by Anand Sharma, Vice President Sales & Marketing, Uttam Group
Over the last decade, CNG has emerged as an immensely popular fuel thanks to its relatively lower cost and carbon emissions compared to fuels such as petrol and diesel. As petrol and diesel prices continue to soar, more and more vehicle and fleet owners are expected to make the switch to CNG. So perhaps, this is an opportune moment to learn more about CNG, particularly its 4 types of tanks; because that is a choice most vehicle owners will soon have to make.
The nature of CNG is such that it expands and contracts depending on the temperature. The amount of CNG that a vehicle tank can hold is therefore dependent on numerous factors such as fueling rate, ambient temperature, pressure rating and tank type. Coming to our subject of focus for this article, CNG tanks can be categorized into 4 types, unimaginatively named Type 1, Type 2, Type 3 and Type 4. The type is determined primarily on the basis of design, method of production and the liner material.
It is important to take a few important factors into consideration before you choose a tank for your vehicle. The type 1 cylinders are made of steel & are in circulation for ages now, these cylinders are heavier & add more dead weight to vehicles on the go as compared to type 3 & 4 fully wrapped composite cylinders with metal & plastic liners respectively which are almost 60 to 70% lighter. This article will shed light on the benefits of Type 3 cylinders and the key differences between Type 3 and Type 4 cylinders.
Understanding Type 3 and Type 4 cylinders
Type 3 is a metal lined cylinder with full exterior composite reinforcement whereas Type 4 is plastic lined full wrapped composite cylinders. It is worthwhile to note that both are carbon composite cylinders but the difference lies in the material used to construct the cylinder. The liner material used for Type 3 cylinder is made from seamless 6061 aluminum alloy as opposed to plastic lined in Type 4 cylinders.
Type 3 cylinders are known to provide weight-efficient gas storage. Fleet operators stand to benefit from Type 3 cylinders through improved fuel economy, extended driving range and substantial reduction in vehicle maintenance costs. Type 3 cylinders are ideal for use in automobiles, light, medium and heavy-duty trucks, buses, gas transport trailers and gas transportation modules.
Type 3 Cylinders - A safer alternative
Let’s dive deeper into a few of the most important benefits of Type 3 cylinders and why these make it a safer alternative
- Seamless aluminum liner
The temperature of gas increases due to the heat of compression in tanks. When the gas is at an elevated temperature, the tank is not filled to its full capacity. A fast fill process results in more heat leaving the storage space underutilized. The aluminum liner used in the Type 3 cylinder can conduct heat away from the internal storage space. Type 3 cylinders facilitate better heat absorption resulting in optimum use of the tank’s storage capacity.
Devoid of any welds or joints, Type 3 cylinders are characterized by gas tight construction with zero permeability. Liners of Type 4 cylinders, on the other hand, are not gas tight and don’t act as an impermeable barrier.
- Impact tolerance
The composite reinforcement in Type 3 cylinders provides about 75 to 90% of the strength to the vessel. The rest of the strength to the cylinder is provided by the liner. As this liner acts as a rigid membrane, it is resistant to damage caused by impact events. The liner mitigates the risk of composite flexing and subsequent breakage. Type 3 cylinders are a superior alternative to Type 4 as the latter is known to be less resistant to damage.
- Heat tolerance
Type 3 cylinders have greater heat resistance. This enables Type 3 cylinders to achieve greater fill compared to Type 4 cylinders. The composite fibers are held intact by the liners. The liner doesn’t wear away and succumb to the heat, which in turn, ensures that the tank has a high degree of heat tolerance; which probably is one quality every vehicle owner would want their fuel tank to have.
- Better fast filling efficiency
In terms of fast filling performance, Type 3 cylinders are superior to Type 4 cylinders. Faster filling results in rapid generation of heat leaving less time to conduct the heat away. Fast fill operations therefore result in a miscalculation of the cylinder’s fuel capacity as the CNG molecules expand due to the heat generated by the rapid movement and compression of CNG in the tank. Type 3 cylinders are an apt choice for fast fill efficiency as the liner conducts heat to Pressure Relief Devices. The aluminum liner in Type 3 cylinders has high thermal conductivity helping one to achieve a more complete fill even during fast fill operations.
- Burst safety factor
Safety is another important aspect that cannot be overlooked at any cost. Type 3 cylinders are unarguably a safer alternative. The aluminum liners in Type 3 cylinders have high resistance to corrosion despite the presence of aggressive components in Natural gas. Type 3 cylinders are built with a burst safety factor to ensure maximum safety of CNG vehicle owners and passengers.
Type 3 & Type 4 both have different material of construction & are made to different standards. Type 3 cylinders are manufactured as per ISO11119-2 & standard for Type 4 cylinder manufacturing is ISO11119-3. With a typical service life of 20 years, type 3 cylinders have seamless impermeable 6061 aluminum alloy that prevents leakage. On the contrary, the permeable liner which is Plastic in Type 4 cylinders make them more prone to leakage.
With a ‘leak before burst’ feature Type 3 cylinders provide excellent safety while also enjoying a better fast fill performance in comparison to Type 4 cylinders. Also type 3 cylinders technology has already completed its 1 life cycle in circulation globally & proven track record stats are available in public domain, whereas type 4 cylinders have been introduced recently in global market technical refinement is yet under process.
In conclusion
Type 3 cylinders are made of high strength carbon fiber and epoxy composite and provide weight efficient gas storage. As the aluminum helps dissipate the heat, the tank can accommodate a larger mass of gas significantly improving the driving range, without affecting the vehicle gross weight. To sum it up, these lightweight, durable Type 3 CNG cylinders come with a host of benefits, and it is undoubtedly a safer and better alternative to other CNG cylinder types.
