MIL-HDBK-1025/10
8.5.2.4 Structure Condition Change Retesting Requirements
WARNING
Pumping out structure water or removing duct line plugs can allow gas into the structure.
a. After a manhole has been pumped, the removal of the water may permit gas to flow
into the manhole. Make the test just above any open ducts. If a test indicates that gas is entering,
ventilate the manhole.
b. Immediately upon the removal of the duct plugs, make a test just above the opened
duct. If gas is entering, ventilate the manhole.
8.5.2.5 Emergency Entrance Into Underground Structures
WARNING
If, in an emergency, it becomes necessary for a worker to enter a manhole
or vault where gas is present, the worker will use an approved self-
contained breathing apparatus (SCBA) and a safety harness with an
attached life line attended by another worker stationed at the manhole or
vault opening.
8.5.3 Ventilation of Underground Structures. There are three methods that can be
used to ventilate a structure. They are the forced air, natural, and sail methods.
8.5.3.1 Forced Air. The forced air method consists of a power blower, blowing air into
the manhole. The blower hose is placed into and on the bottom of the manhole, forcing fresh air
to circulate and oxygen-deficient air to be forced out. This is the best method of ventilating a
manhole.
8.5.3.2 Natural Method. This method consists of taking the manhole cover off and
letting the internal air escape as much as it can. This method of venting a manhole is the least
effective because a gas heavier than air could remain in the bottom of the manhole. Use this
method of venting a manhole only as an emergency measure.
8.5.3.3 Sail Method. The sail method (figure 12), ventilates the manhole by using the
wind. A piece of plywood or some other material is placed over the manhole. The edge of the
plywood facing the wind is lifted up until the plywood forms about a 45 degree angle with the
manhole opening. The wind enters the manhole, forcing possible contaminated air out of the
manhole.
119
Previous Page
