2012-06-29
GLOBE VALVE are named for their exterior shape, which is round and
globular, and not in accordance with any characteristic of the internal
structure of the valve itself, such as is the case with gate valves or
ball valves. Internally, globe valves most often consist of a shaped
plug or disc that mates with a valve seat according to a carefully
defined mathematical relationship of opening percentage to flow
percentage. This provides the flow characteristics best suited to the
flow, level, or pressure control they are used for.
Quick opening valve characteristic
The quick opening flow characteristic provides for maximum change in
flow rate at low valve travels with a nearly linear relationship.
Additional increases in valve travel give sharply reduced changes in
flow rate, and when the valve plug nears the wide open position, the
change in flow rate approaches zero. In a control valve, the quick
opening valve plug is used primarily for on-off service; but it is also
suitable for many applications where a linear valve plug would normally
be specified.
Linear valve characteristic
The linear flow characteristic curve shows that the flow rate is
directly proportional to the valve travel. This proportional
relationship produces a characteristic with a constant slope so that
with constant pressure drop, the valve gain will be the same at all
flows. (Valve gain is the ratio of an incremental change in valve plug
position. Gain is a function of valve size and configuration, system
operating conditions and valve plug characteristic.) The linear valve
plug is commonly specified for liquid level control and for certain flow
control applications requiring constant gain.
Equal percentage valve characteristic
In the equal?percentage flow characteristic, equal increments of valve
travel produce equal percentage changes in the existing flow. The change
in flow rate is always proportional to the flow rate just before the
change in valve plug, disk, or ball position is made. When the valve
plug, disk, or ball is near its seat, the flow is small; with a large
flow, the change in flow rate will be large. Valves with an equal
percentage flow characteristic are generally used on pressure control
applications and on other applications where a large percentage of the
pressure drop is normally absorbed by the system itself, with only a
relatively small percentage available at the control valve. Valves with
an equal percentage characteristic should also be considered where
highly varying pressure drop conditions can be expected
