Page 109 - Know and Understand Centrifugal Pumps

The System

Curve

The system controls the pump

All pumps must be designed to comply with or meet the needs of thc
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.

The elements of the Total Dynamic head (TDH)

The Total Dynamic Head (TDH) of each and every pumping system is
composed of up to four heads or pressures. Not all systems contain all
four heads. Some contain less than four. They are:
1. Hs - the static head, or the change in elevation of the liquid across
the system. It is the difference in the liquid surface level at the
suction source or vessel, subtracted from the liquid surface level
where the pump deposits the liquid. The Hs is measured in feet of
elevation change. Some systems do not have Hs or elevation