TM-5-855-4
6-4. Ventilation.
a. To prevent contaminated air infiltration and to obtain scavenging airflow through the
decontamination area, the hardened structure will be pressurized at all times. A vestibule pressure of 0.3
in. wg behind the blast lock will provide adequate protection against infiltration with winds of 25 mph
(eq 2-16). The pressure drop through the corridor type decontamination facilities is that of the two M2
doors in series or 0.2 in. wg at 400 cfm scavenging air flow. The total pressure is then 0.5 in. wg in the
remainder of the structure.
b. Blast locks and vestibules, discussed in paragraph 6-2, each have a minimum area of the square
feet and are provided with blast doors that are not gastight. Depending on the leakage rate of these doors,
the blast lock is ventilated at some pressure intermediate between that of the vestibule and the outside.
The blast plenum above the blast lock acts as a surge volume to attenuate the overpressure transmitted
inwards during the time it takes for the blast valve to seat, thereby reducing the impact of these blow-by
effects to tolerable levels. Blast plenum design is covered in TM 5-858-5.
(1) A blast valve will be installed in the air supply duct at the penetration point into the blast lock if
additional ventilation air is required.
(2) Moving the antibackdraft valve to the vestibule side and ducting it to an additional blast valve
above the inner blast door eliminates the blast plenum and provides a fully ventilated blast lock at
ambient zero pressure. However, when the inner blast door is opened the vestibule pressurization is lost
and that of the rest of the facility may be compromised by the sudden increase in air loss. As a result this
alternate arrangement is not recommended.
c. Air pressure regulator M-1 shown in figure 6-6 is a slide valve designed for installation in a wall
between two areas of a hardened structure where airflow and pressure differentials are desired when no
permeable door is provided. Such a valve would be installed in the inside wall between the main occupied
area of a structure and the vestibule or between any other areas requiring free airflow and pressure
differentials.
d. Antiback-draft valve M-2 shown in figure 6-7 will not withstand the forces generated by a strong
blast and will be installed upstream from a blast closure. It is used for exhausting air without danger of
reversal of airflows in the event that structure pressurization is lost or sudden outside pressure increases
as a result of high winds. These valves will be located in the outermost inside wall of a hardened
structure as shown on figures 6-1 through 6-4. Similar wall or ceiling mounted, counterbalanced or
spring type adjustable, heavy duty backdraft dampers, serviceable from either side, are commercially
available.
e. To ensure that the required pressures are obtained and maintained, a manometer with necessary
outlets to the outside, the vestibule, the decontamination area, and the main structure will be installed at a
convenient control point.
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