Interview Questions for Understanding of ventilation systems and methane detection

Mine ventilation is all about controlling the air movement underground to create a safe and productive working environment. It’s essentially about diluting and removing harmful gases like methane, and providing enough fresh air for the miners. This is achieved by carefully designing and managing airflow through a network of airways within the mine.

The fundamental principle is to create a controlled airflow pattern that draws fresh air into the mine workings, transports it to the active faces where miners are working, and then carries out the contaminated air – removing methane and other potential hazards. This is often accomplished using a combination of fans, ventilation shafts, and strategically placed stoppings (barriers) to direct the airflow.

Think of it like a giant lung system for the mine. The intake shafts are like the nostrils bringing in fresh air, the airways are like the bronchi distributing it, and the exhaust shafts are like the mouth expelling the used air.

Several technologies are employed for methane detection in mines, each with its own strengths and weaknesses. These include:

• Flame Safety Lamps: These older technologies relied on the flame lengthening or changing color in the presence of methane. While simple, they aren’t very precise and are now largely superseded.
• Portable Methane Detectors: These are widely used and are usually based on catalytic combustion sensors or infrared (IR) sensors. Catalytic sensors measure the heat produced when methane burns, while IR sensors measure the absorption of infrared light by methane molecules. Both offer a quicker and more accurate reading than older technologies.
• Fixed Methane Detectors: These are permanently installed within the mine and continuously monitor methane levels. They often trigger alarms automatically if concentrations exceed a pre-defined threshold, alerting miners and ventilation control centers. They utilize similar sensor technologies to portable detectors but with more robust construction and often linked to a mine’s central monitoring system.
• Laser-based Methane Detectors: These offer high precision and are increasingly common. They measure methane concentration by detecting changes in the wavelength of a laser beam passing through the air.

The choice of technology often depends on factors such as cost, accuracy requirements, and the specific mining environment.

Calibrating a methane detector is crucial for ensuring its accuracy and reliability. This involves adjusting the detector’s readings to match known concentrations of methane. The process typically involves these steps:

1. Zeroing: Exposing the sensor to clean, methane-free air to establish a baseline reading of 0% methane.
2. Spanning: Exposing the sensor to a known concentration of methane (usually provided by a calibration gas cylinder) to check if the sensor’s response is within acceptable limits. The calibration gas will normally provide a precise concentration e.g., 1% methane.
3. Adjustment (if needed): If the detector’s reading during spanning differs significantly from the known concentration, adjustments may be made following the manufacturer’s instructions, often through internal settings or by adjusting calibration screws.
4. Documentation: The calibration results (date, time, zero reading, span reading) should be carefully recorded in a log book to ensure traceability and compliance.

Calibration frequency varies depending on the detector type, manufacturer recommendations, and regulatory requirements, but it’s typically carried out regularly, often monthly or even more frequently depending on usage and environmental conditions.

Safety regulations surrounding methane detection in mines are stringent and vary slightly depending on the jurisdiction. However, common elements include:

• Mandatory Methane Monitoring: Regular monitoring of methane levels is compulsory, often using both fixed and portable detectors. The frequency and locations depend on the risk assessment for the specific mine.
• Alarm Systems: Fixed detectors must be linked to audible and visual alarms that alert miners and control personnel of any dangerous methane build-up.
• Emergency Procedures: Clear procedures must be in place to evacuate the mine or implement other safety measures if methane levels exceed predetermined limits.
• Personnel Training: Miners and other personnel must be thoroughly trained on how to use, maintain, and interpret readings from methane detectors, as well as how to respond to methane emergencies.
• Regular Inspections and Maintenance: All methane detection equipment must be regularly inspected and maintained to ensure its operational readiness and accuracy.

Non-compliance with these regulations can lead to significant fines and legal consequences.

A typical mine ventilation system consists of several key components:

• Intake Airways: These are the passages that bring fresh air into the mine.
• Exhaust Airways: These passages carry the contaminated air out of the mine.
• Fans: These create the airflow, pushing or pulling air through the airways. They can be located either underground or on the surface.
• Ventilation Shafts: These vertical passages connect the surface to the underground workings, acting as either intake or exhaust paths.
• Stoppings and Regulators: These control the airflow by directing it along specific paths and adjusting its volume. Stoppings are barriers that completely block airflow; regulators partially restrict it.
• Overcasts and Undercasts: These allow airways to cross over or under each other without mixing the airflow.
• Monitoring Equipment: This includes methane detectors, anemometers (for measuring air velocity), and other instruments used to monitor ventilation parameters.

