Pipeline pump flow regulation
Exit throttling
For low and medium specific speed pumps, this is the most common and inexpensive method of flow regulation. Usually this method is also limited to use on low and medium specific speed pumps. Partially closing any type of valve on the outlet pipe will increase the system head, so the system head curve will intersect the pipeline pump head curve at a lower flow rate. The outlet throttling moves the operating point to a lower efficiency point and there is a power loss at the throttle. This may be important for large pump installations, and higher investment adjustment methods may be more economically attractive. Throttling to the dead point may cause the fluid in the pump to overheat. Bypass can be used to maintain the necessary minimum flow, or different adjustment methods can be used. This is very important for the aforementioned pumps that handle hot water or volatile liquids.
Throat restriction
If sufficient NPSH is available, some power can be saved in the suction line by throttling. Because the throttling of the outlet can cause the liquid to overheat or vaporize, the jet fuel line pump often uses the inlet of throttling. At very low flow rates, the impellers of these pumps are only partially filled with liquid. Therefore, the input power and temperature rise are approximately 1/30 of the full operating position of the impeller when the throttle is at the outlet. This is equivalent to inlet throttling. Special designs reduce cavitation damage to these pumps to a negligible level, but energy levels also become quite low.
Bypass adjustment
The discharge line from the pipeline pump can be divided into all or part of the flow, and led to the suction port of the pump or other appropriate points through the bypass pipe. One or more flow orifices and suitable control valves can be installed in the bypass. Metering bypass is usually used to reduce the flow of boiler feed pump, mainly to prevent overheating. If the bypass propeller pump excess flow is used to replace the outlet throttling, considerable power can be saved.
Speed adjustment
When using this method to regulate the flow, the required power can be minimized, and overheating during the flow adjustment process can be eliminated. Steam turbines and internal combustion engines can easily adapt to speed regulation with little additional cost. Various mechanical, magnetic, hydraulic transmissions and DC and AC variable speed motors can be used to adjust the speed. Variable speed motors are often too expensive and can only be adjusted with adjustable blades after economic research on special cases proves worthwhile.
After studying the adjustable guide vane installed in front of the impeller, it was found that this method is effective for pump adjustment when the specific speed is 5700 (2.086). The blades produce positive prespin, which reduces head pressure, flow, and efficiency. And for the relatively small regulating effect obtained only by the leaves. Large-scale energy storage pumps for power generation in Europe have successfully used adjustable outlet diffusion blades. Propeller pumps with variable pitch blades have also been successfully studied. With a constant head and a relatively small loss of efficiency, a large range of flow changes can be obtained. However, these methods are too complicated and expensive, so they are very limited in practical applications.
Make-up gas
Adding air to the suction of the pipeline pump is also a method of adjusting the flow rate, which can save some power compared with the outlet throttling. Generally, the presence of air in the liquid being conveyed is not desired, and too much air always has the risk of causing the pump to lose its filling head, so this method is rarely used in practice except for occasions.
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