Understand Globe Valves (BS 1873 & API 602)

 

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GLOBE VALVE

WHAT IS A GLOBE VALVE?

Definition of Globe Valve

A globe valve is a type of valve used for regulating flow in a pipeline, or to shut off the flow, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body (even if for shut-off operations ball valves and gate valves shall always be preferred).

The term 'globe' comes from the valve's spherical body shape, though the actual shape can vary, especially in modern designs. The key function of a globe valve is to start, stop, or throttle the flow of fluid through a pipeline; it achieves this through the precise positioning of the disk against the valve seat, which forms a gap allowing the fluid to pass through in a controlled manner.

Globe valves are bi-directional valves, different from check valves, meaning that the fluid can flow either way within the valve.

Globe valves offer a reliable solution for flow control applications, combining good sealing capabilities with the ability to precisely regulate flow. Their versatility and efficiency make them a valuable component in various piping systems.

Design and Operation

The basic components of a globe valve include the body, seat, stem, disc, and handwheel or actuator. The valve's operation is straightforward:

• Turning the handwheel or activating the actuator moves the stem and the attached disc toward or away from the seat.
• When the disc moves towards the seat, it restricts the flow path, reducing flow or stopping it completely when fully closed.
• Moving the disc away from the seat opens the flow path, allowing fluid to pass through.

Key Features

• Flow Regulation: Globe valves excel at regulating flow rates due to their design, which allows for fine adjustments of the disc position.
• Sealing Capability: The close contact between the disc and seat provides good sealing capabilities, minimizing leaks when the valve is closed.
• Directional: Typically, globe valves are used in applications where the flow direction is important, as they are designed to handle flow in one direction, which helps in reducing pressure drop and preventing backflow.

Applications

Globe valves are used across various industries where flow needs to be precisely controlled, including:

• Water and Wastewater Management: For regulating flow and pressure within distribution networks.
• HVAC Systems: In heating and cooling systems for controlling the flow of hot or cold water.
• Chemical and Petrochemical Industries: For managing the flow of various chemicals in processing plants.
• Steam Services: Such as in power plants, where steam flow must be carefully controlled.

Globe Valves Selection Considerations

Selecting the right globe valve for your application involves careful consideration of various factors to ensure optimal performance, reliability, and safety. Globe valves are used for regulating flow in a piping system, and the correct selection is crucial for effective system operation. Here's a detailed guide on how to select a globe valve:

1. Understand the Application

• Fluid Characteristics: Identify the type of fluid (liquid, gas, corrosive, toxic, etc.) the valve will handle. This determines the material selection for the valve body, trim, and sealing components.
• Pressure and Temperature: Determine the operating pressure and temperature range. This information is crucial for selecting a valve that can withstand the system's conditions without failure.
• Flow Requirements: Consider the flow rate and the necessity for flow regulation or throttling. Globe valves offer excellent throttling capabilities, but the degree of control required will influence the valve design.

2. Select the Proper Valve Size

• Pipe Size: The valve size should correspond to the pipe size in the system to ensure proper fit and avoid unnecessary pressure drops.
• Cv Value: Calculate the valve's flow coefficient (Cv) required based on the flow rate, fluid density, and the pressure drop across the valve. The Cv value helps in selecting a valve size that meets the flow requirements without excessive pressure loss.

3. Choose the Valve Body Material

• Compatibility: The valve material must be compatible with the fluid to prevent corrosion. Common materials include carbon steel, stainless steel, bronze, and special alloys for corrosive applications.
• Temperature and Pressure: The material must also be suitable for the operating temperature and pressure. Refer to ASTM, ASME, and other relevant standards for guidance on material properties under specific conditions.

4. Determine the Valve End Connections

• Connection Type: Globe valves are available with flanged, threaded, butt-weld, and socket-weld end connections. The choice depends on the piping system design, pressure rating, and ease of installation or maintenance.
• Standards Compliance: Ensure the selected end connections comply with industry standards, such as ASME B16.5 for flanged connections, to ensure compatibility and integrity in the piping system.

5. Select the Appropriate Trim Material

• Service Conditions: The trim (disc, seat, and stem) material should be selected based on the fluid type, pressure, temperature, and potential for wear or corrosion. The trim material may differ from the body material to optimize performance and lifespan.
• Leakage Class: Consider the required leakage class based on system requirements. Different trim designs offer varying levels of sealing efficiency, affecting the valve's ability to completely shut off or precisely regulate flow.

6. Actuation Method

• Manual vs. Automated: Decide between manual operation (handwheel or gear operator) and automated actuation (pneumatic, hydraulic, or electric). Automated actuators are preferred for remote control, frequent operation, or where precise control is needed.
• Actuator Sizing: Ensure the actuator is properly sized to operate the valve under all expected conditions, including the maximum differential pressure.

