Installation Methods For Pipeline Pumps

Discharge Throttling
This is the most common and inexpensive method of flow regulation for low- and medium-specific-speed pumps. Typically, its use is limited to pumps in 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 throttle valve. This factor can be significant for large pump installations, where more capital-intensive regulation methods might prove more economically attractive. Throttling to the shut-off point can cause fluid overheating within the pump; this can be mitigated by using a bypass to maintain the necessary minimum flow or by employing alternative regulation methods. This is particularly important for the pumps mentioned earlier that handle hot water or volatile liquids.

Suction Throttling
If sufficient NPSH is available, power savings can be achieved by throttling the suction line. Suction throttling is frequently used for jet engine fuel pumps because discharge throttling would cause the liquid to overheat or vaporize. 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 those observed with a fully filled impeller under discharge throttling conditions. The flow rate of condensate pumps is typically controlled by adjusting the submergence depth, which is functionally equivalent to suction throttling. Special designs can minimize cavitation damage in these pumps to a negligible level, though the energy levels involved are also quite low.

Bypass Regulation
All or part of the flow can be diverted from the pump discharge line through a bypass pipe back to the pump suction or another suitable point. The bypass line may be equipped with one or more flow orifices and appropriate control valves. Metering bypasses are commonly used to reduce the flow rate of boiler feed pumps, primarily to prevent overheating. Significant power savings can be realized by bypassing excess flow-such as with propeller pumps-instead of using discharge throttling.

Speed ​​Regulation
Regulating flow by adjusting speed minimizes power requirements and eliminates the overheating issues associated with other flow regulation methods. 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 DC and AC variable-speed motors, can be used to regulate rotational speed. Variable-speed motors are generally too expensive; consequently, adjustable blade regulation is employed only when an economic analysis justifies it for specific applications.
Studies on adjustable guide vanes installed upstream of the impeller have shown that this method is effective for pump regulation at a specific speed of $n_s = 5700$ (2.086). The vanes generate positive pre-rotation, thereby reducing head, flow rate, and efficiency, although the degree of regulation achieved through the vanes alone is relatively limited. Adjustable outlet diffuser vanes have been successfully applied in large pumped-storage units used for power generation in Europe. Propeller pumps with variable-pitch blades have also been successfully investigated; these allow for a wide range of flow rate variations while maintaining constant head and incurring relatively low efficiency losses. However, these methods are overly complex and costly, which significantly limits their practical application.