Cascade storage system
A cascade storage system is a high pressure gas cylinder system which is used for the refilling of smaller compressed gas cylinders or air cylinders. Each of the large cylinders is filled by a compressor, but the cascade system allows small cylinders to be filled without a compressor. In addition, a cascade system is useful as a reservoir to allow a low-capacity compressor to meet the demand of filling several small cylinders in succession.
In general rupture discs are installed inline between the compressor and the storage units to burst if the maximum safety pressure is exceeded.
Principle of operation
When gas contained in a cylinder at high pressure is allowed to flow to another cylinder containing gas at a lower pressure, the pressures will equalise to a value somewhere between the two initial pressures. The equilibrium pressure is affected by transfer rate as it will be influenced by temperature, but at constant temperature the equilibrium pressure is described by Dalton's law of partial pressures and Boyle's law for ideal gases.
The formula for the equilibrium pressure is:
- P3 = (P1×V1+P2×V2)/(V1+V2)
- where P1 and V1 are the initial pressure and volume of one cylinder
- P2 and V2 the initial pressure and volume of the other cylinder
- and P3 is the equilibrium pressure.
An example could be a 100 litre (internal volume) cylinder (V1) pressurised to 200 bar (P1) filling a 10 litre (internal volume) cylinder (V2) which was unpressurised (P2 = 1 bar) (resulting in both cylinder equalising to approximately 180 bar (P3). If another 100 litre cylinder pressurised this time to 250 bar were then used to "top-up" the 10 litre cylinder, both of these cylinders would equalise to about 240 bar. However, if the higher pressure 100 litre cylinder were used first, the 10 litre cylinder would equalise to about 225 bar and the lower pressure 100 litre cylinder could not be used to top it up. In a cascade storage system, several large cylinders are used to bring a small cylinder up to a desired pressure, by always using the supply cylinder with the lowest usable pressure first, then the cylinder with the next lowest pressure, and so on.
In practice the theoretical transfers can only be achieved if the gases are allowed to reach a temperature equilibrium before disconnection. This requires significant time, and a lower efficiency may be accepted to save time. Actual transfer can be calculated using the general gas equation of state if the temperature of the gas in the cylinder is accurately measured.
A breathing set cylinder may be filled to its working pressure by decanting from larger (often 50 litre) cylinders. (To make this easy the neck of the cylinder of the Siebe Gorman Salvus rebreather had the same thread as an oxygen storage cylinder, but the opposite gender, for direct decanting.) The storage cylinders are available in a variety of sizes, typically from 50 litre internal capacity to well over 100 litres.
Cascade filling is often used for partial pressure blending of breathing gas mixtures for diving, to economize on the relatively expensive oxygen, for nitrox, and the even more expensive helium in trimix or heliox mixtures.
Compressed Natural Gas fueling
Cascade storage is used at Compressed Natural Gas (CNG) fueling stations. Typically three CNG tanks will be used, and a vehicle will first be fueled from one of them, which will result an incomplete fill, perhaps to 2000 PSI for a 3000 PSI tank. The second and third tanks will bring the vehicle's tank closer to 3000 PSI. The station normally has a compressor, which refills the station's tanks, using natural gas from a utility line. This prevents accidentally overfilling the tank, which could happen with a system using a single fueling tank at a higher pressure than the target pressure for the vehicle.
In cascade storage systems for hydrogen storage, as for example at hydrogen stations, fuel dispenser A draws hydrogen from tank A, while dispenser B draws fuel from hydrogen tank B. If dispenser A is over-utilized, tank A will become depleted before tank B. At this point the dispenser A is switched to tank C. Tank C will then supply dispenser A and B and tank A until tank A is filled to the same pressure as tank B and the dispensers are disconnected, after which the control system will close the control valves to switch to its former state. 
- Millar IL; Mouldey PG (2008). "Compressed breathing air – the potential for evil from within.". Diving and Hyperbaric Medicine. (South Pacific Underwater Medicine Society) 38: 145–51. Retrieved 2009-02-28.
- Harlow, V (2002). Oxygen Hacker's Companion. Airspeed Press. ISBN 0-9678873-2-1.
- Specialty Gas Cylinder Dimensions Air Products
- Hydrogen Fueling Station Pag 4. Broken link