EP0203133B1

EP0203133B1 - Secondary life support system

Info

Publication number: EP0203133B1
Application number: EP85905862A
Authority: EP
Inventors: Derek William Clarke; Brian Murray; Donald Lynn Rodocker
Original assignee: Gas Services Offshore Ltd
Current assignee: Gas Services Offshore Ltd
Priority date: 1984-11-23
Filing date: 1985-11-22
Publication date: 1990-04-11
Anticipated expiration: 2005-11-22
Also published as: CN85109648A; NO162063B; EP0203133A1; CN1009816B; GB8429706D0; JPS62501280A; DE3577074D1; AU580829B2; SU1722222A3; ZA858960B; AU5200186A; WO1986003171A1; NO862931L; NO162063C; GB2169209A; BR8507074A; NO862931D0; GB2169209B

Abstract

As one aspect diver's secondary life support system comprising a semi-closed circuit rebreather set connected via one or more hoses to an isolating valve and a helmet- or mask-interface, the rebreather set in a standby mode during normal diving operations being maintained at a pressure in excess of ambient external pressure. A further aspect of the invention is a semi-closed circuit rebreather set for a diver's secondary life support system, the set being attachable by one or more hoses (2) to an isolating valve (4) and helmet- or mask-interface (17), the set comprising at least one counterlung (1), a moisture absorber (10), a carbon dioxide scrubber (11), and a restrictor (6) which is attachable to a pressurised gas bottle (5) so that when the set is actuated gas can bleed into the set at a substantially constant rate. It is a particularly advantageous feature of such a set that it may be maintained at a pressure in excess of ambient external pressure when in a standby mode. By maintaining an over-pressure in the rebreather set in use, the possibility of sea water leakage into the set is avoided.

Description

The present invention relates to a secondary life support (SLS) system designed specifically for use im bail-out by divers, particularly bail-out during deep diving operations. Conventional bail-out equipment of the type shown in US Patent 3680556 (Morgan) comprises compressed breathing gas in a bottle with a connecting hose or hoses and demand regulator valving allowing a diver to breath down the gas from the bottle. This is an open circuit system and the breathed gas is expelled from the divers helmet or mask. In deep diving operations (e.g. 450 metres depth, ⁴⁵ bar pressure) the quantity of breathing gas which can be carried (e.g. 4 litres at 300 bar) is sufficient only for a short period of time (e.g. about 20 to 90 seconds) depending on breathing rate.
Conventional divers primary life support systems operate on a demand principle and are normally supplied with gas from a remote source (e.g. either from the surface or from a diving bell). A diver may require a bail-out if there is a failure in the primary system, e.g. if his umbilical supply line becomes damaged, disconnected or caught up. The bail-out duration should allow sufficient time for the diver to return to safety [e.g. to a diving bell] or for a rescue to be made.
Secondary life support systems of the type shown by DE-A-2104153 (Dragerwerk) and "Mark VIII" of "Machine Design Vol. 40 No. 25" comprise a semi-closed rebreather set according to the first part of claim 1 which includes a breathing circuit with inhale and exhale hoses coupled to the interface of the diver's headgear, at least one counterlung, a moisture separator and a carbon dioxide scrubber, these three components coupled to at least one of said hoses. The breathing circuit is connected to a secondary source of pressurized breathing gas, there being a gas flow restrictor in the connecting conduit means. The system may include means for isolating the secondary gas source from the breathing circuit of the rebreather set in the standby mode and connecting the secondary gas source to the breathing circuit to provide a controlled supply of breathing gas in the actuated mode.
The present invention according to the characterising part of Claim 1 provides such a system characterised by a valve positioned between the breathing circuit of the rebreather and the helmet interface whereby, in the standby mode the diver can breathe from the primary gas supply and is prevented from breathing through the rebreather set as the valve isolates the rebreather set from the helmet interface. In the actuated mode the valve connects the rebreather set and the interface and isolates the interface from the primary gas supply to allow the diver to breathe through the rebreather set. The present invention is further characterised by conduit means for connecting the primary breathing gas supply, via a regulator, to the rebreather set when in standby mode. This is used to maintain within the rebreather set a predetermined breathing gas pressure in excess of ambient external pressure, regardless of the changes in the ambient external pressure, and if the pressure within the rebreather set becomes in excess of said predetermined pressure, excess gas is exhausted via relief valve means. The overpressure may be, for example, about 4 bar but a preferred over-pressure of about 0.1 to 0.2 bar is considered sufficient. Particular advantages of an SLS system described above include:

