|Fast-Fill stations allow vehicles to pull up and refuel in a short period of time. In order to fuel vehicles quickly, gas must be drawn from pre-pressurized storage vessels. Unlike gasoline stations, natural gas must be stored above ground. And because it is stored at high pressures it must be contained in ASME (American Society of Mechanical Engineers) coded vessels. Vessels are 20” in diameter and approximately 23 foot long. Each vessel will hold 10,000 cubic feet at 5,000
psi. They are arranged in a three bank cascade meaning that there is a low bank vessel, a medium bank vessel and a high bank vessel.
As a general rule of thumb, only 40% of the stored gas in a three bank cascade arrangement is available for refueling. This means that a 30,000 cubic foot storage cascade will deliver about 12,000 cubic feet of natural gas quickly. This equates to about 96 equivalent gallons of gasoline.
A priority sequential panel is used to direct compressor discharge to the high, then medium, then low bank. When filling, a vehicle will draw first from the low bank, then the medium bank and top off from the high bank. The
Compressor will replenish the cascade by filling the high bank first, then the medium bank and finally the low bank.
Storage Spheres are an alternative to the long tubes shown previously. Another form of storage that has lost popularity are DOT bottles. They resemble welding bottles and have burst disks instead of ASME relief valve. We do not like these for a number of reasons. First they need to be re-certified every five years, and second they do not have any drain valves.
SAFETY OF CNG
Natural gas is a clean, low cost, domestically available alternative fuel that can power our vehicles. The advantages to our energy independence by reducing foreign oil imports, the reduced cost compared to gasoline and less emissions have been well documented. However, there may still be a misconception about the safety of Compressed Natural Gas (CNG) compared to gasoline.
All fuel sources contain energy that is released through combustion and any fuel can be potentially dangerous if not properly handled. Throughout time we have learned to harness these fuels for heating, light and powering our vehicles.
The fact is that CNG is safer than gasoline as a vehicular fuel based upon two important facts; the physical qualities of natural gas and the structural integrity of the NGV fueling system.
Natural gas or methane is a non toxic gas that is lighter than air. This means that it will not puddle (like gasoline) or sink to the ground like propane, which is heavier than air. Instead, Natural Gas will rise and dissipate in the atmosphere.
Natural gas also has a higher ignition temperature. This means that it is much harder to ignite.
The storage systems used for compressed natural gas are infinitely stronger that the gasoline tanks found on cars and trucks today.
These tanks are designed to hold natural gas at pressures up to 3,600
There are four types of tanks available, these include:
Type 1: This is an all metal - cylinder made of steel. There is no covering, other than paint, on the outside of the cylinder. This is the most common type of cylinder and least expensive. It is also the heaviest.
Type 2: This is a metal cylinder (steel or aluminum) with a partial wrapping that goes around the cylinder. The wrapping is usually made of glass, or carbon, contained in an epoxy or polyester resin. This design weighs less than the type one and costs more.
Type 3: This type of cylinder is fully wrapped with the same kind of material used for the partial wrapping of a Type 2 cylinder. This type of cylinder has a metal liner usually aluminum. They are lightweight and even more expensive than Type 1 or Type 2.
Type 4: This type of cylinder is fully wrapped with the same kind of material used for the partial wrapping of a Type 2 cylinder. This type of cylinder has a plastic liner, it is very light and the most expensive.
All CNG cylinders are constructed to withhold up to 1.25 times their operating design pressure and include valves and other safety devices to prevent leakage.