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- Reasons behind oxygen injection in feedwater circuit for supercritical boiler? - Friday, July 30, 2010 - B.Sarkar [1089]
In super critical boiler, oxygen in controlled amount is injected in condensate and feed water after deaerator.
Though dissolved oxygen has limits in the feed water circuit to discharge in to supercritical boiler to avoid
oxydation, resistance to heat transfer surface and tube damage. My question is why extra effort is paid to
discharge external oxygen in power cycle water even after condensate polisher output if oxygen is harmful
to boiler feed water circuit. Note that there is an oxygen injection control loop which includes PID controller
and a flow control valve in oxygen supply path.
Please explain with a logical approach.
Post a Reply... - Re: Reasons behind oxygen injection in feedwater circuit for supercritical boiler? - Friday, July 30, 2010 - Murphy [21]
The world is not black and white in power plants that use supercritical boilers or units. Since these supercritical units operate at very-high pressures of 3,500-psi and higher, boiler tube corrosion and deposit issues are at an extreme importance.
The injection of oxygen, which is known as oxygenated treatment or OT works if the in feedwater with a pH ranging 8.0 to 8.5 is very-pure . In distilled terms, the injection of oxygen in the feedwater system regulates the formation of an oxide layer on the supercritical units tubes consequentially, reducing a deposit of oxide-forming exfoliation therefore reducing the likelihood of a tube failures.
Can anyone else put a better spin on this?
Post a Reply... - Re: Reasons behind oxygen injection in feedwater circuit for supercritical boiler? - Friday, July 30, 2010 - Jim Watts [764]
It is certainly true that removal of Oxygen from boiler feed water has been the conventional treatment of boilers for many years with unarguable benefits. It will also remain the case for the majority of boilers.
The problem with oxygen removal has only raised its head as steam temperatures have become very high in particular with supercritical. When oxygen is removed by a scavenger such as hydrazine the trace oxygen remaining oxidises steel to magnetite the black scale normally present in most boilers. This is normally considered a protective scale as it slows further attack on metal surfaces. Unfortunately it does not stop completely however and magnetite layers continue to grow eventually needing acid cleaning. In the case of supercritical temperatures the magnetite growth is quite fast and due to the cycling operation of boilers the temperature shocks cause the scale to shed into the bottom bends of tubes or flow on to the turbine.
Experiments carried out mostly in Japan found that it was better to provide sufficient oxyen to the corrosion process to allow the scale to form as Heamatite instead of magntite. This scale is much thinner and stronger and does not cause as many outages or turbine damage as magnetite scale. The only problem is it must be very carefully controlled by providing just enough oxygn and no more or less.
Further info at http://tinyurl.com/2begu6r
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- Re: Reasons behind oxygen injection in feedwater circuit for supercritical boiler? - Friday, July 30, 2010 - Murphy [21]
- Reasons behind oxygen injection in feedwater circuit for supercritical boiler? - Friday, July 30, 2010 - B.Sarkar [1089]
