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 TB 740-97-4
dehumidification machine will be controlled by a humidistat. The humidistat will be set to maintain a 40 percent relative humidity,
plus or minus 5 percent, within the dehumidified zones. The humidistat will be installed within the dehumidified area approximately
three feet above the compartment deck. It will be located well away from incoming dry air. Where a series of vessels or
compartments are linked in a continuing cycle of air circulation, the humidistats will be located within the area from which the moist
air is returned directly to the machines.
c. Sensing Elements. Humidity sensing elements of an approved type will be installed in dehumidified areas for inspection
purposes. The number and location of sensing elements will vary with the configuration of the total area included in a single sealed
zone. The number of elements used will be limited to the number required for a reasonable check on vapor diffusion within the zone.
Sensing elements will be located near the points from which moist air is withdrawn. In no case will they be directly in the path of
incoming dry air.
139. Sealing of Dynamic Dehumidification. a. Requirements. All openings to the zone to be dehumidified must be sealed In a
manner to provide good resistance to the passage of water vapor. The sealing media must be capable of withstanding prolonged
exposure to the elements and forces generated by pressure differentials between the inclosed space and the exterior. Since
appreciable pressure upon the seals can be anticipated, It is important that strippable coating be applied over sufficient surface area
around the opening to develop resistance to dislodgment. If the strippable coating is applied over tape, it should extend well beyond
the tape. Application of bituminous coating over the strippable coating improves resistance to the elements and also lowers the water
vapor transmission rate of the seal. The following basic methods of sealing will be used as applicable:
(1) Edges, seams and Joints. For sealing openings of small surface area, strippable coating will be applied
directly to the vessels. Bituminous coating will be applied over the strippable coating unless the area to be sealed is gasketed or
otherwise reasonably airtight by construction.
(2) openings up to 3 Inches. In general, openings up to 3 inches will be masked with pressure-sensitive tape, then
coated as described in (1) above. In some instances, it may be desirable to substitute a solid plug or blank in place of the tape.
(3) Openings exceeding 3 inches. Openings exceeding 3 inches in length and width will be covered with
galvanized sheet metal or exterior grade plywood blanks. The blanks should be secured by means of hookbolts, strongbacks or other
fasteners which do not require welding to the vessel. Welding for this purpose should be avoided if possible because of the time and
cost of grinding, chipping and replacement of multiple paint coatings at time of activation. Sealing around blanks will be performed
as described in (1) above.
(4) Raised accesses. For escape hatches and similar openings having covers which fit over coaming, sealing will
be performed by trowling the cover and the coaming or by one of the methods described above. Opening mechanisms in covers will
be masked and sealed in accordance with applicable provisions of (1) and (2) above.
(5) Hulls of dry stored vessels. Wherever possible, sealing of hulls of dry stored vessels for dehumidification will
be performed on the exterior of vessels. This will permit opening and drying of injection and suction lines to their termination at the
hull. It will also permit easier inspection and repair of the seals.
(6) Openings on wet stored vessels. Wherever possible, sealing of openings above the main deck will be
performed from the exterior of the vessels. Sealing of piping below and near the water line will be attained by insertion of blank
flanges in the lines as near to the hull as feasible.
b. Testing. Sealing performed for dynamic dehumidification will be tested to insure that the degree of air tightness attained will
permit economical operation of the dehumidifier. A properly sealed area should retain at least 50 percent of a pressure, or vacuum of
1 ounce per square inch, or 2 inches of water for a period of 5 minutes. Air will be introduced into, or withdrawn from, the sealed area
by the use of a blower, air line, or pump. When the desired pressure differential is attained, the blower, air line, or pump will be
turned off. A pressure gage or monometer, connected to the area under test, will be observed for 5 minutes If a pressure loss of
more than 50 percent is noted during that period, the sealing should be checked for deficiencies. If a blower is used to create the air
pressure, it will be necessary to include an airtight shutoff device on the blower line to maintain the pressure or vacuum after the
blower is turned off. If compressed air is used, the air may be introduced more rapidly into a large area by attaching several
compressed air lines to a manifold, with the manifold connected to the area under test. Typical examples of air eductors and
monometer are shown in figures 3 and 4 Figure 3A shows a field fabricated blower suitable for testing small enclosures. Figure 3B
shows the M-S-A Lamb Air-Mover, a product of Mine Safety Appliance Company, used for testing large enclosures.
140. Establishing Limits of Zones. For the purpose of this bulletin a zone will be considered
Change 2
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