Condensate return system in power plant should be well designed to satisfy the steam quality parameters, so the generated steam can drives steam turbine generator and produces expected power output at generator terminals. Condensate from condenser of turbine should not be directly distributed into the boiler through steam drum, condensate should be passed to some equipment to perform treatment on it, and so feedwater quality is suitable with the requirements of boiler water.
Steam drum serves to separate the steam from a mixture of water and steam which enter into drum from evaporator circuit (boiler tube banks), to accommodate the steam produced, and serves as a “buffer tank” so that the steam pressure that will be distributed relatively not fluctuate too much.
This means that the steam drum is a vessel that collects generated steam that has pressure and temperature as desired. Therefore, condensate return should not be distributed directly into steam drum because it will create fluctuations in pressure and temperature increases.
Condensate return system is designed to enter into some equipment first such as LP Feed Heater, Deaeratorand HP Feed Heater, after that together with the new make-up water to be feed water. At the time of steam is changed to be liquid phase in the condensation process, so there is enrichment of oxygen dissolved in the condensate, therefore oxygen scavenger is needed in the deaerator.
Excess oxygen in the boiler system is not permitted because free oxygen would be oxidative, so that it can accelerate corrosion rates.
Additionally oxygen can react with other minerals present in water to form other compounds, which are usually in the form of salt and increase the boiler water hardness. Therefore blow down rate should be higher that affect the use of more chemicals as much water must be removed, and its cycle of the boiler water becomes shorter.
So, if in the condensate return systems, condensate is returned directly to the steam drum (e.g. with check valve protection and other adequate equipment), the possibilities will occur are:
- Flashing, because the condensate temperature is relatively cool meet with relatively hot steam, which can arise uncontrolled pressure suddenly and sporadically.
- The quality of steam in the steam drum is interrupted, which had been saturated or superheated will change, can become wet steam, so that its enthalpy was changed, and can result in performance of equipment supplied by the steam drum is not as expected, and would be fatal if the steam is distributed into steam turbine, the wet steam will damage the turbine blades, therefore cost of repairs will be very expensive.
- Rate of corrosion in boiler systems will increase, because of the excessive free oxygen.
Condensate usually brings solid results from corrosion along path of pipeline, for example in the form of iron oxide. Each boiler manufacturer has its own criteria to determine feedwater quality before entering the boiler.
At low pressure boiler has less restrictive criteria as for medium or high pressure boiler. Could be, if still eligible, condensate will be mixed with make-up water in the feedwater tank without being processed first to remove corrosion products / dissolved solid, but it still at risk as described above.
But if indeed in the condensate return systems, condensate is distributed directly to the drum in the low pressure boiler, steam should be checked if it still qualifies as feedwater and it should be installed the filtration process and some equipment such as pumps, valves (control valve, check valve, or isolation valve) due to condensate certainly has smaller pressure than the pressure in the steam drum.