POSITIVE PRESSURE VENTILATION (PPV)
TACTICAL FIREFIGHTING RESEARCH
PAGE LAST UPDATED ON 06 September 2006
Throughout the 1980s London firefighter Paul GRIMWOOD presented several controversial papers and articles, based mainly upon his own operational research and experiences as a firefighter both in the UK and the USA, that closely examined structural ventilation practices as carried out by firefighters around the world. His proposed concept of 'Tactical Ventilation' - (a term he originally introduced and defined in 1989 through his book FOG ATTACK and several earlier articles in the UK's FIRE magazine) was to encourage an increased awareness of 'Tac-Vent' Ops and PPV and present a safer and more effective tactical process for the ventilation of fire-involved structures by on-scene firefighters, paying particular attention to the influences of air dynamics and fire gas formations. Following work with Warrington Fire Research Consultants (FRDG 6/94) his terminology and concepts were adopted officially by the UK fire service and are now referred to throughout revised Home Office training manuals (1996-97). During the late 1980s the Wiltshire Fire Brigade became the first UK brigade to use PPV in both pre and post fire ops. Under the direction of CFO John Craig they produced the first UK operational note detailing PPV and Paul Grimwood worked with them on the production of this document.
Over the past decade there have been several European research projects exploring the operational aspects and implementation of offensive (pre-fire attack) Positive Pressure Ventilation (PPV). Some of these have been based upon a strong scientific foundation whilst others have been completed by fire officers with a more practical bias. It is worth summarizing some of the important conclusions to these various studies so that we can formulate an updated implementation strategy.
Report 326 by VTT in Finland was a scientific study that reviewed the offensive use of PPV by using quarter scale laboratory tests and a fire spread zone model. The report concluded that the introduction of fresh air into a fire compartment was most likely to accelerate the heat release rate and intensify the fire, particularly where the fire was ventilation limited. It was also suggested that the siting of exhaust outlets in adjacent compartments to the fire compartment may lead to unfavourable conditions, ie; if a window were vented in the wrong room then conditions are likely to rapidly deteriorate within the structure.
The research investigated optimal airflows when utilising PPV against hot-fire conditions and suggested an ideal flow rate should be based upon a maximum figure of 1,440 Cu.m/hr per 10 Cu.metres of space - this suggests flows of around 30,000 Cu.m/hr for the average residential setting using sequential ventilation tactics. A minimum flow-rate was also discussed with the likelihood that under-pressures may lead to smoke or even flames heading in an adverse direction, spreading into hallways and stairshafts adjacent to the fire compartment. The flow-rate calculated by this research suggests a minimum airflow of 20,000 Cu.m/hr for the average house. These actual airflows are only practically achievable by the larger conventional fans or smaller but more powerful turbo bladed fans. Note: These airflows are actual flows as achieved against the mass of static air loads, moving through and out of a structure and not as recorded by US rating tests.
A series of tests in a wide range of structures and an old 737 aircraft at the Fire Service College were supervised by fire officers from Tyne & Wear Fire Brigade (UK) and monitored by scientific personnel based at the Fire Experimental Unit (FEU).
The general objectives of these tests were to determine the effects that un-vented; natural and positive pressure ventilation has on fire spread; as well as evaluating tenability conditions for trapped occupants and firefighters approaching the source of the fire.
The range of tests produced useful data but the limited fire loading of timber pallets and straw prevented anything more than general conclusions being drawn. However, there was some interesting data in the report that demonstrated comparative performance figures of various sized PPV blowers operating against 'hot fire' conditions. The data clearly showed how a 16 inch turbo style unit could outperform the larger 27 inch conventional units in terms of temperature reductions in the fire compartment -
27 inch Conventional PPV reduced compartmental temperature by 85 deg.C in 2.92 minutes.
21 inch Conventional PPV reduced compartmental temperature by 70 deg.C in 2.92 minutes.
18 inch Conventional PPV reduced compartmental temperature by 70 deg.C in 2.92 minutes.
16 inch Turbo PPV reduced compartmental temperature by 90 deg.C in 2.92 minutes.
However, the report failed to point out that the dimensions of exhaust outlets, compartmental temperature starting points and the amounts of water used to extinguish each fire varied widely for the comparisons and any conclusions therefore may only serve as a rough guide.
