The correct answer is that the technician should prioritize inspecting the pressure relief valve (PRV) for proper operation and venting capacity, and verify its compliance with NFPA 52 standards. This is because a malfunctioning PRV is a critical safety hazard in a CNG system. If the PRV is not functioning correctly, over-pressurization of the CNG tank could occur, potentially leading to a catastrophic rupture. NFPA 52 outlines specific requirements for PRV sizing, testing, and maintenance to prevent such occurrences. Ignoring the PRV and focusing solely on leak checks or other components could leave a critical safety vulnerability unaddressed. While leak checks and component inspections are important, the immediate safety of the system relies on the PRV’s ability to vent excess pressure. Furthermore, even if the pressure regulator seems to be the immediate cause of the overpressure situation, the PRV is the last line of defense, and its functionality must be verified before returning the vehicle to service. The technician must ensure the PRV is not blocked, corroded, or otherwise compromised.

The question addresses the function of an excess flow valve (EFV) in a CNG fuel system. An EFV is a critical safety device designed to automatically shut off the flow of CNG in the event of a sudden and significant increase in flow rate, such as that caused by a ruptured fuel line. This prevents a large and uncontrolled release of CNG, which could lead to a fire or explosion. The EFV does not regulate pressure; that’s the function of the pressure regulator. It also doesn’t vent gas under high-pressure conditions; that’s the role of the TPRD (Temperature and Pressure Relief Device). The EFV’s sole purpose is to stop the flow of gas when it exceeds a predetermined threshold, indicating a line break or other major leak. The valve operates mechanically, typically using a spring-loaded mechanism that is sensitive to flow rate.

The critical aspect here is understanding how temperature affects pressure within a closed CNG cylinder, governed by the Gay-Lussac’s Law (a specific case of the ideal gas law where volume and number of moles are constant). The formula is: \[\frac{P_1}{T_1} = \frac{P_2}{T_2}\] where \(P_1\) is the initial pressure, \(T_1\) is the initial temperature, \(P_2\) is the final pressure, and \(T_2\) is the final temperature. Temperatures must be in absolute units (Kelvin or Rankine).

First, convert Celsius to Kelvin:
\(T_1 = 20^\circ C + 273.15 = 293.15 K\)
\(T_2 = 65^\circ C + 273.15 = 338.15 K\)

Now, solve for \(P_2\):
\[P_2 = P_1 \cdot \frac{T_2}{T_1}\]
\[P_2 = 2400 \text{ psi} \cdot \frac{338.15 K}{293.15 K}\]
\[P_2 = 2400 \text{ psi} \cdot 1.1535\]
\[P_2 = 2768.4 \text{ psi}\]

Therefore, the pressure inside the cylinder at \(65^\circ C\) is approximately 2768.4 psi. This calculation highlights the direct relationship between temperature and pressure in a closed system. It’s important to consider this pressure increase to ensure that it does not exceed the safety limits of the cylinder and related components. The increased pressure can cause a rupture in the cylinder if the pressure exceeds the design limit, and temperature changes can also affect the readings from pressure and temperature sensors within the CNG system, potentially causing inaccurate fuel delivery.

A Type 4 CNG cylinder is constructed with a non-metallic liner, typically made of plastic (such as high-density polyethylene – HDPE or polyamide), overwrapped with a carbon fiber composite. The plastic liner acts as a gas-tight barrier, preventing CNG leakage, while the carbon fiber composite provides the structural strength to withstand the high pressures (typically 3000-3600 psi or 200-250 bar) associated with CNG storage. The composite material is designed to bear the load, making the cylinder lighter than all-metal cylinders. The non-metallic liner is not designed to handle structural loads. A metallic liner, like steel or aluminum, would be characteristic of Type 1, Type 2, or Type 3 cylinders, respectively. The service life of a CNG cylinder is determined by factors such as material properties, operating conditions, and regulatory standards (e.g., FMVSS 304). Regular inspections, including visual and hydrostatic testing, are essential to ensure the cylinder’s integrity and safety throughout its service life. The composite overwrap is the key structural component, bearing the pressure load.

The question pertains to the proper inspection and hydrostatic testing requirements for CNG cylinders, referencing the specific standards outlined in 49 CFR § 180.205. This regulation dictates the periodic requalification (hydrostatic testing) of CNG cylinders based on their construction type and service history. Type 1 cylinders, typically made entirely of steel or aluminum, generally require hydrostatic testing every 5 years. However, cylinders used exclusively for dedicated CNG vehicle service and meeting specific performance criteria may qualify for a 10-year retest interval, provided they pass rigorous visual inspections in the intervening years. Type 2, 3, and 4 cylinders, which incorporate composite materials, often have different requalification schedules, typically 3 or 5 years, and are subject to more stringent visual inspection criteria due to the potential for damage to the composite layers. The key is determining if the cylinder meets the criteria for extended testing intervals. Factors include the original cylinder certification, dedicated CNG service, and successful completion of all required visual inspections. Since the cylinder is a Type 1 and dedicated to CNG service, it needs to meet the requirements for the 10 year retest interval. If it doesn’t, it must be tested in 5 years interval.

