(a) Explain the circumstances under which EACH of the following devices fitted to an air compressor may operate;
(1) fusible plug;(4)
(ii) bursting disc.(4)
(b) State where EACH device in part (a) may be fitted.(2)
Question 3.
Although the question asks about air compressors, many answer concerning fusible plugs on receivers. Few actually give the consequences of the devices not being present / not operating.
What is a fusible plug?
A fusible plug is a safety device incorporated into certain pressurized systems, like air compressors and boilers. It functions as a temperature-sensitive plug designed to melt and release pressure if the system’s temperature exceeds a predetermined safe limit.
1. Fusible Plugs Explained – saVRee
2. Fusible Plug | SKYbrary Aviation Safety
Key Components:
- Body: The main body of the plug is typically made of a metal with a relatively high melting point, like brass, bronze, or gunmetal. 1. Fusible plug – Wikipedia en.wikipedia.org
- Core or Filling: A tapered hole is drilled through the body, filled with a metal alloy with a lower melting point, often tin or a tin alloy. 1. Fusible plug – Wikipedia en.wikipedia.org
Working Principle:
- Normal Operation: Under normal operating conditions, the fusible plug remains intact, maintaining the integrity of the pressurized system.
- Overheating: If the system’s temperature rises above the melting point of the core material due to a malfunction or other issue, the core melts. 1. Fusible plug – Wikipedia en.wikipedia.org
- Pressure Release: The melted core creates an opening in the plug, allowing the pressurized fluid (air, steam, etc.) to escape, thus preventing excessive pressure buildup and potential explosions. 1. Main Function of Fusible Plug in Boiler www.thermodyneboilers.com
Applications:
- Air Compressors: Installed in the air discharge line or receiver to protect against overpressure and fire hazards resulting from overheating.
- Boilers: Fitted in the crown sheet (top of the firebox) to protect against low water levels, which can lead to overheating and potential boiler explosions. 1. Fusible plug – Wikipedia en.wikipedia.org
- Other Pressurized Systems: Used in other systems where over-temperature can be a safety concern, such as certain tanks or pressure vessels. 1. Fusible plug – Wikipedia en.wikipedia.org
Advantages:
- Simple and Reliable: It’s a simple, passive device with no moving parts, making it reliable and requiring minimal maintenance.
- Failsafe: It operates even if the primary pressure relief valve fails, providing an additional layer of safety.
- Cost-Effective: It’s a relatively inexpensive safety device.
Disadvantages:
- Single-Use: Once a fusible plug melts, it needs to be replaced. 1. A Fusible Plug: Safeguarding Steam Boilers and Preventing Catastrophic Failures www.thermodyneboilers.com
- Limited to Temperature Sensing: It doesn’t respond to overpressure unless it’s caused by overheating.
- Potential for False Alarms: In rare cases, a fusible plug might melt due to localized overheating or impurities in the core material, even if the overall system is not in a dangerous condition.
Key Points to Remember:
- Melting Point: The melting point of the fusible plug’s core material is carefully selected based on the system’s operating conditions and safety requirements.
- Installation Location: The plug is strategically placed in a location where it will be exposed to the highest temperatures in the system, ensuring it acts promptly in case of overheating. 1. Main Function of Fusible Plug in Boiler www.thermodyneboilers.com
- Regular Inspection: Fusible plugs should be inspected regularly for signs of corrosion or damage and replaced if necessary.
Overall, fusible plugs are vital safety components in many pressurized systems. They provide a last line of defense against overpressure and overheating, protecting equipment, personnel, and the environment from potential catastrophic events.
1. A Fusible Plug: Safeguarding Steam Boilers and Preventing Catastrophic Failures
The circumstances under which a fusible plug fitted to an air compressor may operate
A fusible plug fitted to an air compressor is a safety device designed to operate under specific circumstances, primarily when there’s a risk of overpressure and overheating in the compressor’s discharge system. Let’s explore these circumstances in more detail:
1. Overpressure:
- Primary Safety Valve Failure: The fusible plug acts as a secondary safety mechanism in case the primary pressure relief valve fails to operate correctly. If the pressure in the compressor’s discharge line or air receiver exceeds a critical level, the fusible plug melts, creating an opening to vent the excess pressure and prevent a catastrophic explosion.
2. High Discharge Temperature:
- Ineffective Cooling: The fusible plug also responds to high temperatures. If the compressor’s cooling system malfunctions or becomes inadequate (e.g., due to a blockage or low coolant flow), the compressed air temperature can rise significantly.
- Fire Risk: In extreme cases, the high temperature could lead to auto-ignition of lubricating oil or other flammable materials in the compressor, posing a fire or explosion hazard. The fusible plug melts at a predetermined temperature, venting the hot air and reducing the risk of fire.
