MIL-HDBK-1003/6
of the system pump by means of an interconnecting balanced line. The steam
cushion in the expansion tank is produced by flashing HTW supply and by the
operation of the external steam boiler. The HTW heat energy source for
flashing is provided by means of a small bleed line connected at the HTW
supply leaving the generator. The HTW flows to the expansion tank to maintain
the water in the tank at near generator discharge temperature. The bleed line
also serves as a boiler feedwater source to the external steam boiler. The
external steam boiler adds sufficient additional heat energy to the HTW bleed
supply to produce steam at the desired pressure in the upper portion of the
compression tank. Although the flashing water content in the lower part of
the tank may alone produce a steam cushion at saturation pressure
corresponding to HTW supply temperature, system characteristics require
pressures higher than saturation. The external boiler provides a means for
maintaining system pressures above saturation under all conditions of
operation. A pressure of 25 psi (103.5 kPa) above HTW saturation pressure is
considered to be a minimum differential.
One difficulty experienced in many steam cushioned systems is that
of steam flashing in generator tubes during a cold start-up. The reason for
this flashing is that until steam has been produced, the system operates in an
unpressurized condition. In this state, furnace heat produces steam bubbles
in generator tubes in spite of maintained water circulation. As the steam
bubbles form, they collect and reduce the mass of water circulated in the tube
which further accelerates flashing and reduced heat transfer. This flashing
has resulted in tube failure by overheating.
The external boiler permits operating pressure to be established
prior to firing the generators. This is accomplished by starting the external
boiler and establishing the expansion tank pressure first. Once the system is
at operating pressure there is little interchange of heat between the steam
cushion and the water in the expansion tank. Therefore, the energy
requirements of the external boiler are essentially limited to the tank
surface heat losses and the generation of steam to replace the loss of water
volume experienced during the net contraction of system water. The small
magnitude of heat exchange between the tank water and steam cushion can be
better realized when one considers that the temperature of the steam and water
at their point of contact is the same. Heat transfer, therefore, depends
mainly on conduction between the upper and lower parts of the steam cushion
and upper and lower parts of the water volume. Without mechanical
turbulation, the water temperature levels will tend to stratify with the
warmest (lowest density) on top and the coolest (highest density) on the
bottom. Stratification, therefore, tends to limit heat exchange in a manner
advantageous to the desired process.
Inert Gas. This cycle places the expansion tank on the suction
4.8.1.2
side of the system pumps by means of an interconnecting balance line. No
system water flows through the tank due to action of the pumps. The point of
connection of the balance line to the system return water piping is known as
"the point of no pressure change". This phrase is defined as indicating that
the total pressure value within the piping at that point remains the same
whether the pumps are running or stopped. The "point of no pressure change"
is important to assure a condition that will prevent cavitation at the pump
impeller. Such a condition requires that a pressure be maintained well above
the boiling point of water at any temperatures likely to exist.
The expansion tank is charged with nitrogen to provide the desired
system static pressure level. The charge of gas is a fixed quantity and
assuming no leakage or water absorption would remain without adding or
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