Clean Air: Myths and Facts about CNG

Myth: CNG vehicles are unsafe

Even new (CNG) buses cannot withstand the heat.

No (safety) code (for CNG kits) is notified. CNG vehicles are on the road without this permit - we are playing with people's lives, especially children's lives. I shudder to think how CNG buses will behave in the middle of summer. They are known to overheat ......

10 kg of CNG is like 10 kg of RDX.

Fact

CNG, unlike LPG, is a lighter-than-air gas that dissipates quickly into the environment and is unlikely to gain enough concentration in the air to explode. In addition, CNG has a flash point (the temperature at which a fuel can explode on its own) of 540oC, which is higher than gasoline, which has a flash point of 232-282oC.

In the event of a vehicle crash, CNG tanks are much stronger The U.S. Department of Energy says it is safer than diesel or gasoline fuel tanks.

Probably the biggest safety concern in India is the use of fake gas cylinders, especially when unauthorized cars, cabs or car dealers convert gasoline to CNG. But this problem can be solved if enough effort is made.

It is not surprising that some people wonder if CNG cylinders explode when temperatures reach 45 degrees Celsius in Delhi during the summer. People have a right to be concerned about safety (even if a bus catches fire, it's unlikely to blow a cylinder).

But let's see how this works. When the temperature rises, the gas expands. If kept in a closed space, the pressure increases. The company that makes the gas cylinder should be able to state the pressure that the CNG cylinder can withstand. If in doubt, test the cylinder by heating it in the lab. If flaws are found - not only in normal summer temperatures, but also in extreme situations such as a bus fire - proper safety codes can be set.

Gasoline or diesel tanks can also explode - try throwing a burning match into one of them - but they rarely happen. This is simply because there are safety norms in place. The government can easily take the proper measures to keep people informed.

In principle, CNG is quite safe. Unlike LPG, CNG is a lighter-than-air gas that dissipates quickly into the environment and is unlikely to acquire sufficient concentration in the air to explode. In addition, CNG has a flash point (the temperature at which a fuel may explode on its own) of 540oC, which is higher than gasoline, which has a flash point of 232-282oC.

CNG cylinders are subjected to rigorous abuse testing before being approved by statutory authorities. They are tested to withstand pressures of up to 340 bar, compared to 220 bar working pressure.

No one ever cared that even the hot desert city of Cairo started buying CNG vehicles. Let us not forget that the future of transportation will be fuel cells powered by hydrogen, the ultimate non-polluting fuel, but a more flammable gas with a flash point of 259oC.

Myths about the safety of CNG have spread around the world. A statement from the U.S. Department of Energy says, "The technology to manufacture CNG storage tanks is well known and proven. In the event of a vehicle collision, CNG tanks are stronger and safer than diesel or gasoline tanks. The few CNG tank failures that were carefully studied, which were primarily related to support belt failures or tank wear during normal operation, have been resolved."

CNG buses have some different safety issues than diesel buses, but overall there is no evidence that CNG buses pose a greater risk of fire or explosion than diesel buses. Natural gas buses are equipped with on-board gas detectors and other safety equipment specifically designed to ensure safe operation.

The biggest problem may be the use of fake gas cylinders, especially when unauthorized cars, cabs, or car agents make modifications. In the case of buses, only government-certified agencies can perform the conversion, so the use of fake cylinders is less likely.

Ironically, despite safety concerns, regulators have never cared to evaluate the safety regulations that apply to CNG vehicles.

"Even if the bus catches fire, the cylinders won't explode."

Some cylinders are manufactured in a special way to ensure they can withstand high pressures. This is based on the British Pressure Vessel Code and the American Pressure Vessel Code. This code has various parameters. Engineering parameters include size dimensions, strength, etc. It also specifies special parameters, such as "relief". When a cylinder is manufactured, it undergoes what is called stress and the metal becomes brittle. To get rid of this stress, "relief" is done. Several technical parameters are specified.

These cylinders are manufactured in a special way. For the manufacture of ordinary cylinders, the metal plates are molded and welded, but in the case of compressed natural gas and other gases that maintain pressure, a solid block of metal called a block is pierced from the inside to avoid any cracks. The chances of such cylinders bursting are very rare.

Can CNG cylinders withstand the heat of a hot summer?

CNG engines are similar to petrol engines and generate more heat to produce the same power as compared to diesel engines. This is why the area around the engine and near the driver may get hot. However, there is no safety risk.

What happens to a CNG cylinder or kit when a bus catches fire?

I can assure you that the cylinder will not explode even if the bus catches fire. Yes, this would have terrible consequences for the bus and passengers, but a certified cylinder will not burst.

Is it true that the cylinder can withstand 340 bar pressure and the ignition temperature of CNG is 540 degrees Celsius as reported in the newspaper?