Typical Operating Temperature:
- Fusible Plug Design: Fusible plugs are designed to melt at a specific temperature, usually higher than the set point of the primary pressure relief valve. This ensures the plug only operates in critical situations where the primary safety mechanism has failed.
- Common Temperature Range: The melting point of fusible plugs in air compressors typically ranges from 100°C to 120°C (212°F to 248°F).
Consequences of Fusible Plug Operation:
- Loss of Compressed Air: When the fusible plug melts, the compressed air is vented to the atmosphere, resulting in a loss of pressure in the system. This might lead to a temporary shutdown of pneumatic equipment or engine starting capabilities.
- Compressor Shutdown: In some systems, the fusible plug’s operation might trigger an automatic shutdown of the compressor to prevent further overheating or damage.
- Maintenance and Replacement: The fusible plug needs to be replaced after it has melted. The underlying cause of the overpressure or overheating should also be investigated and rectified before resuming operation.
Importance of Fusible Plugs:
- Ultimate Safety Measure: Fusible plugs provide an additional layer of safety, acting as a last resort to prevent catastrophic overpressure or fire hazards in air compressor systems.
- Critical for Unattended Operation: They are especially important in systems that operate unattended for extended periods, where immediate manual intervention might not be possible.
In summary, a fusible plug operates when the pressure or temperature in the air compressor’s discharge system exceeds its design limits, primarily due to the failure of the primary relief valve or inadequate cooling. By venting the excess pressure or hot air, the fusible plug helps prevent severe damage, fire, or explosions, ensuring the safety of the vessel and its crew.
What is a bursting disc?
A bursting disc, also known as a rupture disc, is a safety device designed to protect pressurized systems from overpressure by rupturing (bursting) at a predetermined pressure. It acts as a one-time-use membrane that fails when the pressure exceeds its designed limit, allowing the pressure to be safely released.
1. What is a rupture disc? – OsecoElfab blog post
Key Characteristics:
- Non-Reclosing: Once a bursting disc ruptures, it cannot be resealed and must be replaced. 1. Rupture disc – Wikipedia en.wikipedia.org
- Precise Burst Pressure: It is manufactured with a specific burst pressure rating, ensuring it will rupture reliably at the desired pressure. 1. Specification of Rupture Disk Burst Pressure www.nationalboard.org
- Fast Response: Bursting discs offer a very quick response time to overpressure, often within milliseconds, making them ideal for protecting against rapid pressure surges. 1. Rupture disc – Wikipedia en.wikipedia.org
- Wide Range of Materials: They can be made from various materials, including metals (stainless steel, nickel alloys), graphite, and even plastics, depending on the application and the properties of the fluid being handled. 1. Rupture Disc Wiki – REMBE® GmbH Safety+Control rembe.com
Working Principle:
- Normal Operation: Under normal operating conditions, the bursting disc remains intact, forming a barrier within the system and containing the pressurized fluid.
- Overpressure: If the pressure in the system exceeds the bursting disc’s rated pressure, the disc ruptures, creating an opening. 1. Pressure Relief Glossary – OsecoElfab www.osecoelfab.com
- Pressure Release: The pressurized fluid is released through the opening, relieving the pressure in the system and preventing potential damage or catastrophic failure. 1. How does a rupture disc work? – YouTube www.youtube.com
Advantages:
- Fast Response: Provides near-instantaneous pressure relief, critical in situations where rapid pressure surges can occur. 1. Rupture disc – Wikipedia en.wikipedia.org
- Accurate Burst Pressure: The precise burst pressure rating ensures reliable and predictable performance.
- Leak-Tight: When intact, the bursting disc provides a leak-tight seal, preventing any fluid leakage from the system.
- Wide Range of Applications: Suitable for various industries and applications handling different fluids and operating conditions.
Disadvantages:
- Non-Reclosing: Once ruptured, the disc must be replaced, leading to system downtime and maintenance costs. 1. How Long Will a Rupture Disk Last zookdisk.com
- Limited Reusability: Unlike relief valves that can reseal after opening, bursting discs are single-use devices. 1. When To Use Safety Relief Valve vs Rupture Disc www.steriflowvalve.com
- Potential for Fatigue: In applications with cyclic pressure fluctuations, the bursting disc may experience fatigue and rupture prematurely.
Applications on Marine Vessels:
- Air Compressors: Protecting air receivers and discharge lines from overpressure.
- Hydraulic Systems: Safeguarding hydraulic power units and accumulators.
- Fuel Systems: Protecting fuel tanks and piping from overpressure.
- Other Pressurized Systems: Used in various other systems where overpressure protection is required, such as boilers, reactors, and storage tanks.
In Summary:
A bursting disc is a reliable and fast-acting safety device that protects pressurized systems from overpressure by rupturing at a predetermined pressure. Its precise burst pressure, quick response, and leak-tight seal make it a valuable component in various marine and industrial applications.