7. Consider Special Features

• Bellows Seal: For toxic or hazardous fluids, consider a globe valve with a bellows seal to prevent stem leakage.
• Cage Guided: Cage-guided trims provide stability to the plugin in high-flow conditions, reducing vibration and wear.

8. Compliance and Certifications

Ensure the globe valve meets relevant industry standards and certifications for safety, performance, and quality. Common standards include ASME, API, and ANSI specifications relevant to globe valves.

9. Consult with Manufacturers or Experts

When in doubt, consult with valve manufacturers or industry experts. They can provide valuable insights and recommendations based on experience with similar applications.

Selecting the right globe valve is a process that requires an understanding of the application requirements, knowledge of valve design principles, and consideration of the operating environment. By systematically addressing these factors, you can ensure the selected globe valve will perform efficiently and reliably in your system.

GLOBE VALVE ADVANTAGES AND DISADVANTAGES

Globe valves, recognized for their ability to regulate flow in piping systems, come with a set of advantages and disadvantages that make them suitable for specific applications while less ideal for others. Understanding these pros and cons is essential for selecting the right valve type for a particular system requirement.

Advantages of Globe Valves

1. Precise Flow Control: One of the primary advantages of globe valves is their exceptional capability to regulate flow. The design allows for fine adjustments of the disc position, providing superior control over the flow rate.

2. Good Sealing Capability: Globe valves offer a tight seal when closed, thanks to the close contact between the disc and the seat. This minimizes leaks and ensures effective isolation.

3. Bi-Directional: While typically used in one direction, globe valves can be designed for bi-directional flow, adding to their versatility in system design.

4. Availability in Various Designs: Globe valves come in several body designs (Z-body, Y-body, and angle body), each offering specific advantages for different applications, such as minimized pressure drop or enhanced flow control.

5. Ease of Maintenance: Many globe valve designs allow for easy access to internal components, facilitating maintenance, repairs, and part replacements without removing the valve from the pipeline.

Disadvantages of Globe Valves

1. Pressure Drop: The biggest drawback of globe valves is the inherent pressure drop across the valve, even when fully open. The flow path through the valve body is not straight, causing changes in flow direction and resistance that result in pressure loss.

2. Size and Weight: Globe valves tend to be larger and heavier than other valve types (like ball valves) of the same nominal size and pressure class, potentially requiring more space and support in a piping system.

3. Higher Operating Torque: Due to the design and the need for a tight seal, globe valves may require a higher torque to operate, especially in higher-pressure applications. This can necessitate larger or more powerful actuators.

4. Cost Considerations: Globe valves can be more expensive than other valve types, particularly in larger sizes, due to their complex design and the materials required for manufacturing.

5. Not Ideal for Slurry Applications: The tortuous flow path in a globe valve can lead to the accumulation of solids or slurries, which can affect the valve's operation and lead to wear or damage.

In conclusion, globe valves are highly effective for applications requiring precise flow control and where the benefits of their design outweigh the potential drawbacks, such as in throttling applications and systems where a high degree of flow regulation is needed. However, for applications where pressure drop is a concern or where a straight-through flow path is required, alternative valve types may be more suitable.

Globe valves have become a necessary instrument in controlling fluid flow. They are used for two major functions as they also operate as on-off valves. These valves have a wide range of applications in several industries, such as water systems, turbine seals, and fuel oil systems.

However, the design of the globe valve results in resistance to fluid flow, and this leads to high-pressure drops in the fluid. Therefore, before selecting a globe valve, factors such as pressure drop at positions within the valve and valve sizing should be considered.

This post discusses the function of the globe valve, its parts, installation and maintenance requirements, and other factors to be considered before buying an industrial globe valve.

What Is A Globe Valve?

Globe valves are valves whose disc or plug moves in a straight line to allow, restrict or regulate the media flow. The vertical movement of the globe valve disc is called linear motion, which is why globe valves are called linear motion valves.

As the disc moves linearly down the valve, the globe valve seat opens to accommodate the discs. The globe valves are primarily used, because of their design, for throttling and regulation of flow through the valve. Still, they can also be used in the on/off services of totally allowing or restricting media.

During passage through the globe valve, the fluid moves up on the side of the seat and against the valve disc before continuing its flow; this cleans the disc and reduces dirt in the valve body.

However, the fluid flow rate and direction are altered and therefore result in a substantial pressure drop as the fluid moves.

