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.

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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.

Sorry I didnt understand on the last comment you gave... "you need to be precise that how much information is necessary for u" as in?

it only means that are you really wanna go deep into the standard, because i am keep repeating all the majjor differences between BS and API602 which u need as a buyer point of view but it seems like you want to go in detail but also doesn't want to read the standard.
kindly read this lines from my last comment :-
"Ok the bore dia. mentioned in drawing is equivalent to full bore valve as per API 602, in BS the bore dia. shall be 48 for 2 inch CL- valves. so that's the one major difference. rest all differences are from design point of view which include stem dia, no. of packing used etc. if u really want to know about that then i already said you need to read standard as i cant copy paste whole standard here."

so my question is are u really want to know every minor details? because it seems like you do want to but looking for any shortcuts where u find all the answer in just few lines.

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