The arrangement and size of these components are determined based on the mine’s geology, layout, and ventilation requirements.

Inadequate ventilation in mines presents a wide range of serious hazards:

• Methane Buildup: A lack of sufficient airflow can lead to the accumulation of methane, creating an explosion risk. Methane is a highly flammable gas, and an explosive mixture can form when its concentration reaches a certain level in the presence of an ignition source.
• Oxygen Deficiency: Poor ventilation can reduce the oxygen levels, making it difficult or even impossible for miners to breathe. This can lead to hypoxia, a condition where the body’s tissues and organs are deprived of oxygen.
• Heat Stress: Without sufficient airflow, heat can build up in the mine, causing heat stroke and other heat-related illnesses.
• Dust Accumulation: Inadequate ventilation can also allow harmful dust particles (e.g., coal dust, silica dust) to accumulate, increasing the risk of respiratory diseases such as silicosis and coal worker’s pneumoconiosis (black lung disease).
• Harmful Gas Accumulation: Besides methane, other harmful gases can build up, like carbon monoxide or hydrogen sulfide, further endangering miners’ health.

Ensuring adequate ventilation is paramount for the safety and health of mine workers.

The primary difference between active and passive ventilation systems lies in how they create airflow:

• Active Ventilation Systems: These use mechanical devices, such as fans, to create and control airflow. This allows for precise control over the air movement, volume, and direction, making it ideal for larger and more complex mines with high ventilation demands. Most modern mines utilize active ventilation.
• Passive Ventilation Systems: These rely on natural forces, such as pressure differences between the mine’s intake and exhaust openings, to create airflow. This approach is simpler and less energy-intensive but offers less control over the ventilation pattern. Passive ventilation may be suitable for small mines or certain aspects of larger operations, but it is rarely the sole method for a significant mine.

Active ventilation systems are generally preferred due to their superior control and ability to handle varying conditions and demands.

Troubleshooting a malfunctioning methane detector involves a systematic approach. First, you must ensure the detector is powered correctly and that the sensor is clean and free from obstructions. A simple visual inspection often reveals obvious issues like a cracked sensor or a loose connection.

Next, check the calibration. Methane detectors require regular calibration to maintain accuracy. A calibration check will reveal if the detector is reading correctly compared to a known standard. If calibration fails, the sensor might need replacement or the whole unit may require servicing by a qualified technician.

If the detector is calibrated and still malfunctions, consider the environment. High humidity or extreme temperatures can affect sensor performance. Also, check for sensor poisoning – certain chemicals can interfere with the sensor’s ability to detect methane. Lastly, consult the detector’s user manual for specific troubleshooting steps and error codes.

For example, if a detector consistently reads zero, it could indicate a power issue, a faulty sensor, or a problem with the internal electronics. Conversely, consistently high readings might point to a leak or sensor contamination. Careful investigation and methodical troubleshooting are essential.

Methane diffusion in underground mines is a complex process influenced by several key factors. The primary factor is the pressure gradient; methane migrates from areas of high pressure to low pressure. This is often driven by the geological structure of the mine and the presence of permeable strata.

Airflow patterns heavily influence diffusion. A well-designed ventilation system can effectively dilute and remove methane, preventing dangerous accumulations. Conversely, stagnant air pockets can allow methane to accumulate to hazardous levels.

Temperature variations also affect diffusion. Warmer temperatures can increase the kinetic energy of methane molecules, accelerating the diffusion rate. The porosity and permeability of the surrounding rock and soil determine how easily methane can travel through the strata. Highly porous and permeable rocks allow for faster diffusion, while dense, impermeable rocks hinder methane movement.

Finally, methane concentration itself plays a role. A high concentration of methane will diffuse more rapidly than a low concentration due to the increased pressure differential.

Think of it like this: imagine a balloon filled with methane underground. The pressure inside wants to equalize with the surrounding air. The rate at which the methane escapes depends on how porous the balloon (rock strata) is, how much air is circulating, and the temperature.