I am not sure of the exact pressure that a cylinder can withstand, but it is at least 1.5 times the storage pressure of CNG, which is 200 bar. the flash point of CNG is 540oC. this is the temperature at which CNG explodes on its own. The flash point of gasoline and diesel fuel is much lower than that of CNG.

Is it dangerous to keep a valve in a CNG vehicle?

It is like keeping the regulator of an LPG cooking cylinder open. The valve should close on its own when not in use. If it does not work properly and the gas continues to leak, then it could be a problem. the CNG kit includes the actual conversion kit, piping, valves and the entire system. The appropriate valve provisions are in the draft document but have not been notified.

Inspection of CNG vehicles

There is no provision for inspection of in-service buses after conversion to CNG. To ensure that the converted bus meets the specifications of the type approved vehicle, it is important to inspect each bus before it is allowed on the road. This inspection can be considered equivalent to a Conformity of Expected Production (COP) inspection for OEM CNG buses. This system is already in place in many countries.

Every converted bus must be inspected for engine and high pressure fuel storage system before it is allowed on the road. This inspection program should then be conducted annually on all operating CNG vehicles to check for emissions and safety compliance.

CNG vehicles, especially those converted from diesel to CNG, should be inspected regularly. Our regular inspection system includes a roadworthiness check and control of the carbon monoxide concentration in the exhaust gas (up to 3% by volume at idle conditions). Based on experience in other countries, this is not sufficient to ensure proper operation of emission control systems for CNG buses equipped with catalytic converters and closed-loop mixture control. In the German demonstration project (BMU Demonstrationsvorhaben Emissionsarme gasbetriebene Nutzfahrzeuge), the following regular inspection procedures for CNG vehicles were very effective.

 Visual inspection of emission-related components, including the exhaust gas emission system, the

Measurement of carbon monoxide, hydrocarbon, carbon dioxide and oxygen levels at engine idle, closed-loop control checks, and determination of nitrogen oxide and carbon monoxide levels at full engine load (as specified in Article 115, Section 2 of the Central Motor Vehicle Regulations) on a simple chassis dynamometer (cost less than $20.000).

Such an inspection system should be introduced immediately as there are already a large number of CNG buses operating in Delhi for more than a year. annual inspection frequency for CNG buses is one year in combination with annual roadworthiness inspection. In addition, all new and retrofitted buses should go through the same process to establish reference values for NOx standards and to verify that the closed-loop air-fuel ratio control is functioning properly before they are put into operation.

As criteria to be met at idle, the Panel recommends the following.

For CO: 0.5% at idle and 0.3% at high idle (1,700 ± 150 rpm for buses and HDTs and 2,650 ± 150 rpm for light duty vehicles and light trucks).

For NOx: Type-specific standards should be defined based on reference values derived from the initial exhaust concentration check.

Other engine modifications needed

The Panel also noted some deficiencies in the engine and engine design of CNG passenger vehicles. More detailed certification rules and guidelines are needed to address these issues.

In the production of the chassis, there is room for improvement in the material of the high-pressure lines, the fixing of the lines to the chassis, the tightening of the couplings, the venting of the pressure relief valves, and the inspection of the gas lines. Experts believe that mixing brass fittings with steel piping should be avoided. In wet conditions, this combination could lead to galvanic corrosion. To avoid potential future corrosion problems, they recommend using stainless steel fittings and piping for high-pressure gas components.

The gas piping should be installed in such a way that sump movements and vibrations are not transmitted to the gas piping, which could cause the piping to break or be damaged by scraping after a period of use. Rubber wall pipe insulators are often used when bridging sump components and the pipe is attached to the sump through the use of rubber gaskets. In addition, when vibration cannot be avoided, the pipe should have sufficient space and length to allow for slight movement.

Experts have observed that gas piping is tested for leaks after installation. This is necessary to identify the presence of leaks in the system. The leak test was performed at each connection using brushed soapy water. Experts found leaks in a number of locations. To reduce the chance of leaks, it can be useful to use Teflon sealing tape in the threaded joints. In addition, it would be wiser to use compression fittings where possible or to use fittings with tapered threads rather than cylindrical threads.

Each cylinder has a pressure relief valve for venting gas when the cylinder is exposed to high temperatures or high internal pressures. Until now, the venting of the valve has not been directional, thus releasing gas close to the cylinder. Since the cylinders are mounted under the bus body, the vented gas may reach areas close to people or may enter the passenger compartment if it does not travel. One solution is to direct the vent from the relief valve to the roof of the bus via a hose or pipe. This will allow the gas to dissipate harmlessly upward with little chance of contact with people or sources of ignition.

When a CNG vehicle has been in use for a period of time, the gas line should be visually inspected. However, in some cases the gas piping used is covered with a protective plastic or rubber coating. This will make visual inspection of the piping very difficult. If stainless steel pipes are used instead of plain steel, there is no need to cover the pipes to protect them from corrosion, which will facilitate the visual inspection.