Report!on PPV Trials

at Oxford Road, Preston, U.K.

10 –14 January, 2000

Richard Stott B.Eng (Hons),

Assistant Divisional Officer

Lancashire Fire and Rescue Service, U.K.

  1.1    Ventilation

  Ventilation as a fire-fighting tactic has been used since time immemorial.  Hence it is nothing new.  Venting (the creation of exhaust/inlet ports for the products of combustion) a fire has been done with caution, following much training and consequently has been instrumental in reducing property damage and increasing comfort levels for fire-fighters at virtually every fire.  The maxim within the Fire Service has been ‘vent, vent early, but only when safe to do so’.  Hence it has always been recognised that caution is the better part of valour but that used correctly, venting is a key tool in the fire-fighter’s toolbox.

  Positive pressure ventilation is a method of using high volumetric flow rate fans to increase the pressure within a compartment so that the products of combustion are encouraged to exit the compartment in order to reach a region of lower pressure. Generally speaking, the mobile fan unit is placed approximately 3 metres from the doorway, i.e. outside the building, and the air is directed in through the doorway. Positive pressure ventilation can be viewed as simply an advanced use of natural ventilation techniques.

  Positive pressure ventilation is widely utilised in the USA and the concept was first introduced there many years ago. The constructional methods used for buildings in USA differ widely to those of the UK, so it is difficult to justify the direct application of the ventilation techniques developed there.

  However, many local authority Fire Services in Great Britain now use positive pressure ventilation in the post-fire scenario. Most however, are wary regarding the use of positive pressure ventilation prior to the fire being extinguished.  This has been termed ‘aggressive positive pressure ventilation’.  In fact, at the time of writing, only one of the fifty-seven local authority brigades has routinely adopted this approach.

In contrast to the above paragraph, it has been decided to utilise the terms offensive and defensive in this report, to parallel the Home Office and LFRS, with the following definitions⁽¹⁾;

  Defensive – ventilating away from the fire, or after the fire is out.

Offensive - ventilating close to the fire to have a direct effect on the fire itself.

  From hereon, positive pressure ventilation will be referred to as ‘ppv’. With regard to domestic ppv use, as it is very unlikely in a single domestic dwelling that a fan will be used away from the fire, the term defensive should be considered to mean use of ppv once the fire has been extinguished.

  1.2      The Trials

  During the week of 10 – 14 January 2000, Lancashire Fire and Rescue Service, supported by the Department of Built Environment, University of Central Lancashire, undertook a series of ppv trials at Oxford Road, Preston.

  The main objectives of the trials were as follows: -

  ·      To examine the possible tactical advantages of offensive ppv

·      To improve knowledge and skills in the use of defensive ppv

·      To further assess fan selection in terms of fan size

·      To produce a comprehensive report, (including stills and video footage), to assist in future training packages and the further development of operational procedures.

·      To provide realistic, live fire observation for personnel involved.

·      To provide comprehensive data for use in risk assessments.

  The trials were originally planned to take place in two-storey properties but were relocated, due to uncontrollable circumstances to single-storey properties.

Plate 1.  Oxford Road, Avenham

Each fire experiment room was furnished with the type of materials that would be expected with a real incident i.e. carpets, curtains, sofas, bedding etc. Consequently, the trials were possibly the most realistic, in terms of fire loading, to have been carried out to date in the U.K.

The trials took place over five days. The chosen approach was to carry out each scenario twice, once using standard fire fighting techniques, (defensive) and once using offensive positive pressure ventilation.

  Although much quantitative data was gathered, the nature of fire, with its many variables, dictates that qualitative data is very important. Consequently, each volunteer has been canvassed for his or her experiences during the trials. This information is included within the report.

  This report!is a précis of a more detailed and technical engineering analysis, which is available if so required.

  1.3      Positive Pressure Ventilation

  Extensive research has been undertaken regarding the subject of positive pressure ventilation and its applications for the Fire Service. Lancashire Fire and Rescue Service and several other Brigades have conducted ‘in house’ experiments to determine the effectiveness of positive pressure ventilation post fire, as a result, the use and safe application at incidents is well documented. 

  Supporters of the use of ppv offensively claim that it has a number of advantages, such as:

  ·        The enhancement of fire-fighter safety due to the improved atmosphere that positive pressure ventilation can create.

  ·        The rapid improvement of conditions remote from the fire-affected compartment will enhance the chances of survival for persons trapped in a building on fire.

  ·        Fire-fighting operations can be expedited as the improved conditions enable the fire attack to take place quicker.

  ·        The correct use of positive pressure ventilation prevents firespread beyond the fire compartment.