i) the use of a semi closed rebreather extends the bail-out duration as compared to an open-circuit system;
ii) by maintaining an over-pressure in the set the possibility of sea water leakage into the set is removed; and
iii) the operation of the set in the standby mode can be controlled so there is neither variation in buoyancy nor in gas over-pressure when the diver changes depth.
iv) isolation of the diver's breath from the rebreather set during normal diving operations prevents unnecessary depletion of the activity of the C0₂ scrubber in the standby mode, and thus provides the maximum effective lifetime of the rebreather set in the actuated mode when it is required in emergency.

Embodiments of SLS bail-out rebreather sets illustrating the above-mentioned features will now be described in detail, by way of example only, with reference to the accompanying Figures in which:-

Figure 1 is a schematic representation of a bail-out rebreather set according to the invention;
Figure 2 shows the predicted work of breathing of a bail-out rebreather set at various breathing rates at 450 MSW and 250 MSW depth; and
Figure 3 is a schematic representation of a further bail-out rebreather set according to the invention.

With reference to Fig. 1, a divers secondary life support system includes a semi-closed circuit bail-out rebreather set having a counterlung 1 connected via a single hose 2 to a helmet 3, with an isolating valve 4 mounted on the helmet. In prior art rebreather sets for use in standard diving operations (e.g. closed circuit oxygen rebreathers of the type shown in US Patent 4362154 (LeMasson)), it is normal to have twin hose connections to the helmet or mask; separate hoses are used for inhalation and exhalation. As shown in Fig. 1, a conventional bottle 5 of approximately 4 litres water volume is used for make-up gas storage at a pressure of e.g. 200 to 300 bar. The outlet pressure of this make-up gas is regulated to a pressure in excess of the ambient external pressure and when the set is in the actuated mode for bail-out the gas is allowed to bleed via a restrictor 6 into the counterlung at a fixed rate of e.g. 1 to 2 litres per minute. The make-up gas preferably has a physiologically high oxygen content of up to about 2.5 bar partial pressure.
In the standby mode the set may be maintained at a predetermined pressure relative to the external ambient pressure; it is particularly preferable to have an over-pressure in the set in the standby mode of up to approximately 4 bar, e.g. generally about 0.1 to 0.2 bar.
Within a back pack 7 the hose 2 splits into separate inhale and exhale hoses 8 and 9, which pass through a moisture absorber 10 and a C0₂ scrubber 11 on the exhale/cycle. In this embodiment the major system components, including the C0₂ scrubber 11, are heated in normal operation by a bleed 12 taken from the normal diver hot water supply. The back pack 7 is preferably insulated against external cold. Heating of the C0₂ scrubber 11 in the standby mode maintains the chemical absorbent (e.g. soda lime) at a temperature at which it will operate efficiently if the set is put into the acuated mode for bail-out. A thermal regenerator consisting of layers of fine wire mesh may be placed upstream of the counterlung to prevent heat loss via the large surface area of the counterlung. When the set is actuated heat is removed from the exhaled gas and, on inhalation, the cold gas is drawn back through the thermal regenerator where it picks up stored heat prior to entering the divers respiratory system.
When bail-out is required, the isolating valve 4 is opened and the counterlung will immediately vent any over-pressure into the helmet. Depending on the nature of the emergency, this immediate supply of gas may be of value in purging the helmet. The helmet mushroom valve will vent any excess quantity of gas introduced in this way, avoiding any over-pressurisation of the helmet.
In the actuated mode exhaled gas, which consists mainly of a diluent, some residual oxygen and carbon dioxide passes via one or more hoses to a chemical absorbent (i.e. soda lime) to remove carbon dioxide and to the counterlung where the gas is mixed with make-up gas containing a physiologically high oxygen content. Gas from the counterlung is reinspired by the diver.
Clearly, the endurance of the set is governed to a large extent by the bleed rate of make-up gas into the counterlung. As is shown later a bleed rate of 1-2 I/min is adequate for respiration rates of up to 75 t/min RMV (Respiratory Minute Volume). Since each breath removes only a fraction of the total oxygen content, given a high initial oxygen partial pressure, the same gas can be rebreathed many times over providing that effective C0₂ scrubbing is provided. In order to maximise the reliability of the set and avoid maintenance problems offshore, electronic devices for controlling oxygen injection have been avoided. Because of the relatively wide range of oxygen levels which can be breathed satisfactorily, a fixed bleed of mixed gas having an oxygen partial pressure of up to about 2.5 bar can be shown to give acceptable oxygen levels at all breathing rates.
For a bail-out set there are a number of areas where it is appropriate to consider particular design criteria in more detail, for example:

