Hot boiler feedwater in conjunction with dissolved residual oxygen – this means corrosion for the plant units from the pipe to the turbine on the one hand and high maintenance and repair costs on the other hand.
To prevent this, oxygen-consuming chemicals are added to the boiler feedwater. The effectiveness of these chemicals must be monitored. A leak in the pipes and tanks also results in oxygen penetration and increased corrosion.
The basic conditions of high pressure and high temperature demand a sensor suitable for daily use, with associated reliable sample conditioning, to safely display the residual oxygen contents in the ppb range.
The measure of the amount of dissolved oxygen gas in the water is used to monitor performance of deaerators, control chemical injection and to detect air leakage into vulnerable parts of the feedwater and condensate system.
Oxygen corrosion and the associated corrosion products represent a great expense to power plant water components. Oxygen pitting is often seen in economizers during operation, while superheaters and reheaters are especially susceptible during standby conditions. All carbon steel components in a system are vulnerable to this type of attack.
Copper alloy corrosion in condensate and feedwater systems is a function of oxygen. Oxygen can cause corrosion fatigue of boiler tubes as well as turbine disks and blades.
Totally eliminating oxygen from the water is virtually impossible. Potential sources of oxygen ingress include leaking turbine/condenser expansion joints, low pressure heater flanges and connections, turbine explosion diaphragms, leaking pipe joints, etc. Because the oxygen will always find a way into the system, the oxygen level is constantly monitored and controlled. This is typically done by means of a mechanical deaerator and/or chemical reaction. Regardless of the type of treatment used, the dissolved oxygen level is always controlled to the parts-per-billion range.
Continuous measurement of dissolved oxygen at several points is critical to the long term reliability of critical components, especially the boiler.
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