Deaerator Performance
As a general rule of thumb, the most common corrosive gases within a boiler system are oxygen, carbon dioxide, and hydrogen sulfide, but how can the performance of a deaerator be determined?
Deaerators are large, insulated pressure vessels that produce preheated boiler feedwater to help ensure efficient steam-plant operation. Deaerators come in numerous sizes and shapes, dependent upon boiler plant design, capacity, and system pressure rating. The units are produced in spray, tray, spray-tray, vacuum, and atmospheric pressure designs. As water enters a deaerating system, the stream is distributed into tiny droplets. This is accomplished by spray nozzles or by a series of trays arranged so that water will flow from one to another in a cascading fashion. Steam is blown through the falling water, quickly heating it to within one or two degrees of boiling temperature. 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 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 proportional to the concentration of oxygen and nitrogen in the atmosphere. In raw water, this corresponds to 8-10 ppm of oxygen. Because of their working mechanics and their ability to use steam pressure and temperature, the deaerator is far more effective than a heated feedwater tank. Regarding a properly operating deaerator, the three key parameters indicative of quality feedwater are dissolved oxygen removal, temperature, and pressure.
Temperature
Since the rate of diffusion of gas molecules increases as temperature rises, it is crucial that the operating temperature range specified is being maintained. Regarding preventative maintenance, it is low-hanging fruit to routinely check the temperature gauges located on deaerators. Most deaerators perform efficiently in the 225-230°F range (Figure 1). Taking several readings with an infrared thermometer can also help confirm the temperature gauge is working correctly. Temperatures can vary due to system pressure and if the boiler demand fluctuates, take temperature readings at varying loads.
Figure 1: Solubility of dissolved oxygen
Dissolved Oxygen Removal
If the deaerator is operating properly, it will remove soluble gases, lowering dissolved oxygen down from 3 - 4 parts per million to within 4 - 10 parts per billion. Each deaerator should include the mechanical oxygen removal specifications in their manual, which can then be verified with testing. To capture a representative sample, a flow of water leaving the deaerator will need to be installed. Collection of a grab sample presents difficulty in that as soon as any sample is taken from the system, it is surrounded by air that contains 21% oxygen. The difference between atmospheric oxygen and the DA operational range is a factor of 108. Collecting an accurate sample requires a sample cooler in conjunction with a method to test the water without the interaction of ambient air. When collecting samples, any chemical methods of oxygen control should be temporarily halted. NOTE – because of the difficulty in testing, a common approach has been to maintain a sufficient concentration of oxygen scavenger which increases treatment costs.
Pressure
Most deaerators are American Society of Mechanical Engineers (ASME) certified pressure vessels that operate at 5 to 15 psig by utilizing a low-pressure steam supply and because of this, quality pressure sensors are essential to ensuring precise control. Deaerator vessels should have the same design pressure as the header pressure of the steam supply header. This will prevent over-pressurizing the vessel, which in turn can lead to deaerator cracking and valve failures. A recommendation to avoid these issues is to install a pressure-reducing valve in the line that feeds the DA tank. Fluctuations in pressure can also indicate faulty water control as the sudden rush of relatively cool water can reduce efficiency.
Significant energy savings may be possible in plants that do not monitor feedwater-system performance closely. If boiler efficiency is not at its peak (and the cause of the inefficiency cannot be identified), a critique of the feedwater system may be revealed. An unscheduled deaerator outage can be a serious issue, affecting process operations, energy costs, maintenance budgets, and the long-term integrity of the entire boiler system. When it comes to deaerator performance, simple understanding and routine maintenance can have a profound effect on the overall boiler plant.
Jed Kosch
List of Resources:
[1] “Boiler Dissolved Oxygen Control.” IC Controls. Accessed July 5, 2021. https://iccontrols.com/wp-content/uploads/art-v1500388_boiler_dissolved_oxygen_control.pdf.
[2] “Evaluating Deaerator Operation.” HPAC Engineering. Accessed July 5, 2021. https://www.hpac.com/heating/article/20927654/evaluating-deaerator-operation.
[3] “Pressurized Deaerators.” Spirax Sarco. Accessed July 5, 2021. https://www.spiraxsarco.com/learn-about-steam/the-boiler-house/pressurised-deaerators.
[4] “Explained: How Does a Deaerator Work? - Boiler Water Treatment.” Water Treatment Basics, August 27, 2020. https://watertreatmentbasics.com/how-does-deaerator-work/#Deaerator_Performence_Issues.
