TYPES OF VALVES
Figure 9 Z-Body Globe Valve
A globe valve is a linear motion valve
used to stop, start, and regulate fluid flow.
A Z-body globe valve is illustrated in
As shown in Figure 9, the globe valve
disk can be totally removed from the
flowpath or it can completely close the
flowpath. The essential principle of globe
valve operation is the perpendicular
movement of the disk away from the seat.
This causes the annular space between the
disk and seat ring to gradually close as the
valve is closed. This characteristic gives
the globe valve good throttling ability,
which permits its use in regulating flow.
Therefore, the globe valve may be used
for both stopping and starting fluid flow
and for regulating flow.
When compared to a gate valve, a globe
valve generally yields much less seat
leakage. This is because the disk-to-seat
ring contact is more at right angles, which
permits the force of closing to tightly seat
Globe valves can be arranged so that the disk closes against or in the same direction of fluid
flow. When the disk closes against the direction of flow, the kinetic energy of the fluid impedes
closing but aids opening of the valve. When the disk closes in the same direction of flow, the
kinetic energy of the fluid aids closing but impedes opening. This characteristic is preferable
to other designs when quick-acting stop valves are necessary.
Globe valves also have drawbacks. The most evident shortcoming of the simple globe valve is
the high head loss from two or more right angle turns of flowing fluid. Obstructions and
discontinuities in the flowpath lead to head loss. In a large high pressure line, the fluid dynamic
effects from pulsations, impacts, and pressure drops can damage trim, stem packing, and
actuators. In addition, large valve sizes require considerable power to operate and are especially
noisy in high pressure applications.
Other drawbacks of globe valves are the large openings necessary for disk assembly, heavier
weight than other valves of the same flow rating, and the cantilevered mounting of the disk to