Air quality monitoring in confined spaces is paramount for worker safety and health. Confined spaces, by their nature, often have limited ventilation and can accumulate harmful gases like methane, carbon monoxide, and oxygen-deficient atmospheres. Without monitoring, workers risk exposure to dangerous levels of these substances, potentially leading to injury, illness, or even death.

Monitoring allows for real-time assessment of air quality, enabling immediate action if hazardous conditions arise. Continuous monitoring provides early warning of potential problems, allowing for corrective measures like increased ventilation or evacuation. Data from monitoring devices provides valuable information for improving ventilation strategies and developing safer work practices.

For instance, in a confined space like a sewer or a storage tank, oxygen levels could become dangerously low due to displacement by other gases. Methane detection is crucial in underground mines to prevent explosions. Continuous monitoring prevents accidents by ensuring the atmosphere remains within safe limits for workers.

Ventilation plays a crucial role in controlling methane explosions by diluting the methane concentration to below the lower explosive limit (LEL). Methane, when mixed with air within a certain concentration range (typically 5-15%), becomes highly flammable and explosive.

Ventilation systems introduce fresh air into the mine, sweeping away methane accumulations and replacing them with oxygen-rich air. This dilution process keeps the methane concentration below the LEL, thereby preventing ignition and explosions. Effective ventilation systems are designed to ensure sufficient airflow to all areas of the mine, particularly in high-risk zones where methane is more likely to accumulate.

The design of the ventilation system needs to account for factors such as mine layout, methane emission rates, and the required airflow volume. Regular maintenance and monitoring of ventilation equipment is essential to ensure its effectiveness in mitigating explosion risks.

Think of it like blowing out candles: the air from your lungs disperses the methane, preventing it from forming an explosive mixture. In a mine, the ventilation system plays this crucial role on a much larger scale.

Interpreting methane concentration readings requires understanding the units and the context. Readings are often expressed as a percentage of the LEL (%LEL) or in parts per million (ppm). A reading of 0% LEL or 0 ppm indicates the absence of methane. Readings above 0% LEL represent a potentially hazardous situation, as they indicate the presence of methane.

For example, a reading of 25% LEL means the methane concentration is 25% of the lower explosive limit. This should trigger immediate action to reduce the methane concentration, potentially by increasing ventilation or evacuating the area.

It’s critical to remember that the interpretation of readings should always be considered within the specific context of the location and the safety regulations in place. Each mine will have its own safety protocols and emergency response plans triggered by specific methane concentration levels.

Different methane detection methods each have their limitations. Catalytic combustion sensors, a common type, are relatively inexpensive and reliable but are susceptible to poisoning by certain chemicals and have limited sensitivity at very low concentrations. They also require frequent calibration.

Infrared (IR) sensors are less susceptible to poisoning but are more expensive and can be affected by dust and other interfering gases. Electrochemical sensors offer good sensitivity and are relatively inexpensive, but they can also be susceptible to poisoning and have limited lifespan.

Gas chromatography is a highly accurate and sensitive method, but it is more complex, expensive, and requires specialized training and equipment. It’s not well suited for continuous real-time monitoring. Each method has its strengths and weaknesses; the choice depends on factors like cost, required sensitivity, environmental conditions, and the level of maintenance available.

Designing a ventilation system for a new mine is a complex engineering process requiring careful consideration of numerous factors. It starts with a detailed geological survey of the mine site to understand the layout, potential methane sources, and the permeability of the rock strata. This information helps determine the airflow requirements and ventilation strategy.

Next, a detailed mine ventilation model is created using specialized software. This model simulates airflow patterns under various conditions, helping optimize the placement and sizing of ventilation equipment like fans, ducts, and regulators. The design must ensure adequate airflow to all working areas, keeping methane concentrations below the LEL. It must also consider factors such as air quality, temperature control, and dust suppression.

The system should also include safety features like emergency ventilation routes and backup power supplies. Following the design phase, construction, installation, and commissioning of the system are carried out, followed by thorough testing and ongoing monitoring to ensure its effectiveness. Regular maintenance and inspections are crucial for long-term reliability and safety.

The process requires expertise in mine ventilation engineering, fluid dynamics, and safety regulations. It often involves collaboration with geologists, mining engineers, and safety personnel to ensure the ventilation system is robust and meets all safety requirements.

Ensuring personnel safety during methane detection is paramount. It’s a multi-layered approach combining robust safety protocols, advanced equipment, and rigorous training. Think of it like a three-legged stool – remove one, and the whole thing collapses.

