CNG Storage Cascade

CNG TUBE SKIDS

A traditional cascade system is a cellular arrangement of multiple CNG cylinders mounted over a vehicle (open to sky, with a stationary cage), that is filled with gas and then transported. The cylinders mounted are mostly classified on the basis of the material of construction and the water capacity held by them.
 

One of the key components in a fast-fill NGV Refueling Station is the CNG Storage. In fact, the inclusion of ASME ground storage vessels really defines a fast-fill refueling application.

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.

The amount of storage you need depends on the amount of vehicles, the amount of gas each vehicle requires, and the time frame in which the vehicles need to be filled.

There are four types of tanks available, designed to hold natural gas at pressures up to 3,600 psi:

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.

ll 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.