Globe Valve Symbol

The globe valve symbol is composed of two triangles meeting at a point. A solid circle is inserted at the meeting point of the triangles, and solid horizontal lines extend from the opposite sides of the triangle.

The horizontal lines show that the globe valve is a two-way inline valve used for throttling and on-off applications.

The globe valve symbol used in piping and instrumentation diagrams (P&ID) is a modification of the valve symbol, which is the same symbol without the solid circle at the meeting point of the two triangles.

The three isometric symbols in the diagram indicate symbols for butt-welding end connections, flanged ends, and socket ends connections.

Types of Globe Valves

Globe valves are of various types and can be differentiated based on their valve body design and body-bonnet connection.

Based On Valve Body Design

Globe valves can be differentiated into three types based on their body design: Angle pattern globe valve, Standard or T-pattern globe valve, and Oblique or Y-pattern globe valve. 

Standard Pattern (T-Pattern) Globe Valve

This valve is a commonly used industrial globe valve. The valve seat is set horizontally so that the disc moves perpendicular to the flow through the pipe flanges. 

The standard pattern globe valve obstructs flow in several places in the valve body, leading to a high-pressure drop during the fluid passage. This pattern offers the highest obstruction to flow among the three patterns, and it also has the lowest flow coefficient.

The T-pattern globe valve is used in extreme throttling cases. For example, they are used in processing plant operations in bypass lines required around control valves. Also, they are used in flow regulation operations where pressure losses in the fluid are not important.

Angle Pattern Globe Valve

This pattern is a modified version of the T-pattern. The angle pattern globe valve has its ends at a right angle, and fluid flows after a 90-degree rotation. Angle pattern globe valves have a higher flow coefficient than the standard pattern globe valves.

Because of the ability of the globe valves to handle slug flow of fluid, it is used commonly with fluids of periodic variations and pressure changes. The angle pattern globe valve offers lower obstruction to fluid flow than the standard valves.

Also, when installed near pipeline ends, they decrease the number of pipe joints required and function as a pipe elbow in changing the direction of the fluid flow.

Oblique Pattern (Y-pattern) Globe Valves

The oblique pattern globe valve, out of the three globe valves, offers the least obstruction to fluid flow. Because it least obstructs flow, this results in a decreased fluid pressure drop. 

The valve seat is positioned at 45 degrees to the stem, contributing to the direct fluid flow through the valve and the reduction in fluid pressure drop.

The Y-pattern globe valves are not easily susceptible to erosion, even after long periods of fluid flow. During early operation periods, the oblique globe valve is used for flow regulation. Their ability to be cleaned off of sludges when operating in closed lines gives them an advantage over some other valves.

Based On Body-Bonnet Connection

Bonnets are components of the globe and gate valve placed on top of the valve body. The bonnet connects the valve body to the manual lever device or the actuators and grants entry into the valve trim components.

Industrial valve manufacturers uncouple the bonnet from the body for repair and maintenance of globe valves. The body-bonnet connection is important for easy maintenance, checking the seal's integrity, valve size selection, etc. 

Below are the types of globe valves based on the bonnet connection to the valve body :

Bolted Bonnet Connection

The bolted bonnet globe valve is also called the screwed-in globe valve. It is the most common and economical connection. The bonnet should not be uncoupled from the valve body often as it may damage the joining areas of the bolts.

The bonnet joints of these valves require large torque in screwing into the valve body and are usually used on valve sizes that are not large.

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Welded Bonnet Connection

The welded bonnet globe valve is created by welding the bonnet to the valve body. The bonnet is usually welded completely to the valve body when made of a weldable material. 

The welded bonnet connection can be applied to most valve sizes and is functional in different operating conditions. 

However, a major disadvantage of the welded bonnet is that uncoupling the connection could damage the bonnet and valve body. Hence, this bonnet connection is used in cases where the valve body can be discarded. 

Flanged Bonnet Connection 

Flanged bonnet globe valves have bolts spread over the connection area. The extra bolts help hold the connection tighter and are more evenly spread than in the bolted connection globe valves. It also allows the flanged bonnet connection to be applied to various sizes of valves and demanding operating conditions.

However, for larger valve sizes, the flanged connection may be welded for safer operations.

Union Ring Bonnet Connection

In the union ring bonnet globe valve, the bonnet is joined to the valve body with a union ring. The contact areas for the joints in this bonnet connection are not affected by frequent unscrewing. However, it is used in valve sizes that are not large.

Pressure Seal Bonnet Connection

This bonnet connection is most suitable for high temperature and pressure conditions. The pressure seal bonnet globe valve uses fluid pressure to tighten the joint between the bonnet and valve body. 

