Globe Valves

A Globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body.

Globe valves are linear motion valves with rounded bodies, from which their name is derived.  They are widely used in industry to regulate fluid flow in both on/off and throttling service.  Globe valves consist of the following moving parts – the disk, the valve stem, and the handwheel. The stem connects the handwheel and the disk. It is threaded and fits into the threads in the valve bonnet.  The location of the valve disk in relation to the valve seat allows or restricts flow.  The direction of fluid flow through the valve changes several times, which increases the pressure drop across the valve. In most cases, globe valves are installed with the stem vertical and the higher-pressure fluid stream connected to the pipe side above the disk, which helps to maintain a tight seal when the valve is fully closed.

When globe valves are open, the fluid flows through the space between the edge of the disk and the seat. These valves are commonly used as a fully open or fully closed on/off valves, but they may be used for throttling as long as fine adjustments are not required.

These linear motion valves can be used in a variety of applications as long as the pressure and temperature limits are not exceeded, and the process does not require special materials to combat corrosion. Globe valves are generally available in a variety of metal and alloy constructions, as well as plastics, to cover this wide range of industrial applications. Proper material compatibility requires knowledge of the type, concentration and temperature of the media being handled. It may be necessary to consult the manufacturer of the valve for specific properties of the materials used in the valve

Globe valves provide the following advantages: they offer precise throttling and control and have high-pressure limits. Conversely, they offer a low coefficient of flow and are not good selections in applications that require cleanliness or sterility.

Globe valves are available in three main body types (although custom designs are available as well): angle design, Y-design and multi-piece design. Angle valves are designed so that the inlet and outlet are perpendicular, for transferring flow from vertical to horizontal. Y-design valves derive linear action from the incline between the axis of the inlet and outlet ports. The bodies of multi-piece design valves are bolted together. The inlet and outlet are not of single piece construction.

Parts of a typical globe valve

Body

The main pressure containing structure of the valve and the most easily identified as it forms the mass of the valve. It contains all of the valve's internal parts that will come in contact with the substance being controlled by the valve. The bonnet is connected to the body and provides the containment of the fluid, gas, or slurry that is being controlled.

Globe valves are typically two-port valves, although three port valves are also produced. Ports are openings in the body for fluid flowing in or out. The two ports may be oriented straight across from each other on the body, or oriented at an angle such as a 90° angle Globe valves with ports at such an angle are called angle globe valves. A globe valve can also have a body in the shape of a y.

Bonnet

Provides leakproof closure for the valve body. The threaded section of stem goes through a hole with matching threads in the bonnet. Globe valves may have a screw-in, union, or bolted bonnet. Screw-in bonnet is the simplest bonnet, offering a durable, pressure-tight seal. Union bonnet is suitable for applications requiring frequent inspection or cleaning. It also gives the body added strength. A bonnet attached with bolts is used for larger or higher pressure applications. Bonnets also contain the packing, which is a wearable material that maintains the seal between the bonnet and the stem during valve cycling operations

Plug or disc (disk)

The closure member of the valve. Plugs are connected to the stem which is slid or screwed up or down to throttle the flow. Plugs are typically of the balance or unbalanced type. Unbalanced plugs are solid and are used with smaller valves or with low pressure drops across the valve. The advantages are simpler design, with one possible leak path at the seat and usually lower cost. The disadvantages are the limited size; with a large unbalanced plug the forces needed to seat and hold the flow off become impractical. Balanced plugs have holes through the plug. Advantages include easier shut off as the plug does not have to overcome static forces. However, a second leak path is created between the plug and the cage, cost is generally higher.

Stem

The stem serves as a connector from the actuator to the inside of the valve and transmits this actuation force. Stems are either smooth for actuator controlled valves or threaded for manual valves. The smooth stems are surrounded by packing material to prevent leaking material from the valve. This packing is a wear material and will have to be replaced during maintenance. With a smooth stem the ends are threaded to allow connection to the plug and the actuator. The stem must not only withstand a large amount of compression force during valve closure, but also have high tensile strength during valve opening. In addition, the stem must be very straight, or have low runout, in order to ensure good valve closure. This minimum runout also minimizes wear of the packing contained in the bonnet, which provides the seal against leakage. The stem may be provided with a shroud over the packing nut to prevent foreign bodies entering the packing material, which would accelerate wear.

Cage

The cage is part of the valve that surrounds the plug and is located inside the body of the valve. Typically, the cage is one of the greatest determiners of flow within the valve. As the plug is moved more of the openings in the cage are exposed and flow is increased and vise versa. The design and layout of the openings can have a large effect on flow of material (the flow characteristics of different materials at temperatures, pressures that are in a range). Cages are also used to guide the plug to the seat of the valve for a good shutoff, substituting the guiding from the bonnet.

Seat ring

The seat ring provides a stable, uniform and replaceable shut off surface. Seat rings are usually held in place by pressure from the fastening of the bonnet to the top of the body. This pushes the cage down on the lip of the seat ring and holds it firmly to the body of the valve. Seat rings may also be threaded and screwed into a thread cut in the same area of the body. However this method makes removal of the seat ring during maintenance difficult if not impossible. Seat rings are also typically beveled at the seating surface to allow for some guiding during the final stages of closing the valve.

Economical globe valves or stop valves with a similar mechanism used in plumbing often have a rubber washer at the bottom of the disc for the seating surface, so that rubber can be compressed against the seat to form a leak-tight seal when shut.

Many globe valves have a class rating that corresponds to the pressure specifications of ANSI 16.34. Bibcocks and sillcocks are variations of globe or stop valves used in plumbing. Needle valves are variations of globe valves where instead of a separate attached disc piece, the internal end of the stem is conically tapered to act as the disc to fit into a matching seat for fine flow adjustment. Other different types of valve usually are called globe style valves because of the shape of the body or the way of closure of the disk. As an example typical swing check valves could be called globe type.