A globe valves is a linear motion valve and are primarily designed to stop, start and regulate flow. The disk of a globe valve can be totally removed from the flowpath or it can completely close the flowpath.
The fundamental principle of the globe valve operation is the perpendicular motion of the disk away from the seat. This ensures that the ring-shaped space between the disk and seat ring gradually close as the valve is closed. This property gives a globe valve reasonably good throttling capability. Therefore, the globe valve can be used for starting and stopping flow and to regulate flow.
Advantages of using globe valves:
Good shutoff capability
Reasonably good throttling capability
The major drawbacks to the use of a globe valve are:
Higher pressure drop compared to a gate valve
Large valve sizes require considerable power or a larger actuator to operate
BODY DESIGNS OF GLOBE VALVES
There are three primary body designs for globe valves, namely: Z-body, Y-body and Angle body.
Z-body design is the most common body type, with a Z-shaped diaphragm.
The horizontal setting of the seat allows the stem and disk to travel perpendicular to the horizontal line.
Y-body design is an alternative for the high pressure drop, inherent in globe valves.
Seat and stem are angled at approximately 45 degrees, what gives a straighter flowpath at full opening.
Angle-body design is a modification of the basic Z-type globe valve.
The ends of this globe valve are at an angle of 90 degrees, and fluid flow occurs with a single 90 degrees turn.
DISKS OF A GLOBE VALVE
The most common disk designs for globe valves are: ball disk, composition disk and the plug disk.
Ball disk design is used primarily in low pressure and low temperature systems. It is capable of throttling flow, but in principle it is applied to stop and start flow.
Composition disk design uses a hard, non-metallic insert ring on the disk, which ensures a tighter closure.
Plug disk design provides better throttling than ball or composition designs. They are available in many different designs and they are all long and tapered.
STEM AND DISK CONNECTIONS OF A GLOBE VALVE
Globe valves uses two methods for connecting the disk and the stem: the T-slot and the disk nut construction. In the T-slot design, the disk slides over the stem, while in the disk nut design, the disk is screwed into the stem.
SEATS OF GLOBE VALVES
Globe valve seats are either integrated or screwed in to the valve body. Many globe valves have backseats inside the bonnet. Back seats provides a seal between the stem and bonnet and prevents system pressure from building against the valve pakking, when the valve is fully open. Back seats are often applied in globe valves.
FLOW DIRECTION OF GLOBE VALVES
For applications with low temperature, globe valves are normally installed so that the pressure is under the disc. This contributes an easy operation and helps protect the packing.
For applications with high temperature steam service, globe valves are installed so that the pressure is above the disk. Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat.