The Typical Globe Valves

Valves are used to regulate or control the flow rate of fluids in the pipelines.Globe Valves are specially design in the spherical body shapes with the two halves of the body that are present separately by an internal baffle.Its consist on movable disc and seat rings in the shape of spherical body. The typical Globe Valves are shown in the pictures below:

 

Construction Parts of Globe Valve:

There are following parts of construction of a Globe Valve.

• Hand wheel
• Gland
• Stem
• Bonnet
• Plug or Disc
• Seat
• Packing
• Rings
• Cage
• Body 

1.Hand Wheel

Handle is used to the purpose of moving.When we have a desired to close or open the valve then we use handle wheel .

2.Body

The body is the main part of any equipment or machine that are used in the Chemical or other industries.

3.Bonnet

The bonnet provides a leak-proof closure for the valve body.The threaded section of the stem goes through a hole with matching threads in the bonnet.Globe vales may have a screw-in, union or bolted bonnet.Screw-in is the simplest bonnet of globe valve.It consist on durable and 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.The bonnet also consists a packing, a wearable material that maintains the seal between the bonnet and stem during the valve cycling.

4.Plug or Disc

Plug or disc is another construction part of the globe valve.Its connected to the stem which is slid or screwed up or down to throttle the flow.Plug are the typically of the balance or unbalanced type.Unbalanced type plugs are solids and are used with smaller valves or low pressure drops across the valve.The advantages are simple design, with one possible leak path at the seat and usually lower cost.The disadvantages are that these are with limited body size, with large and unbalanced plug, the forces needed to seat and hold the flow of fluids often becomes impractical.The balanced plugs have holes through the plug.Advantages include easier shut off as the plug does not have to overcome static forces.Therefore, a second leak path is created between the plug and the cage and that cost is generally very high.

5.The Cage

The cage is also a part of the globe valve that surrounds the plug and is located inside the body of the valve.Special thing of it is that the cage is one of the best and greatest determiners of the flow within 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 materials (the flow characteristics of different materials at pressures,temperatures that are in the range). Cages are also used to guide the plug to the seat of the valve for a good shut off, substituting the guiding from the bonnet. 

6.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 vales or threaded for manual valves.The smooth stems are surrounded with packing material to prevent leaking material from the valve.The packing is a wearable material and will have to be replaced during maintenance.With a smooth stem the ends are threaded to allow connection to the plug and actuator.The stem must not only withstand a large quantity of compression force during valve closure, but its also have high tensile strength during the valve opening.In addition, stem must be very straight, or have a low run-out, in order to ensure good valve closure.The minimum run-out also minimizes wear of the packing contained in the bonnet which provides the seal against leakage.The stem may be provide with a shroud over the packing nut to prevent foreign bodies entering the packing material, which would accelerate the wear.

7.Seat Rings

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 from to the top of the body of valve.This pushes the cage down on the lip of the seat ring and holds it firmly to the body of 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 valves.Economical globe valves or stop valves with a similar mechanism used in plumbing often have a rubber washer at the bottom of the valve disc for the seating surface, so that rubber can be compressed against the seat to form a leak-tight seal when it shut.

Uses for material types:

Globe valves are typically made of metallic alloys, also some synthetic materials are available.These materials are chosen based on pressure, temperature and controlled media properties.Corrosive or erossive process streams mar require a compromise in material selection or exotic alloys or body coatings to minimize these material interactions and extend the life of the valve.