Tag: Valves

Gate valves are classified based on the type of disc, body-bonnet joint, and stem movement . Here’s a breakdown:

I. Based on Disc Type:

• Solid Wedge Gate Valve: This is the most common and basic type, known for its simplicity and strength . It’s a single, solid piece and suitable for most fluids and turbulent flow .
• Flexible Wedge Gate Valve: This has a one-piece disc with a cut around the perimeter to provide flexibility . This design is better for thermal expansion and prevents thermal binding, often used in steam systems .
• Split Wedge (Parallel Disc) Gate Valve: It has two solid pieces held together by a mechanism . This allows each disc to adjust to the seating surface and is suitable for noncondensing gasses and liquids .

II. Based on Body-Bonnet Joint:

• Screwed Bonnet: This is a simple, inexpensive design .
• Bolted Bonnet: This is the most common type, using a gasket to seal the joint .
• Welded Bonnet: It’s a design where disassembly is not required, and it’s lighter than bolted bonnets .
• Pressure-Seal Bonnet: Used for high-pressure and high-temperature applications, where increased pressure improves the seal .

III. Based on Stem Movement:

• Rising Stem (Outside Screw and Yoke – OS&Y): The stem rises when the valve is opened, giving a visual indication of the valve position . The stem threads are outside the valve, protecting them from the fluid .
• Non-Rising Stem (Inside Screw): The stem does not rise or lower; instead, it rotates, making it suitable for tight spaces . The stem threads are exposed to the fluid .

Additional Types:

• Knife Gate Valve: This is designed with a sharp edge to cut through thick fluids and slurries, often used in industries like mining and paper .
• Through-Conduit Gate Valve: This type has a gate that’s fully enclosed when open, which is designed to maintain a smooth, uninterrupted flow path. It’s commonly used in pipelines where pigging is required .

Globe Valve Types based on Body Bonnet Connection

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.

Application of Globe valve
Globe Valves are used in the systems where flow control is required and leak tightness is also important.
It used in high-point vents and low-point drains when leak tightness and safety are major concerns. Otherwise, you can use a gate valve for drain and vent.
It can be used in Feed-water, chemical, air, lube oil and almost all services where pressure drop is not an issue
This valve is also used as an automatic control valve, but in that case, the stem of the valve is a smooth stem rather than threaded and is opened and closed by lifting action of an actuator assembly.

Advantages
Better shut off as compared to gate valve
Good for frequent operation as no fear of wear of seat and disk
Easy to repair, as seat and disk can be accessed from the valve top
Fast operation compares to gate valve due to shorter stroke length
Usually operated by an automatic actuator

Disadvantages
High head loss from two or more right-angle turns of flowing fluid within the valve body.
Obstructions and discontinuities in the flow path lead to a high head loss.
In a large high-pressure line, pulsations and impacts can damage internal trim parts.
A large valve require considerable power to open and create noise while in operation.
It is heavier than other valves of the same pressure rating.
Costlier compared to the gate valve

Depending on the type of body there are three types of globe valves;

• Z types
• Y types
• Angle Types

Z types Globe Valve

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 passes through the bonnet like a 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.

Y types Globe Valve

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 types Globe Valve

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.