To calculate the required relief valve flow capacity, we use the Compressed Gas Association (CGA) S-1.3 standard formula for sizing pressure relief devices for CNG cylinders. This formula considers several factors including the total surface area of the cylinder, the gas constant for nitrogen (since the formula is based on nitrogen venting), the absolute temperature, and a constant related to the gas properties of CNG.

The formula is:
\[Q = C \cdot A \cdot \sqrt{\frac{ZT}{M}}\]
Where:
– \(Q\) is the required flow rate in SCFM (Standard Cubic Feet per Minute).
– \(C\) is a constant, typically 555 when using nitrogen equivalent flow.
– \(A\) is the external surface area of the cylinder in square feet.
– \(Z\) is the compressibility factor of the gas (CNG), which we’ll assume is 1 for simplicity.
– \(T\) is the absolute temperature in degrees Rankine (\(^{\circ}R\)). To convert Fahrenheit to Rankine, use the formula \(^{\circ}R = ^{\circ}F + 460\).
– \(M\) is the molecular weight of the gas (nitrogen equivalent), which is 28.97 lb/mol.

First, convert the temperature from Fahrenheit to Rankine:
\[T = 120^{\circ}F + 460 = 580^{\circ}R\]

Next, calculate the surface area of the cylinder. The surface area \(A\) of a cylinder is given by:
\[A = 2\pi r h + 2\pi r^2\]
Where \(r\) is the radius and \(h\) is the height. Given the diameter is 16 inches, the radius \(r\) is 8 inches or \(8/12\) feet. The height \(h\) is 60 inches or \(60/12\) feet.
\[A = 2\pi \left(\frac{8}{12}\right) \left(\frac{60}{12}\right) + 2\pi \left(\frac{8}{12}\right)^2\]
\[A = 2\pi \left(\frac{2}{3}\right) (5) + 2\pi \left(\frac{4}{9}\right)\]
\[A = \frac{20\pi}{3} + \frac{8\pi}{9} = \frac{60\pi + 8\pi}{9} = \frac{68\pi}{9} \approx 23.76 \, \text{ft}^2\]

Now, plug the values into the flow rate formula:
\[Q = 555 \cdot 23.76 \cdot \sqrt{\frac{1 \cdot 580}{28.97}}\]
\[Q = 555 \cdot 23.76 \cdot \sqrt{\frac{580}{28.97}}\]
\[Q = 555 \cdot 23.76 \cdot \sqrt{20.02}\]
\[Q = 555 \cdot 23.76 \cdot 4.47\]
\[Q \approx 59078.8 \, \text{SCFM}\]

Rounding to the nearest hundred, the required relief valve flow capacity is approximately 59,100 SCFM.

The correct procedure involves isolating the CNG fuel system by closing the manual shut-off valve on the CNG cylinder. This prevents further gas flow into the system. Then, the engine is started and allowed to run until it stalls due to fuel starvation, purging the lines of CNG. After the engine stalls, attempt to restart the engine a few times to ensure all residual CNG is consumed. Finally, disconnect the negative battery cable to prevent any accidental ignition during component removal. This ensures the system is safely depressurized and electrically isolated before any maintenance work begins. Simply disconnecting the battery without purging the system leaves high-pressure CNG in the lines, posing a significant safety risk. Opening the manual shut-off valve after stalling the engine would re-pressurize the system, negating the purging process. Venting the CNG directly into the atmosphere is illegal and environmentally irresponsible, violating EPA regulations. This procedure prioritizes safety by eliminating both fuel and ignition sources before commencing any repair work.

A Type 3 CNG cylinder is a hybrid design. It consists of a thin metallic liner, usually made of aluminum, which provides a gas-tight barrier. This liner is then overwrapped with composite materials, typically carbon fiber, glass fiber, or aramid fiber, bonded by a resin. The composite overwrap bears the majority of the structural load and provides strength. The aluminum liner primarily serves as a permeation barrier to prevent gas leakage. The composite overwrap is not designed to be gas-tight on its own. Regulations and standards like ANSI NGV2 and CSA B51 dictate rigorous testing procedures for CNG cylinders, including burst testing, to ensure they can withstand pressures significantly higher than their service pressure (typically 3600 psi or 24.8 MPa). These tests confirm the structural integrity of the composite overwrap and the leak-tightness of the liner under extreme conditions. Damage to the composite overwrap can compromise the cylinder’s structural integrity, even if the aluminum liner remains intact initially. The composite materials are susceptible to impact damage, abrasion, and degradation from UV exposure or chemical attack, which can lead to premature failure.

