Boiler Dissolved Oxygen Control
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BOILER DISSOLVED OXYGEN CONTROL DISSOLVED OXYGEN CHEMISTRY tiny leak in the equipment will result in high DO and corrosion. Dissolved Oxygen (DO) is a measure of the amount of oxygen, usually thought of as a gas, that is dissolved in The most economical DO removal can be achieved by a liquid such as water. Oxygen is essential to life and is continuously measuring deaerator ppb DO results and also the most common element found taking part in fine tuning deaerator operation for lowest possible ppb corrosion reactions. DO can be thought of as the fuel DO reading before adding the more costly chemical needed for the corrosion process to proceed. It is the scavengers. In addition, even with chemical oxygen corrosion reactions that necessitate trace (boiler) DO scavengers present, DO ppb levels should be measured analyzers; such as IC Controls 865-25 or 869-22 at potential air in-leakage points to prevent localized portable that are designed to measure down to part per high DO and corrosion. billion (ppb or µg/L) levels. 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 DO Corrosion of Boilers ppm (boiler water systems best practice, kept below 5- The presence of dissolved oxygen in water makes for a 10 µg/L.) allows monitoring of leaks from condensers, highly corrosive environment. Mechanically hard, valves and fittings, plus low level precision of ±2% of porous metal oxide deposits with little strength form reading, or 2 digits, clearly shows the performance of rapidly with the most serious aspect of this oxygen oxygen removal equipment and scavengers. Design corrosion being that it occurs in localized cells that considerations include a simple, accurate calibration result in pitting. Pitting is a concentration of corrosion approach plus the capability to communicate with in a small area of the total metal surface that effectively Distributed Control Systems (DCS) systems and drills a small metal oxide filled hole in the metal. Pits evolving technology. can produce large mechanical failures even though only a relatively small amount of metal has been lost and the Galvanic ppb DO advantage. overall corrosion rate is low. Rapid corrosion will progress inside an industrial or utility boiler plus its With IC Controls trace dissolved oxygen measuring water and steam system unless dissolved oxygen can be sensor, galvanic current naturally is zero when oxygen virtually eliminated. Unchecked corrosion eventually is absent. An electrochemical cell similar to a battery it results in expensive repairs or equipment failures and produces current only when oxygen is present. By using subsequent replacement. carefully selected electrodes in contact with an electrolyte, a chemical reaction occurs that uses electrons gained from oxygen molecules to produce a Boiler water DO removal galvanic current directly proportional to the Dissolved Oxygen removal from any steam and water concentration of oxygen present. By comparison 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 Fig. 1: Galvanic Cell percent and ppb is a factor of 100,000,000. Thus, any 29 Centennial Road • Orangeville Ontario L9W 1R1 • Tel:(519)941-8161 • Fax:(519)941-8164 • 1-800-265-9161 www.iccontrols.com • [email protected] Electrolytic DO cells require an external supply to drive so they automatically calibrate the analyzer. To calibrate the chemical reaction, the resulting zero-current can the sensor, simply suspend the probe above water, Fig. drift. This can result in negative ppb readings or offsets 2, and let the analyzer auto calibrate. This calibration in the readings. Luminescent DO sensors use light from technique will give a 100% saturation reading for the an LED source to excite fluorescence which decays in temperature and pressure which the analyzer will proportion to the oxygen present. The delay (or phase display as ppb dissolved oxygen. shift) of the light is then measured and translated to ppm DO. In practice, phase shift sensitivity is reduced in DISSOLVED OXYGEN CONTROL order to improve signal to noise ratio which results in OBJECTIVES AND CHALLENGES certain limitations. Galvanic current, naturally linear to Successful water treatment in closed (or almost closed) oxygen and zero when oxygen is zero, is a big systems such as boiler and steam, hot-water heating or advantage for reliable trace ppb level DO operation. The chilled water systems has three fundamental IC Controls galvanic cell is separated from the sample requirements: 1. Dissolved oxygen concentrations by an oxygen permeable membrane. The cell has a must be controlled to reduce the likelihood of forming silver cathode in close contact with the membrane oxygen corrosion cells that can result in pitting of where oxygen gains electrons (is reduced) to become carbon steel. 2. The pH must be maintained high hydroxyl ions, and a lead anode that is oxidized enough to minimize general corrosion of carbon steel, regenerating the electrolyte and balancing the reaction but not so high that caustic attack can occur. 3. An to complete the galvanic cell. Installed in an 865-25 external-treatment system must be in place to minimize sample panel that controls fouling and permits in-line the ingress of dissolved and suspended solids and/or an calibration, these advantages result in years long internal treatment system must be used to control these accurate ppb DO performance. 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. Fig. 2: ppb DO Calibration Simple Calibration 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 Fig. 3: Boiler Steam and water Cycle standard, and it will correctly read dissolved oxygen in water samples. IC Controls DO analyzers have automatic temperature and barometric pressure sensors, Other systems, such as a closed hot water heating or cooling system, particularly when they are tight (no 29 Centennial Road • Orangeville Ontario L9W 1R1 • Tel:(519)941-8161 • Fax:(519)941-8164 • 1-800-265-9161 www.iccontrols.com • [email protected] oxygen in-leakage points), may achieve their target with the plume is lower or higher, the steam flow should be only the chemical addition. The hot-water heating adjusted. system in Fig. 4 uses only a by-pass feeder for intermittent chemical feed. Fig. 4: Closed Hot-Water Cycle MECHANICAL DEAERATION As new make-up water is fed to the boiler, or condensate is recycled back, it goes through the deaerator or deaerating heater. The principle behind the deaeration process is Henry's Law of Partial Pressures. The solubility of a gas in a liquid is proportional to the partial pressure of the species in the atmosphere in Fig. 5: Deaerating Heater System equilibrium with the liquid. In this case, the make-up water has been in equilibrium with the atmosphere and the solubility of oxygen and nitrogen has been Typically, oxygen can be reduced to 10 µg/L or lower. proportional to the concentration of oxygen and The manufacturers of deaerators such as Ecodyne make nitrogen in the atmosphere. In the raw water, this both scrubber and tray type deaerators. The expected corresponds to 8-10 ppm of oxygen. performance from both types can be 10 µg/L or better under steady load conditions. As the contact between In the deaerator, the water equilibriates with steam. As the steam and liquid water will change with flow, it can oxygen and nitrogen are now excluded, the dissolved be expected that the actual performance will be gases must leave the liquid to enable the equilibrium to somewhat dependent upon the plant load. Of the two be achieved. Deaeration is performed in two stages. types, the scrubber unit can tolerate operation over a a. In the first stage, the feedwater is sprayed into wider load range (typically 20-120% of design) than the the deaerator vessel where it contacts a flow of steam. tray unit (typically 40-100% of design) which can The steam both heats the water to within a degree or tolerate more cold water. The tray unit is generally the two of the steam temperature and removes most of the choice in the bigger units that operate under constant dissolved non-condensable gases. load as in major electrical generating stations. b. In the second stage intimate and vigorous When conditions depart from the design conditions, mixing of the liquid and steam removes the remaining there may be some deterioration in the concentration of non-condensable gases after which the water moves to dissolved oxygen that will be achieved by the deaerator the storage tank until needed by the boiler. The steam and within the system.