• Pre-entry checks: Before anyone enters a potentially hazardous area, we conduct thorough atmospheric monitoring for methane using portable gas detectors. This is like checking the weather forecast before a hike – you wouldn’t go mountain climbing without knowing if a storm is brewing.
• Personal Protective Equipment (PPE): Personnel are always equipped with self-rescuers, which provide breathable air in emergency situations, and appropriate safety clothing. It’s like having a backup parachute during a skydive – crucial for survival if things go wrong.
• Ventilation management: We ensure adequate ventilation to dilute any methane present. This is akin to opening windows in a stuffy room – it prevents the build-up of harmful gases. We constantly monitor the ventilation system’s efficacy.
• Trained personnel: Only trained and certified personnel conduct methane detection and related tasks. Think of this like a pilot needing certifications before flying a plane – it’s a matter of skill, experience, and responsible conduct.
• Emergency response plan: A detailed emergency plan is in place, with clearly defined roles and responsibilities, and regular drills are conducted. This is your fire escape route – well-defined and rehearsed.

By combining these elements, we minimize the risk to personnel during methane detection and related operations.

Ventilation system failures can stem from various causes, often interconnected. Think of it as a domino effect – one problem can trigger a cascade of others.

• Equipment malfunction: Fans can fail due to mechanical wear and tear, electrical issues, or blockage. Imagine a car’s engine failing – it stops the whole system.
• Power outages: Loss of power will immediately shut down electrically powered ventilation systems, creating a hazardous situation. This is like a sudden blackout – it cuts off all power.
• Blockages in ductwork: Debris, water, or even ice accumulation can significantly reduce airflow, rendering the system ineffective. This is similar to a clogged artery – it restricts the flow of blood.
• Inadequate design: A poorly designed ventilation system might not be able to handle the specific requirements of the mining environment. It’s like building a house without considering the weather – it will be inefficient and potentially unsafe.
• Lack of maintenance: Neglecting routine maintenance increases the likelihood of equipment failures and reduced efficiency. It’s like not servicing your car – eventual breakdown is inevitable.

Regular maintenance is crucial for ventilation equipment; it’s about preventing catastrophic failures and ensuring continuous, safe operation. It’s the difference between a well-maintained car and one prone to breakdowns.

• Preventative measures: Regular inspections, lubrication, and component replacements prevent unexpected failures and extend equipment lifespan.
• Safety enhancement: A well-maintained system ensures adequate airflow, minimizing the risk of methane accumulation and other hazards.
• Efficiency gains: Regular maintenance optimizes airflow and reduces energy consumption, minimizing operational costs.
• Compliance: Maintenance schedules ensure compliance with safety regulations and industry best practices. It’s like renewing a driver’s license – a necessary step for legal operation.
• Early problem detection: Routine checks allow early detection of minor issues before they escalate into major problems, saving time and resources.

Ventilation equipment varies depending on the mine’s size and complexity. Think of it as a network – with different components working together.

• Fans: Axial flow fans create high airflow at relatively low pressure; centrifugal fans create high pressure and are more suitable for long, narrow ventilation shafts. They are like the heart of the system – pumping air through the network.
• Ducts: These transport air throughout the mine, typically made of metal or reinforced plastic, designed to withstand the mining environment. They’re the veins of the system – carrying air to different parts of the mine.
• Airlocks: These are used to control airflow between different areas of the mine and prevent the spread of methane. They are the valves controlling the air pressure and flow.
• Monitoring equipment: This includes sensors and control systems that continuously measure air quality, airflow rates, and other parameters. These act as the brain of the system – controlling and monitoring the ventilation network.
• Auxiliary ventilation systems: These can include portable fans, used for localized ventilation in specific areas or during emergency situations. These are emergency backup systems.

Ventilation management differs between coal and metal mines due to varying geological conditions and gas hazards. Coal mines have a higher risk of methane, requiring more robust ventilation strategies. Metal mines may face different challenges, such as dust and heat.

• Coal mines: High-volume, low-pressure ventilation systems are usually employed to dilute methane concentrations below the explosive limit. This is critical because methane is lighter than air and tends to accumulate at high points.
• Metal mines: Ventilation systems prioritize removing heat, dust, and other airborne contaminants. Airflow requirements are usually less stringent than in coal mines unless significant gas hazards are present.
• Specific considerations: Both types of mines need tailored ventilation plans, considering factors like the mine’s layout, depth, and geological conditions. The ventilation plan must address potential hazards specific to the mine.