The circumstances under which a bursting disc fitted to an air compressor may operate
A bursting disc fitted to an air compressor is a safety device designed to operate under specific circumstances, primarily when there is a risk of overpressure in the compressor’s system. Here are the main scenarios in which it might operate:
1. Overpressure in the Air Receiver:
- Primary Safety Valve Failure: If the primary pressure relief valve on the air receiver malfunctions or fails to open, the pressure inside the receiver could continue to rise to dangerous levels. The bursting disc acts as a secondary safety measure, rupturing at a predetermined pressure slightly higher than the relief valve’s set point. This releases the excess pressure and prevents a potential explosion of the receiver.
2. Overpressure in the Discharge Line:
- Blockage or Restriction: If there’s a blockage or significant restriction in the compressor’s discharge line, the pressure can build up rapidly. The bursting disc, installed downstream of the compressor, will rupture if the pressure exceeds its designed limit, protecting the compressor and other components from damage.
3. Intercooler or Aftercooler Failure:
- Tube Rupture: If a tube within the intercooler or aftercooler ruptures, high-pressure air can leak into the water side, potentially causing the water jacket or shell to rupture due to overpressure. A bursting disc on the water side of the intercooler/aftercooler will rupture first, releasing the pressure and preventing catastrophic damage to the heat exchanger.
4. Other Malfunctions:
- Compressor Control System Failure: If the compressor’s control system malfunctions and fails to stop the compressor at the desired pressure, the bursting disc will act as a final safety measure to prevent overpressure in the system.
- External Factors: In rare cases, external factors like fire or impact damage could lead to a rapid pressure increase in the system, causing the bursting disc to rupture.
Key Points to Remember:
- One-Time Use: Once a bursting disc ruptures, it cannot be resealed and must be replaced. This necessitates system shutdown and maintenance to install a new disc.
- Precise Burst Pressure: Bursting discs are manufactured with specific burst pressure ratings to ensure they operate reliably at the desired pressure.
- Fast Response: They offer a very rapid response to overpressure, often within milliseconds, making them effective in protecting against sudden pressure surges.
- Preventative Maintenance: Regular inspection and replacement of bursting discs according to manufacturer recommendations are crucial to ensure their continued effectiveness as a safety device.
In essence, a bursting disc acts as a last line of defense against overpressure in air compressor systems, providing critical protection in scenarios where other safety mechanisms might fail or be overwhelmed. It sacrifices itself to prevent catastrophic damage to the compressor, piping, and other components, ultimately ensuring the safety of the vessel and its crew.
Where might a fusible plug and a bursting disc fitted to an air compressor be fitted?
In an air compressor system, both fusible plugs and bursting discs are strategically located to provide overpressure protection in different areas. Here’s where they might typically be fitted:
Fusible Plug:
- Air Discharge Line: Often fitted on the underside of the air discharge line, close to the compressor’s outlet. This allows it to sense high air temperatures directly from the compressor, which could indicate overheating or a potential fire hazard within the compressor itself. 1. Fusible Plugs – Bonut Engineering Ltd bonut.co.uk
- Air Receiver: May also be installed on the air receiver, typically near the top, to protect against overpressure caused by a malfunctioning relief valve or other system failures. It senses the temperature of the air inside the receiver, triggering if it rises to a dangerous level.
Bursting Disc:
- Air Receiver: Commonly fitted directly on the air receiver, often in conjunction with the pressure relief valve. It acts as a secondary safety measure, rupturing if the pressure exceeds the relief valve’s set point, preventing a catastrophic explosion of the receiver.
- Discharge Line (Aftercooler): May be installed on the water side (coolant side) of the aftercooler to protect against overpressure caused by a ruptured tube in the heat exchanger. This prevents damage to the aftercooler and potential water leakage into the compressed air system.
- Other High-Pressure Sections: Depending on the compressor system’s design and complexity, bursting discs might also be installed in other high-pressure sections of the piping or on specific components like oil separators or filters to protect against overpressure.
Key Points to Remember:
- Redundancy: The combination of a pressure relief valve and a bursting disc or fusible plug provides redundancy in overpressure protection. If the primary relief valve fails, the secondary device activates to prevent a catastrophic event. 1. Rupture disc – Wikipedia en.wikipedia.org
- Material Compatibility: The materials used for the fusible plug or bursting disc must be compatible with the compressed air and operating temperatures to ensure their reliability and prevent corrosion.
- Regular Inspection and Replacement: These safety devices should be regularly inspected and replaced according to manufacturer recommendations to ensure they remain effective in protecting the system.
In essence, the placement of fusible plugs and bursting discs is strategically determined to safeguard critical components and prevent overpressure situations in an air compressor system. Their presence provides an extra layer of safety, ensuring the protection of the vessel, its crew, and the surrounding environment.Sources and related content