To determine the pressure drop across the fuel filter, we first need to calculate the flow rate of CNG required by the engine at the specified conditions. The engine’s power output is 150 horsepower, and we’re given that 1 horsepower requires 9,000 BTU/hour. The heating value of CNG is 1,000 BTU/cubic foot. Therefore, the CNG flow rate can be calculated as follows:

Total BTU/hour required = \(150 \text{ hp} \times 9,000 \frac{\text{BTU}}{\text{hp} \cdot \text{hour}} = 1,350,000 \frac{\text{BTU}}{\text{hour}}\)

CNG flow rate in cubic feet per hour = \(\frac{1,350,000 \frac{\text{BTU}}{\text{hour}}}{1,000 \frac{\text{BTU}}{\text{cubic foot}}} = 1,350 \frac{\text{cubic feet}}{\text{hour}}\)

CNG flow rate in cubic feet per minute (CFM) = \(\frac{1,350 \frac{\text{cubic feet}}{\text{hour}}}{60 \frac{\text{minutes}}{\text{hour}} } = 22.5 \text{ CFM}\)

Now, we use the pressure drop formula: \(\Delta P = K \times Q^2\), where \(K\) is the resistance coefficient and \(Q\) is the flow rate. We are given \(K = 0.05 \frac{\text{psi}}{\text{CFM}^2}\) and we calculated \(Q = 22.5 \text{ CFM}\).

Pressure drop \(\Delta P = 0.05 \frac{\text{psi}}{\text{CFM}^2} \times (22.5 \text{ CFM})^2 = 0.05 \times 506.25 \text{ psi} = 25.3125 \text{ psi}\)

Therefore, the pressure drop across the fuel filter is approximately 25.3 psi. This calculation takes into account the energy requirements of the engine, the heating value of CNG, and the flow characteristics of the fuel filter. Technicians must consider these factors when evaluating the performance and maintenance needs of CNG fuel systems.

A Type 3 CNG cylinder is a hybrid design. It consists of an aluminum liner, which provides the shape and acts as a gas-tight barrier. This liner is then overwrapped with a composite material, typically carbon fiber or a combination of carbon and glass fiber, embedded in a resin matrix. The composite overwrap is the primary load-bearing component, providing the necessary strength to contain the high-pressure CNG. The aluminum liner handles the gas permeation. This design balances weight, cost, and strength, making it lighter than Type 1 and Type 2 cylinders while being more cost-effective than Type 4 cylinders. The composite material’s tensile strength is significantly higher than steel or aluminum, allowing for a lighter cylinder with the same pressure rating. The aluminum liner ensures gas tightness and prevents corrosion from the CNG. The key is the load sharing: the aluminum liner provides a gas-tight seal, while the composite overwrap provides the structural strength to withstand the high pressure. Type 3 cylinders are a compromise between cost, weight, and performance, making them a popular choice for many CNG vehicle applications.

The scenario involves a Type 4 CNG cylinder that has been subjected to a thermal event. Type 4 cylinders consist of a polymer liner fully wrapped with carbon fiber. Thermal events can significantly impact the integrity of the cylinder. Visual inspection is crucial, but it may not reveal all damage, especially to the liner or the carbon fiber layers beneath the surface. While a pressure test might seem appropriate, it could lead to catastrophic failure if the cylinder has been compromised. Hydrostatic testing is generally performed during manufacturing or recertification, not as a routine inspection after a thermal event. Given the potential for hidden damage and the high pressures involved, the safest course of action, aligning with safety regulations and industry best practices, is to retire the cylinder from service to prevent potential hazards. This aligns with safety protocols emphasizing caution when dealing with compromised high-pressure CNG cylinders. Regulations often dictate specific actions following thermal incidents involving CNG cylinders, typically erring on the side of caution. The decision prioritizes safety and adherence to established protocols for handling potentially damaged CNG storage systems.

To determine the ideal CNG cylinder volume for the delivery van, we need to calculate the required energy, convert it to CNG volume, and then account for practical factors like cylinder pressure and usable capacity.

First, calculate the total energy needed:
\[ \text{Energy Needed} = \text{Distance} \times \text{Fuel Consumption Rate} \times \text{Energy Density of Gasoline} \]
\[ \text{Energy Needed} = 450 \text{ miles} \times \frac{1 \text{ gallon}}{20 \text{ miles}} \times 120,000 \frac{\text{BTU}}{\text{gallon}} = 2,700,000 \text{ BTU} \]

Next, convert the energy needed to the equivalent volume of CNG:
\[ \text{CNG Volume (BTU basis)} = \frac{\text{Energy Needed}}{\text{Energy Density of CNG}} \]
\[ \text{CNG Volume (BTU basis)} = \frac{2,700,000 \text{ BTU}}{125,000 \frac{\text{BTU}}{\text{gallon}}} = 21.6 \text{ gasoline gallon equivalent (GGE)} \]

Convert GGE to cubic feet at standard conditions (1 GGE ≈ 127 cubic feet):
\[ \text{CNG Volume (cubic feet)} = 21.6 \text{ GGE} \times 127 \frac{\text{cubic feet}}{\text{GGE}} = 2743.2 \text{ cubic feet} \]

Now, calculate the required cylinder volume considering the pressure and the usable capacity. A typical CNG cylinder stores gas at 3600 psi. The standard cubic foot is measured at 14.7 psi. We need to adjust for this pressure difference.
\[ \text{Volume at Cylinder Pressure} = \frac{\text{Volume at Standard Pressure} \times \text{Standard Pressure}}{\text{Cylinder Pressure}} \]
\[ \text{Volume at Cylinder Pressure} = \frac{2743.2 \text{ cubic feet} \times 14.7 \text{ psi}}{3600 \text{ psi}} = 11.2 \text{ cubic feet} \]

