Syphon drainage is necessary in applications such as submerged coils. Steam condenses on the inside of the coil and must be lifted to the trap, which is located above the bottom of the coil.
For these applications there are two ways to trap.
Differential Condensate Controller is the best trap to select, because it provides continuous controlled scavenging of the syphon line.
If any other trap is used, locate a check valve in the inlet line or internal to the trap. The check valve will prevent backflow out of the trap when the trap valve is closed. The syphon pipe should be one size smaller than the nominal size for the trap selected but never less than 1/2" (15 DN) pipe size.