Cylinders should only be filled with air from diving air compressors or with other breathing gases using gas blending techniques. Both these services should be provided by reliable suppliers. Breathing industrial compressed gases can be lethal because the high pressure increases the effect of any impurities in them.
Special precautions need to be taken with gases other than air:
- oxygen in high concentrations is a major cause of fire and rust.
- oxygen should be very carefully transferred from one cylinders to another and only ever stored in cylinders that are certified and labeled for oxygen use.
- Gas mixtures containing proportions of oxygen other than 21% could be extremely dangerous to divers who are unaware of the proportion of oxygen in them. All cylinders should be labeled with their composition.
Contaminated air at depth can be fatal. Common contaminants are: carbon monoxide a by-product of combustion, carbon dioxide a product of metabolism, oil and lubricants from the compressor.
The blast caused by a sudden release of the gas pressure inside a diving cylinder makes them very dangerous if mismanaged. The greatest risk of explosion exists at filling time and comes from thinning of the walls of the pressure vessel due to corrosion. Another cause of failure is damage or corrosion of the threads and neck of the cylinder where the pillar valve is screwed in. Aluminium cylinders have been observed occasionally to fail explosively, fragmenting the cylinder wall. Steel cylinders usually remain mostly intact, and tend to fail at the neck.
Keeping the cylinder slightly pressurized at all times reduces the possibility of contaminating the inside of the cylinder with corrosive agents, such as sea water, or toxic material, such as oils, poisonous gases, fungi or bacteria.
MACHINERY
Modern diving compressors are three- or four-stage-reciprocating air compressors that are lubricated with high-grade compressor oil (a few use ceramic-lined cylinders with O-rings, not piston rings, requiring no lubrication). Oil-lubricated compressor operators must only use lubricants specified by the compressor's manufacturer. Special filters are used to clean the air of any residual oil (see "Air purity").
The compression process helps remove water from the gas, making it dry, which is good for reducing rust in diving cylinders and freezing of diving regulators, but causes dehydration, a factor in decompression sickness, in divers who breathe the gas.
AIR PURITY
In addition, the compressed air output by the compressor must be filtered to make it fit for use as a breathing gas The following filters remove:
- Water, using silica gel, activated alumina or a molecular sieve.
- Oil, using activated carbon or a molecular sieve.
- Carbon monoxide, using catalyst.
Oil, which must be used to lubricate the compressor's internal parts, can be particularly deadly if it enters the breathing gas and is inhaled as a mist. Petroleum-based oils cannot be absorbed and metabolized by the body and will coat the internal surfaces of the lungs, causing a condition known as lipoid pneumonia and leading to asphyxiation and death. For this reason, compressors must be carefully designed and maintained to ensure that no oil enters the breathing gas. Petroleum-based oils must never be used within the compressor. Instead, vegetable-based or specifically-formulated synthetic oils are used, which can be safely absorbed and metabolized by the body in small quantities, should a malfunction occur.
Carbon monoxide (CO) is a gas that is present in the exhaust gas of internal combustion engines, including those often used to drive compressors. It also comes from the breakdown of lubricating oil when compressors run too hot. CO is odorless, colorless, and tasteless. CO is deadly even in small quantities, because it readily binds with the hemoglobin in red blood cells and thus destroys the blood's ability to carry oxygen. Diving compressors must be carefully designed and placed so that the compressor's intake is located in fresh cold air well away and upstream from any engine exhaust.
Periodically, the gas produced by a compressor must be tested to ensure it meets air purity standards. The following impurities are checked for:
- Carbon dioxide
- Carbon monoxide
- Lubricating-oil vapour
- Nitrogen dioxide
- Odor and taste
- Solid particles
- Water vapor
PRESSURE
Diving compressors generally fall into one of two categories: those used for surface supplied diving and those used for filling scuba diving cylinders.
Surface supplied diving compressors are low-pressure and high-volume. They supply breathing gas directly to a diver, from a control station called a "rack" via a hose called an "umbilical". Their output is generally between 6 and 20 bar /100 and 300 psi. These compressors are often very large and powerful, because they must be able to deliver gas at a sufficient pressure and volume to multiple divers working at depths of several hundred feet as the divers breathe.
Compressors used in scuba are high-pressure and low-volume. They fill diving cylinders and storage flasks or banks of storage flasks. These compressors are generally smaller and less powerful because the volume of gas they deliver is not so critical; a lower volume compressor can be used to fill large storage flasks during the long periods of the day when demand is low. This stored compressed air can be decanted into diving cylinders when needed. Common scuba diving cylinder pressures are 232 bar / 3000 psi and 300 bar / 4500 psi.
FILLING HEAT
If diving cylinders are filled too quickly, the gas inside them becomes hot as a result of adiabatic heating, increasing in pressure, which results in a drop in pressure when the cylinder cools later. Cylinders are often filled at a rate of less than 1 bar (100 kPa or 15 lbf/in²) per second to reduce this increase in temperature. In an attempt to cool the cylinder when filling, some people “wet fill”, immersing their cylinders in a cool water bath. This increases the risk of internal cylinder corrosion by moisture from damp components entering the cylinder during filling.
THE BANK
Often compressors are connected to a bank of large, high-pressure cylinders to store compressed gas, for use at peak times. This allows a cheap and low-powered compressor, which is relatively slow at pumping gas, to fill the bank automatically during idle periods, storing a large volume of pressurized air so that a batch of cylinders can be filled quickly one after the other at peak demand without being delayed by the slow-running compressor. In surface-supplied diving, high-pressure cylinder banks may be used as an emergency backup in case of primary compressor failure, or they may be used as the primary source of breathing gas.
GAS BLENDING
Compressors are often linked to a gas blending panel so nitrox, trimix, heliair or heliox mixes can be created. The panel controls the decanting of oxygen and helium from cylinders bought from commercial gas suppliers.
As it is not possible to decant to a diving cylinder from a storage cylinder that holds gas at a lower pressure than the diving cylinder, the expensive gas in low pressure storage cylinders is not easily consumed and may go to waste when the storage cylinder is returned to the supplier. The cascade system is often used with a bank to economically consume these high cost gases so that the maximum possible gas is removed from the bank. This involves filling a diving cylinder by first decanting from the bank cylinder with the lowest pressure that is higher than the diving cylinder's pressure and then from the next higher-pressure bank cylinder in succession until the diving cylinder is full. The system maximizes the use of low-pressure bank gas and minimizes the use of high-pressure bank gas.
Another method for scavenging expensive low pressure gases is to pump it with a booster pump.
Source: Wikipedia
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