When you find that the individual radiator fails to
heat, either an inoperative steam trap or a radiator that is
not installed correctly can cause the trouble. Repairing
or replacing the steam trap or correcting the improper
installation of the radiator can eliminate these troubles.
When it is the whole distribution system that fails to
heat, clogged or closed receiver vents can cause the
trouble, a flooded return line, the lack of pump capacity,
or air binding the system. These troubles can be
remedied by opening the vents, checking and adjusting
the pump cut-in, replacing the pump, or repairing
inoperative rerun traps.
One common trouble that occurs in this type of
distribution system is the overflow of water from the
receiver vent. An inoperative pump usually causes this
condition. The pump may be causing the flooding
because of its inadequate capacity or because it is
unable to handle the volume of condensate required.
This condition can be corrected by either repairing or
replacing the pump.
Another cause of overflow of water from the
receiver vents is an obstruction in the line between the
condensate receiver and the boiler. The trouble can be
remedied by eliminating the obstruction, regardless of
whether it is a closed valve or a clogged line.
TWO-PIPE VAPOR SYSTEM WITH A
VACUUM PUMP AND A CONDENSATE
The two-pipe vapor distribution system with a
vacuum pump and a condensate return, as shown in
figure 3-8, is similar to the two-pipe vapor system with a
condensate pump. The piping in this system includes
separate steam and return mains.
Most vapor distribution systems with vacuum
pumps and condensate returns are similar. However, it
is seldom that two steam distribution installations are
alike in detail. When installing vapor-heating
distribution systems, it is advisable to refer to the
manufacturer's recommendations, civil engineer
mechanical drawings, and specifications for the proper
When this type of distribution system is operated,
the steam is supplied at the top of the radiator and the air
and condensate discharged through a thermostatic trap
from the bottom of the opposite end of the boiler. All
returns are dry and terminate at the vacuum pump. The
vacuum pump is usually a motor-driven unit, although
low-pressure steam turbines have been successfully
used to a limited extent. The vacuum pump returns the
condensate to the boiler and maintains the vacuum or
subatmospheric pressure in the return system. The
maintenance of a vacuum in the return system (3 to 10
inches of water) enables almost instantaneous filling of
the heating units at low steam pressure (0 to 2 psi) since
air removal is not dependent upon steam pressure.
The vacuum pump withdraws the air and water
from the system, separates the air from the water, expels
the air to the atmosphere, and pumps the water to the
Figure 3-8.A two-pipe vapor system with a vacuum pump.