Steam traps cannot be sized strictly for nominal steady-state heat exchanger condensate flow. During startup, cold metal piping and subcooled process fluids condense steam at 2 to 4 times the running rate. Furthermore, steam pressure across modulating control valves fluctuates constantly. A safety factor (typically 1.5x to 2.0x for continuous-discharge Float & Thermostatic traps, and 2.5x to 3.0x for cyclic Inverted Bucket or Thermodynamic disc traps) ensures adequate discharge capacity during peak startup loads without backing condensate into the heat exchanger.

When high-pressure saturated condensate at pressure P1 passes through a steam trap into a lower pressure return header P2, the sensible enthalpy of the liquid h_f1 exceeds the boiling enthalpy h_f2 of liquid at the lower pressure. This excess thermal energy instantly vaporizes a fraction of the water into flash steam: % Flash = 100 * (h_f1 - h_f2) / h_fg2. For example, discharging 10 bar_g condensate (184°C, h_f=781 kJ/kg) into a 1.5 bar_g header (127°C, h_f=535 kJ/kg, h_fg=2181 kJ/kg) converts 11.3% of the liquid mass into low-pressure flash steam.

Although flash steam represents only 10% to 15% of the condensate mass, low-pressure steam has a specific volume several hundred times greater than water (e.g. 0.88 m³/kg for steam vs 0.001 m³/kg for water at 1.5 bar_g). As a result, flash steam occupies over 95% to 98% of the total volumetric pipe flow. Sizing condensate lines using liquid velocity rules (1-2 m/s) results in catastrophic two-phase sonic choking, extreme backpressure, and violent water hammer.

Group trapping is the piping practice of connecting the condensate drains of multiple steam-using pieces of equipment into a single shared steam trap. Because each equipment unit operates at different thermal duties and pressure drops, the unit with the highest condensing rate experiences the lowest steam pressure. The higher-pressure units back-pressure the lower-pressure unit through the common header, causing condensate waterlogging, loss of temperature control, and corrosive carbonic acid attack.

Heat exchanger stall occurs when a modulating temperature control valve on the steam supply throttles down to maintain process setpoint at reduced load. The steam chest pressure inside the exchanger drops significantly—often below atmospheric pressure or below the return condensate header backpressure. With zero or negative differential pressure across the steam trap, condensate cannot discharge by gravity, flooding the tubes until steam space is lost, causing hunting temperature swings and severe water hammer.