(i) It is well known that the onset of oxygen toxicity is dependent on many factors, including the duration of the exposure. For the present purposes, it is suggested that a maximum oxygen partial pressure of 2.5 bar is acceptable as a design figure for the short durations involved in a bail-out set. Higher values up to 3 bar are probably acceptable, but should be investigated further. It is worth noting that the US Navy decompression tables allow administration of a therapeutic gas mix containing up to 2.5 bar pp0₂ for treatment of decompression sickness.
(ii) The minimum desirable oxygen level is 0.4 bar, although levels down to 0.2 bar are tolerable.
(iii) With regard to C0₂ levels, it is proposed that the design target for a bail-out set be 20 millibar average inspired C0₂ level and 7 millibar end tidal inspired C0₂ level at the end of the scrubber canister duration.

The following describes the results of a series of calculations aimed at predicting the performance of a proposed rebreather set. Separate calculations have been carried out to estimate the following:

The oxygen level occurring in the set under a variety of operating conditions;
The re-inspired carbon dioxide level as a function of breathing rate;
The breathing resistance and work of breathing.
In the following, the calculation procedure is outlined and the results presented.

Oxygen partial pressure

Following some preliminary hand calculations, a computer solution was adopted as being the most suitable way of obtaining a clear picture of oxygen levels in a set under a variety of operating conditions. Essentially, the procedure adopted was to carry out an oxygen balance over the duration of the set. That is, at the start of a run, the counterlung etc. was assumed to be fully charged with gas mix corresponding to that in the storage bottle. Over a short time step, oxygen enters the system via the gas bleed from storage while oxygen leaves the system both through metabolic consumption and through the overboard dump. Thus, the change in oxygen level in short time steps can be computed.
Table 1 shows the results obtained for 4 breathing rates at depths from 100 metres to 450 metres. In each case, the oxygen level falls from the initial value to reach a plateau, depending on breathing rate. Throughout, the maximum oxygen level was around 2.5 bar, at the start of the run. Plateau levels varied from around 2 bar at the lowest breathing rate down to 0.4-0.8 bar at the highest breathing rates. The endurance of the set is determined principally by the rate at which the gas bleed depletes the storage volume. However, some additional time is gained by "breathing down" the gas in the counterlung. In general, the endurance of the set diminishes with depth because of the greater quantity of gas consumed at depth. The shortest endurance calculated was approximately 16 minutes, at a depth of 450 metres, breathing continually at a rate of 75 I/min RMV. At lower breathing rates, at the same depth, this endurance extends to 24 minutes. At shallower depths, the endurance of the set will generally exceed 25 minutes.
The oxygen profiles for a more realistic breathing sequence with a variable RMV show that the oxygen level in the set to vary according to the work rate, with overall endurance figures slightly in excess of that obtained at the maximum RMV.
On this basis, the set can achieve a minimum endurance of 15 minutes at 450 MSW and considerably longer at shallower depths. Although no electronic oxygen partial pressure control is provided it may be shown that the upstream level stays at all times within a band which is acceptable, at least for the short durations required of a bail-out set.