A more realistic scenario consisting of a furnished room fire in a three-bedroom house that was scientifically monitored by personnel from Chiltern Fire provided some interesting data. The objective of these tests was to evaluate PPV against hot fire conditions as well as determining tenability conditions for a casualty situated between the fire and an exhaust outlet in both vented and un-vented situations. Perhaps the most enlightening event of this series of three tests came in test 3 when the implementation of PPV caused a major ignition of the fire gases leading to flashover in the fire compartment.
This paper describes the Home Office’s research programme into PPV which began in 1995, and is now being drawn to a close. The work commenced with a survey of practice in the USA and Europe, undertaken by a consultant accompanied by a team of four junior officers from UK Brigades. FEU trials were conducted looking at various tactical scenarios including domestic fires, basement fires, stairwell fires, and an attempt is made to draw these together to derive general guidance for the UK Fire Service.
The scientific research detailed in report 419 extended that completed in Part 1 (above) and evaluated 28 quarter scale tests in a four storey building. It was further noted that the rate of heat release (RHH) within the fire compartment was seen to double where PPV was used offensively and this significantly raised the heat exposure to lower parts of the room to untenable conditions.However, it was also noted that conditions in adjacent rooms improved dramatically in terms of reductions in temperature, visibility and fire gas analysis. Again, the research determined an optimal overpressure should be aimed for which maintains PPV airflow in the direction of the exhaust point whilst raising HRR (fire intensity) to a readily acceptable and realistic level. The report noted that airflow management may be established by reducing or increasing the RPM of the fan in use, suggesting that excessive over-pressures are likely to push fire into voids or even back out of the fire compartment at advancing firefighters, or to cause the fire to breach structural elements.
"We carried out further tests using 'Chiltern International Fire' testing facilities in High Wycombe and have scientific evidence that proves using PPV offensively causes no further damage to a casualty located between the seat of the fire and the exhaust vent, than that which has already occurred prior to the commencement of firefighting........."
November 29, 2003, a 31-year-old male volunteer fire fighter (the
victim) died while fighting a basement fire in a residential structure.
The victim and another fire fighter were in the basement applying water
to the fire on the ceiling. A Deputy Chief in the basement reported to
Incident Command that the fire was knocked down and requested
ventilation. A positive pressure ventilation fan (PPV) was started at
the front door as the basement windows were vented. Suddenly, thick
black smoke filled the entire basement area as the hoseline became
covered by debris falling from shelving in the basement.
Mr Stott's most recent report describes a series of tests carried out by Lancashire Fire & Rescue Service (UK) into the offensive use of PPV and begins with an interesting review of UK research to date. The Preston fire trials demonstrated similar effects to previous tests in Manchester when excessive over-pressures had been caused (probably by under-sized exhaust outlets), causing flames and smoke to travel out of the fire compartment, moving back against the airflow created by the PPV and on one occasion causing some sort of 'flashback' effect. The Preston trials highlighted the importance of adequate airflow management and purposely demonstrated the undesirable effects caused by ventilating the wrong window/compartment.
The four year long research project undertaken by the University of Le Havre has closely examined the airflows created by PPV ventilators and has studied the air dynamics involved in moving the mass associated with static air loads through and out of various compartments and structures. They have built a full sized test-house and quarter scale multi-storey building to measure various air-flows from an array of ventilators placed in singles and multiple alignments.
The Use of PPV in Under-Ventilated Compartments - Daniel Gojkovic & Lasse Bengtsson
A paper by Daniel Gojkovic of Lund University in Sweden studies the various firefighting tactics used in under-ventilated compartment fires. Using a CFD model Mr Gojkovic compares the operational implications of PPV airflow injections with other tactical options.
The benefits and advantages of using PPV in defensive post-fire situations are proven beyond doubt and the research into its offensive use is prompting us to consider that, with adequate equipment, carefully implemented training and strong protocols upon which to base our guidance documents, there is most certainly potential for advancing this strategy in certain situations.
There is still some debate as to the advantages and disadvantages of conventional versus turbo bladed units and there may be more research in this area in future. Already there have been a number of research findings strongly in favour of the turbo design which benefit greatly from weight to size to performance ratios. The turbo bladed fans have demonstrated extremely high air flows through structures and their ability to reduce compartmental temperatures (as seen above) when used against hot-fire conditions is considered a major advantage. Small stowage space; easier manual handling and lowest noise outputs are other benefits. However, there is still controversy over 'air seals' at door entry points despite research from Le Havre University and the FEU at Moreton stating that an air seal is never truly in existence and the practical experience of the Preston and Manchester trials proves that backflows are as likely to occur with conventional airflows as they are with turbo streams.
