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.
