Steps to boiler chemical cleaning and treatment selection

Title: 7 Steps to Boiler Chemical Cleaning and Treatment Selection in Thermal Power Plant H1: 7 Steps to Boiler Chemical Cleaning and Treatment Selection in Thermal Power Plant H2: Boiler Chemical Cleaning and Treatment Selection involves the use of inhibited acid to dissolve the scales.

Description: Chemical cleaning of boiler usually consists of a combination of the following stages.    

Mechanical Cleaning: Mechanical cleaning and water flushing can remove loose scale and other debris from the boiler through wiping, brushing, blowing, and friction method.

Alkaline Treatment: This treatment usually remove oil and hydrocarbons that might interfere with the dissolution of the scale by acid solvent.

Solvent Cleaning: This is the process in which inhibited acid is used to remove scale from the boiler.

Neutralization & Passivation: This is designed to remove any last traces of iron oxide. In Neutralization & Passivation is executed by replacing the active metal with a wellpassivated surfaces layer.

Selection of Treatment: Selection of a specific treatment or series of treatments is essential as fouling composition, quantity and distribution will vary between one boilers to another. Final selection of chemical cleaning treatment with due reference to other constraints including cost, disposal problems and cleaning time available. This selection segment criteria provides a guide to the selection of treatments.  

The treatment must be compatible with the material of construction of the equipment and safe to use.

The foulant (Unwanted material on solid surfaces) must show sufficient solubility in the treatment selected so when it is considered together with any insoluble matter that may reasonably be expected to detach fully during the process, it will achieve the desired degree of cleaning.

With these chemical cleaning treatment criteria satisfied, make the final selection with respect to other constraints including cost, cleaning time, and disposal problems.

Hot Alkaline Treatment Selection In case boiler is filled with oil, grease, carbon or other organic compounds, they must be remove by chemical cleaning treatment. This selection is depends on the degree of contamination. Use hot alkaline treatment for boiler chemical cleaning when organic deposits interfere with solvent cleaning.

Hot Alkaline Treatment Selection Treatment Soda ash (Na2CO3) degreasing Caustic degreasing (NaOH) Potassium permanganate (KMnO4) degreasing

Sulfate Conversion Treatment Copper Removal with Ammonium Bicarbonate, Air or Oxygen

Selection Criteria Mild treatment used where contamination is primarily light oil and grease, with less than 5 percent organic contamination Used for all new boilers, where mill scale is present or where organic contamination is 5 to 30 percent Used where organic contamination is heavy (> 30 percent) and carbonized. Should only be employed where fouling type and quantity render it necessary, since costs, disposal problems and complication of subsequent solvent cleaning are greater.

Where scales contain considerable quantities of calcium sulfate (i.e. > 10 percent) This will tend to increase scale solubility during a subsequent solvent cleaning If copper concentration in the scale deposit is greater than 10 percent, a separate treatment is necessary to dissolve copper as much as possible before solvent cleaning

Control Parameters of Alkaline Degreasing with Soda Ash Chemical Concentration Sodium carbonate 0.5 to 1.0% by weight Sodium metasilicate 0.5 to 1.0% by weight Trisodium phosphate 0.5 to 1.0% by weight Surfactant 0.1 to 0.2% by volume Antifoam (If required) 0.05 to 0.1% by volume Temperature Limit 155 oC Circulation Rate Normal boiler operational circulation Residence Time 18 to 24 hours Corrosion Rate < 2 mpy Control Parameters of Alkaline Degreasing with Caustic Chemical Concentration Sodium hydroxide 1.0 to 2.0% by weight

Trisodium phosphate Surfactant Antifoam (If required) Temperature Limit Circulation Rate Residence Time Corrosion Rate

0.5 to 1.0% by weight 0.1 to 0.3% by volume 0.05 to 0.1% by volume 155 oC Normal boiler operational circulation 18 to 24 hours < 2 mpy