  The literature review undertaken for this project demonstrates that although research into positive pressure ventilation has been fragmented, the overall picture is one of support!for the technique. There is a distinct lack of formal academic evidence to support!this however and consequently many sceptical Chief Fire Officers, this being demonstrated by the reticence of UK Fire Brigades in offensive positive pressure ventilation use. Most of the reports reviewed indicate the need for further research.

  Two Fire Brigades have already undertaken offensive ppv experiments, Tyne and Wear Metropolitan Fire Brigade and Greater Manchester County Fire Brigade.  The results of the two sets of experiments undertaken by Tyne & Wear indicate that the use of ppv does not seriously intensify fire spread ⁽²⁾. In contrast to this the report!of offensive ppv experiments in Greater Manchester indicate a rapid acceleration of the fires involved on operation of the fans ⁽³⁾.

  Clearly from the differing results produced by two large Metropolitan Brigades the use of offensive ppv warrants further investigation and research. It is universally accepted that without proper training and correct tactical deployment at incidents, ppv could prove potentially fatal for fire fighters and members of the public.

  When assessing the use of positive pressure ventilation there is a tendency to forget that pressurisation has been incorporated within building designs for several decades. Heating ventilation and air conditioning systems can be designed so that, in the event of a fire, they can be used as a smoke control system. Separate heat and smoke control systems are also designed for the larger building and for certain industrial applications, therefore the concept of ppv and fire application is not a new one.

  1.4 Previous U.K. Research -- Overview

  A major concern amongst fire officers and the research establishment in this country is the fact that ppv may prove ineffective when used offensively, possibly even dangerous. Ventilation techniques for domestic dwellings and small industrial units in the UK have relied on the available openings of the building involved in fire, windows, doors, or occasionally roof lights are used when appropriate to exhaust the contaminants. The problem foreseen initially with ppv in UK construction buildings is the lack of available or adequate exhaust points to compensate for the amount of air entering the building from the ventilation units.  Flashover, backdraught and firespread were all perceived problems associated with the fans in early assessments.  

  Early in 1994, Warrington fire research consultants undertook a survey of ventilation when used as a fire-fighting tactic; the report!from the survey was published later that year. The report!concluded that further research needed to be undertaken on the subject of ppv ventilation since little or no research existed in the UK. As a result of the Warrington report!the Fire Experimental Unit at Moreton in Marsh undertook a further research project, this work to be backed up by a survey of the expertise available in the USA and other countries ⁽⁴⁾.

  The research results indicated that ppv to be particularly suited to fire attack in small domestic dwellings however, caution was expressed regarding its use (ppv) due to the ‘significant amount of additional training’ that would be required.

  Two trials of note were conducted by the Fire Experimental Unit using the Fire Service College’s facilities; the first using fires on the ground floor and the second experiment consisted of fires in a basement.

The results of these experiments demonstrated the need for a systematic ventilation procedure when using ppv. It was found that any compartment where an outside window was not opened was effectively sealed by the ppv, preventing any smoke clearance. Where ppv was not used the compartment cleared gradually, but this did not happen with ppv. The main conclusion from this experiment is that if using ppv for smoke clearance, rooms should be cleared one at a time and once a room is clear of smoke, the windows should then be closed. The report!further concludes that these results were due to the limited flow from the portable ventilation units and that ppv was not as effective in clearing rooms with upwind windows.

  Another very important point to be concluded from the Experimental Unit’s experiments was the fact that ppv can easily be overcome by a strong wind therefore its use is limited to certain scenarios.

  A comprehensive research document was produced by J G Rimen ⁽⁵⁾, which researched the use of positive pressure ventilation in domestic properties. The document describes a series of trials conducted in still air conditions and a further series of trials conducted in a four bedroom detached house. J.G Rimen had listened to the views of Fire Officers and others in the building industry and acknowledged the sceptics who regarded increasing the oxygen content to fires via ppv as dangerous.

  A series of trials were undertaken in a ‘mock’ detached dwelling, the primary objective being to determine basic rules for the use of ppv at incidents. The results obtained during the trials were recorded using a data logger and the results subsequently analysed.

  The report!concluded that an inlet / outlet ratio of 2 / 1 is the ideal ratio for the use of ppv and this is what Brigades should aim for. If this ratio is unable to be obtained then it would be still advantageous to have the inlet opening larger than the outlet opening. Experiments undertaken also suggest an optimum distance of 1.0m from the inlet opening for the fan and that it was not necessary to seal the opening with the fan, this last conclusion contradicts current working practises which have been adopted from other research. Another conclusion from the report!indicates that the most efficient method of using the ppv fan is when it is placed so that air is projected horizontally into the building.

  The research conclusions indicate that ppv has the capability of rapidly improving the environment at fires, but can also make things worse and that it is virtually impossible to predict with any certainty the effect of the fan in a given circumstance. This particular conclusion from the report!was used to back up the evidence of those individuals opposed to the adoption of ppv for fire service operations.

