Condensate Return Lines and Condensate Pumps
Between the trap and the feed tank, condensate travels as a mixture of water and flash steam. Lines sized as if they carried only water back up, hammer and pressurize the traps feeding them. Where condensate cannot drain by gravity, a pump lifts it.
You'll learn
- Distinguish drain lines before the trap from return lines after it
- Size return lines for two-phase flow
- Understand lift and back pressure
- Compare electric and pressure-powered condensate pumps
Before this: Flash steam, Trap sizing and testing
Before the trap and after it
Upstream of the trap, the drain line carries water at steam pressure and is sized on water velocity — GyanTosh’s code uses 1.5 m/s for short runs and 1 m/s for long ones. Downstream, the pressure has dropped and part of the condensate has flashed. The return line now carries a little water and a great deal of steam by volume.
Ideally condensate falls by gravity to a receiver, from which it is pumped back to the feed tank. Where equipment sits low, or the return must rise to an overhead main, it has to be lifted.
Sizing for the flash, not the water
- Discharged from the trap to a line at atmospheric pressure.
- Flash: 13.3% = 133 kg/h of steam, occupying 222 m³/h.
- Water: 867 kg/h, about 0.91 m³/h.
Undersized return lines raise back pressure on every trap discharging into them, which cuts trap capacity, floods equipment upstream and drives the steam-water mixture fast enough to hammer. Common returns from traps at different pressures make it worse.
Lifting condensate and pumping it
A trap can lift condensate only with its own differential pressure, roughly 1 kg/cm² for every 10 m of lift plus line losses. That works on a steadily loaded main drain, but not on a temperature-controlled heater, where the steam pressure in the coil can fall below what the lift needs and condensate stops draining altogether — stall, covered in Module 09.
- Electric condensate pumps: a receiver with a vent and a level-controlled centrifugal pump. Simple; the pump must suit near-boiling water, so it needs enough suction head to avoid cavitation.
- Pressure-powered pumps: a vessel that fills by gravity and is emptied by steam or compressed air pressure, with no motor or seals. They handle hot condensate without cavitation and can be combined with a trap to drain stalled equipment.
Key takeaways
- Drain lines before the trap carry water; return lines after it carry water and a large volume of flash.
- Flash from 7 kg/cm²(g) condensate is hundreds of times the water’s volume: size returns for the flash.
- Lift costs about 1 kg/cm² per 10 m; temperature-controlled equipment may not have it.
- Electric pumps need suction head; pressure-powered pumps handle hot condensate and stall.