Boiler Dissolved Oxygen Control
DISSOLVED OXYGEN CHEMISTRY
Dissolved Oxygen (DO) is a measure of the amount of oxygen, usually thought of as a gas, that is dissolved in a liquid such as water. Oxygen is essential to life and is also the most common element found taking part in corrosion reactions. DO can be thought of as the fuel needed for the corrosion process to proceed. It is the corrosion reactions that necessitate trace (boiler) DO analyzers; such as IC Controls 865-25 or 869-22 portable that are designed to measure down to part per billion (ppb or μg/L) levels.
DO Corrosion of Boilers
The presence of dissolved oxygen in water makes for a highly corrosive environment. Mechanically hard, porous metal oxide deposits with little strength form rapidly with the most serious aspect of this oxygen corrosion being that it occurs in localized cells that result in pitting. Pitting is a concentration of corrosion in a small area of the total metal surface that effectively drills a small metal oxide filled hole in the metal. Pits can produce large mechanical failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is low. Rapid corrosion will progress inside an industrial or utility boiler plus its water and steam system unless dissolved oxygen can be virtually eliminated. Unchecked corrosion eventually results in expensive repairs or equipment failures and subsequent replacement.
Boiler water DO removal
Dissolved Oxygen removal from any steam and water system is of major importance. The first step is typically mechanical deaeration which is economical and serves to also eliminate other corrosive gases such as ammonia and carbon dioxide. A properly operated deaerator can reduce dissolved oxygen to as low as 10 μg/L (10 ppb). Note: 1 μg/L = 1 ppb. For complete oxygen removal down to low ppb levels a second step, chemical oxygen removal, is needed to assist the deaerator. It is important to monitor the actual DO level since air contains 21% oxygen and the difference between the atmospheric percent and ppb is a factor of 100,000,000. Thus, any tiny leak in the equipment will result in high DO and corrosion.
The most economical DO removal can be achieved by continuously measuring deaerator ppb DO results and fine tuning deaerator operation for lowest possible ppb DO reading before adding the more costly chemical scavengers. In addition, even with chemical oxygen scavengers present, DO ppb levels should be measured at potential air in-leakage points to prevent localized high DO and corrosion.
The IC CONTROLS model 865-25 monitors ppb dissolved oxygen continuously in these critical steam and water circuits. The operating range of 0.1 ppb to 10 ppm (boiler water systems best practice, kept below 5- 10 μg/L.) allows monitoring of leaks from condensers, valves and fittings, plus low level precision of ±2% of reading, or 2 digits, clearly shows the performance of oxygen removal equipment and scavengers. Design considerations include a simple, accurate calibration approach plus the capability to communicate with DCS systems and evolving technology.
Galvanic ppb DO advantage.
With IC Controls trace dissolved oxygen measuring sensor, galvanic current naturally is zero when oxygen is absent. An electrochemical cell similar to a battery it produces current only when oxygen is present. By using carefully selected electrodes in contact with an electrolyte, a chemical reaction occurs that uses electrons gained from oxygen molecules to produce a galvanic current directly proportional to the concentration of oxygen present. By comparison Electrolytic DO cells require an external supply to drive the chemical reaction, the resulting zero-current can drift. This can result in negative ppb readings or offsets in the readings. Luminescent DO sensors use a LED light pulse (flash) to excite fluorescence which diesaway (faster with oxygen) and the delay (or phase) after the pulse is measured, then read as ppm DO. At ppb DO levels, tiny pulse variations (current) and photo-diode sensitivity to almost no change in die-away rate, combined, result in muddy inconsistent readings. Galvanic current, naturally linear to oxygen and zero when oxygen is zero, is a big advantage for reliable trace ppb level DO operation. The IC Controls galvanic cell is separated from the sample by an oxygen permeable membrane. The cell has a silver cathode in close contact with the membrane where oxygen gains electrons (is reduced) to become hydroxyl ions, and a lead anode that is oxidized regenerating the electrolyte and balancing the reaction to complete the galvanic cell. Installed in an 865-25 sample panel that controls fouling and permits in-line calibration, these advantages result in years long accurate ppb DO performance.
Calibration is easy
At any given temperature and barometric pressure the partial pressure of oxygen in water-saturated air is exactly the same as it is in air-saturated water. Thus a galvanic sensor can be calibrated in water-saturated air, using the 20.9% oxygen available in air as the full-scale standard, and it will correctly read dissolved oxygen in water samples. IC Controls DO analyzers have automatic temperature and barometric pressure sensors, so they automatically calibrate the analyzer. To calibrate the sensor, simply suspend the probe above water, Fig. 2, and let the analyzer auto calibrate. This calibration technique will give a 100% saturation reading for the temperature and pressure which the analyzer will display as ppb dissolved oxygen.
DISSOLVED OXYGEN CONTROL OBJECTIVES
Successful water treatment in closed (or almost closed) systems such as boiler and steam, hot-water heating or chilled water systems has three fundamental requirements: 1. Dissolved oxygen concentrations must be controlled to reduce the likelihood of forming oxygen corrosion cells that can result in pitting of carbon steel. 2. The pH must be maintained high enough to minimize general corrosion of carbon steel, but not so high that caustic attack can occur. 3. An external-treatment system must be in place to minimize the ingress of dissolved and suspended solids and/or an internal treatment system must be used to control these materials.
Looking at the first item, dissolved oxygen control,. target values specified by most manufacturers and utilities have called for oxygen concentrations in boiler feedwater or closed heating/cooling systems to be kept below 5-10 μg/L (5-10 ppb). Achieving this low level of oxygen may require a combination of mechanical and chemical methods in steam and water systems such as the boiler system illustrated in Fig. 3. Other systems, such as a closed hot water heating or cooling system, particularly when they are tight (no oxygen in-leakage points), may achieve their target with only the chemical addition. The hot-water heating system in Fig. 4 uses only a by-pass feeder for intermittent chemical feed.