Condensate and Flash steam recovery: Design issues and considerations
Contrary to popular belief, condensate lines do not always contain only water. This is because condensate is discharged at the same pressure as steam in the equipment. When the pressure drops below its saturation pressure, flash steam is formed. The presence of flash steam is a significant factor that needs to be considered in the design of condensate lines as condensate lines often have bi-phase flow i.e. both flash steam and condensate flowing through it.
Designing condensate lines to accommodate this bi-phase flow presents several challenges:
- Flash steam occupies a greater volume of space in the pipelines than condensate i.e. greater than 95% and pipelines need to be correctly selected to accommodate this volume.
- The presence of flash steam can introduce additional pressure drop along the condensate lines.
- While designing pipelines with flash steam, velocities need to be kept low. Higher velocities can lead to water hammer and damage to the pipelines.
- Care also needs to be taken to avoid formation of condensate slugs which can damage the pipelines.
Poor design can manifest into water hammer, corrosion, heat loss, higher batch times, non-attainment of process parameters, leakage losses. Design issues thus have far reaching effects, impacting not only energy conservation and sustainability but personnel safety and process efficiency as well.
While right sizing of condensate lines is important, line layout is equally crucial.
Some common aspects to consider while designing the condensate recovery network
Do not connect traps of different pressures to the same common condensate return line
In practical situations, equipment using different steam pressures are placed alongside each other. To simplify the implementation of condensate recovery piping, it is a common practice to connect the steam traps from all of these equipment to the same common condensate return line.
However, since condensate is at the same pressure as steam, when high pressure and low pressure condensate mix in the same return line, the downstream pressure increases. As a result the differential pressure across the trap decreases. This has a greater impact on the steam trap which has a lower upstream pressure, reducing its capacity to discharge condensate. Consequently, condensate is not effectively evacuated from the low pressure equipment, resulting in water logging. This not only hampers heat transfer but also poses a risk of equipment damage and leaks caused by water hammering.
Steam traps should be installed as close to the equipment as possible
The pipeline from the equipment outlet up to the steam trap should be kept as short as possible. One may believe that as long as there is a steam trap, locating it a few meters away from the process equipment outlet would not matter.
However, something as trivial as this can lead to poor productivity in your process. Steam traps are designed to discharge condensate and trap steam. However, when the steam trap is located at a greater distance from the trap, the trap can get steam locked i.e. steam can flow over the condensate and collect in the line blocking the flow of condensate to the trap. In such a situation the steam trap remains closed.
Condensate will only reach the trap once the steam that is blocking it condenses. Up to this point condensate formed in the equipment will continue to back up in the line and process equipment. This will hamper heat transfer and affect the process.
The steam trap should therefore be located as close to the process equipment as possible.
The steam trap should always be installed below the equipment or line being drained
To ensure that condensate flows by gravity to the trap, the trap should always be located below the equipment or line being drained. For equipment drain lines it is recommended that the pipeline from the equipment outlet falls vertically for at least about 10 pipe diameters to the steam trap.
This ensures that condensate does not accumulate in the bottom of the equipment preventing corrosion and damage due to water hammer.
Vertical lift of the discharge line after the steam trap should be avoided
When the discharge line after the trap is vertically lifted, it introduces back pressure on the steam trap which can interfere with the proper operation of the steam trap and reduce its capacity to discharge condensate.
It is ideally recommended to have a vertical drop in the condensate line immediately after the equipment outlet and a gradual slope in subsequent lines to facilitate flow of condensate to the condensate pump by gravity.
For traps which have a blast discharge, the connection to the common return line should not be vertical
Both thermodynamic and inverted bucket traps discharge condensate intermittently with significant force (blast). Repeated blast discharge in the pipeline can lead to erosion of the material of the line consequently resulting in leaks. Giving the pipeline a slight angle at the point of connection can reduce the intensity of the discharge, preventing erosion of the pipeline. This slight angled junction on the branch side is known as a swept junction or a swept tee.
Trap discharge lines of temperature controlled equipment should not be drained into flooded condensate collection lines
When a temperature control valve installed on the steam supply line to a process equipment throttles, it reduces the differential pressure across the steam trap. This can lead to stalling. Typically, the discharge lines of such equipment should never be connected to a flooded collection line as this can introduce additional backpressure on the steam trap and adversely affect its discharge capacity.
Condensate pipelines should also never be routed underground
Condensate lines are prone to corrosion and leakage and should never be buried underground. Typically, leakages in condensate pipelines which have been buried underground go unnoticed. This makes troubleshooting and maintenance difficult.