If, for some reason, swept tees cannot be used, a float-thermostatic trap with its continuous discharge action is a better option (Figure 14.2.7). The flooded line will absorb the dissipated energy from the (relatively small) continuous flow from the float-thermostatic trap, more easily.
If the pressure difference between the steam and condensate mains is very high, then a diffuser will help to cushion the discharge, reducing both erosion and noise.
Another alternative is to use a thermostatic trap that holds back condensate until it cools below the steam saturation temperature; this reduces the amount of flash steam formed (Figure 14.2.8).
To avoid waterlogging the steam main, the use of a generous collecting pocket on the main, plus a cooling leg of 2 to 3 m of unlagged pipe to the trap is essential. The cooling leg stores condensate while it is cooling to the discharge temperature.
If there is any danger of waterlogging the steam main, thermostatic traps should not be used.
Temperature controlled plant with steam traps draining into flooded lines
Processes using temperature control provide an example where the supply steam pressure is throttled across a control valve. The effect of this is to reduce steam trap capacity to a point where the condensate flow can stop completely, and the system is said to have stalled. The subject of stall is discussed in greater depth in Block 13.
Stall occurs as a result of insufficient steam pressure to purge the steam plant of condensate, and is more likely when the plant has a high turndown from full-load to part load.
Not all temperature controlled systems will stall, but the backpressure caused by the condensate system could have an adverse effect on the performance of the trap. This in turn, might impair the heat transfer capability of the process (Figure 14.2.9).
Condensate drain lines should, therefore, be configured so that condensate cannot flood the main into which they are draining as depicted in Figure 14.2.10.
Discharge lines at different pressures
Condensate from more than one temperature controlled process may join a common line, as long as this line is:
• Designed to slope in the direction of flow to a collection point.
• Sized to cater for the cumulative effects of any flash steam from each of the branch lines at full-load.
The concept of connecting the discharges from traps at different pressures is sometimes misunderstood.
If the branch lines and the common line are correctly sized, the pressures downstream of each trap will be virtually the same. However, if these lines are undersized, the flow of condensate and flash steam will be restricted, due to a build up of backpressure caused by an increased resistance to flow within the pipe. Condensate flowing from traps draining the lower pressure systems will tend to be the more restricted.
Each part of the discharge piping system should be sized to carry any flash steam present at acceptable steam velocities. The discharge from a high-pressure trap will not interfere with that from a low-pressure trap if the discharge lines and common line are properly sized and sloped in the direction of flow. Module 14.3, ‘Sizing of condensate return lines’ gives further details.
Pumped return lines
Flash steam may, at some point, be separated from the condensate and used in a recovery system, or simply vented to atmosphere from a suitable receiver (Figure 14.2.11). The residual hot condensate from the latter can be pumped on to a suitable collecting tank such as a boiler feedtank. When the pump is served from a vented receiver, the pumped return line will be fully flooded with condensate at temperatures below 100°C, which means flash steam is less likely to occur in the line.
Flow in a pumped return line is intermittent, as the pump starts and stops according to its needs. The pump discharge rate will be higher than the rate at which condensate enters the pump. It is, therefore, the pump discharge rate which determines the size of the pump discharge line, and not the rate at which condensate enters the pump.
The pumping of condensate is discussed in further detail in Module 14.4, ‘Pumping condensate from vented receivers’.