Account for the usable capacity (80%):
\[ \text{Total Cylinder Volume} = \frac{\text{Volume at Cylinder Pressure}}{\text{Usable Capacity}} \]
\[ \text{Total Cylinder Volume} = \frac{11.2 \text{ cubic feet}}{0.80} = 14 \text{ cubic feet} \]

Convert cubic feet to gallons (1 cubic foot ≈ 7.48 gallons):
\[ \text{Total Cylinder Volume (gallons)} = 14 \text{ cubic feet} \times 7.48 \frac{\text{gallons}}{\text{cubic foot}} = 104.72 \text{ gallons} \]

Therefore, the ideal CNG cylinder volume for the delivery van, considering the energy requirements, pressure, and usable capacity, is approximately 105 gallons.

When inspecting a Type 4 CNG cylinder, technicians must prioritize a comprehensive visual examination of the overwrap. Delamination, characterized by the separation of composite layers, significantly compromises the cylinder’s structural integrity and its ability to withstand high pressures. This can be identified through bulges, soft spots, or visible cracks on the surface. A cylinder exhibiting these signs should be immediately removed from service.

Fiber blooming, while visually concerning, is a less critical issue than delamination. Fiber blooming refers to the fraying or separation of fibers on the surface of the composite material. Minor fiber blooming might be acceptable within manufacturer’s specifications, but extensive blooming indicates potential degradation.

Surface scratches, if shallow and not penetrating the overwrap, are generally acceptable. However, deep scratches that expose the underlying material are a cause for concern. The technician must evaluate the depth and extent of any scratch to determine its impact on the cylinder’s structural integrity.

End boss corrosion is a critical issue because the end boss provides structural support and a sealing surface for the cylinder valve. Corrosion can weaken the end boss, leading to potential leaks or failure. Any sign of corrosion warrants further investigation and possible cylinder rejection.

The technician’s primary concern should be delamination of the composite overwrap because it directly impacts the cylinder’s ability to safely contain CNG at high pressure. Delamination poses an immediate and significant risk of cylinder failure.

A Type 4 CNG cylinder undergoes significantly different stress patterns compared to Types 1, 2, and 3. A Type 4 cylinder consists of a non-metallic liner (typically plastic, such as high-density polyethylene) wrapped with a carbon fiber composite. The liner primarily serves as a gas barrier, while the carbon fiber composite bears the majority of the structural load caused by the high-pressure CNG. During pressurization, the liner experiences hoop stress (circumferential stress) and axial stress, but its contribution to the overall strength is minimal. The carbon fiber composite is specifically designed to withstand high tensile stresses in the hoop direction, which is the primary stress direction in a cylindrical pressure vessel. The composite layers are carefully oriented during manufacturing to maximize their strength in this direction. Unlike metallic cylinders, Type 4 cylinders do not experience significant yielding or plastic deformation under normal operating pressures. Instead, the composite material exhibits linear elastic behavior until failure. The failure mode is typically characterized by fiber breakage or delamination of the composite layers, rather than necking or bulging as seen in metals. Regulations like FMVSS 304 dictate rigorous burst testing to ensure a significant safety margin above the working pressure. Therefore, understanding the stress distribution and material behavior of each type of CNG cylinder is crucial for proper inspection and maintenance.

The question involves calculating the expected change in CNG tank pressure due to a temperature change, considering the ideal gas law and the tank’s volume.

First, we need to apply the ideal gas law, which is \(PV = nRT\), where:
– \(P\) is the pressure,
– \(V\) is the volume,
– \(n\) is the number of moles,
– \(R\) is the ideal gas constant, and
– \(T\) is the temperature in Kelvin.

Since the number of moles \(n\), the volume \(V\), and the ideal gas constant \(R\) remain constant, we can write the relationship as:
\[\frac{P_1}{T_1} = \frac{P_2}{T_2}\]
where \(P_1\) and \(T_1\) are the initial pressure and temperature, and \(P_2\) and \(T_2\) are the final pressure and temperature.

Given:
– Initial pressure, \(P_1 = 3000\) psi
– Initial temperature, \(T_1 = 25\)°C = 298.15 K (converting Celsius to Kelvin by adding 273.15)
– Final temperature, \(T_2 = 65\)°C = 338.15 K

We want to find the final pressure \(P_2\). Rearranging the formula, we get:
\[P_2 = P_1 \cdot \frac{T_2}{T_1}\]
\[P_2 = 3000 \cdot \frac{338.15}{298.15}\]
\[P_2 = 3000 \cdot 1.1341\]
\[P_2 = 3402.3 \text{ psi}\]

The change in pressure, \(\Delta P = P_2 – P_1\):
\[\Delta P = 3402.3 – 3000\]
\[\Delta P = 402.3 \text{ psi}\]

Therefore, the expected change in pressure is approximately 402.3 psi. This calculation demonstrates how temperature affects the pressure inside a CNG tank, a crucial consideration for safety and performance. Technicians must understand these principles to accurately assess tank conditions and prevent over-pressurization. The ideal gas law provides a foundational understanding of the relationship between pressure, volume, temperature, and the amount of gas, which is vital for diagnosing issues related to CNG fuel systems. Furthermore, understanding these calculations is essential for adhering to safety regulations and ensuring the reliable operation of CNG-powered vehicles. This also relates to the thermal protection devices that are installed in CNG tanks.

