|
Paul GRIMWOOD |
Positive Pressure Ventilation in Firefighting. Taken and Updated from FOG ATTACK ____________________________________________________ |
|
|
HOME FLASHOVER STRATEGY
& TACTICS - BLAINA CFBT
OPEN FORUM
|
Described as the 'wave of the future' in 1984; 'a giant step forward in firefighter safety'; and, 'the greatest innovation since the introduction of breathing apparatus', it is certain that Positive Pressure Ventilation (PPV) made a big impression upon firefighters in the 1980s. During the 1980s
several fire departments and fan manufacturers in the USA advanced the
concepts further still and the idea of a pre-attack application was
developed. This approach (since termed Positive Pressure Attack (PPA))
was researched by the Fire & Rescue Services Division of the North
Carolina Department of Insurance in 1988/9, focusing upon the use of PPV
as a viable strategy in structural firefighting terms. The study
addressed some highly relevant points, such as - Can PPV be used as an
attack tool during fire suppressive efforts?; Does PPV decrease the
carbon monoxide levels inside the structure?; Should PPV be used before
water is applied to the fire?; Does PPV create a safer environment for
firefighters and victims?; and, Does PPV improve visibility within a
fire involved structure?. The test results
showed - * CO levels on the
fire floor were increasing from the moment the fire originated and
peaked at three minutes. * CO levels on the
floor above the fire did not register until almost two minutes after the
fire had started. However, after four minutes the CO levels were higher
at this location than on the fire floor. * The ability of PPV
to reduce CO levels throughout the structure was outstanding,
particularly in areas furthest from the fire. General Operating Principles As it applies to
firefighting operations, the term tactical ventilation is a
strategy that may be defined as - 'an intervention by firefighters to open up a fire-involved building, releasing the products
of combustion from within to gain tactical advantage during the overall
approach to the firefighting and rescue operation'. It may also refer to
tactics used to 'close-down' a structure (anti-ventilation) in an
attempt to control the 'air-tracks' into/from the fire compartment. Such
strategies should be employed with a specific purpose or intent and
should not be randomly utilised. Any such action should also be
pre-planned and clear guidelines should exist in the form of written
Standard Operating Procedures (SOPs) forming a foundation for effective
and safe ventilation practice. This pre-plan should take into account
the following principles - Any attempt to
ventilate a fire involved structure
must be co-ordinated with interior attack and
rescue teams. This requires an effective communication
link between these crews and the fireground commander who is ultimately
responsible for ventilation actions. Any openings in the structure must
be made with precision to serve a specific purpose,
ensuring they do not cause the fire to spread. An element of anticipation
is necessary in preparation of likely outcomes and any required actions,
eg; a charged hoseline should be laid in advance to cover openings made
where exposures may be at risk. Basic Principles In its purest sense,
the implementation of PPV entails the siting of a 'fan' (also termed smoke
ejector or blower), or multiple fans in various
configurations, so that the airflow created by the fans is directed into
the structure, creating a positive pressure therein. Important features
of the techniques are - (a) fan capability (b) fan placements and
configurations (c) discharge openings (size and location) (d) wind
direction and effects (e) sequential ventilation. Fan
Capability The Groupe
Leader organisation have established themselves as a major force in
the field of PPV. They are an established supplier of PPV fans to fire
authorities throughout europe and the USA and are one of the only
manufacturers able to offer both 'turbo' and 'conventional' units across
such a wide range of sizes. They also offer both petrol driven and
electric powered fans with a wide range of options in the build and
design aspects. A fan's true
performance is measured in the amount of air (Cu.metres/hour or
minute; or Cu.feet/minute) it can transport into, through and out of
a structure. The methods of obtaining fan capability measurements
will vary and therefore, manufacturer's literature may not give a true
reflection of any particular equipment's ability to move air (and
smoke). It is for this reason that prospective purchasers should
evaluate carefully any particular claims made by manufacturers in terms
of air movements achieved by their equipment. As an example, the AMCA
range of tests do not record air movements through and out
of a building but rather concentrate on air flow at a certain distance
from the fan. This form of testing does not take into account the
natural back pressures encountered as an air-flow enters a series
of compartments and may not present a truly reliable guide as to any
particular fan's power and ability to move air into and through a
building. In contrast, all
Groupe Leader fans have undergone strict third party testing at the
University of Le Havre PPV research facility where airflows are
scientifically monitored in a purpose built compartmented structure. The
University are pursuing a European standard for PPV whereby fans
can be tested and rated on their true ability to move weighted air mass
through and out of a structure against some element of back pressure. In the past it has
been common to refer to fan sizes by blade length measured in inches.
