How mechanical deaerators help avoid steam boiler corrosion

There are a number of factors in achieving proper and adequate deaeration of steam plants, to prevent corrosion, writes Robert Jewell. This column appears in the January issue of EPM.
By Robert Jewell | December 18, 2015

One cause of corrosion in steam plants that power the ethanol process is the presence of dissolved gases in boiler feedwater, in particular oxygen and carbon dioxide. Oxygen is the more aggressive, causing corrosion in the form of pitting. The corrosion is generally localized and occurs over a relatively small area, but it can result in serious and costly damage. It can occur even where overall system corrosion rates are found to be very low. Elevated temperatures in particular accessories such as feedwater economizers can accelerate and magnify pitting where sufficient concentrations of oxygen exist.

Mechanical deaeration using a pressure deaerator is a common method for managing and removing oxygen, although it is usually supplemented by the addition of chemical oxygen scavengers. A number of equally important factors are simultaneously involved in achieving proper and adequate deaeration.

One factor is the solubility of a gas in a liquid. Water is sprayed into the deaerating section where steam is also injected. As the feedwater is heated to within a few degrees of the saturated steam temperature, the solubility of the oxygen in the feedwater is reduced and most of the dissolved oxygen in the water is released. This is because the solubility of a gas in a liquid decreases with increasing liquid temperature. In other words, increasing the temperature of the water will reduce the amount of gas the water can hold.

Another factor is the partial pressure of the gas at the water surface. Steam is injected, a portion of which is also vented, and serves to reduce the partial pressure of the oxygen in the deaerator. The solubility of a gas in water is proportional to the partial pressure of the gas at the water surface. This solubility is manipulated by the venting rate. Insufficient venting will result in an increased partial pressure of the specific gas, hindering its release. The vented steam carries with it the oxygen and other noncondensable gasses that were released from the water, lowering the partial pressure of oxygen at the surface, thus allowing more oxygen to be released.

Yet another factor is the thorough mixing and scrubbing of the water and steam, which improves the efficiency of oxygen removal. A deaerator is typically a counterflow design that facilitates greater contact of the steam to the water, efficient heating and vigorous scrubbing of the water with the steam.
Pegging steam, as it’s referred to in this application, is used for mechanical pressure deaeration.

Steam is most often chosen for the purge gas because it reduces of the solubility and partial pressure of oxygen. Also a relatively small amount of steam must be vented. A portion of the steam used to scrub the water is condensed, resulting in less than the full amount injected being vented.

The more consistent and stable the operating conditions are, the more effective the mechanical deaeration will be. Other considerations include:

• Minimizing pressure fluctuations in the deaerator that potentially contribute to reoxygenation of the feedwater.

• Using dedicated pressure regulating valves to maintain the deaerator at a constant, controlled pressure.

• Returning high-temperature condensate streams to the deaerator to allow them to flash and be adequately deaerated.

• Not using flash steam from high temperature returns as pegging steam because the pressure or volume may be variable or inadequate.

• Using pressure-regulated steam injection to achieve adequate deaeration.

• Minimizing temperature fluctuations. The more constant pressure and temperature, the more efficient and effective the deaerator will be.

• Avoiding erratic or intermittent excessive makeup water flow rates.

• Monitoring the operating parameters for proper functioning. Lower than normal temperatures are an indicator of insufficient steam flow, insufficient venting, excessive feedwater flow or excessive makeup rates, to name a few. 

• Monitoring the storage section temperature, which should be within 5 degrees Fahrenheit of the deaeration section temperature. Low storage-section temperatures may indicate problems with faulty nozzles, poor feedwater distribution, or shifted, plugged, or broken trays.

Mechanical deaerators are essential in the removal of dissolved gases and perform a vital function in the safe, efficient and effective production of steam. Knowledge of how deaerators work, and how they should be operated, maintained and monitored is necessary if they are to perform effectively and efficiently in the prevention of oxygen-related corrosion, and ensure safety in operations and the protection of assets. 

Author: Robert Jewell
Energy Systems Chief Engineer,
Chippewa Valley Ethanol Company