Further to a previous answer. Most boiler feed water pumps are multistage centrifugals. These pumps have a minimum flow requirement that must be maintained to prevent imbalance to the rotor that can lead to thrust problems and damage to the pump. The minimum flow bypass is designed to deal with this problem.
A recirculation line, or what is called a 'mini-flow', is basically a spill-back mechanism. This is usually accomplished by running a line from the discharge of a pump, prior to the main control valve, back to the suction of the pump, with its own flow, or pressure control valve. This system is designed to maintain a minimum flow threshold to the suction of the pump. If the flow to the pump suction falls too low, the discharge pressure approaches 'dead head' conditions, where the developed head is too high. Also, there is a flow such that the highest efficiency is reached, and a spill-back offers a way of maintaining the 'perfect' flow. In the special case where a pump is delivering boiler feed water, you definitely want this pump to work well and last a long time running reliably. Losing water flow to a fired or waste heat boiler is extremely dangerous!!! If safety systems fail, you're looking a potentially lethal buildup of pressure and heating. To summarize, a boiler feed water supply pump needs to operate at a preset flow rate, and a spill back allows an additional level of flow control to the suction of the pump. Hope this helps...
