MIL-HDBK-1003/6
4.8.3.2
Steam Pressurization Disadvantages
a) Full flow of heated water through tank, which requires
additional insulation.
b) Inflexibility of expansion vessel location, which can require
higher generator rooms.
4.8.3.3
Inert Gas Pressurization Advantages
a) Relatively cool expansion vessel expansion tank which is
located so that convection currents are restricted and system water flow will
not enter the tank.
b)
Flexibility in locating expansion vessel.
c) System pressure that can be designed for any value within
recommended limits.
4.8.3.4
Inert Gas Pressurization Disadvantages
a) Higher system pressures. In order to ensure that system
pressure will not drop below saturation pressure, a safety factor must be
included in design considerations. This may necessitate the use of higher
pressure rated fittings and valves.
b)
Inert gas costs.
Expansion Vessels. All systems should have dual expansion tanks to
4.9
allow for inspection and maintenance while the system is on-line.
Purpose. Expansion vessels absorb the expansion and contraction of
4.9.1
system water due to load variation and ensure, at all times, a controlled
system pressure above that corresponding to the saturation temperature. They
may also act as a momentary source of water supply in case of a system
malfunction.
Sizing. Under no circumstances where heating is the basic load
4.9.2
should the expansion volume be based on cold water conditions. The expansion
volume should be based on normal design supply and return line differentials.
In a steam-pressurized system the diameter of the expansion vessel should be 6
ft (1.83 m) or more with an aspect ratio of 3.5 to 1.
See Figure
4.9.3
Expansion Vessel Volume for Steam Pressurized Systems.
11.
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