Short-duration breathing apparatus
Short-duration breathing apparatus consists of a small compressed air cylinder and a polythene hood which may be rapidly placed over the head. Their duration is limited to about 15 minutes of comparatively non-exertive effort and the sets must be used only for emergency escape purposes. Depending on the cargoes specified on the ship's Certificate of Fitness, short-duration breathing apparatus may be provided in
accommodation spaces for each crew member. Such equipment may also be supplied for inspections of gas-free enclosed spaces, as an aid in case a hazardous atmosphere is encountered, although, in cases of known danger, it is recommened that compressed air breathing apparatus be worn.
Fresh air respirators
Fresh air respirators consist of a helmet or face mask linked by a flexible hose (maximum length 40 metres) through which air is supplied by a manual bellows or rotary blower. The equipment is simple to operate and maintain and its operational duration is limited only by the stamina of the bellows or blower operators. However, movement of the user is limited by the weight and length of hose and great care must be taken to ensure that the hose does not become trapped or kinked.
Users of such equipment should always wear a safety line for communication and rescue.
While this respirator has been largely superseded by the self-contained or air line compressed air breathing apparatus, it will be found on many ships as a backup to that equipment.
Compressed air breathing apparatus
Compressed air breathing apparatus may be adapted into two forms. It may be the self contained type (SCBA) or the air-line version (ALBA).
In theself-contained (SCBA) version, the wearer carries air for breathing in a compressed air cylinder at an initial pressure of up to 300 bars. The pressure is reduced at the outlet to about 5 bars and fed to the face mask through a demand valve. This provides a slight positive pressure within the mask. The working duration of the equipment depends upon the capacity of the air cylinder and respiratory demand. A pressure gauge and an alarm are provided to warn of low air supply pressure.
A typical set, providing approximately 30 minutes operation with physical exertion, may weigh about 13 kg and the bulk of the cylinder on the back of the wearer imposes some restriction on manoeuvrability in confined spaces. When properly adjusted, the SCBA is simple and automatic in operation. However, maintenance requires care and skill. To ensure serviceability, all such breathing sets should be checked monthly and used during exercises. This should be done using special exercise air cylinders in order to keep the operational cylinders always fully charged or, alternatively, an air compressor may be used for immediate refilling.
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Although demand valves are designed to maintain a slight positive pressure within the face mask, it should not be assumed that this feature will prevent a contaminated atmosphere leaking into an ill-fitting mask. It is essential that, before entry into a dangerous space, the air tightness of the mask on the wearer's face be thoroughly checked in accordance with the manufacturer's instructions. Tests have shown that it is virtually impossible to ensure continued leak tightness in operational conditions on a bearded face.
Most compressed air breathing sets may be used in the air-line version (ALBA) whereby the compressed air cylinder and a pressure reducing valve are placed outside the contaminated atmosphere and connected to the face mask and demand valve by a trailed air hose. At the expense of decreased range and the need for extra care in guiding the trailing air hose, the wearer is relieved of the bulk of the air cylinder. Also, operational duration may be extended by the use of larger air cylinders or special cylinder changeover arrangements.
Canister filter respirators versus SCBA
Canister filter respirators consist of a mask which has a replaceable canister filter attached. In this type of equipment, contaminated air is drawn in by the normal breathing of the wearer and toxic elements are filtered out. They are simple to operate and maintain, can be put on quickly and are used as personal protection for emergency escape purposes on ships certified for carrying toxic cargoes. They are, however, only suitable for relatively low concentrations of toxic gas. Once used, there is no simple means of assessing the remaining capacity of the filter. Filter materials are specific to a limited range of gases and, of course, the respirator gives no protection in atmospheres of reduced oxygen content. For these reasons, the requirements of the Gas Codes for emergency escape protection is now almost exclusively met by lightweight self-contained breathing apparatus.
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