Why does the minimum flow control valve open when the pump inlet flow decreases?
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Why does the minimum flow control valve open when the pump inlet flow decreases?
This is a control valve that returns fluid from the pump outlet to the pump inlet; it works on the same principle as anti-surge systems in compressors
If the inlet flow is insufficient, the outlet flow must be used to compensate, in order to prevent cavitation at the pump inlet.
The purpose of designing a minimum flow line for high-pressure pumps is to prevent overpressure at the outlet of high-speed or multi-stage centrifugal pumps, which could damage the pump’s seals and shaft seals. Therefore, a minimum return line is activated when the flow rate at the pump outlet is low, ensuring that it remains above the designed minimum flow rate.
The outlet of the boiler feed water pump is usually equipped with a minimum flow protection valve. Multistage centrifugal pumps of types BB3, BB4, BB5 have a minimum safe flow rate; when the main flow rate falls below this value, the minimum flow protection valve opens a bypass circuit so that the combined flow from the main stream and the bypass is at least equal to the pump’s minimum safe flow rate. Generally, this minimum safe flow rate is around 30% of the normal flow rate. The purpose of this is to prevent the sealing elements from being damaged due to vaporization of the fluid inside the pump when the flow rate is too low, especially at the impeller’s inlet ring
The minimum flow rate is divided into the minimum stable flow rate and the minimum thermal control flow rate
If the flow measurement point for the minimum flow control valve is located on the inlet pipe, then once the control valve is set to automatic mode, if the flow deviates from the set value, the valve will automatically open wider