Reserve storage of not less than 30 seconds supply to all pumps
connected to the tank.
Space for water expansion due to temperature changes. This depends
on the supply and return temperature limits, and on the total amount
of water in the system.
S p a c e for operation of level alarms and overflow. Normally, l-foot
depth is reserved for this purpose.
Space pressurization above the overflow level. When steam
pressurization is used, this space amounts to 20 percent of the
volume reserved for water expansion.
T h e ratio of diameter to length is kept to 3.5 approximately,
with a minimum tank diameter of 6 feet.
The following connections are usually
4.1 Expansion Tank Connections.
provided for expansion tanks.
D r a i n i n g and filling connections used to completely drain the tank
o r to fill it with water after a shutdown.
Supply piping for steam pressurized tanks. The boiler water should
be fed horizontally below tank water level through independent pipe
leads from each boiler.
Pump suction connections usually provided with vortex eliminators.
S a f e t y valves for protection against overpressure.
Blowoff connections to rapidly remove large quantities of water
resulting from volume expansion when starting up a system, to reduce
water concentration, and to aid in the removal of sludge.
Connections for thermometers, pressure gauges, water gauge glasses,
l e v e l controls, and alarms for proper operation and control.
5 PRESSURIZATION. The design boiler pressure must be high enough to produce
saturated water at a temperature substantially above the design supply water
temperature. T h i s is required to prevent a flashing of water into steam
should the system flow pressure drop. The temperature difference, 20F to
30F approximately, can be obtained by cooling the water below the saturation
t e m p e r a t u r e before it enters the circulation system pump. Additionally, the
expansion tank of a steam pressurized system is usually located about 16 feet
above the boiler outlet header, and this additional head provides a safe
margin above the saturation temperature. The two basic methods of system
pressurization that are used to keep the HTW in its liquid state are the steam
cushioned system, and the inert gas pressurized system.
5.1 Steam Cushioned System. In this system, the steam present at saturated
water temperature conditions in the expansion tank is used directly