Calculation of Deck Foam System

1. WHAT IS FOAM SYSTEM?
Foam for fire protection purposes is an aggregate of air-filled bubbles formed from aqueous solutions, and is lower in density than the lightest flammable liquids. It is mainly used to form a coherent floating blanket on flammable and combustible liquids to prevent or to extinguish fires by excluding air and cooling the fuel.
It also prevents re-ignition by suppressing formation of flammable vapours. It has the property of adhering to surfaces, providing a degree of exposure protection from adjacent fires.

Foam is used as a fire prevention, control, or extinguishing agent for flammable liquid in tanks or processing areas. Foam solution for these hazards may be supplied by fixed systems or portable foam generating systems.

SOLAS Rules.
For ships carrying chemicals or oils in bulk, SOLAS/IMO require a fixed deck foam system
for extinguishing fires on deck or in tanks.

2. FOAM TYPES
The principal use of foam is to extinguish burning flammable or combustible liquid spills or tank fires by developing a coherent coolant blanket. Foam is the only permanent extinguishing agent used for fires of this type. Its application allows fire fighters to extinguish fires progressively. A foam blanket covering a liquid surface is capable of preventing vapour transmission for some time, depending on its stability and thickness.

Fuel spills may be rendered safe by foam blanketing. The blanket may be removed after a suitable period of time.
Foam is used to diminish or halt the generation of flammable vapours from non-burning liquids or solids, and to cut off access to air for combustion. The water content of foam cools and diminishes oxygen by steam displacement.
Foam is also used to fill cavities or enclosures where toxic or flammable gases may collect. Foam solutions are conductive and therefore not recommended to be used for electrical fires.

Foam Concentrate Types
1. Protein foam concentrate. It is diluted with water to form 3% to 6% solutions depending on the type and, in general, it is only used for crude oil fires.

2. Fluoroprotein foam concentrate is very similar to protein foam concentrates. It may also deposit a vaporisation preventing film on the surface of a liquid fuel. It is diluted with water to form 3% to 6% solutions depending on the type, and is used for crude oil or refined oil products where a higher degree of protection is preferred.

3. Special ‘alcohol type’ foam concentrate forms a foam that has an insoluble barrier in the bubble structure which resists breakdown at the interface of the fuel and foam blanket. It is used for fighting fires in water solution and certain flammable or combustible liquids and solvents that are destructive to regular foam. Mainly used for protection onboard chemical tankers.

4. Synthetic foam concentrate includes: AFFF and medium and high expansion foam concentrates are used to produce foam or foam-to-solution volume ratios from 20:1 to approx. 1000:1 and are used for local protection and engine room hi-ex systems.

3. DESIGN FIGURES
Oil Tankers. The foam system capacity shall be a minimum of the largest of the entire cargo tank deck covered with 0.6 l/ m2/min. or 6.0 l/m2/min. for the largest cargo tank.

Chemical Tankers. The foam system capacity shall be a minimum of the largest of the entire cargo tank deck covered with 2.0 l/m2/min. or 20 l/m2/min. for the largest cargo tank.

P&ID of Foam System

4. SYSTEM DESCRIPTION
All foam systems, consist of a water supply, foam liquid storage, a proportioning device and a distribution system.

The water supply pump(s) provide(s) a certain capacity of seawater to the deck foam system, and is/are supplied by the ship’s fire pumps.
The foam liquid is stored in a tank. The tank must be complete with vent, contents gauge, and access manhole.

High PressureFoam Pump c/w Foam Tank.

The foam is delivered via a high pressure foam liquid pump to the automatic foam liquid proportionator, which will accurately proportionate foam liquid at 3% to 6% to the seawater flow, irrespective of flow rate or pressure.

For satisfactory operation of the proportionator, foam liquid must be supplied with a minimum pressure of at least 10 metres head higher than the inlet water pressure under all load conditions. The electrically driven foam liquid pump is provided for this purpose.

Foam Proportioner

Foam solution is supplied to the deck monitors and hand lines by the deck main fitted with isolating valves. Each monitor is isolated from the main supply pipe by means of butterfly valves, which are normally closed.

 Foam Monitor

5. SYSTEM ANALYSIS & CALCULATION
5.1 DIMENSION OF PROTECTED AREA
5.1.1 Cargo Tanks Deck Area (A1)
Cargo tanks Deck area (A1) means the maximum breadth of the ship (B1) multiplied by the
total longitudinal extent of the cargo tank spaces (L1).