Detailed airflow modeling and simulations are often used to optimize ventilation strategies for both coal and metal mines, ensuring worker safety and efficient operation.

Emergency procedures for methane leaks are critical and must be practiced regularly. Think of it as a fire drill – preparing for the worst-case scenario.

• Immediate evacuation: Personnel must immediately evacuate the affected area, following designated escape routes.
• Alarm activation: Emergency alarms should be activated to alert others in the mine.
• Ventilation adjustments: Ventilation systems should be adjusted to increase airflow in the affected area and dilute the methane concentration.
• Emergency services notification: Emergency services (e.g., mine rescue teams) should be notified immediately.
• Post-incident investigation: A thorough investigation should be conducted to determine the cause of the leak and prevent future occurrences.

Effective communication and clear roles and responsibilities during an emergency are paramount for a successful response.

Assessing methane accumulation risk involves a combination of monitoring and modeling. It’s like predicting a hurricane – you use various data points to estimate the probability and impact.

• Atmospheric monitoring: Regularly measure methane concentrations using fixed and portable gas detectors throughout the mine. This provides real-time data on methane levels.
• Ventilation system analysis: Evaluate the effectiveness of the ventilation system in diluting methane. This involves checking airflow rates, pressure differentials, and system integrity.
• Geological assessment: Understand the geological conditions of the mine, identifying potential methane sources and pathways. This helps in predicting potential accumulation areas.
• Risk modeling: Use computer models to simulate methane dispersion under various scenarios, helping to predict potential accumulation zones and identify high-risk areas. This is critical for proactive mitigation.
• Historical data analysis: Review historical methane levels and incident reports to identify patterns and trends, improving risk assessment accuracy.

By combining these methods, a comprehensive risk assessment can be developed, allowing for the implementation of appropriate safety measures.

Airflow modeling in ventilation design is crucial for predicting and optimizing airflow patterns within a mine or other enclosed space. It uses computational fluid dynamics (CFD) or simpler network models to simulate how air moves through the network of airways, considering factors like fan performance, airway resistances, and leakage. Think of it like mapping the ‘roads’ for air to travel through the mine. A good model helps us design a ventilation system that effectively removes methane and other harmful gases, while also maintaining comfortable working conditions for miners.

For example, a CFD model might show that a particular ventilation layout creates stagnant zones where methane can accumulate. This allows engineers to adjust the fan settings, add new airways, or change the location of return airways to improve airflow and reduce methane pockets. Simpler network models, using equations and network diagrams, are useful for initial design and quick analysis while CFD offers more detailed visualization for complex scenarios.

Controlling methane concentration in mines requires a multi-pronged approach. The primary strategies include:

• Ventilation: This is the most important method. By strategically placing fans and designing the ventilation network, we can dilute methane concentrations below the explosive limit. This involves carefully planning airflow patterns to ensure that methane is efficiently extracted from the working areas.
• Methane Drainage: This involves proactively extracting methane from the coal seam before it enters the mine workings. This can involve drilling boreholes to intercept and remove methane before mining begins.
• Sealing: Abandoned or inactive sections of the mine are sealed to prevent methane from migrating into working areas. This acts as a barrier to methane movement.
• Monitoring: Continuous monitoring of methane levels using sensors is essential to detect leaks or changes in concentration. This allows for timely intervention and prevents dangerous situations.
• Inerting: In some situations, inert gases like nitrogen are used to displace methane and reduce its concentration in the air. This is generally a last resort measure.

Each strategy complements the others. A robust methane control plan incorporates all these strategies depending on the mine’s specific geology and operational conditions.

Calculating airflow rates in a ventilation network involves applying basic principles of fluid mechanics, often using the Atkinson-Lambert equation or more sophisticated network analysis software. The Atkinson-Lambert equation, a simplified version, describes the relationship between pressure drop and airflow rate in a mine airway.

