Section 4 Gas Distribution and Supply

4.1.1 The gas distribution systems are to be comprised of piping and components essential to the distribution of gases for normal and emergency operation, and are to be painted with colours appropriate to their function, and marked with the appropriate flow direction.

4.1.2 Each service in the life-support system to a bell or compression chamber is to be capable of being supplied from two sources unless otherwise approved. The arrangements are to be such that loss of any one supply will not render the other supply inoperable.

4.1.3 For wet submersibles, sufficient additional breathing gases should be available to enable decompression stops to be carried out with submersibles operating below 10 m. Such supplies should be readily connectable to breathing apparatus.

4.1.4 Filters or automatic pressure reducers are to be so arranged that they are capable of being removed without affecting the vital gas supplies.

4.1.5 Special attention is to be paid to the oxygen supply, CO2 removal (which may be by chemical means or molecular sieves), emergency breathing arrangements and monitoring devices, particularly if ‘locking in’ and ‘locking out’ are arranged. The CO2 removal system should be capable of operating for a survival period as specified in Pt 5, Ch 4, 8.9 Emergency life-support, in addition to the planned dive times.

4.1.6 Oils used in compressors for gases intended for breathing purposes are to conform to the requirements of the appropriate National or International Authority Standards. Mineral-based oil is not to be used in the compressor itself or where it is likely to contaminate the compressed gases. Consideration may be given to the incorporation of absorption type purifiers.

4.1.7 Means should be provided in the gas mix distribution system to prevent the escape of ‘on board’ gas to the sea in the event of a break in the umbilical gas supply.

4.1.8 The gas supply is to be designed so that a pressure increase up to 2 bar in the living compartment of the compression chamber can be effected at a rate of at least 2 bar/min followed by a rate of 1 bar/per min.

4.1.9 The gas venting system is to be designed so that the pressure in a compression chamber or diving bell can be reduced to 1 bar at a rate of at least 1 bar/per min.

4.1.10 Sets of breathing apparatus which, activated by respiration, supply breathing gas to persons exposed to excess pressures and also remove exhaust gas independently of the chamber atmosphere are to be designed for a gas flow equal to 3 times the required breathing rate per minute (AMV). The required breathing rate per minute depends on the proposed activity and the environmental conditions. When designing the supply and exhaust facilities for breathing masks, the number of persons simultaneously connected to the system is to be allowed for as follows (see Table 4.4.1):

4.1.11 The gas circulating systems are to be so designed that the chamber conditions are maintained.

4.1.13  Where pure oxygen or gas containing more than 23 per cent O₂ by volume is supplied to the chamber, a separate piping system is to be provided for this purpose.

4.1.14 Valves in gas systems are to be so arranged that a valve leakage cannot lead to an unintended mixture of gases and oxygen or oxygen-like gas cannot penetrate into lines intended for other gases. Intersections between oxygen and non-oxygen systems are to be isolated by twin shutoff valves with venting valves in between.

4.2.1 When considering materials to be used in the system due consideration should be given to the effects of electrolytic action and chilling of materials.

4.2.2 All piping and fittings used in the supply and distribution systems should be made from material suitable for the intended service.

4.2.3 Each independent gas supply should be equipped with a gauge capable of monitoring supply pressure when the supply valve is open. Each independent gas supply pressure hull penetration should be equipped with a quick acting shut-off valve. Taper cocks are not acceptable.

4.2.4 Pipes should be formed to eliminate distortion of unions; and the piping system should be designed to have the minimum number of unions.

4.2.5 Non-destructive testing is to be carried out upon completion of all welded pipes.

4.2.6 Pipes, valves, fittings and cylinders subject to external pressure should be capable of withstanding maximum design depth pressure with zero internal system pressure.

4.2.7 Piping, cylinders, valves etc. are to be adequately secured. Cylinders, valves and external piping should be adequately protected from damage.