A Type 4 CNG cylinder is constructed with a non-load bearing liner, typically made of plastic (such as high-density polyethylene – HDPE or polyamide), overwrapped with a carbon fiber reinforced polymer (CFRP). The plastic liner ensures gas tightness, preventing permeation, while the CFRP provides the structural strength to withstand high pressures. The composite material is specifically designed to handle the stress from high-pressure CNG. The pressure rating of a CNG cylinder is critical to its safe operation. Regulations and standards (like ANSI NGV2) dictate the allowable service pressure and test pressure. The service pressure is the normal operating pressure (typically 3600 psi or 24.8 MPa). The burst pressure is the minimum pressure at which the cylinder must rupture during testing, and it’s significantly higher than the service pressure to ensure safety. Hydrostatic testing is a crucial part of cylinder inspection, where the cylinder is pressurized with water to a specified test pressure (typically 1.5 times the service pressure) to detect leaks or structural weaknesses. A cylinder failing hydrostatic testing must be removed from service. The service life of a CNG cylinder is limited by its design and the potential for degradation of the composite material. Visual inspections are performed to check for external damage, such as cuts, gouges, or fiber unraveling. Any indication of damage or failure during inspection requires the cylinder to be taken out of service.

The question addresses the complexities of CNG fuel system leak detection, particularly in scenarios involving composite fuel lines. While soap solution is a traditional method, it’s not always effective on composite materials due to their non-uniform surface and potential for CNG to permeate the material without forming visible bubbles. Electronic leak detectors are more sensitive and can detect smaller leaks, but their accuracy can be affected by ambient conditions and proper calibration. Ultrasonic leak detectors are particularly useful in noisy environments as they detect the sound of the leak itself. The key lies in understanding the limitations of each method in relation to the specific material of the fuel line. A technician must consider the potential for false negatives with soap solution on composite lines, and choose a method that accounts for the material’s permeability and the environment’s noise level. Therefore, the most reliable approach combines multiple methods, starting with a sensitive electronic or ultrasonic detector to identify potential leak areas, followed by careful soap solution application to confirm and pinpoint the leak if possible. This multifaceted approach minimizes the risk of overlooking a dangerous CNG leak.

To determine the remaining service life of the Type NGV2 CNG cylinder, we need to consider the total service life, the time already used, and any extensions granted based on successful requalification. The original service life is 15 years. The cylinder has been in service for 9 years. A successful requalification extended the service life by 3 years.

First, calculate the total potential service life:
\[
\text{Total Service Life} = \text{Original Service Life} + \text{Extension}
\]
\[
\text{Total Service Life} = 15 \text{ years} + 3 \text{ years} = 18 \text{ years}
\]

Next, calculate the remaining service life:
\[
\text{Remaining Service Life} = \text{Total Service Life} – \text{Years in Service}
\]
\[
\text{Remaining Service Life} = 18 \text{ years} – 9 \text{ years} = 9 \text{ years}
\]

Therefore, the remaining service life of the CNG cylinder is 9 years. This calculation highlights the importance of understanding the regulations surrounding CNG cylinder lifespan and the impact of requalification on extending that lifespan. Regulations such as those from DOT (Department of Transportation) and CSA (Canadian Standards Association) mandate periodic inspections and requalifications to ensure the ongoing safety and integrity of CNG cylinders. Technicians must be aware of these standards and how they affect the service life of cylinders they are inspecting or servicing.

The question addresses a scenario involving a Type 4 CNG cylinder that has been subjected to a significant impact. A Type 4 cylinder consists of a polymer liner fully wrapped with carbon fiber. The key to answering this question lies in understanding the structural properties of Type 4 cylinders and the implications of damage. While a minor scratch might only affect the surface coating, a significant impact can compromise the carbon fiber wrapping, which is the primary load-bearing component. Any damage to the carbon fiber necessitates immediate removal from service. A slight bulge indicates that the liner and/or the carbon fiber wrapping has been deformed, this is a clear indication of structural compromise. Hydrostatic testing is a destructive test and not suitable for field inspection. While a certified inspector’s opinion is valuable, the presence of a bulge is a definitive indication of damage, regardless of the inspector’s immediate assessment. The cylinder must be taken out of service immediately.

