Basic
Setup
Remember
the PPV influencing factors
There
are four basic influencing factors that affect the performance of positive
pressure ventilation. They include fresh air available, the intake
opening, the exit size, and the area being pressurized. Each influences
PPV in a different way.
1.
Fresh air
– There needs to be enough, otherwise smoke will only circulate. This is
only a problem if the fan has to be taken inside.
2.
Intake opening
– The opening should be completely sealed by the cone of air.
3.
Vent size
– The number and size of ventilation openings must be balanced with the
fan and building size. If there are not enough exits available, smoke will
blow back out the intake. If there are too many exits, control and
effectiveness will be lost.
4.
Area being pressurized
– The larger the area being pressurized, the more PPV that will be
necessary. PPV can be increased by adding fans or by decreasing exits.
Eliminate smoke blowing back out the intake
Smoke
that blows back out the intake is commonly called turbulence. Turbulence
is eliminated by one of the following techniques.
1.
Completely seal the door
with the cone of air – This
forces smoke to move away from the fan.
2.
Increase the number
of vents – If the door is
completely sealed and turbulence still exists, then there are not enough
vents or doors are closed between the fan and the vents that exist.
Remember, on a typical residential house, there can be two to four vents
and good PPV will still be maintained.
Make vents, and then start PPV
This rule is based on an
old misconception that the structure should be pressurized before any
vents are made. This technique will only allow for the uncontrolled spread
of smoke and fire and should be avoided.
Clear vent openings as indicated by fire conditions
When making vents, make
sure they are completely cleared as appropriate. During fire attack, all
glass, panes, curtains and screens should be removed.
Block open any doors used for the intake and
vent openings
This action will prevent doors from being blown or
pulled shut, thereby hampering the ventilation effort.
During overhaul,
systematically remove smoke
This is a basic PPV skill. Open and close doors and
windows to clear one area at time. This increases the efficiency of PPV.
Do not use PPV fans as smoke
ejectors
PPV fans are not designed for negative pressure
ventilation. It is irresponsible and wasteful to burn them up.
Refuel fans at the truck
This rule is for safety reasons and it also benefits
the fire investigator when gasoline residue found near an entrance does
not have to be attributed to the fire department.
PPV and Fire Attack
Never use PPV when backdraft is a possibility.
If
backdraft conditions are present, PPV will increase the potential for such
an event and should not be considered.
Consider the effects of PPV on fire behavior
PPV
creates a path of air between the fan and any vents. Three basic rules are
based on this concept.
1.
Fire within the path of air will be pulled towards any vents.
2.
Fire not within the path of air will be affected based on proximity to the
path of air – The closer to the path of air; the more it will spread to
the vent.
3.
If no vents exist, the fire will spread towards the fan.
Make
vents as close as possible to the fire
Vents near the fire allow for the escape of heat,
gases, and smoke without increasing fire spread inside the structure.
Remember
the PPV indicators during size-up
There are four indicators that should be detected
during size-up that help to determine when PPV can be used during fire
attack.
1.
Fire location
– This is necessary to ensure that vents are placed in the correct
location
2.
Vents can be established –
If vents cannot be established, PPV should not be started
3.
Victim location known
– It is undesirable to push fire towards the location of any victims. If
the fire area itself is vented, this will be avoided.
4.
Fire attack can be initiated
– PPV like all ventilation, requires immediate supporting fire attack
If any of the above indicators cannot be answered
during size-up, PPV should be delayed.
Consider
the ABC’s of when to delay PPV
There are three conditions when PPV should be
delayed.
1.
A – Application of water is
difficult – If fire attack is
delayed, stop PPV
2.
B – Balloon construction –
If the fire has breached the walls, PPV can increase the spread of fire
into the attic.
3.
C –
Concealed fires in attics and voids – Fires in these areas can be
difficult to access and PPV may increase their spread while firefighters
work to reach them.
Do not panic if flames blow out with force.
This is normal when PPV is used before
knockdown. Just be sure that fire attack will be taking place shortly.
Never attack a fire through a vent
opening – Especially if firefighters are inside!
This is one of the cardinal sins of
firefighting. This tactic will disrupt ventilation, spread the fire in the
opposite direction and threaten the lives of any firefighters operating on
the interior.
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