Pressurized Thermal Deaerators(PTD) Design and Operating Considerations

Deaerator(PTD) is an effecient dissolved gases stripping equipment that protects the entire steam generation system from pitting corrosion. PTDs have rapidaly evolved as boiler manufacturers lay more emphsasis on PTD for steam generation reliability. From a conventional spray-tray type and its enhanced variants, sophisticated designs are implemented: bubbling, spray bubbling,jet bubbling. In a spray type the water droplets Vs size require a large area to achieve target final drop temperature. Example: I kg mass of water dispersed into 0.2 mm size droplets produce 238,853,503 droplets and require 3000 dm2 area. Higher higher require lesser area but are less efficient in degassification. 2.0 mm max drop size or less provides the minimum dissolved oxygen (DO) at PTD outlet.The novel designs overcome these limitations by utilizing the classic mass and heat transfer properties of water/steam while adhering to Henry-Dalton's law of gas solubilities, using Bunsen's coefficients, which enables to fix the most optimum temperature for degassification of both DO and CO2. These novel designs operate at pressures of 2-4 bar a more than conventional DAs which operate at 1.2 to 1.5 bara. Oxygen solubility equation is shown. Mass flow rate of stripping steam & vent rate are vital to deaeration efficacy. Typical stripping steam requirement for a combined "cold" (60 C or below) CBFW flow into deaerator will be 7-9% of the total CBFW mass flow. The venting rate will be0.2 to 0.4% of CBFW flow. The deaerator temperature will be 1-2 degC above saturation conditions. The temperature differential between DA and BFW storage tank must be <2.0 deg C to insure steady state and mitigate oxygen ingress due to higher solubility at lower temperatures. During load swings adjustments of steam/venting must be made to insure optimum operation at all times. Sizing the piping for steam inlet and vent outlet is critical. Proper standard steam velocities must be considered. Typical steam velocity (m/s) standards are: steam 25-30; Vent 10-15. A reliable online DO meter and a lab DO analyser must be employed to periodically evaluate mechanical deaeration efficacy. Thermo-mechanical deaeration will not be able to reduce DO concentrations to the specified levels of <7 microgram/l. Hence a chemical oxygen scavenger slightly higher than stoichiometric concentration is added to the deaerator outlet at equalization legs connected to BFW storage tank. DA+BFW storage tank is at an elevation typically 20-30 meters or higher in many cases, to provide sufficient NPSH to BFW overcoming the vapor pressure of water due to stripping. Higher elevation also reduces relative solubility of gases. The typical water residence time in storage tank is 25 to 30 minutes.The venting rate must be controlled in order not to allow steam condensation at < 1 meter near the tip of the vent pipe to avoid back flow of oxygen laden condensate trickling into DA. Water chemistry monitoring is vital. pH, Sp Conductivity, DO, Concentration of oxygen scavenger, are the main parameters.