Carbon dioxide levels

Based on tests carried out using modern, high performance sodalime (MP United Drug Co. 797 Grade) the C0₂ scrubber works effectively for the duration required using 1-2 litres of sodalime. However, some C0₂ will be re-inspired because of the dead volume in:

(i) the oral nasal
(ii) the inhale/exhale hose
(iii) the isolating valve block

The results of calculations of C0₂ partial pressure versus breathing rate indicate that, except at the lowest breathing rates, the reinspired C0₂ level is satisfactory. Not surprisingly, at low tidal volumes the average inspired C0₂ level is elevated although still within the design target of 20 millibar. At worst, this will result in a minor degree of hyper-ventilation and, as such, gives no cause for concern over the short durations encountered in bail-out. At the higher work rates, because of the increased tidal volume, average inspired C0₂ levels should be low.

Work of breathing

Four sources of breathing resistance have been identified.
Frictional losses in the inhale/exhale hose;
Frictional losses in the C0₂ scrubber;
Mushroom valves;
Inertial (at end tidal conditions) and drag effects (at maximum velocity) in the water surrounding the counterlung.
The rebreather hose calculation is based on conventional pipe friction theory. The CO₂ scrubber calculations are based on tests carried out on a survival kit scrubber, charged with MPUD 797 Grade Sodalime. Results have been scaled to 450 MSW and the higher work rates associated with the present equipment. The hydrodynamic losses in the counterlung have been based on plausible assumptions having regard to its geometry.
Figure 2 summarises the results of this calculation. The open and closed circles on the Figure represent results for a set of 450 and.250 MSW depth respectively. The dashed line represents a recommended limit for work of breathing and the upper full line represents an upper limit. No data has been presented for peak inhale/exhale resistance since this will depend on the biasing applied to the set. However, with regard to work of breathing it may be seen that the predicted values are modest at low work rates and acceptable at the highest work rate of 75 I/min RMV. It is reasonable to suppose that it is easier to obtain satisfactory work of breathing values in a bail-out rebreather rather than a conventional rebreather because of the lower quantities of C0₂ absorbent involved.
The technical appraisal carried out has confirmed the feasibility of the bail-out rebreather set. Despite the absence of electronic control systems oxygen levels appear acceptable at all work rates, at least for the short exposures involved in bail-out. Similarly, CO_Z levels and work of breathing are not found to be excessive.
A second embodiment of a secondary life support system is shown in Fig. 3. The semi-closed circuit bail-out rebreather set, when in a standby mode for diving operations, is maintained at a pressure 0.2 bar in excess of the ambient external pressure. Counterlungs 1 which are physically restrained to prevent inflation by the overpressure whilst in the standby mode are mounted on the divers shoulders. This minimises hydrostatic effects on the breathing circuit when the set is in the actuated mode. On actuation, the counterlungs 1 are released and are inflated (or partially inflated) by the over-pressure within the set. In the event of emergency the diver is required to actuate the rebreather set by two non-sequential actions;

i) rotate the isolating valve 4 which simultaneously presents the mouthpiece 16 in front of the divers mouth, and
ii) expose and pull the actuation cord 13 which will release the counterlungs 1 and transfer the operational mode of the SLS regulator 14 to change the source of gas supply from the divers umbilical 15 to the gas storage bottles 5.

With the rebreather set actuated and in the bail-out mode gas will flow at a controlled rate from the gas storage bottles 5 via the SLS regulator 14 and the restrictor 6 to the scrubber/thermal regenerator housing of the back pack 7 to replenish the gas within the rebreather set.
When the set is actuated the diver will accept the mouthpiece 16 and breath naturally on it. The expired gas will pass through the mouthpiece 16, helmet interface 17 and be directed by the exhale valve 18 into the exhale hose 9. Within the back pack 7, the expired gas will flow to the plenum below the scrubber canister 11 where even distribution is achieved. The gas then passes through the C0₂ scrubber canister 11 which is charged with sodalime granules for removal of C0₂ from the expired breath. From here the gas passes through a thermal regenerator 19, consisting of a number of layers of fine wire mesh which, due to their large surface area, absorb heat allowing relatively cold gas to pass via the hoses 20 to the shoulder mounted counterlungs 1. On inhalation gas is drawn from the counterlungs 1 via the hoses 20 to the thermal regenerator 19 which returns the heat which was stored on the exhale part of the cycle. The gas is then ducted from the backpack 7 and through the inhale hose 8, passed through the inhale valve 30, to the helmet interface 17 and via the mouthpiece 16 to the diver. If a diver, using the rebreather set in the standby mode for diving operations, changes depth upward a pressure differential will occur and the excess pressure in the rebreather set generated as a result of this will be exhausted via a relief valve 31. In the event of the reverse situation whereby a downward depth change is made then additional gas will flow automatically into the set via the SLS regulator 14.
In the standby mode for diving operations hot water is fed to the rebreather set and directed into a hot water jacket 21 around the scrubber/thermal regenerator to preheat and hold the temperature within the scrubber/thermal regenerator at an acceptable level. Heat will be transferred to the breathing gas from the thermal regenerator/scrubber after the actuated mode has been selected even in the worst case situation where the hot water supply to the rebreather set is terminated.
Initially, when the rebreather set has just been actuated for bail-out, a slight negative pressure may occur on account of the diver inhaling and this situation will induce the operation of the demand valve 27 which will immediately inject gas into the backpack 7 and provide the required positive pressure for optimum working of the set.
A moisture absorber is incorporated within the backpack 7 primarily to collect suspended moisture from the divers expired breath.
Additional features of this embodiment of a secondary life support system bail-out rebreather set shown in Fig. 3 are as follows: a pressure gauge 22; a filter 23, a blow-out plug 24; a dip tube 25; a charging connection 26; a primary life support system exhaust valve 28; and an oral nasal mask 29.

