Section 7 Carbon Dioxide (CO2) Removal Systems
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Construction & Classification of Submersibles & Diving Systems, July 2022 - Part 5 Main and Auxiliary Machinery, Systems and Equipment - Chapter 4 Life Support Systems - Section 7 Carbon Dioxide (CO2) Removal Systems

Section 7 Carbon Dioxide (CO2) Removal Systems

Parent topic: Chapter 4 Life Support Systems

7.1 General Requirements

7.1.1 CO2 may be removed either by a chemical which cannot be regenerated such as soda lime granules, or lithium hydroxide, or else by a chemical which can be regenerated such as monoethanolamine (MEA) or molecular sieves.

7.1.2 It is recommended that the CO2 removal system is operable for the planned dive time plus a period of time consistent with the emergency rescue plan. Please refer to Pt 5, Ch 4, 8 Air and Gas for Breathing Purposes.

7.1.3 A regeneration system should be designed with an instantaneous removal rate of 45 litres of CO2 per man hour.

7.1.4 Absorbents should be contained in hermetically sealed packages until required for use. Such seals should be periodically checked. It should also be noted that some absorbents, including lithium hydroxide, have a limited shelf life.

7.1.5 The efficiency of granular carbon dioxide absorbents is detrimentally influenced by low temperature and relative humidity levels of lower than 70 per cent, when absorbent capacity is reduced. Temperature and humidity tests have shown that lithium hydroxide is the most efficient absorber, particularly at low temperatures, with sodasorb as the next best.

7.1.6 Non-corrosive, non-toxic materials should be used for the construction of the CO2 scrubber system. With alkali absorbents such as lithium hydroxide and potassium super-oxide, uncoated aluminium must not be used.

7.1.7 Back up equipment must be provided. The CO2 scrubbing system should be designed to function in case of a power failure, or should be capable of functioning as a passive system. A spare motor blower assembly is to be carried where possible. The back-up system could be one of the following:
  1. A separate power source to motor blower;
  2. A hand or foot-operated drive for the circulating blower;
  3. A passive system designed to operate without forced air circulation;
  4. Lung powered
  5. An independent in chamber CO2 removal system.

7.1.8 The acceptance of alternative means for CO2 removal other than those referenced in this section will be subject to review of supporting data demonstrating satisfactory performance under the intended service conditions.

7.2 Expiration rate

7.2.1 CO2 expiration rates vary over a wide range depending on the degree of activity. For submersibles the minimum rate per man should be taken as 17.5 litres per hour at 20 oC.

7.3 Allowable Concentration

7.3.1 Throughout the normal dive time the design goal should be a maximum CO2 partial atmospheric pressure level of 5.0 mbar. At the end of the emergency life support period the maximum CO2 partial pressure should not exceed 10 mbar.

7.4 Expendable methods of CO2 removal

7.4.1 Lithium hydroxide:
  1. This is a strong alkali and extreme care should be taken to avoid contact with eyes or skin.
  2. Canisters should be replaceable as complete units and should not be refilled on board. The removal system should be located in such a position as to prevent any of the chemical from falling on crew members. When used in an absorbent canister an efficient thick dust filter should be used.
7.4.2 Soda lime:
  1. This is a weak alkali and should be handled with some caution.
  2. A dust filter or fine mesh similar to that recommended for lithium hydroxide should be used. A `high moisture' soda lime should be used since ordinary soda lime will not perform adequately at medium to low humidity levels.
7.4.3 Other chemicals may be considered on the basis of satisfactory performance e.g. potassium superoxide. Superoxide can perform the dual functions of oxygen supply and CO2 removal. Great care should be taken in handling these chemicals and contact with water must be avoided to prevent the sudden release of oxygen and risk of explosion. Grease contamination must also be avoided.
Note the use of barium lime is not recommended.

7.5 Regenerative methods

7.5.1 Molecular sieve. This is a solid absorbent which is capable of removing carbon dioxide from an air stream and which can be regenerated. The entering air stream must be free of organics and moisture since these reduce the ability of the molecular sieve to remove CO2. Any desiccant beds such as silica gel or activated alumina which may be used should be placed in the regeneration flow before the molecular sieve to prevent moisture carry-over to the sieve beds. A means of disposing of the CO2 must be provided.

7.5.2 Liquid Reagents. A liquid scrubbing system using aqueous solution to remove carbon dioxide which can be regenerated. Contact with the eyes and skin must be prevented. Moisture and organic material carry-over (atmosphere contamination) should be controlled.

7.5.3 Other chemicals may be considered on the basis of satisfactory performance. Great care should be taken in handling these chemicals and contact with water must be avoided to prevent the sudden release of oxygen and risk of explosion. Grease contamination must also be avoided.

7.6 Dust Filtration

7.6.1 Dust filtration or a fine mesh shall be provided. Filter materials should be fire-resistant; oil impregnated filters should be avoided.

7.7 Removal of Hydrogen, Carbon Monoxide and Other Contaminants

7.7.1 Means shall be provided for the removal of all identified types of gaseous and vaporous contaminants from the submersible’s atmosphere.

7.7.2 Hydrogen concentration shall not exceed 10% LEL at any point in the enclosed Space.

7.7.3 The concentration levels of toxic and volatile compounds are not to exceed the values determined as guided by UK HSE EH75 for hyperbaric environment and HSE EH 040 as applicable. Alternatively, the limits specified in equivalent recognised national or international standards will be considered

7.8 Control and sensing

7.8.1 A simple on-off system for the circulating blower for the CO2 scrubbing system is sufficient; an automatic control is not required, but an indication light for an inoperative blower motor, or loss of air flow, is recommended.

7.8.2 A continuous indicating analyser for CO2 is desirable, although an intermittent indicator, such as an analyser tube, is acceptable. If a continuous indicating analyser is provided a backup indicator should be available.

7.9 Testing

7.9.1 The CO2 removal system should be tested to confirm system performance. All test data should be fully documented.

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