Figure 3-10.A typical automatic air vent.
prevents the proper heating of the radiator or other
steam equipment. Also, steam that is permitted to blow
through a defective trap results in heat loss.
Types of Traps
Traps are generally classified according to their
operation. The most common types of traps are float,
bucket thermostatic, float thermostatic, impulse,
thermodynamic, throttling, and bimetallic element.
FLOAT TRAP.The float trap normally consists
of a body, float, linkage, seat, and valve. A typical float
trap is shown in figure 3-11. As water enters the trap, the
float rises, opens the valve, and allows the accumulation
of water to flow into the return lines that take it to the
boiler. When the water has run out, the float falls, closes
the valve, and traps the steam.
RADIATOR AIR VENTS
The maintenance to be done on a float trap is of a
simple nature. One of the most common difficulties is
that of the float getting water in it and not rising. In this
case, the float must be replaced. The valve sometimes
gets plugged or worn and has to be cleaned or replaced.
There are two types of radiator air vents: automatic
and manually operated. A typical automatic air vent is
shown in figure 3-10. Air vents are installed to remove
air from the radiators, because air keeps the radiator
from heating properly.
The type of air vent shown consists of a hermeti-
cally sealed bellows, a valve disk and seal, and a vent
body. The bellows contains a volatile liquid with a
boiling point 10°F or lower than that of water. So, when
this liquid is heated to a temperature 10oF below the
steam and water temperature, the liquid volatilizes,
expands, and closes the valve. When air surrounds the
bellows, the air is cooler than the steam. This causes the
bellows to contract, to open the valve, and to allow the
air to escape. This cycle then starts over again.
The type that is operated manually is usually a small
valve that has a slotted screw incorporated in the stem
and a little spout on one side for the discharged air.
These manual vents are normally installed in the same
place in the distribution system as automatic vents.
Steam traps are designed to retain the steam in a
radiator or other using device until it changes into
condensate. After the steam has turned into condensate,
the trap releases the water so it can enter the return lines.
However, it keeps the steam coming into the radiator
from escaping. The trap performs an important
function, since the excessive accumulation of water
BALL-FLOAT TRAP.In a ball-float trap, the
valve of the trap is connected to the float so the valve
opens when the float rises. When the trap is in operation,
the steam and water that may be mixed with it flow into
the float chamber. As the water level rises, the float is
lifted, thereby lifting the valve plug and opening the
The condensate drains out and the float moves
down to a lower position, closes the valve. The
condensate that passes out of the trap is returned to the
Figure 3-11.A typical float trap.