The question addresses the correct procedure for purging a CNG fuel system before performing maintenance. The safest and most environmentally responsible method is to vent the CNG in a controlled manner using a certified CNG recovery system. This system captures the vented gas, preventing its release into the atmosphere and allowing for safe disposal or reuse. Simply opening the cylinder valve vents the gas uncontrolled, which is dangerous and illegal in many jurisdictions. Using a vacuum pump is not effective for purging CNG. While nitrogen can be used to displace CNG, it still requires a controlled venting process. Using a certified recovery system is the best practice for safety and environmental compliance.

To determine the remaining service life, we first need to calculate the cumulative wall stress experienced by the cylinder. The formula for calculating the stress (\(\sigma\)) in a thin-walled pressure vessel (like a CNG cylinder) is: \[\sigma = \frac{P \cdot r}{t}\] where \(P\) is the pressure, \(r\) is the radius, and \(t\) is the wall thickness.

Given: Initial pressure (\(P_1\)) = 3600 psi, radius (\(r\)) = 5 inches, thickness (\(t\)) = 0.25 inches.
The initial stress (\(\sigma_1\)) is: \[\sigma_1 = \frac{3600 \cdot 5}{0.25} = 72000 \text{ psi}\]

The cylinder experiences 2000 cycles at this pressure. The cumulative stress for these cycles is: \[2000 \cdot \sigma_1 = 2000 \cdot 72000 = 144,000,000 \text{ psi-cycles}\]

Next, the cylinder experiences 1500 cycles at a reduced pressure (\(P_2\)) = 2400 psi.
The stress at the reduced pressure (\(\sigma_2\)) is: \[\sigma_2 = \frac{2400 \cdot 5}{0.25} = 48000 \text{ psi}\]

The cumulative stress for these cycles is: \[1500 \cdot \sigma_2 = 1500 \cdot 48000 = 72,000,000 \text{ psi-cycles}\]

The total cumulative stress (\(\sigma_{\text{total}}\)) is: \[\sigma_{\text{total}} = 144,000,000 + 72,000,000 = 216,000,000 \text{ psi-cycles}\]

The cylinder’s design life is 5000 cycles at 3600 psi, which corresponds to a design stress (\(\sigma_{\text{design}}\)): \[\sigma_{\text{design}} = 5000 \cdot 72000 = 360,000,000 \text{ psi-cycles}\]

The remaining service life can be estimated by comparing the total cumulative stress to the design stress. The fraction of life consumed is: \[\text{Fraction of life consumed} = \frac{\sigma_{\text{total}}}{\sigma_{\text{design}}} = \frac{216,000,000}{360,000,000} = 0.6\]

Therefore, the remaining service life is: \[\text{Remaining life} = (1 – 0.6) \cdot 5000 = 0.4 \cdot 5000 = 2000 \text{ cycles}\]

This calculation assumes a linear relationship between stress and fatigue life, which is a simplification. In reality, fatigue life is influenced by various factors, including material properties, manufacturing processes, and operating conditions. The calculation also uses the thin-walled pressure vessel formula, appropriate when the wall thickness is small compared to the radius. Furthermore, it is crucial to consider regulatory standards and manufacturer’s specifications for CNG cylinders, which may impose more conservative estimates of service life based on actual testing and safety factors. Regular inspections, including visual and hydrostatic testing, are essential to ensure the ongoing integrity and safety of CNG cylinders.

The correct procedure involves several steps. First, ensure the vehicle is in a well-ventilated area and that all ignition sources are removed. Next, locate the CNG cylinder valve and close it tightly. Following this, start the engine and allow it to run until it stalls due to fuel starvation. This process depletes the CNG within the fuel lines. Once the engine stalls, attempt to restart it a few times to ensure all residual CNG is used. Finally, disconnect the negative battery cable to prevent any accidental ignition during subsequent maintenance or repairs. This comprehensive approach minimizes the risk of CNG leaks and ensures a safer working environment. Simply opening the cylinder valve without running the engine can lead to a significant and uncontrolled release of CNG, posing a serious fire or explosion hazard. Likewise, only disconnecting the battery cable is insufficient, as CNG remains in the system. Venting the system into the atmosphere without proper precautions is environmentally irresponsible and potentially dangerous.

The question addresses the nuanced relationship between oxygen sensor readings and their impact on CNG fuel trim. While oxygen sensors primarily measure oxygen content in the exhaust, their readings are crucial for the ECM to make fuel adjustments, regardless of the fuel type (gasoline or CNG). The ECM uses these readings to maintain a stoichiometric air-fuel ratio (ideally 14.7:1 for gasoline, but slightly different for CNG due to its different chemical composition, typically around 16-17:1).

A lean exhaust condition (excess oxygen) detected by the oxygen sensor will cause the ECM to increase fuel delivery (positive fuel trim). Conversely, a rich exhaust condition (low oxygen) will cause the ECM to decrease fuel delivery (negative fuel trim). The key is understanding that the ECM *interprets* the oxygen sensor signal and *adjusts* the fuel trim accordingly. The ECM doesn’t “know” it’s running on CNG; it only reacts to the oxygen sensor’s feedback to achieve the target air-fuel ratio.

Therefore, if the oxygen sensor indicates a lean condition while the engine is running on CNG, the ECM will respond by increasing the CNG injector pulse width to enrich the mixture. The magnitude of the fuel trim adjustment will depend on the severity of the lean condition and the ECM’s programming.

