With reference to main air receivers:
(a) list FOUR safety features, stating the purpose of EACH:(8)
(b) state the pressure at which the relief valve should lift.(2)
What is a main air receiver?
A main air receiver on a marine vessel is a large, pressure-resistant vessel that acts as a storage tank for compressed air. It’s a critical component of the ship’s compressed air system, serving several essential functions:
1. Understanding Air Receiver Tanks as Pressure Vessels – Red River
- Storage of Compressed Air:
- Main Air Compressor Supply: The main air receiver receives compressed air from the ship’s main air compressor. 1. Ship’s Service Air System – TSPS Engineering Manual – Massachusetts Maritime Academy weh.maritime.edu
- High-Pressure Storage: It stores this air at a high pressure, typically around 20-30 bar, ready for use when needed.
- Engine Starting:
- Starting Air Reservoir: The primary purpose of the main air receiver is to store enough compressed air to start the main engine and auxiliary engines multiple times without needing to run the compressor.
- Instantaneous Demand: Starting engines requires a sudden and large volume of high-pressure air, which the receiver can readily provide.
- Other Applications:
- Control Air Supply: Besides engine starting, the main air receiver can also supply air to other pneumatic systems on board, such as control valves, engine control mechanisms, and various pneumatic tools. However, the pressure is often reduced using pressure-reducing valves for these applications.
- Emergency Backup: In some cases, the main air receiver might serve as an emergency backup air supply for critical systems.
Key Features and Design:
- Construction: Main air receivers are typically made of welded steel and designed to withstand high pressures.
- Safety Features: They are equipped with:
- Pressure Gauge: To monitor the internal pressure. 1. What is an air receiver used for? – Industrial Air Systems NZ www.industrialair.co.nz
- Safety Relief Valve: To release excess pressure in case of a malfunction or overcharging.
- Drain Valve: To remove condensate (water) that accumulates inside the receiver.
- Manhole or Inspection Openings: For internal inspection and cleaning.
Number and Capacity:
- Multiple Receivers: Most vessels have at least two main air receivers to ensure redundancy and sufficient air storage capacity.
- Capacity: The total capacity of the receivers is determined based on the engine starting requirements and other pneumatic system demands. Regulations like SOLAS often specify the minimum required capacity for main air receivers.
Importance:
- Engine Starting: The main air receiver is vital for starting the ship’s engines, which is critical for propulsion, power generation, and other essential operations. 1. Main Air Reservoir Tank | WOO GYEONG – Komarine www.komarine.com
- System Stability: It helps maintain stable air pressure in the system, preventing fluctuations that could affect the performance of pneumatic equipment. 1. What Is an Air Receiver? | Anglian Compressors angliancompressors.com
- Emergency Preparedness: It provides a backup air supply in case of compressor failure or other emergencies.
In summary, the main air receiver is a crucial component of a vessel’s compressed air system, responsible for storing high-pressure air primarily for engine starting but also for various other pneumatic applications on board.
Safety Features
Let’s dive deeper into the safety features typically found on a main air receiver:
- Pressure Gauge:
- Function: It provides a visual indication of the pressure inside the air receiver. Crew members can monitor the pressure to ensure it remains within safe operating limits and to identify any potential issues like overcharging or leaks. 1. What is a Pressure Gauge? | Tameson.com tameson.com
- Placement: Typically mounted on the receiver’s top or side, easily visible for regular checks.
- Accuracy and Calibration: The pressure gauge should be regularly calibrated and maintained to ensure accurate readings. 1. Accuracy in Pressure Gauges & Calibration Guide – GIC – General Instruments Consortium generalinstruments.co.in
- Safety Relief Valve:
- Function: It’s a spring-loaded valve designed to open automatically and release excess pressure if the pressure in the receiver exceeds a preset safety limit. This prevents overpressure, which could lead to catastrophic failure and potential explosion of the receiver. 1. Machinery and Device Safety – University of North Florida www.unf.edu
- Set Pressure: The relief valve is set to open at a pressure slightly above the normal operating pressure of the receiver, providing a safety margin.
- Discharge Piping: The valve is connected to a discharge pipe that safely vents the released air to the atmosphere or a designated safe location.
- Testing and Maintenance: The relief valve should be regularly tested and inspected to ensure it functions correctly and opens at the correct pressure setting. 1. Safety Relief Valves: The Pressure Is On – Rasmussen Mechanical Services www.rasmech.com
- Drain Valve:
- Function: It allows for the removal of condensate (water) that accumulates inside the air receiver.
