Condensate Recovery Piping

Condensate that is discharged from steam traps is handled in one of two ways. It is either drained out of the system when leaving the trap, which can result in wasted heat energy and water; or it flows into piping to be transported elsewhere, ideally for recovery.

Piping for Two-Phase Flow

The piping used to transport condensate is typically called condensate recovery, or condensate return piping. The design of such piping requires significant specialization because condensate recovery piping must be designed for two-phase flow. The design should not be based on calculations for piping that transports water because these are not suitable for two-phase flow.

Two-phase flow refers to flow in which vapor such as steam (either flash steam, live steam, or a mix of both) flows through piping together with liquid condensate. Although flowing together, this does not necessarily mean that the liquid and vapor are flowing in distinct separate layers. The flow pattern within the piping can also be mixed, as illustrated in the below animation.

Piping Considerations for Flow Patterns of Two-phase Flow

The flow pattern inside condensate recovery piping varies greatly according to the condensate flow rate, pressure, pressure drop, and resulting vapor percentage.

Why is steam present in condensate recovery piping?

Considering steam vapor when designing condensate recovery piping may at first seem counterintuitive, but is in fact necessary.

This is due to a phenomenon know as flash evaporation, which occurs when condensate formed at a high pressure is suddenly introduced into a low pressure system such as a condensate recovery line on the outlet side of a steam trap. In such a case, since the inlet pressure to the steam trap is higher than the outlet, part of the condensate instantly flashes into steam when discharged through the trap.

For more on flash steam evaporation, please read the article:

• Flash Steam

How Flash Steam Amount Influences Pipe Size

As the differential pressure across the trap increases, a larger amount of condensate flashes to steam, requiring the use of larger-sized condensate return lines.

At lower pressures, the specific volume of saturated steam can be more than 1,000 times that of saturated condensate. Even at higher pressures, this volumetric ratio can be more than 90 to 1. The proportion of steam to condensate will therefore vary depending on the amount of flash steam or flashing rate, and this in turn can greatly affect piping design requirements.

If no flash steam occurs, the piping design velocity and pressure drop calculations can be similar to those for simple water transport piping. If the amount of flash steam is large, the design becomes almost identical to that of steam piping. As such, designing condensate recovery piping first requires calculating the amount of flash steam and then sizing the pipe to accommodate both water and steam flow for the required velocity and pressure drop design parameters.

Example of Condensate Recovery Piping

The flash steam ratio (by weight) for an inlet pressure of 10 barg [145 psig] and an outlet pressure 6 barg [87 psig] is roughly 4%, or 1: 24. However, when specific volumes are compared, the ratio is about 10 : 1, flash to condensate. In terms of occupied space, the inside of the condensate return line is therefore often mostly comprised of flash steam.

Example of Condensate Recovered using a Drain Header

If a drain header is installed near the outlet of a pump enabling steam to be separated from condensate, then condensate and steam can be transported using separate piping. In such a case, it may be possible to design the condensate piping using standards for water distribution piping.

Design Methods for Condensate Recovery Piping

TLV sizes condensate recovery piping based on the amount of flash steam and condensate that can be present in a return piping system.

The calculation uses specific volumes to estimate volume ratios of condensate and steam at a given pressure and then determine a maximum allowable flow velocity. Piping is then sized based on the allowable velocity and pressure drop parameters.

Other factors that may be considered when sizing condensate recovery piping are:

• The presence of live steam in the piping
• The long term effects of corrosion or mud in the system, possibly reducing the internal cross-section piping area

Both of these may have the effect of increasing velocity, pressure drop, and system back pressure. For a more detailed explanation of the calculation for sizing condensate recovery piping, readers can refer to TLV’s technical handbook entitled, “Condensate Drainage and Recovery”.