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Minimum Steam Trap Differential Pressure

I have a question that I've always wondered about, and has been a major factor in designing AHU steam preheat coils.

The typical minimum operating differential pressure on a F&T steam trap is 0.5 psig. So when the control valve is closed and the vacuum break opens, you will need 14" of water column to push through a trap with an outlet at atmospheric pressure.

Is it true that the 14" of water column will always remain upstream of the trap? If so, water will back up into a steam coil when a control valve is closed if the trap isn't a minimum 14" below the coil.


    EBEBRATT-Ed Member Posts: 12,007
    I like to drop the steam trap as low as possible below the coil. Especially preheat coils.The 12-14" dimension is minimum although you will find many installed with a lesser drop. The actual back pressure in the return line is usually not known and varies during normal operation as does the trap inlet pressure. The capacity of the trap is based on differential pressure. The 12-14" recommended may or may not be enough depending on job conditions. When the control valve is closed no additional water can enter the coil (no steam no condensate). The 12-14" works on most jobs.

    I usually use a 2-1 safety factor on the traps capacity and use a min. 12-14" drop. Assuming little or no backpressure this will usually be fine.