A1 = B1 x L1 
 
Where :
B1 (m) = Breadth of the cargo tanks deck area 
L1 (m) = Length of the cargo tanks deck area

5.1.2 Area of Largest Single Tank (A2)

A2 = B2 x L2 
Where :
B2 (m) = Breadth of largest single tank
L2 (m) = Length of the largest single tank


5.2 FOAM SOLUTION SUPPLY RATE
The rate of supple foam solution shall not be less than the greatest of the following:
  5.2.1 Q1 = 0.6 l/min.pr.m2 of cargo tanks deck area A1
Q2 = 0.6 l/min. x A1
 
5.2.2 Q2 = 6 /min. pr. m2 deck area of largest single tank A2
Q2 = 6 l/min. x A2
 
5.2.3 Q3 = 3 l/min. pr. m2 of the area protected by the largest monitor,
such area being entirely forward of the monitor, but not less than 1250 l/min. 
Q3 = 1250 l/min.
 
5.2.4 Choose the largest figure, Q1, Q2 or Q3:
Required supply rate Q (l/min) = Largest figure of Q1, Q2 or Q3
 
5.3 FOAM LIQUID AMOUNT (F) AND TANK SIZE 
5.3.1 Sufficient foam concentrate shall be supplied to ensure at least 20 minutes of foam
generation in tankers with an inert gas installation or 30 minutes in tankers not fitted with
such equipment (check carefully special requirements from authorities, class or customer.)
  F (liter) = Q x M x T / 100

Where :
M (%)              = Mixing ratio to sea water, 3%-6%
T (minute)      = Operation time as per item 5.3.1
Q (l/minute)  = Required supply rate from item 5.2.4 

Nearest standard foam liquid Tank size standard should be taken nearest or higher of above calculation.

5.4 MONITOR CAPACITY (Qm)
5.4.1 At least 50% (Qmin) of the foam solution supply rate as per item 5.2.4 shall be delivered from each monitor.

Qmin (l/min) = Q x 0.5 

Then we can select the type of monitor base on above calculation.
For Example, choosen monitor as below :

Inlet design pressure at monitors: 5 bar
Monitor capacity (Qm) = 3000 l/min (MAX)
Throwing length in still air (R0) = 52 m

5.4.2 The distance from the monitor to the farthest extremity of the protected area forward of that monitor shall not be more than 75% of the monitor throw length in still air. (R.).
 
R = R0 x 0.75 
    = 52 m x 0.75
    = 39 m

5.5 DISTANCE BETWEEN MONITORS 
5.5.1 The capacity of any monitor (Qm) shall be least 3 l/min. of foam solution pr m2 deck area protected by that monitor, such area being entirely forward of the monitor.

5.5.2 Distance between monitors according to requirements as described in item 5.5.1:
5.5.2.1 Distance from poop front monitors to aft tanks deck monitor P1a:
 
P1a (m) = Qmin [l/min] / ( 3 x B/2 [m]) 
 
5.2.2.2 Distance between tanks deck monitors P2a:
 
P2a (m) = Qmin [l/min] / ( 3 x B [m])
 
5.5.3 Distance between monitors according to throwing length R: 
5.5.3.1 Distance from poop front monitors to aft deck monitor P1b :
 
P1b (m) = (R^2-(B/4)^2)^0.5
  6.3.2 Distance between tanks deck monitors P2b:
 
P2b = (R^2-(B/2)^2)^0.5

5.5.4 Smallest P1 and P2 
For P1 use the smallest figure of P1a and P1b
For P2 use the smallest figure of P2a and P2b

5.6 NUMBER OF MONITORS (Nom)

         Nom = (L1 – P1) / P2 +2 

5.7 FOAM PROPORTIONER
Supply rate Q (l/min.)as per item 5.2.4.
Mixing ratio M (%) as per item 5.3
 
Water flow rate W (l/min) = Q x ( 100 – M ) / 100
 
Base on Water flow calcution, Type and diameter of Foam Proportioner will be selected.
Water flow range on actual proportioner as above selected foam proportioner.

5.8 FOAM LIQUID PUMP (C)
Foam liquid pump capacity 
 
C (m3/hr) = Q[l/min.] x M x 60 / ( 100 x 1000 )
 
Pressure to be min. 1 BAR higher than max. water pressure at Foam Proportioner.
Minimum required pressure: 10 BAR

5.9 MINIMUM CAPACITY OF FIRE PUMP 
Supply rate Q as per item 5.2.4.
2 x water jets Qh = 800 l/min
Min. fire pump capacity :
 
Cf (m3/hr) = ( Q + Qh ) [l/min] x 60 min x ( 100 – M ) / ( 1000 x 100 )
 
Minimum required pressure: 9 BAR.

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