These valves are designed as either inside screw rising-stem or outside-screw
rising-stem. Small valves generally are of the inside-screw type, while in
larger sizes the outside-screw type is preferred. In most designs the disks are
free to rotate on the stems; this prevents galling between the disk and the
seat.
In the larger sizes, with conical seats, this swivel may permit enough
misalignment to prevent proper sealing between the disk and the seat. When the
valve is close to an elbow on the upstream side, the swivel also permits uneven
distribution of the fluid to spin the disk on the stem. Guides above the disk,
below the disk, or both are used to prevent misalignment and spinning.
Misalignment can also be prevented by the use of spherical seats and
designing the disk so that the pressure point of the stem on the disk is at the
center of the sphere. In some designs, spinning and misalignment are prevented
by rigidly attaching the disk to the stem, preventing rotation of the stem by
lugs which ride along the yoke, and using a yoke bushing as in outside
screw-and-yoke gate valves. Large Globe Valves should be installed with stems
vertical. Globe valves are preferably installed with the higher-pressure side
connected to the top of the disk. Exceptions occur (1) when blocked flow caused
by separation of the disk from the stem would damage equipment or (2) when the
valve is installed in seldom-used vertical drain lines in which accumulation of
rust, scale, or sludge might prevent opening the valve.
Pressure drop through globe valves is much greater than that for gate valves.
In Y-type Globe Valves, the stem and seat are at about 45