Claims

1. A diver's secondary life support system operable in a standby mode during normal diving operations and in an actuated mode for bail-out on failure of a diver's primary breathing gas supply from a diver's primary life support system wherein the primary life support system includes a diver's headgear having an interface (17), the secondary life support system comprising:

a semi-closed circuit rebreather set including a breathing circuit, said breathing circuit including inhale and exhale hoses (8, 9) coupled to the interface (17) on the diver's head gear, at least one counterlung (1), a a moisture separator (10) and a carbon dioxide scrubber (11), said counterlung, moisture separator and carbon dioxide scrubber being coupled to at least one of said hoses;

a secondary source of pressurized breathing gas (5);

conduit means for coupling the secondary gas source to the breathing circuit of the rebreather set;

a gas flow restrictor (6) in said conduit means;

means for isolating the secondary gas source from the breathing circuit of the rebreather set in the standby mode and connecting the secondary gas source to the rebreather set in gas flow communication therewith to provide a controlled supply of breathing gas thereto in the actuated mode; characterised by;

a valve (4) between the breathing circuit and the interface (17) whereby in the standby mode the diver can breath from the primary gas supply and the valve isolates the breathing circuit of the rebreather set from the interface on the diver's headgear to prevent the diver breathing through the rebreather set and in the actuated mode the valve connects said rebreather set and said interface and isolates the diver's breathing gas supply from the primary life support system to allow the diver to breathe through the rebreather set;

means for connecting the breathing circuit to the diver's primary breathing gas supply in the standby mode via a

regulator (14) for maintkining within the rebreather set a predetermined breathing gas pressure in excess of ambient external pressure regardless of the changes in the ambient external pressure; and

relief valve means (3) for exhausting breathing gas from the rebreather set in excess of said predetermined breathing gas pressure.

2. A divers secondary life support system according to Claim 1 wherein the counterlung is inflatable and including means for physically restraining the counterlung so as to prevent inflation when in the standby mode and for allowing inflation of the counterlung in the actuated mode by said predetermined breathing gas pressure.

3. A secondary life support system according to Claim 2, including means for mounting the counterlung on a divers shoulder.

4. A diver secondary life support system according to Claim 1 including means for bleeding gas from the secondary gas source into the breathing circuit of the rebreather set via the gas flow restrictor at a constant rate in the actuated mode.

5. A divers secondary life support systs according to Claim 1 including jacket means (21) for circulation of hot water in order to maintain the rebreather set at an elevated temperature in the standby mode.

6. A divers secondary life support system according to Claim 1 including a thermal regenerator, comprising layers of wire mesh, disposed in the breathing circuit to remove heat from exhaled gas and to warm inhaled gas.

7. A divers secondary life support system according to Claim 1 including a mouthpiece (16) coupled to said hoses (8, 9) and means for presenting the mouthpiece in front of the divers mouth in response to actuation of the valve (4) to place the secondary life support system in the actuated mode.