ΔP = KQn

Where:

• ΔP is the pressure drop across the airway
• Q is the airflow rate
• K is a resistance factor (depends on airway dimensions and roughness)
• n is an exponent (typically between 1.8 and 2, depending on the flow regime)

For complex networks with multiple splits and branches, we use specialized ventilation network analysis software that solves the network’s equations simultaneously to determine airflow rates in each branch. These programs consider fan performance curves, airway resistances, and other factors to provide a more realistic simulation. The inputs required include airway dimensions, roughness factors, fan characteristics, and boundary conditions (e.g., pressure at the intake and exhaust).

Legal requirements for methane monitoring and reporting vary considerably depending on the jurisdiction. However, common threads include:

• Regular Monitoring: Frequent monitoring of methane levels in all working areas, often using fixed and portable sensors. The frequency is usually specified by the regulations and depends on the risk level of the mine.
• Reporting: Detailed records of methane measurements must be maintained and reported to the relevant regulatory authority. This includes reporting any significant methane incidents or exceedances of pre-defined thresholds.
• Emergency Procedures: Mines need established emergency procedures to address methane emergencies. These procedures should detail how to safely evacuate the mine, shut down equipment, and deal with potential explosions.
• Training: Mine personnel must receive training on methane hazards, monitoring procedures, and emergency response protocols.
• Ventilation Plans: A detailed ventilation plan is required, detailing the layout and control of the ventilation system. This plan must be reviewed and updated regularly.

It is crucial to stay up-to-date with the specific legal requirements in the region where the mine operates, as these regulations are constantly reviewed and updated to improve safety.

Instrumentation and control systems are fundamental to effective ventilation management. They provide the means to monitor key parameters, make adjustments, and maintain safe working conditions. These systems typically include:

• Methane sensors: These continuously monitor methane concentrations at various points throughout the mine, providing real-time data to the control system.
• Airflow sensors: These measure airflow rates in key airways, allowing for the verification of the ventilation plan’s effectiveness.
• Pressure sensors: These monitor pressure differentials within the ventilation network, aiding in identifying blockages or leaks.
• Fan controllers: These allow for remote control of ventilation fans, enabling adjustments to airflow rates based on methane levels or other operational requirements.
• Supervisory Control and Data Acquisition (SCADA) Systems: These central systems integrate data from all sensors and controllers, providing a comprehensive overview of the ventilation system’s status and allowing for remote monitoring and control.

Data from these systems are used for both real-time control and historical analysis, helping to optimize ventilation strategies and identify potential problems before they escalate.

Ensuring compliance involves a proactive and multi-faceted approach. Key aspects include:

• Regular Inspections: Regular inspections of the ventilation system and monitoring equipment are crucial to detect any issues or potential problems early. This often involves both internal and external audits.
• Calibration and Maintenance: All instrumentation and control systems must undergo regular calibration and maintenance to ensure accuracy and reliability. This ensures the data used for decision-making is trustworthy.
• Training and Competency: Personnel involved in ventilation management and monitoring must receive adequate training and demonstrate competency in their roles. This includes understanding safety regulations and procedures.
• Documentation: Maintaining complete and accurate records of all monitoring data, inspections, maintenance, and training is essential for demonstrating compliance. This record-keeping is vital for audits and investigations.
• Staying Updated: Keeping abreast of changes in relevant safety standards and regulations is paramount. This requires regular review of updates from regulatory bodies and industry best practices.

Compliance is not a one-time event, but an ongoing process requiring vigilance and commitment to maintaining safe working conditions.

I’ve had experience troubleshooting several complex ventilation problems, including situations where unexpected methane releases occurred. One memorable case involved a mine where a newly opened section experienced a sudden increase in methane concentration despite seemingly adequate ventilation. Through meticulous analysis of the ventilation network data, combined with on-site investigations, we identified a previously unknown geological fault that created a pathway for methane to enter the mine workings. This was not reflected in the initial geological model. We implemented several solutions, including re-routing the airflow to divert away from the fault and installing additional ventilation equipment in the affected area. We also improved the geological survey data to avoid future problems in similar scenarios. Thorough investigation, data analysis, and a multi-disciplinary approach were key to resolving this situation safely and effectively.

Another example involved a situation where a critical fan malfunctioned, leading to a disruption in the airflow patterns and a rise in methane levels in a section of the mine. Our quick response, utilising backup systems and implementing emergency ventilation procedures, successfully mitigated the situation until the main fan was repaired. This highlighted the importance of redundant systems and well-rehearsed emergency procedures.