'When the inlet and outlet vents are first opened, the buoyancy of the hot smoke and gases may be sufficient to overcome the PPV fan for a short period. The hot gases will flow out of both vents. This appears to occur because the PPV fan does not seal the whole of the door opening equally. The majority of the air flow is along the axis of the fan. A simple test with a piece of foil or tell-tales fixed to a ventilation opening will show a flow in through the top of the doorway when there is no fire. However, the smoke and hot gases from a fire can overcome this and flow outwards'.
Fire Experimental Unit - UK - 1996.
2005 PPV Research - USA
Fire departments may use ventilation blowers or fans to pressurize a structure prior tosuppressing a fire. This pressurization or positive pressure ventilation (PPV) tactic can assist in the venting of smoke and high temperature combustion products and make attacking the fire easier than without PPV. However, this tactic also provides additional oxygen to the fire and can increase the rate of heat and energy being released. PPV has not been characterized carefully enough to establish specific guidelines for optimum use.
This study examined gas temperatures, gas velocities and total heat release rate in a seriesof fires in a furnished room. The use of the PPV fan created slightly lower gas temperatures in the fire room and significantly lower gas temperatures in the adjacent corridor. The gas velocities at the window plane were much higher in the PPV case than in the naturally ventilated scenario. This higher velocity improved visibility significantly. PPV caused an increase in heat release rate for 200 seconds following initiation of ventilation but the heat release rate then declined at a faster rate than that of the naturally ventilated experiment.
Following Publication of DCOL 14/1999 it appeared there was some mis-interpretation by fire brigades of a paragraph that appeared on page four. This guidance suggested that 'for best effect the cone of air (from the PPV fan) should just cover the entrance opening' to seal the entrance doorway. This paragraph led some to believe that the Home Office guidance note was not in favour of 'turbo' bladed fans where a cone 'seal' is not part of the operating principles.
However, in a later statement dated 8th December 1999, the operating principles of 'turbo' fans have been fully endorsed by HMI Alan Wells and supported by Dr. Martin Thomas of the FRDG. It was clarified that the cone principle is only a requirement when using 'conventional' fans and the guidance in the DCOL does not purport to make any judgement about the relative merits of different types of fan.
Offensive PPV is not for all situations but the potential benefits are arguably achievable by us all.
An English translation of the French text that discusses different aspects of the types of air-stream which were studied in order to increase the efficiency of PPV as a firefighting tactic. These different aspects are the consequences of the fan design. These aspects are mainly the annular jet, the effect of the initial velocity field of a jet, the swirl effect of a jet and the effect of a wall on a jet. These portable fans are formed of three main parts : an engine on which an axial turbine, a propeller, is directly connected and a protection grid just after the turbine. Moreover, the construction sizes must be as compact as possible because of the necessity to be able to stow the PPV unit inside fire vehicles.
These experiments have clearly shown that the efficiency of fans specially designed for fire fighting use must be defined with two characteristic curves; one is the flow-rate as a function of the distance from the opening and the second one is the pressure versus flow-rate curve as a function of each position.
This research project investigates PPV useage in the UK and poses the question - Why are UK fire brigades not using PPV to its fullest potential. The author, an Assistant Chief Fire Officer in Cornwall County Fire Brigade, visited the USA to study brigades that do use PPV and PPA and his research includes extensive statistical analysis based on information he received in questionnaire mail-shot returns. Download this searching 130 page pdf report HERE
The research indicated that the PPV system could be effective in venting a stair-shaft in a high-rise building.However, the testing was limited to a ten-storey structure. It is recommended that non-fire field tests be conducted on higher buildings to extend the results.
The system was very effective in clearing smoke from the building in situations simulating post-test salvage operations. However, smoke could be diverted into other parts of the building through openings. Care should be taken to ensure doors to the stair shaft above the fire floor are closed prior to venting.
The research indicated that PPV could be used to improve conditions on the fire floor and provide improved accessibility during a fire attack. However, the PPV system can provide increased combustion air to the fire. This has the potential to allow the fire to grow. The use of PPV for a fire attack should be used with caution. Adjoining compartments and vent exposures should be protected by charged hose-lines. Report HERE
3 US Firefighters Killed in two basement fires where PPV was used
PPV RESEARCH IN TUNNELS