This has resulted in some misconceptions in terms of airflow as smaller
bladed fans are sometimes capable of creating airflows through a
structure that are in excess of those provided by larger bladed fans!
This is due to the turbo/conventional designs now common to modern
blowers. The shorter blades of the Groupe Leader 'turbo' impeller
style blowers are designed to deliver a much narrower high-velocity cone
of air to the structure's air entry point than the longer blades of the
more traditional 'conventional' units. This fast moving narrow
cone of air serves to entrain additional air (venturi) into the cone
beyond the fan before entering the air entry point. This unique feature
ensures that maximal use is made of available airflows at the entry
point with over 90 percent of air from the cone entering the building.
The Groupe Leader conventional style blowers will produce a slower
moving widening cone of air that strikes the surrounds of any air entry
point to create the feeling of providing an air seal. This
desired effect is in fact non-existent and it has been shown in
scientific tests in Le Havre and at the FEU Moreton (UK) that no true
air seal can be created with up to 50 percent of the airflow from the
fan failing to enter the structure. However, both styles of blower are
well suited to ventilating fire involved structures with the larger
conventional units producing similar airflows to the smaller turbo units
when CM/Hr is measured at the exit point from a structure.
Interestingly, a national scientific research project in Finland
suggested that air flows of 96 - 144 Cubic metres/hour were necessary
from a fan per Cu.m of space to be ventilated for optimum
effects. Therefore, an average 250 Cu.m of residential structure
(remember - the entire building will not require pressurising)
demonstrates a through flow requirement of around 24,000 - 36,000
Cu.metres/hr. Any through-flow that falls short of these estimates is
likely to prove ineffective and flows in excess may lead to
‘overkill’ causing the fire to intensify. Size and weight are
also important considerations where stowage space is at a premium and
manual handling regulations require compliance. The demands that risk
assessments pay close attention to noise output associated with
equipment is a further consideration. It is worthy of note that Groupe
Leader's fans have been thoroughly evaluated and tested by third party
technicians and fire authorities throughout the UK and the equipment is
constantly seen to outperform the competition in terms of airflow
capability in relation to size and weight factors whilst recording the
lowest noise outputs of all PPV fans. Wind Direction & Effects Early research in the
USA suggested that wind speeds of 25 mph (40 kph) could be overcome by
opposing PPV airflows where necessary. However, more recent research in
the UK might suggest that opposing
head wind speeds as low as 6 mph (10 kph) may serve to counter
the effects of PPV and create a situation where it is difficult to
overcome the natural airflow. In this case, exhaust points should
be smaller than air inlet points (ie; 2-1 inlet to outlet ratio)
to assist the PPV airflow to gain momentum (velocity) against any head
wind. Similarly, crosswinds may affect the stability of PPV airstreams
causing the outputs of fans to be disrupted. It is worthy of note that
the high velocity airflows associated with turbo style fans are
far less likely to be affected by adverse wind effects. The research
also suggests that tail winds in excess of 12.3 mph (20 kph) may not be
assisted further in their natural ventilation capability when supported
by conventional airflows from PPV blowers. Therefore, the wind strength
and direction should be noted and taken into consideration whenever PPV
is utilised on the fireground. Sequential Ventilation Where contaminated
areas requiring ventilation form definite compartments within a
structure, the process of sequential ventilation will achieve the
best results. This entails providing the maximum amount of
pressurised air from a blower to ventilate each area in turn. Such an
effect is obtained by opening and closing doors within, to direct the
inflow of air towards designated channels. PPV - Extremely Versatile The techniques
associated with PPV have become extremely versatile and are not
restricted to clearing compartments of smoke. In addition to the
applications already discussed PPV may be used to: 1)
Reduce levels of carbon monoxide, and other toxic and irritant
gases, during the 2)
Create pressurised stairways or clear smoke in high-rise
buildings to assist firefighting efforts or persons escaping from the
structure. 3)
Clear the structure's facade of smoke to assist exterior rescue
operations and 4)
To divert smoke away from firefighters during firefighting in the
open - car fires etc. 5)
Control and abate certain airbourne contaminants such as
anhydrous ammonia. 6)
Confining the spread of fire in 'defensive' applications, for
example, in 'strip' 7)
Controlling and assisting the extinguishment of chimney fires. 8) Larger PPV units are available to control and ease firefighting efforts in tunnel fires. Reducing the hazards associated with after-fire
'Overhaul' (Mop-Up) Operations Deborah Wallace
detailed a large number of cases in her book In the Mouth of the
Dragon (Avery Publishing NY.USA) and developed upon the notion that
modern plastics play a far bigger part in causing fire deaths than is
currently realised. A close analysis of fires also suggested that short
term exposures to fire gases resulted in long term effects for
survivors. It is commonplace for plastics to decompose in fires,
discharging amounts of toxic gases, corrosive irritants, asphyxiants and
organic chemicals into the atmosphere. Those that are subjected to even
small amounts of such emmissions may suffer damage to internal organs
such as the heart, brain, kidney and the liver, as well as amounts of
respiratory tissue death, lung edema and haemorrhage, chemical pneumonia
and bronchitis, susceptibility to respiratory infections, permanent
abnormal lung functions, skin scarring and sensitisation, eye damage,
and neurological and vascular reactions. Organic chemicals usually
affect the nervous system - Phthalates are heart poisons - Benzene
causes blood cell abnormalities including leukemia, and many orgainics
poison the liver and may cause cancer.......the list is endless! What is
perhaps more relevant is the fact that many of these contaminants are
still present in dangerous amounts even after the fire has been
extinguished and firefighters are often standing and working in what
appear to be 'clear' zones without the protection of breathing
apparatus! It is commonplace for firefighters to experience sore
throats, tight chests with some pain, headaches, nausea, irritated eyes
and lung congestion following heavy 'mop-up' operations at fires - it is
part of the job - but is it necessary?! Studies have suggested that in
certain cases there may be longer term effects - even cancer and heart
disease. The potential for PPV
to clear stagnating fire gases and contaminants from a compartment
during the 'mop-up' phase is a well established and used technique and
environmental monitoring is utilised to record the reduction in air
pollution. Some fire departments in the USA have established a policy of
removing breathing apparatus for interior 'mop-up' at recorded levels of
35 ppm CO or below. In contrast, the Ottawa Fire Service in Canada have
established 5 ppm as the safe limit. The use of PPV during
'mop-up' also serves to assist firefighters by showing up hidden embers
and areas that remain smouldering. It is suggested that such operations
are augmented by thermal image cameras to locate hotspots safely and
effectively. PPV and the High-rise Fire Situation The famous fire at
the MGM Grand Hotel in Las Vegas clearly demonstrated that the movement
of smoke and toxic gases throughout a high-rise building may often
present a greater hazard to life and firefighting efforts than the
spread of fire itself. The problems associated with venting smoke from
tall buildings are unique and special attention should be paid to the
techniques utilised to achieve such objectives. A symposium held in
North Carolina, USA brought together PPV specialists from all over the
USA to present views and share experiences.The symposium included live
demonstrations of the effectiveness of PPV in a 32-storey office tower
under construction. The demonstrations utilised a ground level corridor,
the emergency stairwell and two floors (levels 20 and 28). By using a
rooftop discharge opening it took natural air movements (stack action)
15 minutes to clear the 30 metre long ground floor corridor of smoke
whilst two PPV blowers (placed in-line) were able to achieve the same in
just seven minutes. Further tests by the Charlotte Fire department
involved smoke-logging upper floor areas of 644 sq.metres and 1,288
sq.metres respectively. Various fan placements were evaluated to compare
their effectiveness, including in-line and parallel configurations
forcing air into the base of the emergency stairwell. During the course
of one operation a fan was sited at an upper level to boost the
air-flow. This arrangement led to a noticeable build-up of exhaust fumes
on the involved floor although CO monitoring equipment was unable to
detect any measurable amount. The various fan placements all effected
adequate smoke clearance times, forcing smoke to leave the involved
floors at an average rate of 46 sq.metres per minute. When smoke enters a
stairshaft in a tall building it will generally rise to the upper levels
and either mushroom at the top of the shaft, where it is unable
to escape from the structure, or stratify at a mid-point within
the shaft where the smoke has cooled. This straification generally
serves as a 'lid' for other products of combustion which tend to bank
down below the stratified layers. The principles of PPV may be harnessed
in several ways to assist firefighters in the high-rise situation. Upper
levels may be cross-ventilated by various configurations of blowers
sited at ground level. Tests have shown this set-up to be effective
generally up to 25 storeys. Above this level additional fans will
normally be needed to boost the air-flow on the involved floors.
Vertical ventilation may also be effected within a stairshaft by
multiple fans configured at ground level. Where the intention is to
prevent contaminants from entering the stairshaft (pressurisation) the
operation will prove most effective with no openings at the top.
However, where the objective is to clear a smoke logged shaft, the head
of the stairs will require an opening to exhaust the contaminants to the
exterior. Many high-rise buildings already have pressurised stairshafts
built-in to maintain smoke free escape routes. The same principles that
apply to the overall effectiveness of these systems also apply to PPV
operations in tall structures. The potential for success is dependant on
restricting leakage paths from from the area to be pressurised. Past experience has
demonstrated just how difficult it is to keep stairshafts free of smoke
where pressurised ventilation does not exist. As firefighters gain
access to the fire floors and 'lay-in' hoselines from rising main
systems lobby doors are often forced to remain open, allowing smoke to
contaminate the shaft. This creates difficulties for firefighters
working on upper floors and is particularly a hazard where occupants
remain trapped above the fire floor/s. The Los Angeles Fire Department
experienced such problems during the massive fire at the Interstate Bank
in 1988 and have since written the use of PPV under such circumstances
into their standard operating procedures covering high-rise
firefighting. A recent FEU scientific (UK) report (11/97) suggested that
it may be more productive to open windows from the fire floor upwards
while ascending than by opening the highest vent only where the aim was
to clear a stairshaft of smoke. However, it should also be noted that
air-conditioned high-rise stairshafts rarely have naturally opening
windows and any such ventilation openings would need to be created
through breakage. The use of PPV to Clear Smoke from a
Structure's facade On occasions, the responding fire force is faced
with large amounts of dark smoke issuing from openings on the structural
facade as they arrive on scene. The true extent of the situation as it
is evolving may be masked by the quantity of smoke as trapped occupants
cling to ledges several floors above ground. Overhead power and
telephone lines, street lighting and projections from the structure may
not be immediately apparent to firefighters and the entire situation may
be hindering prompt ladder placements enabling rescuers to reach those
in immediate peril. The use
of PPV to create a false wind across the face of a structure may, if
used in this way, clear smoke from the facade and allow lighting to
facilitate priority ladder placements and assist the incident commander
in making a reliable and accurate assessment of the situation. Control and Abatement of Certain
Airbourne Contaminants, including Anhydrous Ammonia The Watsonville Fire department, in California
USA, conducted a series of tests using a condemned industrial
refrigeration warehouse, to evaluate alternative techniques used to
safely control an accidental release of anhydrous ammonia (NH3) into the
atmosphere. The tests focused on the use of PPV to ventilate the
warehouse of dangerous vapours and also assessed various methods of
dispersing such vapours as they exited from the structure. Anhydrous ammonia is such a common chemical that
every firefighter ought to be aware of it's hazards. It's uses range
from water purification, to refrigant, to fertiliser, among others. When
released in large uncontrollable amounts it can be very dangerous - the
vapours are both flammable and explosive, asphyxiant and may cause
irritation or burns to the flesh. However, it's pungent odour is
detectable at five parts per million (ppm) when it becomes an asphyxiant.
The flammable range is between 16 to 25 percent, ie; 160,000 ppm to
250,000 ppm. The tests were conducted in a 250,000 cu.ft
warehouse where ammonia was discharged from a tank to attain inside
readings of 10,000 to 12,000 ppm. Ammonia concentrations were monitored
at various points within the structure, as well as at the exit point and
several hundred metres downwind. The water run-off at the exit point was
also monitored for its pH factor. Test One:
With the exhaust doorway some 24 metres from the intake opening , the
use of two 27 inch bladed PPV units reduced the vapour levels within the
compartment from 12,000 ppm to 4,000 ppm in 14 minutes. At the exit
point a three-quarter inch PVC pipe, with seven 12 lpm spray heads had
been set up around the doorway. The objective was to evaluate the
effectiveness of a low-flow spray unit on the NH3 vapours. Readings up
to 30 percent of the IDLH (Immediately Dangerous to Life &
Health) were registered just outside the exit point and strong
ammonia vapours were detected 150
metres downwind. Water run-off samples indicated a pH of 10. Test Two:
A concentration of 9,000 ppm was reduced to 5,000 ppm in 12 minutes by using one
27 ins fan. A second fan was sited at the point of exit and proved
extremely effective in directing vented vapours away from adjacent
buildings. The same PVC spray-pipe was utilised in this test and the
results reinforced the need for a larger water spray. Test Three:
A 3,780 lpm monitor was used as a spray stream at the exit
point in place of the PVC pipe. Inside the warehouse the vapours
were reduced 11,100 ppm to 3,500 ppm within nine minutes by using one 27
ins PPV fan. Air monitoring near the exit point, and downwind, still