Globe Valve Advantages and Disadvantages 

Globe valves are usually compared to the gate and ball valves. While the gate and ball valves function as on-off valves, the globe valve functions as a throttling device in on-off operations. Here are some of its other advantages in piping operations:

Advantages 

Good Throttling Ability 

The vertical movement of the valve disc to the valve seat encourages the good throttling ability of the globe valve. As the valve is closed, the space between the disc and seat reduces gradually over a short distance, contributing to good flow regulation.

Maintenance and Structure 

The globe valve has a simpler construction than the gate valve. With one sealing surface and minimal sealing area, the globe valve reduces construction costs and saves production material. 

Maintenance of the globe valve is easy, and it offers a better production process because the valve saves cost and material.

Good Sealing Performance

There is little or no sliding between the valve seat and disc surfaces, leading to less friction between the two surfaces. Therefore, the sealing surfaces experience less wear over a long period and provide good sealing operations.

Also, because the valve seat and disc do not wear easily, the service life of the globe valve is longer than some valves.

Other advantages of the globe valve include:

• The installation of globe valves does not require much height as the valves have a thick structure.
• Globe valves offer different patterns and types for a wider selection of suitable valves.
• Globe valves have fewer seat leakages than when compared to the gate valve.

Disadvantages 

These are some disadvantages experienced while using the globe valve:

Large Torque Requirements

The fluid pressure is overcome by the valve disc from the opposite end, requiring large torque to close this valve. Also, to lift the globe valve disc and open the valve, large torque is required.

High Flow Resistance And Pressure Losses

Shutoff valves offer the highest resistance to flow. This resistance occurs because there are various obstructions to fluid flow in the valve. The high resistance also leads to high-pressure losses in the fluid.

Restricted Flow Direction (Unidirectional)

Globe valves restrict the movement of flow to one direction. Fluids can only move from the bottom up or down without changing direction.

Other disadvantages of globe valves include:

• They are difficult to install.
• They require large openings for the arrangement of discs.
• They are power-consuming and noisy.

Globe Valve Functions

Globe Valves function in on-off operations as blocking or isolation valves and in throttling operations to regulate flow. They are largely designed to operate as flow regulators but are also functional when fully open or closed to restrict fluid flow.

As a result of the small height of the globe valve compared to the gate valve, the globe valve disc moves a short distance to the valve seat when opening or closing. The ability of the valve to decrease the space between the valve seat and disc gradually makes it suitable for throttling operations. 

Also, globe valves are less likely to be damaged by fluid flow when in a partially open position; this contributes to its effectiveness as a throttling device.

When used in on-off operations, the globe valve disc design is carefully selected to ensure sealing reliability. Linear motion valves such as gate, needle, and globe valves can be applied in many operations with suitable conditions. 

Globe Valve Parts 

The parts of the globe valve include the actuator, stem, disc (plug), valve body, seat, and bonnet.

Globe Valve Disc

The valve disc is the main part of the globe valve responsible for allowing, restricting, or controlling fluid flow within the pipeline. The valve disc pulls up or moves down in reaction to the stem rotation. 

Globe valve discs are designed in three ways: Plug disc design, Composition disc design, and Ball disc design.

Plug Disc Design

These discs have an end that narrows to a flat surface. They are better for use in flow regulation than the ball and composition disc.

Composition Disc Design

Composition discs are suited for water systems because they are hardly eroded. They have a wider surface than the plug and ball disc and are replaceable.

Ball Disc Design

This design is mostly used for on-off operations. It has a narrowing lower end and is usually used in low pressure and temperature conditions. 

Globe Valve Seat

The valve seat is the sealing surface for the disc. It is made to accommodate the valve disc as it drops down to close the valve. The globe valve seat can be made with the valve body or screwed. The valve disc connects with the seat at a right angle, resulting in a more reliable seal.

A globe valve back seat is the sealing point for the stem and bonnet above the disc. It holds the media pressure from reaching the valve packing as the valve disc rests on it when fully open.

Globe Valve Stem

The stem connects the disc with the valve actuator. It is usually joined to the bonnet, and the disc can be connected to the stem in two ways: slipping over the stem or screwing into it.

For faster action, the globe valve stem is designed to allow the disc to move in the direction of the fluid as it closes.

Globe Valve Body 

This part of the globe valve protects the internal components of the machine. The globe valve is designed to have a simple but solid body construction.

Globe Valve Actuator 

The valve actuator rotates the stem, which controls the movement of the valve disc. The opening and closing of the valve are determined first by the actuator. The globe valve actuator is classified into two types based on the requirements of the valves. The types are:

• Manually Operated Actuators: These involve using hand wheels or cranks to control the stem.
• Automatically Operated Actuators: These involve using electric, pneumatic or hydraulic systems to control the stem.

