GLOBE VALVES

                                              Globe Valves

A globe valve is a linear motion valve used to stop, start, and regulate fluid flow.

The globe valve disk can be totally removed from the flow path or it can completely close the flow path. 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 the disk.

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 referable 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 the stem.

Globe Valve Body Designs

The three primary body designs for globe valves are Z-body, Y-body, and Angle.

Z-Body Design

The simplest design and most common for water applications is the Z-body. The Z-body is illustrated in Figure 9. For this body design, the Z-shaped diaphragm or partition across the globular body contains the seat. The horizontal setting of the seat allows the stem and disk to travel at right angles to the pipe axis. The stem passes through the bonnet which is attached to a large opening at the top of the valve body. This provides a symmetrical form that simplifies manufacture, installation, and repair

Y-Body Design

This design is a remedy for the high pressure drop inherent in globe valves. The seat and stem are angled at approximately 45°.The angle yields a straighter flowpath (at full opening) and provides the stem, bonnet, and packing a relatively pressure- resistant envelope.

Y-body globe valves are best suited for high pressure and other severe services. In small sizes for intermittent flows, the pressure loss may not be as important as the other considerations favoring the Y-body design. Hence, the flow passage of small Y-body globe valves is not as carefully streamlined as that for larger valves.

Angle Valve Design

The angle body globe valve design,is a simple modification of the basic globe valve. Having ends at right angles, the diaphragm can be a simple flat plate. Fluid is able to flow through with only a single 90° turn and discharge downward more symmetrically than the discharge from an ordinary globe. A particular advantage of the angle body design is that it can function as both a valve and a piping elbow.

For moderate conditions of pressure, temperature, and flow, the angle valve closely resembles the ordinary globe. The angle valve's discharge conditions are favorable with respect to fluid dynamics and erosion.

Globe Valve Disks

Most globe valves use one of three basic disk designs: the ball disk, the composition disk, and the plug disk.

Ball Disk

The ball disk fits on a tapered, flat-surfaced seat. The ball disk design is used primarily in relatively low pressure and low temperature systems. It is capable of throttling flow, but is primarily used to stop and start flow.

Composition Disk

The composition disk design uses a hard, nonmetallic insert ring on the disk. The insert ring creates a tighter closure. Composition disks are primarily used in steam and hot water applications. They resist erosion and are sufficiently resilient to close on solid particles without damaging the valve. Composition disks are replaceable.

Plug Disk

Because of its configuration, the plug disk provides better throttling than ball or composition designs. Plug disks are available in a variety of specific configurations. In general, they are all long and tapered.

Globe Valve Disk and Stem Connections

Globe valves employ two methods for connecting disk and stem: T-slot construction and disk nut construction. In the T-slot design, the disk slips over the stem. In the disk nut design, the disk is screwed into the stem.

Globe Valve Seats

Globe valve seats are either integral with or screwed into the valve body. Many globe valves have backseats. A backseat is a seating arrangement that provides a seal between the stem and bonnet. When the valve is fully open, the disk seats against the backseat. The backseat design prevents system pressure from building against the valve packing.

Globe Valve Direction of Flow

For low temperature applications, globe and angle valves are ordinarily installed so that pressure is under the disk. This promotes easy operation, helps protect the packing, and eliminates a certain amount of erosive action to the seat and disk faces. For high temperature steam service,globe valves are installed so that pressure is above the disk. Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat