Hydraulic Pump Arrangement. As expansion occurs, an automatic
valve releases water to a receiver. As the water contracts, a hydraulic pump
is activated, pumping water from the receiver back into the system. This
system is impracticable because small volume changes result in large pressure
changes. Also, oxygen can be introduced into the system.
Weighted Plunger Arrangement. This system utilizes a separate
vertical expansion tank, a weighted plunger, and necessary seals. The plunger
rises and falls as the volume of water changes and is independent of
temperature and volume changes, maintaining a constant pressure on the system.
Seal problems, however, have been serious, and this system has not proved
practicable for HTW.
Pressurization of the Boiler Drum. Pressure is maintained in the
boiler drum at the desired pressure level with expansion of the water taken up
in the rising water line of the drum and any excess amount being relieved to a
lightly pressurized vessel. A boiler tube failure would cause immediate
pressure loss, with subsequent flashing and water hammer.
Combination Inert Gas and Steam Pressurization System. In this
system a separate expansion vessel, inert-gas tank, external heat source, feed
pumps, and necessary controls are used. As water expands, the inert gas
pressurized tank allows water to enter an externally heated spill tank under
its own pressurization. Pressure in the expansion vessel is kept constant by
releasing and refeeding water into the system. This system is complex,
costly, and difficult to maintain, and is therefore unacceptable.
Air Pressurization System. The use of air as a pressurization
agent is unacceptable since corrosive oxygen is placed into the system.
Advantages and Disadvantages of Steam and Inert Gas Systems. There
is little advantage of one system over the other. The choice of
pressurization is usually an economic choice rather than one of technical
Steam Pressurization Advantages
a) Lower operating pressures due to pressure-temperature
Constant system pressure in expansion vessel.
Controls usually fewer and less complex.
d) Large amount of thermal storage in expansion vessel which aids
in peak load absorption.