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A cascade filling system is a high-pressure
gas cylinder storage system that is used for the refilling of smaller compressed gas cylinders.
In some applications, each of the large cylinders is filled by a
compressor, otherwise they may be filled remotely and replaced when the pressure is too low for effective transfer. 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 close succession, with longer intermediate periods during which the storage cylinders can be recharged.
Principle of operation
When gas in a cylinder at high pressure is allowed to flow to another cylinder containing gas at a lower pressure, the pressures will equalize 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 a constant temperature, the equilibrium pressure is described by
Dalton's law
Dalton's law (also called Dalton's law of partial pressures) states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases. This empirical law was observed by Joh ...
of partial pressures and
Boyle's law
Boyle's law, also referred to as the Boyle–Mariotte law, or Mariotte's law (especially in France), is an experimental gas law that describes the relationship between pressure and volume of a confined gas. Boyle's law has been stated as:
The ...
for ideal gases.
The formula for the equilibrium pressure is:
: P
3 = (P
1×V
1+P
2×V
2)/(V
1+V
2)
: where P
1 and V
1 are the initial pressure and volume of one cylinder
: P
2 and V
2 the initial pressure and volume of the other cylinder
: and P
3 is the equilibrium pressure.
An example could be a 100-litre (internal volume) cylinder (V
1) pressurised to 200 bar (P
1) filling a 10-litre (internal volume) cylinder (V
2) which was unpressurised (P
2 = 1 bar) (resulting in both cylinder equalising to approximately 180 bar (P
3). If another 100-liter cylinder pressurized this time to 250 bar were then used to "top-up" the 10-liter cylinder, both of these cylinders would equalize to about 240 bar. However, if the higher pressure 100-liter cylinder were used first, the 10-liter cylinder would equalize to about 225 bar and the lower pressure 100-liter 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 the 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.
Uses
Breathing sets
A breathing set cylinder may be filled to its working pressure by decanting from larger (often 50 liters) cylinders. (To make this easy the neck of the cylinder of the
Siebe Gorman Salvus rebreather
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's breathing, exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. ...
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.
In the more general case, a high-pressure hose known as a filling whip is used to connect the filling panel or storage cylinder to the receiving cylinder.
Cascade filling is often used for partial pressure blending of
breathing gas
A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed ...
mixtures for
diving
Diving most often refers to:
* Diving (sport), the sport of jumping into deep water
* Underwater diving, human activity underwater for recreational or occupational purposes
Diving or Dive may also refer to:
Sports
* Dive (American football), a ...
, to economize on the relatively expensive oxygen, for
nitrox
Nitrox refers to any breathing gas, gas mixture composed (excepting trace gases) of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual applicatio ...
, and the even more expensive helium in
trimix or
heliox
Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because mixture generates less resistance than atmospheric air when passing through the airways of the lung ...
mixtures.
Compressed natural gas fueling
Cascade storage is used at
compressed natural gas
Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in cy ...
(CNG) fueling stations. Typically three CNG tanks will be used, and a vehicle will first be fueled from one of them, which will result in an incomplete fill, perhaps to 2000 psig for a 3000 psig 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.
Hydrogen storage
In cascade storage systems for
hydrogen storage
Hydrogen storage can be accomplished by several existing methods of holding hydrogen for later use. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand ...
, for example at
hydrogen station
A hydrogen station is a storage or filling station for Hydrogen. The hydrogen is dispensed by weight. There are two filling pressures in common use. H70 or 700 bar, and the older standard H35 or 350 bar. As of 2021 around 550 filling stations were ...
s,
fuel dispenser
A gasoline pump is a machine at a filling station that is used to pump gasoline (petrol), diesel, or other types of liquid fuel into vehicles. Gasoline pumps are also known as bowsers or petrol bowsers (in Australia), petrol pumps (in Common ...
A draws hydrogen from tank A, while dispenser B draws fuel from
hydrogen tank
A hydrogen tank (other names- cartridge or canister) is used for hydrogen storage. The first type IV hydrogen tanks for compressed hydrogen at were demonstrated in 2001, the first fuel cell vehicles on the road with type IV tanks are the Toyota ...
B. If dispenser A is over-utilized, tank A will become depleted before tank B. At this point dispenser A is switched to tank C. Tank C will then supply dispensers 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.
Hydrogen Fueling Station Pag 4.
Broken link
Arrangement of system
The storage cylinders may be used independently in sequence using a portable transfer whip with a pressure gauge and manual bleed valve, to transfill the receiving cylinder until the appropriate fill pressure has been reached, or the storage cylinders may be connected to a manifold system and a filling control panel with one or more filling whips.
Ideally, each storage cylinder has an independent connection to the filling panel with a contents pressure gauge and supply valve dedicated to that cylinder, and a filling gauge connected to the filling whip, so the operator can see at a glance the next higher storage cylinder pressure compared to the receiving cylinder pressure.
The storage cylinders may be filled remotely and connected to the manifold by a flexible hose when in use, or maybe permanently connected and refilled by a compressor through a dedicated filling system, which may be automated or manually controlled.
An over-pressure safety valve is usually installed inline between the compressor and the storage units to protect the cylinders from overfilling, and each cylinder may also be protected by a rupture disc.
References
External links
*
{{Underwater diving, divsup
Breathing gases
Industrial breathing sets
Diving support equipment
Gas technologies
Hydrogen storage
Pressure vessels