The question involves calculating the remaining service life of a Type 3 CNG cylinder based on its operating pressure, test pressure, and initial service life. The key is to understand that the remaining service life is reduced proportionally to the over-pressurization during testing. First, calculate the over-pressure ratio:
\[
\text{Over-Pressure Ratio} = \frac{\text{Test Pressure}}{\text{Operating Pressure}} = \frac{3600 \text{ psi}}{3000 \text{ psi}} = 1.2
\]
This means the cylinder was tested at 1.2 times its normal operating pressure. Now, determine the reduction in service life due to this over-pressure. The cylinder’s service life is reduced proportionally to the over-pressure ratio minus 1:
\[
\text{Service Life Reduction Ratio} = \text{Over-Pressure Ratio} – 1 = 1.2 – 1 = 0.2
\]
This indicates a 20% reduction in the cylinder’s remaining service life for each test cycle. Given that the cylinder has undergone 3 test cycles, the total service life reduction is:
\[
\text{Total Service Life Reduction} = \text{Service Life Reduction Ratio} \times \text{Number of Test Cycles} = 0.2 \times 3 = 0.6
\]
This means the cylinder has lost 60% of its service life due to testing. Now, calculate the remaining service life:
\[
\text{Remaining Service Life} = \text{Original Service Life} \times (1 – \text{Total Service Life Reduction}) = 15 \text{ years} \times (1 – 0.6) = 15 \text{ years} \times 0.4 = 6 \text{ years}
\]
Therefore, the remaining service life of the CNG cylinder is 6 years. This calculation highlights the importance of pressure testing and its impact on the lifespan of CNG cylinders, ensuring safety and regulatory compliance.

The correct answer is the scenario where the technician uses an ultrasonic leak detector after a visual inspection and soap solution test have yielded no results, and the system has been purged according to safety protocols. This approach adheres to a systematic methodology for leak detection, starting with less sensitive methods and progressing to more sophisticated techniques. Regulations and standards mandate a thorough approach to leak detection to ensure safety and environmental protection. Visual inspection and soap solution tests are initial steps, but they may not detect small leaks. An ultrasonic leak detector can identify leaks by sensing the high-frequency sounds produced by escaping gas, even if the leak is too small to create visible bubbles or be heard by the human ear. Purging the system before using an electronic leak detector is a critical safety precaution to prevent the detector from being overwhelmed by a large concentration of gas, which could damage the sensor or give false readings. Furthermore, this ensures that the leak detection process is accurate and reliable, aligning with industry best practices and regulatory requirements for CNG systems. Using the ultrasonic leak detector after these steps ensures a comprehensive and safe leak detection process.

The correct answer is that the system is likely experiencing a leak downstream of the primary regulator, causing a pressure drop that the secondary regulator cannot compensate for, coupled with a potential issue of the ECM not properly interpreting the pressure sensor data due to a calibration fault. The primary regulator reduces the high tank pressure (e.g., 2400-3600 psi) to an intermediate pressure (e.g., 300-900 psi). The secondary regulator then further reduces this to a pressure suitable for the injectors (e.g., 60-120 psi). If a leak exists after the primary regulator but before the secondary, the intermediate pressure will drop. The secondary regulator will attempt to maintain its output pressure, but it is limited by the reduced input pressure. If the ECM relies on a pressure sensor downstream of the secondary regulator and the sensor readings are inaccurate due to a calibration error, the ECM will not be able to correctly adjust the fuel trim, leading to lean running and potential misfires. The system will try to compensate, but the underlying leak and sensor issue will prevent proper operation. A malfunctioning relief valve would cause a rapid pressure drop and likely trigger safety mechanisms. While a clogged fuel filter can cause issues, it is less likely to cause the specific combination of symptoms described. Injector failure would typically manifest as misfires on specific cylinders. The combination of low pressure and inaccurate sensor data points to a more complex problem.

The ideal gas law is \(PV = nRT\), where \(P\) is pressure, \(V\) is volume, \(n\) is the number of moles, \(R\) is the ideal gas constant, and \(T\) is temperature. We can rearrange this to find the number of moles: \(n = \frac{PV}{RT}\).

First, convert the temperature from Celsius to Kelvin: \(T = 25 + 273.15 = 298.15 \, \text{K}\). The ideal gas constant \(R\) is \(8.314 \, \text{J/(mol·K)}\). The pressure needs to be converted from kPa to Pa: \(P = 22000 \, \text{kPa} = 22000 \times 1000 \, \text{Pa} = 22 \times 10^6 \, \text{Pa}\).

Now we can calculate the number of moles in the tank:
\[n = \frac{PV}{RT} = \frac{(22 \times 10^6 \, \text{Pa}) \times (0.06 \, \text{m}^3)}{(8.314 \, \text{J/(mol·K)}) \times (298.15 \, \text{K})}\]
\[n = \frac{1.32 \times 10^6}{2478.77} \approx 532.52 \, \text{moles}\]

Next, we need to find the mass of CNG in the tank. The molar mass of CNG (primarily methane, CH4) is approximately \(16 \, \text{g/mol}\) or \(0.016 \, \text{kg/mol}\).

