that as long as the pressure at the pump’s outlet is higher than the system pressure, the fluid can be pumped into the system? If the flow rate is reduced by closing the outlet valve in order to increase the pump’s head (i.e., the outlet pressure), will this have any impact on the system?
The operating point of the pump is determined by the pump’s performance curve and the characteristics curve of the system. If the back pressure in the system is greater than the pump’s outlet pressure, the pump is unable to deliver the fluid.
This post was last edited by 3983596_FPPZ on 2019-2-25 07:57. The pump has a flow-rate vs. head curve (different outlet pressures at various flow rates); as the flow rate increases, the outlet pressure decreases, resulting in a decreasing curve. The pump also has a flow-rate vs. head (resistance) curve (different resistances at various flow rates); as the flow rate increases, the flow velocity increases and the resistance increases, resulting in an increasing curve. While the pump is operating, the two systems influence and interact with each other, and their conditions change continuously. The two systems automatically reach an equilibrium state (the intersection point of the two curves), at which point the flow rates in both curves are equal, and the pressure at the pump outlet equals the backpressure of the system (the resistance). If the two curves do not intersect, then the pump will not be able to meet the requirements of the device and will not function properly
Overall, your understanding is correct. There is a small detail: the outlet pressure of the pump must be greater than (system pressure + pipeline pressure loss) in order to inject the medium. Reducing the flow rate by closing the valve can increase the pump’s outlet pressure, with no impact on the system. It’s similar to water flowing in a pipe: if you block part of it, the flow rate decreases, but the water sprays higher and farther away for the same reason.
Is there no situation where the pump outlet pressure is greater than the system pressure (resistance) of the device?
By adjusting the outlet valve, the characteristic curve of the device is changed, resulting in a new intersection with the pump’s performance curve, which represents the pump’s operating point. As long as they can intersect, it indicates that the pump is still able to deliver fluid to the system.
The equipment operates properly and requires a pressure difference. That is, the pressure entering the device minus the pressure at the device’s outlet. Economic operation is precisely the pipe resistance of the equipment, that is, the pressure difference. Regulating through valves is the most energy-intensive method; it is recommended to install an inverter for regulation. If the operating conditions are stable, it is recommended to choose an appropriate pump rather than using variable frequency control.