Impulse Steam Trap:

Impulse steam traps, described in Figure, pass steam and condensate by a strainer before entering the trap. A circular baffle keeps the entering steam and condensate from impinging on the cylinder or on the disk. An impulse category of steam trap is dependent on the principle which hot water under pressure tends to flash inside steam while the pressure is decreased.

Figure: Impulse Steam Trap

The only moving category within the steam trap is the disk. A flange close the top of the disk acts as a piston. As elaborates in Figure, a working surface above the flange is larger than the working surface below the flange.