SCBA is used to fight with fire and enable us to enter into enclosed spaces where there is less oxygen.
Basically, there are two types of SCBAs; Closed-Circuit and Open Circuit. I will be focusing on more Open Circuit System since it is the most common type on board ships.
1. Closed Circuit SCBA
- These are also known as ''rebreather''.
- The exhaled gas is filtered and recirculated.
- It allows more working time.
- However, these are heavier.
2. Open Circuit SCBA
- The air to breathe is stored in the cylinder.
- The compressed air is passed 1st stage reducer where presure is reduced to medium pressure. Another branch of pipeline from the cylinder goes directly to the pressure gauge and Warning Device without any reduction in pressure.
- Thereafter, the medium-pressured air is passed the 2nd stage reducer (a.k.a. Lung Demand Valve (LDV)) where pressure is redeced down to slightly above the ambient pressure.
- The exhaled is is vented out to the environment from the mask.
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| BLOCK DIAGRAM OF OPEN CIRCUIT SCBA |
Components of Open-Circuit SCBA
1. BA Cylinders
There are two types of BA Cylinders, steel and carbon-composite cylinders.
Steel BA cylinders are commonly used. They shall be hydro-tested at least once every 5 years (MSC Circ.1432)
Carbon Compsosite Cylinders consist of a alimunium liner over carbon fibers. These may be accepted by Administration if the followings are satisfied:
- Max. charging rate: 27 bar/minute
- If the cylinder is abrased or cut damaged to the carbon composite layer, it must be rendered as unserviceable.
- There must be label on the cylinder that displays vital safety information and should be clearly marked.
- Cylinders should be provided with a suitable protective cover for better protection against contact damage which should be removed prior to each charging for visual check.
- They are subject to hydrostatic test as per Administration requirement.
MSC Circ. 1432, MSIS 12 Chapter 9
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| Draeger Carbon-Composite Cylinder Source: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.draeger.com/Products/Content/compressed-air-breathing-cylinders-all-pi-9105727-en-master.pdf |
The working principle is the same with First Stage Regulator in EEBD.
When the clinder is turned on, high pressure air will start to flow to the valve seat. When air is passed after the spring, the air accumulated between low pressure side of the pipeline and LDV will trigger the spring to close the valve.
Spring load is adjusted by maker.
Outlet pressure < spring load force: Valve Opens (spring in neutral postion) - air flows.
Outlet pressure> spring load force: Valve closes (spring is compressed) - air stops.
Spring load is adjusted by maker.
Outlet pressure < spring load force: Valve Opens (spring in neutral postion) - air flows.
Outlet pressure> spring load force: Valve closes (spring is compressed) - air stops.
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| First Stage Regulator of SCBA |
It shall be activated automatically and warn that air in the cylinder is reduced to no less that 200 litres
( at around 55 +/- 5 barg - red rection at pressure gauge) (FFS Code 2.1.2).
(200 l) / (40 l/min) = 5 minutes. That means, if warning device is activated, we must understand that aprroximately 5 minute remains to go!
4. Lung Demand Valve (LDV)
LDV is also known as ''Second Stage Regulator'' which decreases medium pressure to slightly above the atmospheric pressure. LDVs are activated by breathe of user and/or purge button.
LDV opeartion is automated depending on the face mask pressure. If we asssume that there is no leak in the face mask, user breathes govern the Lung Demand Valve operation.
The activation of LDV can be either ''Negative LDV'' or ''Positive LDV''.
- In Negative LDV operation, LDV activates the flow when the pressure in the fack mask is reduced below ambient pressure. In case of a leak in the face mask, the hazardous gas may be inhaled.
- The Postive LDV relies on the principle that face mask pressure is kept as always slightly above the atmospheric pressure. In case of a leak in the face mask, the cylinder will run off easily to try keeping the face mask pressure above the atmospheric pressure at all times.
Please see the video below the working of Positive Pressure LDV.
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| PARTS OF LDV |
Donning/Doffing button, also known as Reset Button is to cut air supply during donning or doffing.
Purge button activates air flow. It is used to supply air to the user if the user's hardy breathing or to purge the air remaining in the line when cylinder vavle is closed.
Some LDVs are also equipped with ''By-Pass Button''. This button is used to by-pass the LDV and give medium supply to the face mask in case LDV mulfanctions.
Donning Procedure of SCBA
Press OFF button (red) to cut off air supply to
the facemask. Hear ''click''sound.
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Pressure Test |
Open cylinder valve and close.Check for the pressure. The pressure range should be 270-330 barg. Leak Test
 Disconnect facemask from second stage regulator. Breath in the exit port and let the pressure drop slowly (Adjust pressure drop by blocking exit port by your hand). The warning signal
should be heard at around 50 barg. |
- Face Mask Seal Test
Disconnect 2nd stage regulator.
Plug in the end tube with your finger and breath in.
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The face mask should create vacuum over your face.
The donning test procedure above is just a sample. Please check your SCBA Manual for the correct procedure.
Some Important Rules Related with SCBAs
BAs shall be Self Closing type for which volume of contained air in the cylinder shall be at least 1200 l or be functioning of at least 30 min. ((1200 l)/(40 l/min) = 30 min))
SCBA shall be fitted with an audible and a usual or other device which will alert the user before volume of the air in the cylinder has been reduced to no less than 200 l. (This warning device simply consists of a whistle. However, there is also PASS Device (Personal Alert Safety System) which is electronic equipment and able to sense the motion of the user. ıf no motion is detected for 30 seconds, the alarm is generated. As you understand it is an advanced device and rarely seen on board ships.)
-FFS Code 2.1.2
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