An impulse trap (see Figure 10-35) operates with a moving valve actuated by a control cylinder. When the trap is handling condensation, the pressure required to lift the valve is greater than the reduced pressure in the control cylinder, and consequently the valve opens, allowing a free discharge of condensation. As the remaining condensation approaches steam temperature, flashing results, flow through the valve orifice is choked, and the pressure builds up in the control chamber, closing the valve.