registered high levels of ammonia vapour. This seemed to suggest that a venturi
effect was being created from the monitor causing a poor abvsorption
rate by the spray stream. Test Four:
In this test two hoselines were positioned approximately 30
metres downwind from the exhaust opening, using 45mm lines with
combination spray nozzles on a narrow pattern, rotating the spray
streams in large circles. One 27 ins fan was able to reduce the vapour
levels from 12,000 ppm to 6,000 ppm in 20 minutes within the structure
and air monitoring downwind showed a significant reduction than
previously in ammonia concentration. Similar tests by the US Army demonstrated that
HAZMAT vapours could be reduced by 47-72 percent of their original
concentrations within compartments by using standard PPV blowers for
just ten minutes. These figures showed rates of improvement (ROI) of
22-43 times the rate associated with natural ventilation. Based on these tests it was concluded that PPV
was a safe and effective method for ventilating ammonia vapours from a
closed compartment. PPV in a Defensive Mode The use of PPV fans to confine a fire has become
an established defensive strategy whereby compartments surrounding or
aside of the fire compartment itself are pressurised with airflows to
prevent fire and contaminants from spreading beyond the compartment of
origin. PPV and Chimney Fires Several fire authorities in the USA have
developed techniques to assist firefighters in extinguishing chimney
fires that avoids the need for firefighters to operate from rooftops
during inclement weather conditions. These techniques entail using a PPV
fan run at 1/2 to full RPM in unison with a dry chemical extinguisher
discharged up into the chimney shaft in 1-2 second bursts. A thermal
image camera should be used to assess the situation and prevent any fire
extension into walls, flooring or roof voids. Pre-attack PPV - Operating Principles The basic principles for initiating and using PPV
have already been discussed and when used in the pre-attack mode the
operating principles remain the same, with one or two minor adjustments.
Much emphasis should be placed upon promoting a general awareness of
potential shortfalls where incorrect applications may be made. *
A pre-written document should exist providing guidelines on
application technique in *
Always create an exhaust point for smoke and gases before
directing the PPV *
Where fans are correctly sited (not too close to the entry point)
any churning effects of the smoke and gases should be avoided and
disruption of the thermal layers kept
to a minimum. *
The application of PPV will most likely cause a compartment fire
to burn with *
Where PPV is used to ventilate T shaped compartments from
the base of the T a 'swirling' effect in the smoke patterns has
been noted that reduces the effect of gases leaving the compartment
efficiently. *
As smoke gases and flames depart at the exhaust point there is a
danger that local *
Where a fan is sited too close to the point of air entry and/or
the exhaust point is unable to function to full capacity a 'blowback' of
flame/gases may appear at the entry point. Try either moving the fan
back; improving the exhaust action - or turn the fan off.
*
The immediate siting and operation of a PPV fan prior to the
entry point being *
Any attempt to move smouldering mattresses or chairs while PPV is
in operation may * Before initiating pre-attack PPV -
(a) Know where the fire is located.
(b) Lay hoselines in readiness.
(c) Ensure the fan is ready.
(d) Create an exhaust point (near area of involvement). *
Pre attack PPV should NOT be initiated where -
(b) Where dusts or powders may be disturbed.
(c) Where the fire's location has not been established.
(d) Points of fire exposure are not covered by protective
hoselines.
(e) Where the fire is known to be spreading beyond the
compartment of origin.
(f) When it is recognised that internal layout is not suited to
optimum airflows. Conclusion With 90 percent of structure fires being held to
the room of origin there is much potential for the concept of pre-attack
PPV to flourish. However, the use of PPV is not solely restricted to
pre-fire-attack situations and the ever increasing scope of operations
that may be effected by such a strategy
makes PPV a most versatile and effective technique in modern
firefighting terms. Further Information on PPV - Click Here Paul Grimwood August 2000
Warning: All firefighters should be aware that the techniques and methods of applying water to compartment fires presented on this website require extensive training by qualified flashover instructors and any attempt to follow this style of firefighting without such training may be ineffective and potentially dangerous. E-MAIL YOUR COMMENTS TO Firetactics@aol.com Copyright Firetactics.Com 1999 - 2000 - All Rights Reserved - Firetactics.Com is a non-commercial organisation based in the UK whose aims are to increase firefighter safety & effectiveness on a global basis. This website serves as an information portal for associated fire behaviour training programmes. Any profit from sales of publications on the site go directly to support Burns Charities located in the UK, Australia and the USA. |
