5.1 Introduction and Application
A globe valve can be used full open, full closed, or for throttling within limits of the disc and seat configuration.
Globe valves are normally metal or hard-seated, but they can be furnished with resilient disc inserts or seats that are suited for compressed air, compressed gas, or fluids that contain small particles of foreign material.
Globe valves have fewer operability problems as compared to gate valves and normally provide excellent seat leak tightness, but they do so at the penalty of a higher pressure drop. Globe valves also require large actuators.
5.2 Design
There are three basic body shapes in globe valves:
• Standard pattern (also called T-pattern), which is the most common shape (Figure 5-1).
• Angle pattern (Figure 5-2).
• Y-pattern (Figure 5-3), in which the stem is inclined at an angle (for example, 45°) with respect to the pipe axis. The Y-pattern body is designed to reduce the flow resistance of the globe valve. The flow resistance of the angle valve is between that of the standard globe and Y-pattern.
Figure 5-1
T-Pattern Globe Valve
Figure 5-2
Angle-Pattern Globe Valve
Figure 5-3
Y-Pattern Globe Valve
In contrast to the gate valve’s disc-to-seat sealing action (which is accompanied by sliding and friction), the globe valve plug or disc approaches or moves away from the seat in a direction perpendicular to the seat plane without sliding. Thus, relatively high seat contact stresses can be developed to get very tight shut-off without galling the seating surfaces.
Globe valve stems are either a rising and nonrotating design or a rising and rotating design. Some rising and rotating stem globe valves have an integral stem-to-disc connection that causes sliding at the seat face during the final closing action.
Globe valves are available in a wide variety of materials with both metal-to-metal seating components and soft seating options. Due to its relatively short stroke to achieve the full open position (as compared to gate valves), globe valves can easily incorporate diaphragm or bellows-type stem seals to provide zero external leakage.
Other options available in globe valves are dual and balanced plug designs to reduce actuator force requirements, cage guiding, and anti-cavitation and noise control trims for high pressure drop applications and gas services. Special details pertinent to control applications are discussed further in Section 6.
Guidance of the plug in the mid-travel position can be achieved by either a stem guide or plug guide, as discussed in Section 2.2.4. Plug guidance is preferred for larger valves to avoid stem/plug vibrations due to fluid dynamic forces.
For globe valves with unbalanced discs, the major component of required stem thrust is the differential pressure load on the disc, which in turn depends on the differential
pressure area. Testing has shown that, for common valve designs under incompressible flow conditions, there are two possible areas that need to be considered: (1) the area based on disc seating diameter (seat-based valves), and (2) the area based on disc guide diameter (guide-based valves).
The required thrust prediction methodology for globe valves is given in Reference 2.3 along with a detailed criterion for determining whether a valve is seat-based or guide-based. Figures 5-1, 5-2, and 5-3 show typical seat-based designs. A guide-based design is shown in Figure 5-4.
Figure 5-4
Velan 2" (5.1 cm), 1500# Globe Valve (Guide-Based) Model: Figure No. 137132
5.3 Installation Practices
Installation practices noted for gate valves in Section 4.3 apply to globe valves as well.
Additionally, installation of globe valves with the stems vertically upright is even more critical than for gate valves, as proper guidance of the disc into the seat is needed to achieve tight closure. If the stem and valve body are other than vertical, the disc tends to cock or go off center, and the disc seating surface, when going into the seat, will have little tendency to compensate to effect the correct seating angle. Y-pattern globe valves are normally provided with improved guidance to permit valve operation with the stem at an angle to vertical.
Flow Direction
Globe valves are normally installed so that flow is from under the seat. In some
applications, however, it is more important that stem packing be isolated from pressure or vacuum from the downstream side when the valve is closed. For example, a valve used as a containment isolation valve in a line with normal flow into the containment, but with containment accident flow out of the containment, should be installed with normal flow over the seat. In addition, valves connected directly to a vacuum
condenser should be installed so that the packing is not exposed to vacuum when the valve is shut.
5.4 Operation Practices and Precautions
The operation practices and precautions for gate valves indicated in Section 4.4 also apply to globe valves.
Globe valves may be used for rough throttling. However, if the valve was not specified for throttling, cavitation, chattering, and vibration may occur in the throttled position.
5.5 Common Problems
Most problems noted for gate valves in Section 4.6 also apply to globe valves.
• Globe valves improperly applied for throttling will cause damage to the valve and/or adjacent piping (for example, erosion, cavitation damage, flow-induced vibration, and high wear).
• Thrust requirements for some globe valves may be greater than predicted by valve vendors. The required thrust for globe valves can be alternatively calculated using EPRI’s PPM [2.1, 2.3].
• Some globe valve designs may experience stem guide or plug damage due to high side loads caused by high midstroke differential pressures. This most often occurs under high flow rates such as during blowdown events.
• Typical problems of isolation globe valves include stem/disc separation, sticking solenoids, pneumatic system failures, and seat leakage.
• Antirotation arms in globe valves cause several problems including binding, rotating, and breaking.
• Similar to gate valves, globe valves operated by motor actuators are susceptible to load-sensitive behavior (see Section 4.5).
• Metal diaphragm sealed globe valves (Figure 2-25) do not have their stem connected directly to their disc and rely on a spring to open the valve when the stem is
withdrawn. Therefore, these valves should not be used as throttle/control valves, nor should they be used in dirty service.
5.6 Maintenance Methods
Always follow the manufacturer’s instructions.
The maintenance methods for gate valves discussed in Section 4.6 generally apply to globe valves.
Threaded-in seat rings that do not have a provision for seal welding tend to loosen, causing seat leakage. Extreme caution should be used to avoid thread damage while re-installing the seats.
Re-surfacing the seats of a large Y-pattern globe whose stem is not vertical is very difficult due to gravity effects causing tool misalignment and setup difficulty. Use extreme care and proper tooling when performing this maintenance activity (see Reference 1.1 for detailed guidance on globe valve repair).