When set temperature of the process fluid is reached, the steam pressure inside the jaacke ted ke ttle may reduce to 0 PSIG or even go into Vacuum.
To promote condensate drainage, the steam trap is placed a ce rtai n dis t ance below the process equipment. 2.3 ft. will provide 1 psig of condensate head pressure.
As long as the trap discharges into a gravity return line (at 0 psig), there will be 1 psi differential pressure and condensate may freely drain.