Sizing pipe after steam traps

Undersized condensate return-lines create one of the most common problems encountered with process steam traps. Hot condensate passing through a trap orifice loses pressure, which lowers the enthalpy of the condensate. This enthalpy change causes some of the condensate to flash into steam. The volume of the resulting two-phase mixture is usually many times that of the upstream condensate. 

The downstream piping must be adequately sized to effectively handle this volume. An undersized condensate returnline results in a high flash-steam velocity, which may cause waterhammer (due to wave formation), hydrodynamic noise, premature erosion, and high backpressure. The latter condition reduces the available working differential pressure and, hence, the condensate removal capability of the steam trap. In fact, with some traps, excessive backpressure causes partial or full failure. 

Due to the much greater volume of flash steam compared with unflashed condensate, sizing of the return line is based solely on the flash steam. It is assumed that all flashing occurs across the steam trap and that the resulting vapor-liquid mixture can be evaluated at the end-pressure conditions. To ensure that the condensate line does not have an appreciable pressure-drop, a low flash-steam velocity is assumed (50 ft/s) [i]. 

The flash-steam volumetric flowrate is then determined  

Based on the assumed velocity, the required cross-sectional area is calculated as  

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