A float trap (see Figure 10-19) is operated by the rise and fall of a float connected to a discharge valve. The change of condensation level in the trap determines the level of the float. When the trap is empty, the float is at its lowest position and the discharge valve is closed. As the condensation level in the trap rises, the float also rises and gradually opens the valve. The pressure of the steam then pushes the condensation out of the valve. Because the opening of the valve is proportional to the flow of condensation through the trap, the discharge of condensation from the trap is generally continuous. On some float traps, a gauge glass is used to indicate the height of the condensation in the trap chamber.
One of the principal disadvantages of a float trap is the tendency of the valve to malfunction. Valve malfunctions can result from the sticking of moving parts or excess steam leakage due to unequal expansion of the valve and seat.
Float traps are designed for steam pressures ranging from vacuum conditions to 200 psig and are used to drain condensation from heating systems, steam headers, steam separators, laundry equipment, and other steam process equipment. When used in heating systems, a float trap should be equipped with a thermostatic air vent.