(a) Sketch a globe type screw lift valve suitable for sea water service.(7)
(b) State, with reasons, the materials used for the valve sketched in part (a).(3)
A globe type screw lift valve
A globe type screw lift valve suitable for seawater service is a specific type of globe valve designed to handle the corrosive and demanding conditions associated with seawater applications on board vessels.
1. Globe Valves – Screw Lift (SL)
Here are its key features:
- Globe Valve Design: It retains the basic globe valve design, with a disc or plug that moves perpendicular to the flow path, providing good throttling capabilities and tight shut-off. 1. Globe Valves – RFS Hydraulics www.rfshydraulics.com
- Screw Lift Mechanism: The valve’s stem has threads that engage with threads in the valve bonnet. Turning the handwheel or actuator rotates the stem, causing it to move up or down, thereby lifting or lowering the disc/plug to control flow.
- Seawater-Resistant Materials: The valve body, bonnet, disc/plug, and other internal components are constructed from materials that resist corrosion in seawater. These materials typically include:
- Bronze: Especially silicon bronze or aluminum bronze, which form protective oxide layers in seawater.
- Stainless Steel: Particularly grades like 316 stainless steel, which offer excellent corrosion resistance. 1. Does stainless steel rust? – thyssenkrupp Materials (UK) www.thyssenkrupp-materials.co.uk
- Copper-Nickel Alloys: These alloys are known for their resistance to seawater corrosion and biofouling. 1. Biofouling Resistance of Cupronickel: Basics and Experience www.copper.org
- Suitable Packing and Gaskets: The packing around the stem and any gaskets used in the valve should also be made from materials resistant to seawater and compatible with the intended operating temperatures and pressures.
Why is it suitable for seawater service?
- Corrosion Resistance: The primary reason is its construction from materials that can withstand the corrosive effects of seawater, ensuring the valve’s longevity and reliable operation.
- Tight Shut-off: The globe valve design provides a tight seal when closed, preventing seawater leakage, which is crucial for maintaining system integrity and preventing flooding or contamination. 1. The Complete Guide to Understanding Globe Valves | Tameson.com tameson.com
- Throttling Capability: The screw lift mechanism allows for precise control of seawater flow, making it suitable for applications where flow regulation is necessary.
- Durability: These valves are designed to be robust and durable, capable of handling the pressures and potential mechanical stresses associated with seawater systems.
Typical Applications:
- Seawater Cooling Systems: Controlling the flow of seawater to cool engines, generators, or other machinery.
- Seawater Ballast Systems: Regulating the intake and discharge of ballast water.
- Firefighting Systems: Controlling the flow of seawater to the fire main system.
- Other Seawater Systems: Used in various other systems requiring seawater intake or discharge, such as deck wash systems or certain sanitation systems.
Maintenance:
Regular maintenance, including inspections, operational checks, lubrication, and packing adjustments or replacements, is essential to ensure the continued reliability and longevity of globe type screw lift valves in seawater service.
In conclusion, a globe type screw lift valve specifically designed for seawater service is a valuable component in marine applications, offering a combination of corrosion resistance, tight shut-off, flow control, and durability to handle the demanding conditions of seawater systems.
Essential Components
When sketching a cross-section of a globe type screw lift valve for seawater service, you’ll want to include these main components and features:
Essential Components:
- Body: The main cylindrical housing of the valve, showing its thickness and the inlet and outlet ports.
- Bonnet: The cap or cover on top of the body, housing the stem and internal mechanisms.
- Stem: The threaded rod that extends through the bonnet and connects to the disc/plug. It’s typically shown in the center of the valve.
- Disc/Plug: The movable part that controls the flow, shown either in the open or closed position. In a globe valve, it’s usually a disc that moves perpendicular to the flow path.
- Seat Ring: The ring-shaped sealing surface within the valve body against which the disc/plug seats to create a tight seal.
Additional Features to Consider:
- Packing: Indicate the packing material around the stem within the bonnet to prevent leakage.
- Handwheel or Actuator: If manually operated, show the handwheel attached to the top of the stem. If remotely operated, indicate the presence of an actuator (hydraulic, pneumatic, or electric) connected to the stem.
- Flow Direction Arrow: Show the direction of flow through the valve with an arrow.
Important Details to Highlight:
- Material Indication: Label the materials used for the body, bonnet, disc/plug, and seat rings (e.g., bronze, stainless steel).
- Valve Position: Clearly indicate whether the valve is shown in the open or closed position.
- Sealing Surfaces: Emphasize the contact points between the disc/plug and seat rings to show how the valve creates a tight seal.
- Flow Path: Illustrate the path that the fluid would take through the valve, highlighting any changes in direction or restrictions.
Example Sketch (Simplified)
---------- Bonnet ----------
\ / \ /
\ / \ /
------- Stem --------
| |
| |
| Disc | (Closed Position)
| |
| |
---|-------------------| Seat Ring
| |
| Valve Body |
| |
--|----------------------|--
| |
[Inlet] [Outlet]
Tips for Sketching:
- Use a Ruler: Straight lines and consistent proportions will make your sketch clearer and more professional.
- Label Components: Clearly label each component to avoid confusion.
- Use Cross-Hatching: Use cross-hatching or shading to differentiate between different materials or to show solid parts versus open spaces.