Globe Valve Bonnet

Bonnets are components of the globe valve placed on top of the valve body. The threaded stem passes through the bonnet through a hole, and the bonnet connects the valve body to the manual lever device or the actuators.

How Does The Globe Valve Work?

The function of the globe valve is to allow, restrict, or control the flow of media through the pipe from one area to another. To accomplish this, the globe valve uses a valve disc and a valve seat.

The globe valve seat is positioned parallel to and at the center of the pipe. When the handwheel is turned or the actuator is operated automatically, the valve stem is rotated, and the valve disc moves. 

For on-off operations, the globe valve disc, controlled by the stem, is either lowered fully to seal with the valve seat and hence block the flow of media or raised fully to allow the flow of media. When the valve disc is raised fully, the fluid flow is at its highest rate, and when it is lowered fully, fluid flow is shut off completely.

For throttling operations, the disc is not fully raised, and the fluid flow occurs in direct proportion to the lifting of the valve disc.

Installation and Maintenance Of Globe Valves

Installation 

Proper installation of globe valves decreases premature wear and tear and ensures proper operation of the valves. Upright installation of the globe valves and waste removal inside the valve are some proper installation practices. Some others are:

• The internal areas of the valve should be cleaned of fluids before installation.
• The valve load should be supported as pipe clampings on the valve may damage it.
• Enough space should be provided for the installation of the valve.
• When working with flanged connections or bolts, the bolts should be tightened diagonally.
• Check the bolts by increasing the pressure inside the valve. Valve pressure is also increased to confirm the reliability of the seal.
• The materials of the globe valve components should be checked to ensure no damage was done during transportation or storage of the components.
• The pressure rating of the valve should conform to the requirements.
• The parts of the pipe flanges to contact the valves should be clean and free of residue.

Maintenance 

Maintenance of globe valves increases their service life and ensures they work at optimal conditions. Here are some maintenance practices that enable globe valves to last longer:

• The valve should be inspected regularly to ensure leakages are quickly noticed and repaired.
• The valve should be cycled regularly when not in use.
• If the packing needs to be adjusted, the gland bolts should be carefully tightened as over-tightening may damage the glands. (the gland keeps the packing in a stuffing box) 

What To Consider Before Buying Globe Valves

Globe valves are selected based on their suitability to conditions such as temperature, pressure, type of fluid, etc. Therefore selecting the suitable globe valve is based on the valve size, material type, pressure rating, standard, and design of the valve, to select the appropriate globe valve.

Material

Various materials such as cast iron, steel, and plastics, are used in producing globe valves because of their application in most industries. Selecting an appropriate globe valve requires knowledge of the qualities of the media passing through the valve.

Globe valves made with stainless steel are usually used for high-temperature conditions as it is resistant to seizing and galling. Manufacturers should be consulted to know the specific globe valve for different conditions.

Design

Globe valve designs are classified into two types: Valve Body Design and Disc Design. The globe valve body design includes the standard pattern, angle pattern, and oblique pattern. 

The standard pattern valve offers the most obstruction to flow, leading to a higher fluid pressure drop in this valve, while the angle pattern valve is suitable for fluids with slug flow. The oblique pattern valves are hardly susceptible to erosion. (These patterns have been explained in detail under globe valve body types)

The globe valve disc design includes the composition disc, plug disc, and ball disc. The composition disc is best suited for water system operations, while the plug discs have the best flow regulation of the three designs. The ball disc is appropriate for low temperature and pressure conditions. (These, too, have already been discussed above under globe valve parts)

Valve Sizes 

Valve sizing is essential for throttling valves as the sizes determine how fluid flows through the valve. Selecting the globe valve size requires research and knowledge of the requirements. Usually, the manufacturer sizes the valve for a buyer. Sizing a globe valve is different for each of its functions.

As a throttle valve, the globe valve size is important for calculating the pressure drop at specific positions in the valve.

While functioning in on-off operations, the pressure drop is not monitored as the globe valve functions to allow or restrict fluid. However, a smaller valve size may restrict the flow, but it is a more economical option than the larger valve sizes.

Standards

Below are some important standards used in petrochemical industries for the globe valve:

• BS : cast steel valves
• API 603: stainless steel valves
• API 602/BS : forged steel valves
• API 598 and BS EN -1: valves testing
• ASME B16.10: face to face dimensions for valves
• ASME B16.5 and ASME B16.47: flanged connections
• ASME B16.25: butt weld connections design

Conclusion

This post discussed the function of the globe valve, its parts, installation and maintenance requirements, and factors to be considered before buying an industrial globe valve.

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