Cascade storage system

A cascade storage system is a cascade storage tank system which is used for the refilling of smaller compressed gas cylinders or air cylinders. Each of the large cylinders will be filled by a compressor, but the cascade system allows small cylinders to be filled without the presence of 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 rupture if the maximum safety pressure is reached.

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. An example could be a 100 litre (internal volume) cylinder pressurised to 200 bar filling a 10 litre (internal volume) cylinder which was unpressurised resulting in both cylinder equalising to approximately 180 bar. 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 cylinder with the lowest pressure first, then the cylinder with the next lowest pressure, and so on.

Uses

Breathing sets

A breathing set may be filled to its working pressure by decanting from big (often blowtorch-sized) cylinders. (To make this easy the neck of the cylinder of the Siebe Gorman Salvus rebreather had the same thread as a blowtorch oxygen cylinder, but the opposite gender, for direct decanting.) The large cylinders are available in a variety of sizes, typically from 50 litre internal capacity to well over 100 litres. [ [http://www.airproducts.com/NR/rdonlyres/7CCE748B-35BA-45B2-93D4-48AE3D3EEDF3/0/reference_cylinder_information.pdf Specialty Gas Cylinder Dimensions] Air Products]

The big cylinders are put into an order of ascending pressure, say A B C, etc. The small cylinder to be filled is connected to big cylinder A, then the gas is allowed to flow into the small cylinder. This is repeated with B, then C, etc., until the small cylinder is as full as needed or as full as possible. The result is that A drains faster than B, which drains faster than C; A is used for the first bulk of the filling of the small cylinder, which is then topped up from B, then from C, which keep a higher pressure for longer.

Hydrogen storage

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. [ [http://www.hydrogencontest.org/pdf/UMissouri_2004.pdf Hydrogen Fueling Station Pag 4.] ]

References