Working principle Deaerator A deaerator is a device that is widely used for the removal of oxygen and other dissolved / non condensable gases from the feed water to steam-generating boilers. In particular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Water also combines with any dissolved c arbon dioxide to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less.
There are two basic types of deaerators, the tray-type and the spraytype The tray-type (also called the cascade-type ) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feed water storage tank our deaerator is a tray type deaerator. Tray type deaerators are generally considered the superior choice for most applications. These units use stainless steel for all internal surfaces which come in contact with undeaerated water. Residence time for undeaerated water inside a tray type deaerator is longer, providing more efficient deaeration, particularly where wide load swings occur. A large diameter hinged door affords easy access to internal trays and spray tubes for maintenance and replacement. Although tray type deaerators require a larger initial investment, the benefits in efficiency and reduced cost of maintenance tend to pay for themselves quickly • The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feed water storage tank.
Why deaerate boiler feedwater? There are many advantages to deaerating water prior to boiler input, but they all boil down to reduced cost operation. • Water is heated during deaeration to near the temperature of the boiler water, thus minimizing the risk of thermal shock damage to a high-value boiler system. • The deaerating process removes dissolved / no condensable gases (oxygen and carbon dioxide) which tend to act as insulators inhibiting the transfer of heat within the boiler. • Removal of oxygen and carbon dioxide reduces corrosion within the boiler and piping, extending the life expectancy of the system and reducing maintenance cost. • Higher temperature feed water reduces the chance of pressure drop within the boiler which can occur when cold water is added. • Recycling of steam from vents and flash steam from traps that would otherwise be vented to the atmosphere can result in appreciable energy savings. • Mechanical deaeration of a feed water deaerator can cut the amount of chemical consumables (hydrazine) used for water conditioning for a continuing operating cost saving. * The deaerator shell is work as a water reservoir for BFP suction and provides the sufficient value of NPSH for safe operation of BFP
Construction details DEAERATOR DATA Design pressure Storage tank design metal temperature Hydraulic test pressure Insulation thickness Deaerator exit water temperature Capacity DM Water inflow quantity (make up) SU/Normal Deaerator inlet water temperature, SU/Normal
3.0 Ksc & full vacuum 320oC 4.5 Kscg 90 mm 126oC 202 Te/hr 84 / 200 Te/hr 26 / 119oC
Deaerator FW header vent flow (Te/hr) No. of water sprays Maximum capacity of each spray No. of trays Tray thickness Tray material Spray nozzles Storage capacity at 2/3rd height Shell internal diameter Shell thickness Shell length Gross volume PSV-96 Deaerator inlet LP Steam line PSV-97 Deaerator vessel
2% 26 8 Te/hr 3 3 mm SS 304 26 41 m3 2800 mm 16 mm 8400 mm 50 m3 3.0 Kg/Cm2 2.5 Kg/Cm2
Working principle The removal of dissolved gases from boiler feedwater is an essential process in a steam system. The presence of dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH levels and the production of corrosive carbonic acid. Low pH levels in feedwater causes severe acid attack throughout the boiler system. While dissolved gases and low pH levels in the feedwater can be controlled or removed by the addition of chemicals, it is more economical and thermally efficient to remove these gases mechanically. This mechanical process is known as deaeration and will increase the life of a steam system dramatically. Deaeration is based on two scientific principles. The first principle can be described by Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the deaerator. With these principles in mind, Sterling Deaerator Company employs a two-stage system of heating and deaerating feedwater. This system reduces dissolved oxygen concentration to less than 0.005 cc/liter (7 ppb), and completely eliminates the carbon dioxide concentration.
Oxygen Scavengers . Q. What is an oxygen scavenger? A. Oxygen scavengers are chemicals added to boiler feedwater or condensate return to remove residual dissolved oxygen.
Q. Why use an oxygen scavenger? A. Dissolved oxygen can cause extensive corrosion damage at the high temperatures and pressures encountered in utility boilers and must be removed. Widely used oxygen scavenger is hydrazine (N 2H4). Hydrazine does not produce corrosive gases at high temperatures and pressures, and in application, reacts with oxygen to form nitrogen and water: N2H4 + O2 2 H2O + N2 In calculating the theoretical requirement of hydrazine for scavenging oxygen, a value of 1 part per part oxygen is obtained: 32 g/mole hydrazine = 1 32 g/mole O2 In operation, a 100% excess of hydrazine is used. Boiler residuals of 1 ppm hydrazine are typically maintained. Hydrazine does not contribute solids to the system, so boiler blowdown, or the mechanical removal of solids from the after-boiler section as sludge, is reduced. It also promotes the formation of the protective magnetite film on the boiler tubes and drum, and converts red iron dust (hematite) to magnetite. It is because of these passivation effects that an excess of scavenger to oxygen is required when changing a boiler system form a non-passivating scavenger to one which passivates. Hydrazine is not without limitations. It is not considered “volatile”, so it does not leave the boiler with the steam to scavenge oxygen and passivate metal throughout the system. In boilers operating above 400˚F (205˚C), it can degrade to ammonia and volatilize with steam, and, in the presence of oxygen, attack metals containing copper: →
2 N2H4
→
2 NH3 + N2 + H2
Procedure for starting and isolation of deaerators Trouble Shooting
