Condensate return lines between the equipment drain point and steam trap should be carefully engineered to ensure minimum back pressure is created on the equipment steam space.
Condensate is valuable resource. An effective condensate recovery system, collecting the hot condensate from the steam using equipments and returning it to the boiler feed system needs to be carefully designed. Condensate return lines can be classified broadly into three categories: drain lines to traps, trap discharge lines and pumped return lines.
For drain lines to traps, the condensate must flow from the steam drain outlet on the equipment, to the trap. The steam space of the equipment and the body of the trap being at the same pressure, gravity can be relied on to induce flow. The lines from the drainage points to the traps can be laid with a slight fall, say 1/70.
To minimize the risk of steam locking, these lines should be kept short, with the traps as close as possible to the equipment discharge point. When selecting the pipe size, the quantity of condensate to be considered is not necessarily the normal full load of the equipment being drained. At plant startup the condensing rate may be up to twice the running load or in a few cases even more. Further the line to every trap very often has to carry the air which is being displaced by the incoming steam. Thus this line between the equipment drain point and the steam trap has to be designed considering these factors.
If care is not observed in this designing, the line may be undersized thereby increasing back pressure on the equipment, hampering condensate evacuation. Usually sizing the line for a flow rate of twice the running load should deliver acceptable results.