The mass \(m\) of CNG is:
\[m = n \times \text{molar mass} = 532.52 \, \text{moles} \times 0.016 \, \text{kg/mol} \approx 8.52 \, \text{kg}\]

The pressure drop is \(1500 \, \text{kPa}\), so the final pressure is \(22000 – 1500 = 20500 \, \text{kPa} = 20.5 \times 10^6 \, \text{Pa}\).

Now, we recalculate the number of moles at the new pressure:
\[n_{new} = \frac{P_{new}V}{RT} = \frac{(20.5 \times 10^6 \, \text{Pa}) \times (0.06 \, \text{m}^3)}{(8.314 \, \text{J/(mol·K)}) \times (298.15 \, \text{K})}\]
\[n_{new} = \frac{1.23 \times 10^6}{2478.77} \approx 496.22 \, \text{moles}\]

The difference in moles is \(532.52 – 496.22 = 36.3 \, \text{moles}\).

The mass of CNG dispensed is:
\[m_{dispensed} = 36.3 \, \text{moles} \times 0.016 \, \text{kg/mol} \approx 0.5808 \, \text{kg}\]

Therefore, the mass of CNG dispensed is approximately \(0.58 \, \text{kg}\).

The correct procedure for purging a CNG fuel system involves several critical steps to ensure safety and prevent the formation of a flammable mixture. The first step is to isolate the CNG tank by closing the manual shut-off valve, if equipped, to prevent further gas flow into the system. Next, the engine should be started and allowed to run until it stalls due to fuel starvation, depleting the CNG remaining in the fuel lines and regulator. After the engine stalls, open the CNG fuel line at a point downstream of the regulator, typically at an injector or fuel rail connection. This allows any remaining CNG to vent slowly into the atmosphere. It is crucial to perform this venting in a well-ventilated area, away from any ignition sources, to prevent the accumulation of flammable gas. Finally, after venting, the system should be checked for zero pressure using a calibrated pressure gauge before commencing any repair work. The purpose of each step is to remove CNG in a controlled way, minimizing the risk of accidental ignition. This procedure is compliant with safety standards outlined in NFPA 52 and CSA B109.1.

The scenario describes a Type 4 CNG cylinder that has experienced a significant impact. Type 4 cylinders consist of a polymer liner fully wrapped with carbon fiber. The primary safety concern is the potential for damage to the carbon fiber wrap, which is the main structural component responsible for containing the high-pressure CNG. While the plastic liner might also be damaged, its failure is secondary to the carbon fiber’s integrity. A compromised carbon fiber wrap can lead to catastrophic failure, even if the liner appears intact. Regulations and industry best practices mandate that any cylinder suspected of impact damage be removed from service and inspected by a qualified technician or hydrostatic testing facility. Visual inspection alone may not be sufficient to detect internal damage within the composite layers. Hydrostatic testing is often required to verify the cylinder’s structural integrity after a suspected impact event. Simply continuing to use the cylinder without proper inspection poses a significant safety risk due to the potential for a sudden and forceful rupture. Repairing a damaged Type 4 cylinder is generally not recommended or permitted due to the complex nature of the composite structure and the difficulty in ensuring a reliable repair.

To determine the remaining service life of a Type 3 CNG cylinder, we need to consider the allowable strain limit, the current strain reading, and the annual strain increase rate. The cylinder’s initial strain was 0, and the allowable strain limit is 0.2%. The current strain reading is 0.12%, and the annual strain increase rate is 0.015%.

First, calculate the remaining allowable strain:
\[Remaining\ Strain = Allowable\ Strain\ Limit – Current\ Strain\]
\[Remaining\ Strain = 0.2\% – 0.12\% = 0.08\%\]

Next, calculate the remaining service life in years by dividing the remaining allowable strain by the annual strain increase rate:
\[Remaining\ Service\ Life = \frac{Remaining\ Strain}{Annual\ Strain\ Increase\ Rate}\]
\[Remaining\ Service\ Life = \frac{0.08\%}{0.015\%/year} = 5.33\ years\]

Since CNG cylinder service life is typically assessed in whole years, we must consider the implications of the fractional year. Rounding to the nearest whole year is generally not recommended for safety-critical assessments. Instead, it’s prudent to round down to ensure the cylinder is retired before exceeding its allowable strain limit. Therefore, the remaining service life is 5 years.

This calculation is crucial because exceeding the strain limit can compromise the structural integrity of the cylinder, leading to potential failure and safety hazards. Regular inspections and strain measurements are essential for ensuring the safe operation of CNG vehicles. Furthermore, understanding material properties and their degradation over time is vital for predicting the remaining service life of CNG cylinders. Factors such as temperature variations, pressure cycles, and environmental conditions can influence the strain increase rate, necessitating more frequent inspections in harsh operating environments. The inspection procedures must adhere to relevant industry standards and regulatory requirements to maintain safety and compliance.