The exact calculation of the line size would be complex and probably inaccurate due to the number of variables. Few problems will arise if the trap discharge lines are sized on flash steam velocities of 15 to 20 m/s.
1) The flash steam would pass unhindered above the condensate through correctly sized trap discharge lines sloping in the direction of flow and being open-ended or vented at a receiver (Figure 6). The minimum recommended slope is 1 in 70 (150mm drop every 10 s). A simple visual check will usually confirm if the line is sloping - if no slope is apparent it is not sloping enough!
2) In unavoidable situations, non-pumped rising lines should be fitted with non – return valve and length should be kept as short as possible to prevent condensate falling back down to the trap. Risers discharging into the top of overhead return lines stops condensate draining back to the riser. This assists the passage of flash steam up the riser.
It is sensible to consider using a slightly larger riser, which will produce a lower flash steam velocity. This will reduce the risk of water hammer and noise caused by steam trying to force a path through the liquid condensate in the riser.
IMPORTANT: A rising line should only be used where the process steam pressure is guaranteed to be higher than the condensate backpressure at the trap outlet. If not, the process will waterlog unless a pumping trap or pump-trap combination is used to provide proper drainage against the backpressure.
3) Common return lines should also slope down and be non-flooded (above Figure 7). To avoid flash steam occurring in long return lines, hot condensate from trap discharge lines should drain into vented receivers (or flash vessels where appropriate), from where it can be pumped on to its final destination, via a flooded line at a lower temperature.