Figure 3-8.Herringbone gear pump.
Valves must be accurate in the control of fluid flow
and pressure and the sequence of operation. Leakage
between the valve element and the valve seat is
reduced to a negligible quantity by precision-
machined surfaces, resulting in carefully controlled
clearances. This is one of the very important reasons
for minimizing contamination in the system.
Contamination causes valves to stick, plugs small
orifices, and causes abrasions of the valve seating
surfaces which will result in leakage between the valve
element and valve seat when the valve is closed. Any of
these can result in inefficient operation or complete
stoppage of the equipment.
Valves may be controlled manually, electrically,
pneumatically, mechanically, hydraulically, or by
combinations of two or more methods. Factors that
determine the method of control include the purpose of
the valve, the design and purpose of the system, the
location of the valve within the system, and the
availability of the source of power.
Valves are classified according to their use: flow
control, pressure control, and directional control.
Some of these valves have multiple functions that fall
into more than one classification.