The system controls the pump

All pumps must be designed to comply with or meet the needs of the system. The needs of the system are recognized using the term 'Total Dynamic Head', TDH. The pump reacts to a change in the system. For example, in a small system, this could be the changes in tank levels, pressures, or resistances in the piping. In a large system, an example would be potable water pumps designed for an urban area consisting of 200 homes. If after 5 years the same urban area has 1,000 homes, then the characteristics of the system have changed. New added piping adds friction head (Hf). There could be new variations in the levels in holding tanks, affecting the static head (Hs). The increase in flow will affect the pressure head (Hp), and the increased flow in old, scaled piping will change the velocity head (Hv). New demands in the system will move the pumps on their curves. Because of this, we say that the system controls the pump. And if the system makes the pump do what it cannot do, then the pump becomes problematic, and will spend too much time in the shop with failed bearings and seals.

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