Maintaining high TDS levels in the boiler drum results in water carryover
Maintaining high TDS levels in the boiler drum results in water carryover and foaming. This leads to wet steam and lower heat transfer efficiencies to the process.
When steam is generated all impurities present in the boiler feedwater do not boil off. The concentration of these impurities keeps increasing as steam is formed.
As the concentration of impurities increases, the steam bubbles rising from the bottom of the drum tend to become more stable, failing to burst as they reach the water surface in the boiler.
There comes a point where a substantial part of the steam space in the boiler becomes filled with bubbles and this foam is carried over into the steam main.
Presence of moisture reduces the heat exchange rate, increasing the process time. Thus it is also wastage of fuel as only steam participates in heat exchange in the process. High TDS levels in the boiler drum also lead to increased water side scaling.
E.g. The incremental fuel consumed due to high level of TDS maintained in the boiler is illustrated below.
Boiler design pressure : 10 bar g (145 psi) | Operating pressure : 8 bar g (116 psi) |
Boiler efficiency : 70% | Fuel (Coal) GCV : 3000 Kcal / kg |
Feedwater (softened) TDS : 800 ppm | Blowdown TDS : 8000 ppm |
Steam demand : 2000 kg/hr (4410 lb/hr) | |
At a TDS level of 8000 ppm, the steam would be 85% dry, i.e. there would be 15% moisture | |
Under given conditions, the plant would lose 167 tons (334,000 lbs) of coal | |
Increased moisture in steam is also undesirable as it contains a high level of dissolved and possibly suspended solids. These solids contaminate and cause damage to control valves, heat exchangers and steam traps. Modern blowdown control systems are available to ensure TDS levels in the boiler are always maintained within set parameters.