Deaerator Operation

Being born on Earth, I grew up unaware of the true nature of oxygen. Submerged in a sea of N and O, I was ignorant of how unique oxygen is in the universe as a whole. Except on our home planet, oxygen does 

In particular, all must be removed from BFW. This is done either with a chemical scavenger or with steam stripping. Stripping air out of BFW is relatively inexpensive compared to the use of chemicals. 

A deaerator (Fig. 22.6) is used to remove oxygen. The more efficient the deaerator, the less scavenger chemical treatment is required. Residual in the deaerator effluent BFW is determined by a field titration kit, and the oxygen scavenger chemical addition rate is adjusted accordingly. 

The steam and cold deaerator makeup water mix in the small stripping section shown in Fig. 22.6. Most of the steam condenses (typically 90 percent) to heat the cold water (120°F) to its boiling point temperature (250°F) at the deaearator operating pressure of 15 psig. The rest is vented to the atmosphere. This vented steam is being used to strip out the residual in the cold-water feed. 

Note that there is sometimes a second, hotter feed to the deaerator, in addition to the cold-water feed to the stripping section. This secondary feed is steam condensate, which having already been steam, bypasses the stripping section. The exception to this generalization is steam condensate recovered from turbine exhaust vacuum 

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The humble deaerator, operated by the Utility Department, is an interesting and important component of any process facility. Oxygen is a highly corrosive element, and if left in the boiler feedwater, would rapidly oxidize the boiler s tubes. 

Deodorization. The neutralized/bleached oil is pumped into a deaerator operated under a pressure of 500 Pa to evacuate entrained air. From the deaerator, the oil passes through a shell and tube economizer and is heated to a temperature of 240°C by means of a thermal oil heater. The stripper and deodorizing column operates under a pressure of 600-1000 Pa volatile components such as low-molecular-weight fatty acids, ketones, aldehydes, and other odoriferous substances are stripped off by live steam. The rising vapors laden with volatile components pass through a cyclone scrubber where fresh fatty acid oil is sprayed on top of the vessel to recover outgoing fatty acids. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Typically, for any given pressure, industrial packaged boilers operate at higher heat-flux rates than field-erected boilers, This requires that the package boiler FW quality should be substantially better (i.e., lower overall TDS and lower levels of silica and sodium). Appropriate MU water pretreatment may, for example, necessitate the use of twin bed and mixed bed demineralization ion exchange, or RO and mixed bed (in addition to mechanical deaeration and other processes). 

NOTE The operational efficiency of a deaerator is governed by its potential to reduce the partial pressure of oxygen in the FW to a minimum. The equilibrium concentration of any gas (such as Of) dissolved in a liquid (i.e., the gas s solubility) is proportional to the partial pressure of that gas in contact with the liquid. This relationship is expressed by Henry s Law ... 

Pressure Deaerator Troubleshooting If pressure DA equipment is properly maintained and carefully set up, operators should be able to produce FW containing dissolved oxygen (DO) levels of less than 0.005 to 0.01 cubic centimeters per liter (cc/1) 02. 

It is rare, however, that other types of organizations (such as the average industrial facility) can afford this level of DO monitoring vigilance. It is also rare (utility stations apart) that deaerators receive the kind of preventative maintenance and general attention necessary to ensure that they operate constantly at peak performance. The inevitable... 

Most DAs are set to operate at 5 psig with an exiting boiler FW temperature of 227 °F (108.3 °C). In practice, however, the stored water discharged from the base of the deaerator drops by several degrees during its journey to the boiler as the result of some heat loss to the surroundings. 

This last design (chloride anion exchange) is very often specified when some form of dealkalization plant is under consideration. Selection usually is made on the grounds of operator safety and reduced risk of boiler corrosion. Although it may have the attraction of not creating any potential for acid handling or acid introduction into the FW line or not requiring a deaerator, these are mere diversions. 

All large industrial WT and power boiler plants utilize deaerators and supplement the DO removal process by means of a suitable chemical oxygen scavenger. Many midsize factories operating FT boilers or FT-WT boiler combinations also employ mechanical deaeration. This is especially common in the United States, Germany, and several other industrialized countries where boiler design custom and practice virtually dictate that a deaerator be included in almost every midsize and larger boiler plant facility. 

For these medium to large operations, apart from taking measures to ensure the continuous efficiency of deaeration and oxygen scavenging, little more can be said because all that can be done practically to remove DO is being done. The problems of inadequate FW deaeration, the resultant corrosion risks entailed, and the consequent need for constant vigilance primarily affects those owners and operators of small to midsize steam-raising boiler plants where mechanical deaeration is not available. 

It should be noted, however, that oxygen corrosion of operational boilers supplied with mechanically deaerated FW, supplemented by the use of an appropriate oxygen scavenger chemical, and under a constant load is relatively rare. This position is not necessarily the same in idle boilers, low-load boilers, or chemically cleaned boilers, and despite all best efforts oxygen corrosion may take place. 

NOTE Although FT boilers usually do not operate under knife-edge control conditions (as do some large power WT boilers), it is evident that chelant programs generally are not suitable for any FT plant operating without benefit of a deaerator. 

All boiler operations utilize some form of basic chemical oxygen scavenging, and most larger facilities also employ mechanical deaeration prior to chemical polishing. Alternatives to these essentially independent techniques exist and have been steadily gaining in popularity for application in larger boiler plants and in other types of ultrapure water applications. 

The residual oxygen content remaining after deaeration varies depending on equipment design and operational efficiency, but suffice to say that, unless the deaerator is scrupulously well maintained, its per-... 

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