Undersized pipes create large pressure drops and high velocities, whilst oversized pipes are unnecessary and costly. This calculator allows steam and condensate pipes to be sized correctly. Steam pipes can be sized on pressure drop and velocity, and up to three steam trap discharge lines and associated common lines can be sized. Discharge lines from mechanical and electrical condensate pumps can also be sized.


  • Sizing New Pipes - Design new steam pipes correctly on pressure drop and velocity methods.

  • Checking Existing Pipes - The capacity of existing pipework can be checked using pressure drop and velocity methods, and the maximum steam flow can be determined for the chosen sized pipe.

  • Start Up & Running Losses - For any steam main, determine the appropriate size of warm-up valve and number of steam traps, in line with good practice.


  • Single Discharge Line - Size a steam trap discharge line based on condensate and flash steam flow.

  • Two Discharge Lines - Size the discharge lines from two steam traps plus the associated common line based on condensate and flash steam flow.

  • Three Discharge Lines - Size the discharge lines from three steam traps plus the associated common lines based on condensate and flash steam flow.

  • Pumped Return Lines - Size the pump discharge line from a mechanical or electrical condensate pump to the downstream condensate system.

Drain Lines to Traps

It is recommended that a condensate drain line running from a steam-using appliance to the steam trap should be sized on either the inlet connection of the correctly sized steam trap or the outlet connection of the appliance, whichever is the larger.

It is often the case that the appliance connection and the trap inlet connection are of different size. It is also possible that the appliance outlet connection is smaller than it should be. Where the drain line is short, typically less than 1 metre, this is not a problem, but with longer drain lines, friction losses will be higher and can reduce the capacity of the pipe. It is recommended that drain lines are kept as short as possible, generally less than 10 metres maximum.

It is important to select and size the steam-trapping device correctly; which means, not only must the trap be able to pass the condensing rate at full load, but also the stall load if the appliance has temperature control. It should never be taken for granted that the steam-trapping device will have the same size inlet connection as the appliance condensate outlet connection.

Please refer to our Steam Engineering Tutorial about Sizing Condensate Return Lines for further detail on drain lines.