4.3.1 All valves must be designed to a recognised code or standard.

4.3.2 The maximum permissible working pressure of the valves should be readily identifiable.

4.3.3 Valves or cocks liable to seizure due to changes in temperature or otherwise should not be used.

4.3.4 Valves should be so designed that when the valve is shut the valve gland is not under internal pressure.

4.3.5 Valve rotation should be anticlockwise for opening.

4.3.6 Valves should be clearly marked to indicate their purpose.

4.3.7 Wherever practicable valves should indicate clearly when they are open or shut.

4.3.8 The valve spindle should not rely on the gland to retain the spindle in the valve body.

4.3.9 All valves to be constructed so as to prevent the possibility of the valve or glands being slackened back or loosened when the valves are operated.

4.4.1 For the design requirements of oxygen supply and piping refer to Pt 5, Ch 4, 5 Oxygen Distribution and Supply.

4.5.1 At least one breathing mask is to be provided for each occupant inside each separately pressurized chamber compartment. One spare breathing mask should be provided inside each separately pressurised chamber compartment. An additional spare mask should be included for every ten occupants.

4.5.2 The BIBS are to be connected to the BIBS gas supply and exhaust gas system either permanently or by plug and socket connectors.

4.5.3 The exhaust gas (exhalation line) side of the BIBS is to be protected against any inadmissible pressure drop or inadmissible pressure difference.

4.5.4 The supply of gas to the chamber is to be arranged so as to ensure a homogeneous gas distribution inside the chamber is achieved as quickly as possible.

4.5.5 The open end of the exhaust pipes are to terminate in a safe location.

4.6.1 Besides their normal breathing gas supply diving bells and divers in the water must also carry an independent reserve gas supply.

4.6.2 The supply of breathing gas to the diving bell is to be designed in such a way that, should the diving bell umbilical fail, the reserve chamber supply can be switched manually or automatically to the divers without flowing back into the chamber umbilical.

4.6.3 The divers' umbilical system is to be so designed that each diver has his own independent supply.

4.6.4 In the diving bell at least one breathing mask is to be provided for each diver, plus one spare, and this must be connected to both the normal and the reserve gas supply. Divers' masks and helmets with a gas supply may be recognized as breathing masks.

4.6.5  Automatic pressure reducers are to be provided for the breathing masks.

4.6.6 The emergency oxygen supply is to be fitted with a dosage system to enable the oxygen in the diving bell to be maintained at the correct partial pressure.

4.6.7 The diving bell is to be equipped with two independent exhaust gas lines, which must be arranged to avoid flooding of the electrical equipment. An exhaust gas valve is to be mounted close to the divers' exit.

4.6.8  For the requirements of leak testing and cleaning please refer to Pt 5, Ch 4, 5 Oxygen Distribution and Supply.

4.7.1 An emergency breathing system should be fitted for use in case of fire or smoke in manned compartment(s). This may be a Built-in Breathing System (BIBS) supplied from breathing gas cylinders, or a portable breathing apparatus using its own supply. In some cases both systems may be employed. In either system a face mask connected to the breathing gas supply should be provided for each diver. The system shall be operable for sufficient time to enable an autonomous submersible craft to surface from its maximum operating depth and be able to open the hatch and for all other submersible to be recovered from maximum operating depth; this time shall be a minimum of 30 minutes. Special consideration will be given to craft operating at depths greater than 1,000 metres.

4.7.2 An open circuit system is recommended. If a closed circuit breathing system is used, the resultant pressure build up in the Chamber must be considered.

4.7.3  Either a full-face mask or an oral-nasal mask with eye protection should be provided. It should be possible to communicate whilst wearing breathing masks. Masks should be easily donned, close fitting, and comfortable for the duration of the emergency. The equipment should be designed so that the crew can exit from the submersible craft without first removing the face mask.

4.7.4 Sufficient breathing systems should be provided so that an emergency mask for each member of the crew can be reached in a minimum of time.

4.7.5 Portable breathing apparatus should be examined regularly for possible leakage.

4.7.6 The dive control station and local control stations for handling systems are to be provided with emergency breathing devices with communication equipment to enable the personnel to perform their duties in smoky/polluted environments. The emergency breathing devices are to be capable of functioning for at least 30 minutes. The emergency breathing devices may be self-contained or umbilical supplied.

4.7.7 Diving bells are to be provided with emergency breathing gas using a Built-in Breathing System (BIBS). The BIBS is to be independent of the normal surface supply.