Condensate Recovery Piping
Downstream of Steam Traps
The condensate that is discharged from steam traps is handled in one of two ways. It is either drained out of the system at the point where it leaves the trap or it flows into piping to be transported elsewhere.
Steam Condensate Transport Piping
Piping that transports condensate is called condensate recovery piping or condensate return lines.
Piping designed for the transport of water is not necessarily suitable for use as condensate recovery piping. The design of condensate recovery piping requires a bit of specialization.
Condensate recovery piping must be designed for two-phase flow. Two-phase flow in this instance refers to flow in which a vapor, steam, flows through piping together with a liquid, condensate. Please note that this does not mean that the liquid and the vapor are flowing in separate layers inside the piping.
Changes in Flow Patterns of Two-phase Flow, Depending on Flow Rates and Percentage of Vapor
Steam is present even though it's for condensate?
So why do we have to take steam into consideration even though it's a condensate recovery line?
We must do so because of something called the "re-evaporation phenomenon." It is also known as "flashing phenomenon" and this is what occurs when condensate that is at a high pressure suddenly encounters a low pressure. Part of the condensate instantly changes back to steam, or 'flashes' back to steam. For more on flash steam, see the Flash Steam tutorial.
The specific volume of saturated steam is more than 1000 times that of saturated condensate. This means that even with the same amount of condensate, the proportion of steam to condensate changes depending on the flash rate, and this in turn results in large differences in the necessary design. If no flash steam occurs, the design can be viewed as being that of water transport piping. If the amount of flash steam is large, the design becomes almost identical to that of steam piping.
Design Methods for Condensate Recovery Piping
As previously stated, two-phase flow occurs when some of the condensate becomes flash steam, and the required piping design for this type of flow therefore falls 'somewhere between piping for water and piping for steam.' This is very vague, however, so TLV bases its determination of whether the design should be close to that for water or close to that for steam on the amount of flash steam and the flash rate. The concept of equivalent specific volume is used in this determination. This can be thought of as something like an average volume.
For actual design of condensate return piping, the upper allowable limit for flow velocity is determined based on the equivalent specific volume, and this limit is used to determine the diameter of the piping. The actual procedure is somewhat complicated so we will not go into it here. If interested, please refer to our technical handbook entitled Condensate Drainage and Recovery.
