Flow Rate Regulation Of Pipeline Pumps

Discharge Throttling
For low- and medium-specific-speed pumps, this is the most common and inexpensive method of flow regulation. Typically, its use is limited to pumps within these specific-speed ranges. Partially closing any type of valve in the discharge line increases the system head; consequently, the system head curve intersects the pump head curve at a lower flow rate. Discharge throttling shifts the operating point to a lower efficiency level and results in power loss across the throttling valve. This factor can be significant for large pump installations, where alternative regulation methods involving higher initial investment might prove more economically attractive. Throttling down to the shut-off point can cause fluid overheating within the pump; this can be mitigated by using a bypass line to maintain the necessary minimum flow or by employing a different regulation method. Such considerations are crucial for the pumps mentioned earlier that handle hot water or volatile liquids.
Suction Throttling
If sufficient Net Positive Suction Head (NPSH) is available, power savings can be achieved through throttling in the suction line. Suction throttling is frequently employed for jet engine fuel pumps to avoid the overheating or vaporization of the liquid that can result from discharge throttling. At very low flow rates, the impellers of these pumps are only partially filled with liquid; consequently, the power input and temperature rise are approximately one-thirtieth of the values ​​observed when the impeller is fully primed (as in discharge throttling). The flow rate of condensate pumps is typically controlled by adjusting the submergence depth, a method equivalent to suction throttling. Specialized designs can minimize cavitation damage in these pumps to negligible levels, although the energy levels involved are relatively low.
Bypass Regulation
All or part of the flow can be diverted from the pump's discharge line through a bypass pipe back to the suction inlet or another suitable point. The bypass line may be equipped with one or more flow-restricting orifices and appropriate control valves. Metering bypasses are commonly used to reduce the flow rate of boiler feed pumps, primarily to prevent overheating. Bypassing excess flow-as opposed to using discharge throttling-can result in significant power savings. Speed ​​Regulation
Regulating flow by adjusting rotational speed minimizes power requirements and eliminates overheating during the flow control process. Steam turbines and internal combustion engines can easily accommodate speed regulation at minimal additional cost. Various mechanical, magnetic, and hydraulic variable-speed drives, as well as variable-speed DC and AC motors, can be used to adjust speed. However, variable-speed motors are generally quite expensive; adjustable-blade regulation is typically employed only when an economic analysis of the specific application justifies it.