e) In the expansion vessel, the pump intake line should be located
to prevent short circuiting of the cooler return water, and a minimum of 6
inches (152 mm) below the water line of the expansion vessel.
f) Provisions should be made to prevent vortexes from forming at
the inlet of the pump intake line. This will aid in preventing cavitation in
g) The expansion vessel must be located at a level above the
generator outlet. An expansion vessel located below the boiler outlet can
flood, causing the steam space to form outside the expansion vessel.
Design Criteria for Gas Pressurized Systems
a) The pump suction line should extend a minimum of 6 inches
(152 mm) above the expansion vessel bottom to prevent sludge from entering the
b) The line between the expansion vessel and return main should be
the same size as the return main.
c) The expansion vessel should be located on the inlet side of the
generator and on the suction side of the pump.
Pumps. The three primary considerations in pump selection are
suitability for HTW services, leak-tightness and NPSH.
Pump Types. Single stage centrifugal pumps are normally used to
circulate HTW through the distribution system and through the generator. The
pumps selected for this service must be designed especially for HTW in order
to secure efficient, reliable operation with a minimum of maintenance. The
head characteristic of the system circulating pumps should be flat in order to
deliver nearly constant head throughout the range of operating capacities.
Generator circulating pumps shall have steep head characteristics to
accommodate the controlled circulation concept. At the same time it is
necessary that the maximum head should occur at shutoff and should fall off
gradually up to maximum capacity, suffering a decrease in pressure no more
than 15 percent below the shutoff pressure at the maximum operating capacity.
Circulating pumps are located in the return line of the system in order to
maintain the highest possible positive suction head.
To further improve and guarantee efficient operation of these
pumps, a mixing connection is provided so that a portion of the system water
mixes with the water from the generator, lowering its temperature to avoid the
danger of flashing at the pump suction. The suction intake of HTW pumps must
be carefully designed to avoid sudden changes in velocity or direction which
might contribute to flashing and inefficient operation. Adequate water
cooling must be provided for all pumps. Mechanical seals are recommended.