- Simplify: Focus on the essential components and features. You don’t need to include every nut and bolt.
Remember:
- The specific design of a globe type screw lift valve can vary, so refer to any provided diagrams or specifications for accuracy.
- The sketch should be clear and easy to understand, conveying the basic structure and function of the valve.
By following these guidelines, you can create a helpful sketch that demonstrates your understanding of a globe type screw lift valve suitable for seawater service.
Basic Operation
The fundamental principle of a globe valve is to control the flow of fluid by raising or lowering a disc (or plug) in relation to a seat within the valve body. The “screw lift” mechanism is how this movement is achieved.
1. The Complete Guide to Understanding Globe Valves | Tameson.co.uk
2. How to Rebuild a Gate Valve – AQE Machinery Corporation
- Open Position:
- The handwheel or actuator is turned counter-clockwise, rotating the threaded stem.
- As the stem rotates, it also moves upwards, lifting the disc away from the seat.
- This creates an opening between the disc and seat, allowing fluid to flow through the valve. 1. The Complete Guide to Understanding Globe Valves | Tameson.co.uk tameson.co.uk
- The further the stem is lifted (and the handwheel turned), the larger the opening and the greater the flow rate.
- Closed Position:
- The handwheel or actuator is turned clockwise, rotating the stem downwards.
- This lowers the disc onto the seat, creating a tight seal that prevents fluid flow.
Specific Features Relevant to Seawater Service:
- Materials: The valve body, bonnet, disc, seat, and stem are made from corrosion-resistant materials like bronze or stainless steel to withstand the harsh marine environment.
- Packing: The packing around the stem is typically a material like graphite or PTFE that resists seawater corrosion and provides a good seal to prevent leakage along the stem.
- Seat Design: The seat and disc are often designed with a metal-to-metal seal for durability and tight shut-off in seawater applications.
Advantages of the Screw Lift Mechanism:
- Precise Control: The threaded stem allows for fine control of the valve opening, enabling accurate regulation of flow rate.
- Tight Shut-Off: The disc can be firmly pressed against the seat, providing a reliable seal even in high-pressure seawater systems.
- Visual Indication: The position of the handwheel or actuator visually indicates the valve’s open or closed state.
Considerations:
- Higher Pressure Drop: Compared to ball or butterfly valves, globe valves generally have a higher pressure drop across the valve due to the flow path changes. 1. Butterfly Valves vs Globe Valves, Blog – Valworx www.valworx.com
- Slower Operation: The screw lift mechanism can be slower to operate than the quarter-turn action of ball or butterfly valves.
In summary: The globe type screw lift valve operates by using a threaded stem to raise or lower a disc in relation to a seat, controlling the flow of seawater. Its design, coupled with suitable materials, makes it ideal for applications requiring precise flow control and tight shut-off in marine environments.
Materials
The main components of a globe type screw lift valve suitable for seawater service are typically made from the following materials:
- Body and Bonnet:
- Bronze: This is a common choice, especially silicon bronze or aluminum bronze, due to its excellent corrosion resistance in seawater. It’s also strong and durable enough to handle the pressures in marine systems. 1. Why Use Silicon Bronze? | theatomworks – TAW Fasteners Ltd. www.tawfasteners.co.uk
- Stainless Steel: Certain grades of stainless steel, such as 316 or 316L, are also used for their superior corrosion resistance and strength. They can be more expensive than bronze but offer exceptional longevity.
- Stem:
- Stainless Steel: The stem is usually made from stainless steel for its strength, corrosion resistance, and smooth operation within the packing. 1. Selecting the Right Materials for Your Globe Valves | Tameson.com tameson.com
- Bronze: In some cases, bronze may also be used, especially in smaller valves or when compatibility with other bronze components is desired.
- Disc/Plug:
- Bronze or Stainless Steel: Similar to the body and bonnet, the disc or plug is typically made from bronze or stainless steel for corrosion resistance and durability.
- Bronze-Faced Cast Iron: For larger valves, the disc may be made of cast iron with a bronze facing on the seating surface to provide a good seal and wear resistance.
- Seat Ring:
- Bronze or Stainless Steel: The seat ring is generally made from the same material as the disc or plug to ensure compatibility and a good seal.
- Packing:
- Flexible Graphite: This is a popular choice for its excellent chemical resistance, high-temperature tolerance, and low friction, enabling smooth stem operation.
- PTFE (Teflon): PTFE is also widely used for its chemical inertness, low friction, and resistance to a wide range of temperatures. 1. PTFE or Teflon uses, properties & limitations. – Fluorocarbon fluorocarbon.co.uk
- Other Internal Components:
- Bolts, Nuts, and Washers: Stainless steel is typically used for these components due to its corrosion resistance. 1. Stainless Steel Grades -Nuts, Bolts, Screws, Washers, Studs – Lightning Bolt & Supply Inc. lightningboltandsupply.com
- Pins and Bushings: These are often made of bronze or stainless steel for wear resistance and smooth movement.
The specific material choices might vary depending on the manufacturer, the intended application, and any specific requirements or regulations. However, prioritizing corrosion resistance, strength, and compatibility with seawater is paramount in selecting materials for these valves.
Remember, even with the use of corrosion-resistant materials, regular maintenance and inspection are crucial to ensure the longevity and proper function of globe type screw lift valves in seawater service.