Acid Boiler Cleaning

Acid cleaning can be considered one of the most difficult steps within the boiler cleaning process. Its purpose is to remove deposits that cannot be removed by alkaline cleaning, and this removal can be done by solubilizing or displacing the deposit.

Depending on the chemical nature of the deposit, different inorganic or organic acids are used. In the table below are some of the most frequently used acids for removing deposits:

Tab 1 – Acids used for boiler cleaning

As the acids used for cleaning are very reactive and can corrode the metallic surface of the boiler tubes, it is necessary to use specific inhibitors, so that these acids cause the least possible attack on the metallic surface that was cleaned, ensuring that during the removal of the deposited material there is the least risk of damage to the metallic structure of the boiler.

The acid recirculation and action time can vary to values higher or lower than 72 hours, due to the amount of fouling present.

The final steps in the acid cleaning process are neutralization and passivation, which has great importance in preventing the metal surface from the medium.

In neutralization, a caustic soda solution is usually employed. In the passivation step, the passivation agent sodium nitrite is used, which aims to convert the free metal (Fe ⁺³ ) into oxides (Fe ₃ O ₄ ), known as magnetite, forming a protective film.

The following is a simplified scheme of a chemical cleaning process in boilers

Figure 2 – Chemical cleaning scheme in boilers

PERFORMING ACID CLEANING

In March 2022 the EP Engenharia technical team performed the chemical cleaning in a boiler of a petroleum sector customer, with thermal capacity of 1,300 Kg/h of steam generation.

The application of acid cleaning in this boiler was necessary because the boiler had been out of operation for a long period due to problems in the equipment, which underwent recovery and refurbishment.

The following steps were performed:

• General inspection of the boiler installations;
• Insulation (racketing) of all measuring instrument and steam outlet sockets;
• Setting up the recirculation and product addition system;
• Isolation of the area;
• Adding clean water up to 1/3 of the total boiler volume;
• Adding hydrochloric acid and inhibitor to the recirculation tank;
• Pre-recirculation of the acid solution;
• Adding water up to the boiler operating level;
• Recirculation of the acid solution;
• Rinse with water – recirculation and discard;
• Neutralization with caustic soda solution;
• Passivation with sodium nitrite;
• Hibernation of the boiler according to standard procedure.

Photographic records of the acid cleaning process.

Figure 3 – Boiler and recirculation tank