Chapter 6
Steam Traps—How They Work
Float and thermostatic Traps
Float and thermostatic traps are mechanical
units that operate on both density and temperature
principles.
The float valve operates on the density
principle. A lever connects a ball float to a
valve and seat. Condensate enters through the
inlet near the top of the trap, once it reaches a
certain level the float rises, opening the valve
and draining condensate. Condensate drains
from the port near the bottom of the trap, leaving
a water seal (formed by the condensate) to
prevent live steam loss.
Since the discharge valve is under water, it is
not capable of venting air and non-condensable
gasses.
The thermostatic element operates based
on temperature. When the trap is filled with
steam the thermostatic valve is closed. When
the accumulation of air and non-condensable
gases causes a significant temperature drop,
a thermostatic air vent in the top of the trap
opens and discharges them.
A typical operating sequence is as follows:
1. The thermostatic air vent is open at startup
to discharge large volumes of air to the
condensate Return. As steam enters the trap
body the air vent closes.
2. The float is closed at start-up and stays in
the closed position while steam is in the trap
body.
3. When the steam condenses, the hot condensate
lifts the float, moving the valve head
off the seat and opening the trap to discharge
condensate.
4. As condensate discharges, steam enters
the trap body, the float falls and drives the
valve head into the valve seat closing the trap.
36 National Oilheat Research Alliance
For detailed info see: https://www.youtube.
com/watch?v=fbJbg2kKx4Q
Heat Emitter (Radiator) Traps
Individual radiator traps operate on temperature
only.
1. At start up when air and condensate are
cooler, the thermostatic element (diaphragm)
contracts, pulling the valve head off the valve
seat.
2. The trap then opens and discharges air
and cool condensate.
3. As steam enters the heat emitter the
condensate gets hotter, the element expands
and drives the valve head into the valve seat,
closing the trap.
4. The trap then stays closed until the condensate
cools enough to contract the element
and open the trap.