  Others adopted a totally opposing opinion within the UK fire service for ppv application as a result of J.G Rimen’s report. The report!states that ‘each fire situation, and specifically whether or not to deploy ppv, would need to be considered on its particular merits’, in other words risk assessment. Those in favour of ppv use argue that a ventilation fan is just another piece of equipment available to the incident commander and with adequate training and correct incident assessment, it (ppv) is safe to use.

Fire Authorities in the UK have acknowledged the training requirement for ppv; two metropolitan Brigades have conducted fire experiments to assess the training implications and effectiveness of ppv.

Tyne and Wear Metropolitan Fire Brigade acquired the use of two derelict semi-detached properties; these were subsequently used for tactical ventilation trials in real fire situations during November 1996. The properties used were identical in construction and layout, which enabled fires to be repeated and compared.

Cribs were placed in various rooms within the houses and holes drilled in the walls at various heights to facilitate thermocouples. Temperature readings for all the exercises were recorded and used to compare the effect of ppv in different locations of the houses. Wooden pallets, straw, diesel and paraffin produced the heat and smoke for the experiments with maximum temperatures reaching around 800°C.

The experiments from Tyne and Wear are summarised as follows ⁽⁶⁾:

·        Experiments carried out indicated ppv would overcome wind strengths of 8-10m/s, the experiments carried out by the experimental unit at Moreton showed ppv to be ineffective against a 2m/s wind.

·        Ppv did not seriously intensify or spread a fire however some localised flaming did occur.

·        Ppv improves visibility in a short time scale.

·        Ppv reduces temperatures dramatically in a short period of after application.

·        Effective use of ppv improves the working environment and reduces the risk of flashover.

The Tyne and Wear report!recommends the defensive use of ppv and appears positive regarding the use of ppv offensively.

  On 1^st May 1998 Greater Manchester Fire Service carried out a series of tests using disused terraced properties. The objectives for the tests were to document and develop the behaviour of fire using offensive positive pressure ventilation, the results to be used for the formulation of Brigade training policy. The tests were carried out with assistance from the University of Central Lancashire, who produced the report⁽⁷⁾ from which this information is drawn.

  Three main tests were carried out; a bedroom fire, with a vent made prior to application of the fan, a bedroom fire without a vent, a lounge fire with vent made after application of the fan.

Equipment from the University Of Central Lancashire was used to measure temperatures, gas compositions and fire growth in a similar manner to the Tyne and Wear experiments. Video cameras erected outside the buildings observed the smoke movement from the exhaust points whilst a thermal imaging camera was used to show heat transfer and fire spread internally.

  With these tests the fire was not confined to a crib and sufficient materials were present to support!flashover. The fire load consisted of upholstered furniture and chipboard that were arranged to simulate the furniture and fittings of a typical room.

  During all the tests, the application of PPV produced higher temperatures within the fire compartment after an initial decrease. It should be mentioned, however, that two of the tests were deliberately arranged so that ppv was used incorrectly.

  In the first test, the compartment (internal) door was open until it was considered the fire had almost reached the fully developed stage, and then the door was closed. The graph indicated that the fire had become strongly ventilation controlled by these actions, dropping from a temperature of 700⁰c to just over 200. The room was then vented, by breaking the window and opening the internal door, prior to applying the ppv fan.

The same criticism can be applied to each of the tests carried out i.e. it cannot be concluded that ppv accelerated the fire any more than natural ventilation alone would achieve.

During all the tests a visible ‘flashback’ was reported on application of ppv. The timing of this event for each test is unclear and no evidence is apparent on the graphs. No reference had been made to this effect before and consequently it is an interesting area for consideration.

There are a number of possible reasons for this effect:

The data from Tyne and Wear does highlight a small localised increase in temperatures after ppv application, but does not mention the possibility of a ‘flashback’ on initial application, the report!does acknowledge the potential danger of offensive use.

In July 1998, Tyne and Wear Fire Brigade carried out a further set of trials at the Fire Service College. The trials were conducted in conjunction with the Fire Experimental Unit of the Home Office. The brief for these tests was to establish the effects of ppv on a casualty situated between the fire and the vent, and the effect of ppv on fire spread, two issues perceived at the time to be holding ppv use back.

The report⁽⁸⁾ of the trials, produced by the Fire Research and Development Group, concludes; ‘ppv produced no measurable difference in fire spread from that observed under natural ventilation alone. However ventilation, whether natural or forced, did appear to increase fire spread.’

This was an interesting conclusion which again conflicts with the findings from GMC.

Note:  the performance of positive pressure ventilation fans has improved rapidly since their inception. Many of the early trials used fans that were of low output and consequently some results may be misleading.