- Moisture Removal: Compressed air naturally contains moisture, which can condense inside the receiver due to temperature changes. This condensate can cause corrosion, damage pneumatic equipment, and freeze in cold climates. 1. Water in Compressed Air Calculations – Air Compressor Guide www.air-compressor-guide.com
- Manual or Automatic: The drain valve can be either manually operated or automatic (timed or float-operated). 1. How can Automatic Drain Valves Improve the Efficiency of Your Compressed Air System? www.elgi.com
- Regular Draining: Regular draining is crucial to prevent moisture-related problems.
- Fusible Plug (if applicable):
- Function: A fusible plug is a secondary safety device that melts at a specific temperature, typically higher than the relief valve’s set point.
- Backup Protection: It acts as a backup safety measure in case the relief valve fails to operate, preventing overpressure by releasing air when the temperature inside the receiver reaches a critical level.
- Placement: Usually located on the top of the receiver.
- Replacement: Once a fusible plug has melted, it needs to be replaced.
- Manhole or Inspection Openings:
- Function: These openings allow access to the interior of the receiver for inspection, cleaning, and maintenance.
- Internal Inspection: Periodic inspections are necessary to check for corrosion, debris buildup, or any damage to the internal surfaces of the receiver.
- Cleaning: The receiver should be cleaned periodically to remove any accumulated rust, scale, or other contaminants that could affect the air quality or damage pneumatic equipment.
Other Safety Features:
- Low-Pressure Alarm: An alarm that sounds when the pressure in the receiver drops below a certain level, indicating a potential problem with the compressor or air leaks in the system. 1. Low Pressure Alarm: 25–150 psi | Nuvair nuvair.com
- High-Temperature Alarm: An alarm that activates if the temperature inside the receiver exceeds a safe limit, warning of potential overheating or fire hazards.
These safety features work together to ensure the safe operation of the main air receiver, preventing overpressure, managing condensate, and allowing for periodic inspection and maintenance.
Set Pressure
The pressure at which a relief valve should lift (its set pressure) depends on the specific system it’s protecting and its design parameters. However, some general guidelines and considerations apply:
General Rule:
- The relief valve’s set pressure should be higher than the normal operating pressure of the system but lower than the maximum allowable working pressure (MAWP) of the weakest component in the system.
Specifics for Fuel Supply Pump Systems:
- Typical Set Pressure: In most fuel supply pump systems, the relief valve is set to open at a pressure around 10-15% above the normal operating pressure of the pump. This provides a safety margin while preventing unnecessary activation of the relief valve.
- Manufacturer’s Recommendations: It’s crucial to consult the pump and system manufacturer’s recommendations for specific pressure settings and tolerances.
- Regulatory Requirements: Some classification societies or flag state regulations may also specify minimum or maximum relief valve set pressures for certain systems.
Example:
If a fuel supply pump operates at a normal pressure of 5 bar, the relief valve might be set to open at around 5.5 to 5.75 bar.
Factors Influencing Set Pressure:
- System Design: The MAWP of the weakest component in the system is a key factor in determining the relief valve set pressure.
- Fluid Properties: The properties of the fluid being pumped, such as its compressibility and temperature, can also influence the set pressure.
- Flow Rate: The relief valve must have sufficient flow capacity to handle the maximum potential flow in case of overpressure.
Importance of Correct Set Pressure:
- Too Low: If the set pressure is too low, the relief valve will open frequently during normal operation, causing unnecessary pressure fluctuations, energy waste, and potential system instability.
- Too High: If the set pressure is too high, the valve might not open in time to prevent overpressure, leading to potential damage or even catastrophic failure of the system.
Testing and Maintenance:
- Periodic Testing: Relief valves should be periodically tested to ensure they open at the correct set pressure and have sufficient flow capacity. 1. How Often Should Pressure Relief Valves Be Tested? www.everlastingvalveusa.com
- Calibration and Adjustment: The set pressure might need to be adjusted over time due to wear or changes in system conditions.
In Conclusion:
The correct set pressure for a relief valve is critical for ensuring the safety and proper function of the system it protects. It’s essential to follow manufacturer’s recommendations, regulatory requirements, and sound engineering practices when determining and setting the relief valve pressure.