POSITIVE-PRESSURE VENTILATION IN A TEST SETTING

POSITIVE-PRESSURE VENTILATION IN A TEST SETTING

STRATEGY & TACTICS

FIREFIGHTERS ARE exposed to various products of combustion during the routine performance of their duties. Although many scientific studies have examined the toxic components of smoke, few have sought to identify suppression strategies that significantly reduce or eliminate the smoke-related hazards in a structural fire situation.

The Fire and Rescue Services Division of the North Carolina Department of Insurance recently completed the first in a series of research projects focusing on the use of positive-pressure ventilation (PPV), which has drawn considerable attention in its use as a tactic in structural firefighting. The study attempts to answer the following questions: Can PPV be used as an attack tool in fire suppression? Does PPV decrease the carbon monoxide levels inside the structure? Should PPV be used before the interior fire attack? Does PPV create a safer environment for firefighters and victims? Does PPV increase firefighter visibility in the structure?

Positive-pressure ventilation is a relatively new technology in which stir from outside a burning structure is drawn through a large-volume fan and forced into unburned areas of the structure. This creates a differential between the outside atmospheric pressure and the now higher (positive) pressure in the building. Smoke and gases within the structure, displaced by the outside air that’s forced inside under higher-thanatmospheric pressure, seek an outlet to a lower pressure. An open window or door near the fire provides the outlet.

METHODOLOGY

The Department of Insurance research was conducted at the Chapel Hill, N.C. Fire Department Training Center. The fire department’s two-story masonry burn building was set up resemble a dwelling structure, with two rooms on each floor totaling 1,291 square feet of floor space.

The positive-pressure fan is in place and operating as fire attack commences.

(Photos courtesy of North Carolina Department of Insurance.)

Five fires were started in each three fire suppression scenarios, for total of 15. In Scenario 1, the firefight was conducted with natural ventilation (without the use of a fan); in Scenario PPV was started before applying water to the fire; and in Scenario 3, PPV was begun after water application.

Protocols were established to ensure that all of the fires were as identical as they possibly could be. Each fire was ignited in the lower-floor burn room and was fueled with five dry oak pellets, one-half bale of straw, and two liters Number Two diesel fuel. The fires were set by the same person, the attack crew remained the same, and the nozzle was advanced from the same place for each attack and its flow time recorded. In each case the fan was focused at the lower-level door on the opposite end of the structure from the fire room; it was placed six feet from the door so that the entire door opening was in the cone of air moving from the fan. No ventilation was provided for upper-level rooms; horizontal ventilation was accomplished through a window in the burn room. Furthermore, the fires were not set in any predetermined scenario sequence, and complete ventilation and cleanup followed each experiment.

Test instruments measured carbon monoxide levels, temperature, and oxygen levels in various positions in the burn building. These monitoring positions were chosen to simulate the probable positions of firefighters crouching or standing and of potential fire victims at bed level. Readings from each monitoring position were collected every 20 seconds during the tests and continued for a minimum of 600 seconds.

Instruments used to monitor the carbon monoxide levels included a Neotronix Exotox 40 and two Exotox 50’s; MSA models 360 and 70; and a Gas Tech. All instruments had remote sampling capabilities. The fan used was Tempest model TGB 24″—4-cycle. Omega HH-52 thermocouples were used to track the temperature.

SCENARIO 1: NO FAN

  • Fire ignited in burn room.
  • Two minutes after temperature in burn room reaches 500° F, attack team enters building.
  • Two minutes after attack team’s entrance, water applied to fire.
  • One minute after attack, window in burn room opened for horizontal ventilation.
  • Total time from 500° F to water application is 4 minutes.
  • Total time elapsed from entrance of firefighters to ventilation is 5 minutes.

SCENARIO 2: FAN BEFORE ATTACK

  • Fire ignited in burn room.
  • Two minutes after temperature in burn room reaches 500° F, fan started, burn room window opened and attack team enters building.
  • Two minutes after attack team’s entrance, water applied to fire.
  • Total time elapsed from entrance of firefighters to ventilation is 2 minutes.
  • Total time from 500° F to water application is 4 minutes.

SCENARIO 3: FAN AFTER ATTACK

  • Fire ignited in burn room.
  • Two minutes after temperature in burn room reaches 500° F, attack team enters building.
  • Two minutes after entry, water applied to fire.
  • One minute after water application, fan started and burn room window opened for ventilation.
  • Total time from 500° F to water application is 4 minutes.
  • Total time from 500° F to ventilation is 5 minutes.
A firefighter vents the rear of the fireas fan is started. Results are impressive

RESULTS

Carbon monoxide (CO) generated by the fire quickly accumulated throughout the structure. In Scenario 1, CO levels in the upper rooms remained high in spite of natural horizontal ventilation of the fire room. PPV initiated after the fire attack was already in progress (after water application, Scenario 3) reduced CO levels, but not significantly. However, PPV applied before the fire attack (Scenario 2) significantly reduced CO levels both near to and remote from the fire. There was no evidence that PPV at the lower level the building forced carbon monoxide into the upper level.

Graph 1 shows the breakdown, by scenario, of the average CO levels measured at the six monitoring positions throughout the entire structure. During those tests when PPV was performed prior to the fire attack, toxic levels remained significantly lower at each location.

The conclusion that CO levels were reduced throughout the entire structure in Scenario 2 tests is supported by Graphs 2 and 3. Graph 2 shows the CO levels in the downstairs room adjacent to the fire room at 2 1/2 feet above floor level; Graph 3 depicts the toxic levels in the upstairs room farthest from the burn room. The most striking effects of preattack PPV on CO levels were in areas remote from the fire. This implies that, under similar conditions, firefighters performing search and rescue, salvage, and overhaul would be exposed to dangerous levels of CO for shorter periods of time.

The purpose of advancements in firefighting technology is to make firefighting safer and more effective, thereby elevating protection of life, property, and the environment. The only ways to judge these advancements are through careful, unbiased scientific testing and practical application in actual conditions. Is PPV an attack tool that increases fireground safety and efficiency?

Standard firefighting strategy involves quick suppression or isolation of the fire with immediate search of the remainder of the structure. During these operations visibility is poor and breathing air supplies are limited. Many firefighters have suffered injuries and have died during such activities after becoming disoriented and exhausting their air supplies. Furthermore, at least half of civilian deaths in residential fires are caused by carbon monoxide intoxication. Many of these deaths occur before suppression operations begin; however, some occur while firefighters struggle to search an unfamiliar place in nearzero visibility.

This study suggests that in some situations preattack PPV may effectively reduce carbon monoxide levels in the structure, particularly in areas remote to the fire. It could reduce the toxicity to such an extent as to keep the victims alive long enough for firefighters, moving more quickly through reduced smoke conditions, to find them.

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