During a blast, hot gasses under extreme pressures can enter through the air vents. In order to attenuate this shock wave, a blast valve (also known as an explosion protection valve) must be placed into all air pipes, vents and other penetrations to the shelter. Blast valves instantly shut off the flow of air when a shock wave enters the air vent. This not only protects the other components of the ventilation system from damage, but prevents injury to shelter occupants.
Some shelter manufacturers do not wish to go to the trouble and expense of installing underground shelter blast valves into their air vents, addressing the problem instead by using a very undersized (less than 2-inches in diameter) air pipe. Small diameter air pipes cause a ‘choking’ effect and they hope that only a limited amount of blast will enter the shelter through this undersized pipe…but this also means that only a limited amount of fresh air will enter the shelter as well. This small, open-ended air pipe also allows for continual loss of air pressure and for exchange of air from the outside environment. It is mandatory that continual positive pressure be assumed for the proper protection from war gases.
All Utah Shelter System’s ventilation pipes are constructed from 6-inch diameter schedule 40 steel pipe, which will not burn, crack, or easily distort. This pipe size assures a generous flow of air. All shelters supplied by Utah Shelter Systems are fitted with blast valves that have been tested and certified by ANDAIR’s legitimate government laboratory, thereby assuring our customer adequate protection from these serious life threatening effects.
Rock Cribs: In areas of high blast potential, in order to prevent damage from flying debris and blast, air intake and exhaust vents are sometimes placed into below grade rock-filled pits called, ‘rock cribs’. The four to eight inch diameter rocks inside the crib diffuse the shock wave and enable the blast valves inside the shelter to withstand significantly more overpressure.
Air Volume: How often and for how long the air handling unit should be operated depends on the size of the shelter, the number of occupants, and the capacity of the unit. Assuming a family of six in a 3700 cubic foot shelter, they may wish to operate the air system for an hour every six hours or so to freshen the shelter atmosphere. This schedule is not very close to the level of desperation, but it does maintain good morale and comfort for shelter occupants. In order to maintain acceptable air quality in a shelter housing twenty to thirty occupants, the ventilator should be run for twenty minutes and then rested for twenty minutes. Fifty occupants is considered the absolute limit with a single VA-150 and would require continuous operation to support life. This assumes a steel or concrete shelter with un-insulated walls that will absorb body heat and keep temperatures under control. The combination of insulated walls, lots of occupants, and inadequate ventilation will cause temperatures to soar to intolerable levels in a short period of time. With this many people in a shelter, there will be a larger number of volunteers to operate the system (imagine the consequences of losing power with an electricity-dependant air handling unit under these conditions). The “ventilation officer” will also have the job of listening to the radio (perhaps on headphones) to stay informed about an ongoing situation. For this reason, we suggest that you position the radios near the ventilation unit.