One of the biggest mistakes made when sizing condensate return lines is to size them based on water flow in GPM. There are formulas and charts for determining liquid pipe sizing based on gallons per minute. However, this is not the correct way to size condensate return lines as they carry two-phase flow. Because hot condensate is discharged from higher pressure steam traps to lower pressure condensate systems, a portion of the condensate–typically 10%-15%–will re-evaporate, or flash into steam. And steam, being a gas, takes up much more space than water. The specific volume of water at 0 psig is 0.16 cubic feet. The specific volume of steam at 0 psig is 26.8 cubic feet. So, as we can see, if a portion of the condensate flashes to steam the volume it requires is quite substantial. If condensate return lines are sized for water flow alone they will not be able to accommodate the increase in volume and the flow will be reduced substantially.
The specifics on condensate return line sizing varies with the system, the primary factors being the pressure of the steam system and the pressure of the condensate system. The resulting pressure differential along with flow, in pounds per hour, determines the amount of flash steam as a percentage. The condensate flow is then multiplied by the percent of flash to determine the flow rate of flash steam. The condensate return line size is based on the amount of flash steam flow at a given pressure in the effective pipe size that will keep the flow between 4,000 and 6,000 feet per minute.
There is no easy, one-size-fits-all approach to condensate line sizing. The complete system should be considered, including motive pressure, back pressure, lift, pressure differential, and pipe lengths. Considering all aspects of the system, an optimal return system can be designed.
A key aspect of condensate line sizing is understanding flash steam. Flash steam will occur when there is a pressure drop. And there will typically always be a pressure drop. Pressure differential is the motive for steam flow, as pressure always flows from higher to lower seeking equalization. So flash steam is a given in a condensate return system. In terms of condensate lines flash steam can be a big problem if not allowed for. However, if given proper consideration in the design process it can be another component of a smooth operation. But even though we allow for it in line sizing, we often still have to deal with it before everything is properly returned to the boiler. The next installment in this series will consider flash steam recovery and utilization.