Boiler Blog | Nationwide Boiler Inc.

The removal of dissolved oxygen from boiler feedwater is absolutely necessary to protect your boiler equipment from severe corrosion. To ensure trouble-free boiler operation, a good deaerator is essential.

Deaerators in industrial steam systems are mechanical devices used to remove air and other dissolved gases from boiler feedwater in order to protect the system from corrosion. Dissolved oxygen in boiler feedwater will attach to the walls of metal piping and other metallic equipment and will form oxides (rust). It also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. A dissolved oxygen level of 5 parts per billion (PPB) or lower is needed to prevent corrosion in most high pressure boilers, accomplished by reducing the concentration of dissolved oxygen and carbon dioxide to a level where corrosion is minimized.

The two major types of deaerators are the tray type and the spray type. In both cases, the major portion of gas removal is accomplished by spraying cold makeup water into a steam environment.

Tray-type deaerating heaters release dissolved gases in the incoming water by reducing it to a fine spray as it cascades over several rows of trays. The steam that makes intimate contact with the water droplets then scrubs the dissolved gases by its counter-current flow. The steam heats the water to within 3-5º F of the steam saturation temperature and it should remove all but the very last traces of oxygen. The deaerated water then falls to the storage space below, where a steam blanket protects it from recontamination.

Nozzles and trays should be inspected regularly to insure that they are free of deposits and are in their proper position.

Spray-type deaerating heaters work on the same general philosophy as the tray-type, but differ in their operation. Spring-loaded nozzles located in the top of the unit spray the water into a steam atmosphere that heats it. The steam heats the water, and at the elevated temperature the solubility of oxygen is extremely low and most of the dissolved gases are removed from the system by venting. The spray will reduce the dissolved oxygen content to 20-50 ppb, while the scrubber or trays further reduce the oxygen content to approximately 7 ppb or less.

During normal operation, the vent valve must be open to maintain a continuous plume of vented vapors and steam at least 18 inches long. If this valve is throttled too much, air and non-condensable gases will accumulate in the deaerator. This is known as air blanketing and can be remedied by increasing the vent rate.

For optimum oxygen removal, the water in the storage section must be heated to within 5º F of the temperature of the steam at saturation conditions. From inlet to outlet, the water is deaerated in less than 10 seconds. Call us today is you have additional questions about deaerators and how important they are to your entire system.

Deaeration of boiler feed water is primarily known to remove dissolved oxygen from the water, however, there are four additional advantages of utilizing a deaerator: (1) carbon dioxide removal, (2) improved operation, (3) improved heat transfer, and (4) energy savings. In this month’s edition of Boiler Basics 101, we review the importance of including deaerators as part of your boiler plant.

Corrosion in boilers from dissolved gases leads to reduced heat transfer and efficiency losses. Both oxygen and carbon dioxide, if not removed from the water entering a boiler, will cause corrosion. Dissolved oxygen will attach to the metallic components of a steam system and form oxides, or rust, on boiler heat transfer surfaces. If carbon dioxide is present with the oxygen, the two gases together can be up to 40 percent more corrosive than if they were acting individually. Removing non-condensable gases and limiting or avoiding corrosion will greatly improve heat transfer.

Deaerators also serve the purpose of pre-heating the water before it enters the boiler. This process saves energy by recovering flash and exhaust steam from plant returns, energy that would normally be lost to the atmosphere, and utilizing it to pre-heat the feedwater. This recovered steam can account for 20 percent of the fuel typically required to provide heat for that process. Pre-heating the feedwater will also greatly reduce the chance of thermal shock caused by the expansion and contraction of heating surfaces and will ultimately improve the operation of your boiler.

Deaerators and atmospheric feedwater systems both have the same purpose and are made up of several individual devices including feed pumps, a corrosion resistant receiver tank, and a control panel. A deaerator does its job by mixing steam with soft water inside a pressurized tank, removing oxygen molecules which are then vented into the atmosphere. Atmospheric feedwater systems are non-pressurized and perform the same function while operating at a lower (atmospheric) pressure. A notable disadvantage of operating at a lower pressure is that a lesser amount of dissolved gases are removed. Although atmospheric systems are more cost effective, process requirements will dictate which system is ideal for your application.

Overall, if boiler feed water is not properly deaerated, corrosion will occur, and a boiler will operate less efficiently with a higher possibility of facing costly downtime and boiler repairs. Nationwide Boiler provides new deaerators and maintains a rental fleet of deaerating boiler feedwater systems for both low and high pressure applications. Our systems range in size for boilers 24,000 to 225,000 lb/hr, and are configured one of two ways: a skid-mounted deaerator with feed pump stand or complete mobile feedwater system including the deaerator, feed water pumps, water softener, and chemical feed system, all pre-piped and wired and installed inside of a trailer-mounted van.  

Be sure to check out our previous Boiler Basics 101 blogs and stay tuned for the next edition!