Causes of tube corrosion in the economizer of the steam generation system

Steam Generation System

A typical steam generation system consists of a pre-boiler section (deaerators, piping, pumps, stage heaters, and economizer), a steam generation section (including boilers, superheaters, and reheaters), a post-boiler section (including process equipment, steam pipes, and condensate traps), and a condensate section (including pipes, flash tanks, pumps, and condensate storage tanks).

1. Pre-boiler Section
2. Steam Generation Section
3. Post-boiler Section
4. Condensation Section

Corrosion in Steam Generation Systems

Corrosion in steam generation systems causes many problems in refineries, gas plants, and petrochemical plants. Boiler corrosion is a function of the concentration of oxygen, carbon dioxide, and ammonia and the control of these species, and in some cases, due to the presence of hydroxide and chloride anions or high concentrations of chelating chemicals in the boiler water. However, in more advanced steam generation systems, control of water-soluble gases such as oxygen and carbon dioxide usually results in reduced corrosion of the inner wall of the tubes used in steam generation systems. Pitting corrosion, alkali embrittlement, alkali gouging, chelating attacks, and hydrogen embrittlement are the most common corrosion mechanisms in steam generation systems.

The main factor of boiler corrosion

The results of many studies show that the control of boiler corrosion is largely influenced by the quality of the feed water and the conditions of deposition. Oxygen is the main factor of boiler corrosion. If the pH of the boiler is controlled, the amount of corrosion upstream and inside the boiler will be controlled. Oxygen corrosion and corrosion products in the boiler cause heavy boiler maintenance costs. The failure of expensive equipment and the deposition of corrosion products lead to a decrease in the efficiency of the boiler system. Therefore, monitoring and controlling the oxygen concentration in the boiler is of particular importance. In practice, in order to prevent oxygen corrosion in the boiler, the oxygen concentration of the boiler water must be reduced to a few ppb and controlled at these low values.

Oxygen Corrosion

The economizer and feed water heater are the first areas to suffer from oxygen corrosion in an operating system. In cases where the dissolved oxygen content in the water is high, oxygen corrosion may also affect other parts of the boiler system. In most cases, the damage caused by oxygen corrosion is very high even if the oxygen content exceeds the permissible limit for a short period of time.

Function of Economizers

The economizer is a key factor in increasing the thermal efficiency of the boiler due to the use of energy from the gas leaving the boiler system. The economizer preheats the boiler feed water. Economizers also reduce the potential for thermal shock and severe fluctuations in the temperature of the water entering the drum or water walls. In economizers, the heat transfer coefficient on the gas side is much lower than on the water side, which is compensated for by the use of finned tubes in economizers. The ultimate goal of designing an economizer is to achieve the desired heat transfer at the lowest cost.

Corrosion and failure of economizer tubes (similar to other equipment used in the boiler system) is divided into two categories: water side 1 and flame side 2. Of course, the main corrosion problems are observed on the water side. In general, abrasion by gas flow, corrosion in low temperature water, fatigue corrosion and pitting corrosion due to the presence of oxygen are the main causes of failure of economizer tubes.

When steel is exposed to water containing dissolved oxygen, reaction (1) takes place.

2Fe + H2O + O2 = Fe2O3 + H2 (1)

Controlling Oxygen Corrosion in Boilers

In a working boiler system, this type of corrosion can be easily controlled by reducing the dissolved oxygen concentration in the water entering the system. However, sometimes the presence of dissolved oxygen due to improper operation of the deaerator system, inappropriate chemicals used to remove oxygen, or air leakage into the system causes oxygen corrosion.

Mechanical Deoxygenation of Water

Due to the high operating pressure of the boiler, the only way for oxygen to enter is through the feed water. The initial deoxygenation of the feed water is carried out by mechanical deaerators. At this stage, the feed water is brought into contact with steam and the gases dissolved in it are separated and removed with the steam. Modern deaerators must be able to reduce the amount of dissolved oxygen in the water. Subsequently, deoxygenating chemicals are injected to further reduce the dissolved oxygen concentration. The mechanical removal of oxygen must be carried out before its removal by chemicals. In fact, most of the deoxygenation of water is carried out by mechanical methods.

How to inject oxygen scavengers into the boiler?

Since oxygen scavengers require time to function, it is necessary to have a time interval between the injection of oxygen scavengers and the consumption of water in the boiler. For this reason, in order to provide the maximum time for the oxygen scavenger to function, its injection should be immediately after mechanical deaeration. Usually, the injection is done in the deaerator storage tank. In addition, the use of chemicals with catalytic ability to remove oxygen is recommended to reduce its operating time. Oxygen scavengers are used in catalyzed form (cobalt and manganese salts are usually used for sulfites and organic catalysts are usually used for volatile oxygen scavengers).

Source:Analysis of the causes of tube corrosion in the economizer of a steam generation system. Akram Nouri Delavar, Mohsen Sabouri, Amir Pasha, Mehdi Javanmardi. Specialized Scientific Extension Quarterly / Winter 2014 / Issue 4