GLOBE VALVES

The simplest design and most common type is a Z-body. The Z-shaped partition inside the globular body contains the seat. The horizontal seating arrangement of the seat allows the stem and disk to travel at a perpendicular to the pipe axis resulting in a very high-pressure loss.

The valve seat is easily accessible through the bonnet which is attached to a large opening at the top of the valve body. Stem pass through the bonnet like gate valve.

This design simplifies manufacturing, installation, and repair. This type of valve is used where pressure drop is not a concern and throttling is required.

The Y-type design is a solution for the high-pressure drop problem in Z-type valves. In this type, seat and stem are angled at approximately 45° to the pipe axis.

Y-body valves are used in high pressure and other critical services where pressure drop is concerned.

Angle globe valve turns the flow direction by 90 degrees without using an elbow and one extra pipe weld. Disk open against the flow. This type of globe valve can be used in the fluctuating flow condition also, as they are capable of handling the slugging effect.

 The body is 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 mostly in straight-flow configuration. Ports are openings in the body for fluid flowing in or out. The two ports may be oriented straight across from each other or anywhere on the body, or oriented at an angle (such as a 90°). Globe valves with ports at such an angle are called angle globe valves. 

Globe valves are mainly used for corrosive or high viscous fluids which solidify at room temperature. This is because straight valves are designed so that the outlet pipe is in line with the inlet pipe and the fluid has a good chance of staying there in the case of horizontal piping. 

In the case of angle valves, the outlet pipe is directed towards the bottom. This allows the fluid to drain off. In turn, this prevents clogging and/or corrosion of the valve components over a period of time. 

A globe valve can also have a body in the shape of a "Y". This will allow the construction of the valve to be straight at the bottom as opposed to the conventional pot type construction (to arrange bottom seat) in case of other valves. This will again allow the fluid to pass through without difficulty and minimizes fluid clogging/corrosion in the long term.

 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. 

 MFC offers the following bonnet design for the globe valves:

Screwed bonnet

This is the simplest design available and it is used for inexpensive valves.

Bolted-bonnet

This is the most popular design and used in a large number of globe valves. This requires a gasket to seal the joint between the body and bonnet.

Welded-Bonnet

This is a popular design where disassembly is not required.  They are lighter in weight than their bolted-bonnet counterparts.

Pressure-Seal Bonnet

This type is used extensively for high-pressure high-temperature applications.   The higher the body cavity pressure, the greater the force on the gasket in a pressure -seal valve 

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 often becomes impractical.

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 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 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.

The seat ring provides a stable, uniform and replaceable shut off surface. Seat are usually screwed in or torqued. This pushes the cage down on the lip of the seat and holds it firmly to the body of the valve. 

Seat 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.