Control Parameters of Alkaline Degreasing with Permanganate Chemical Concentration Sodium hydroxide 1.0 to 2.0% by weight Potassium Permanganate 1.0 to 3.0% by weight Temperature Limit 100 oC Circulation Rate Normal boiler operational circulation Residence Time 6 to 12 hours Corrosion Rate < 2 mpy Control Parameters of Sulfate Conversion Treatment Chemical Concentration Sodium carbonate 1.0 to 5.0% by weight Surfactant 0.1 to 0.2% by volume Temperature Limit 95 oC Circulation Rate Normal boiler operational circulation Residence Time 12 to 24 hours Corrosion Rate < 2 mpy Control Parameters of Copper Removal with Ammonium Bicarbonate, Air, or Oxygen Chemical Concentration Ammonium bicarbonate 1.6 kg/kg of copper to be removed Aqua ammonia 2.4 liter/kg of copper to removed, and adjust pH to 9.5 Air or Oxygen 1.3 to 1.5 cubic meters per minute Temperature 50 °C to 60 °C Residence Time 2 to 4 hours Corrosion Rate < 2 mpy Solvent/Acid Cleaning Selection – Hydrochloric Acid Hydrochloric acid solution with inhibitors and complexors are the most widely used chemical cleaning solvent to remove internal deposits of a boiler. Hydrochloric Acid solvent is not compatible

with stainless steel. A separate copper removal treatment process is required prior to using the hydrochloric acid solvent.

Hydrochloric acid solution with inhibitors is a widely used solvent since, 1. Hydrochloric Acid has good solubility with a wide variety of scale.

2. This solvent is economical and easy to handle.

Chemical Hydrochloric acid Inhibitor Surfactant Ammonium bifluoride Thiourea Oxalic acid Temperature Circulation Rate Residence Time Corrosion Rates Total dissolved Iron

Control Parameters of Hydrochloric Acid Cleaning Concentration 3.5 to 7.5 % by weight 0.2 to 0.3 % by volume 0.1 to 0.2 % by volume 0.0 to 1.0 % by weight 0.0 to 1.5 % by weight (at 5 kg/kg of copper to be removed) 1.0 % by weight 70 °C to 82°C 1200 liters/minute to 4500 liters/minute 8 to 18 hours < 600 mpy 10,000 mg/L maximum

Solvent/Acid Cleaning Selection – Citric Acid Citric Acid solvent is compatible with alloy steels and require low chloride solvent. Citric Acid has a limited effect on calcium salts as well as more expensive than hydrochloric acid. To handle citric acid solvent usually requires higher temperature and longer contacts times.

Reasons for Selecting Citric Acid Treatment on Boiler:   

Citric Acid solvent contains aesthetic material of construction.

The effective potential of citric acid solvent is remove copper from high copper-content scales.

This solvent can reduce cleaning time by eliminating the need to drain, flush and refill the boiler.

Control Parameters of Citric Acid Cleaning Chemical Concentration 1. Iron Removal Phase Citric acid 2.5 to 5.0 % by weight Inhibitor 0.2 to 0.3 % by volume Ammonia To pH 3.5 to 4.0 Residence Time 4 to 8 hours Total dissolved Fe < 10,000 mg/L Corrosion Rates < 660 mpy Circulation Rate 1200 to 4500

Chemical Concentration 2. Copper Removal and Passivation Phase Ammonia To pH 9.5 Ammonium 1.0 % by weight bicarbonate Sodium nitrite 0.5 % by weight Residence Time 4 to 8 hours Total dissolved Fe < 10,000 mg/L Corrosion Rates < 660 mpy Circulation Rate 1200 to 4500

Temperature Limit

liters/minute 79 °C to 93 °C

Temperature Limit

liters/minute 45 °C to 50 °C

Ethylene-diamine-tetraacetic-acid (EDTA): Ethylene-diamine-teraacetic-acid is also known by several other names and are generally expensive in comparison with other solvent like Citric and Hydrochloric acid. EDTA chemical used for both industrial and medical purposes as it prevents metal ion impurities from modifying EDTA requires high temperature to achieve satisfactory cleaning. Properly controlled conditions of EDTA may result in oxide removal, neutralization and passivation by using single solution.

EDTA have a potential to eliminate temporary circulating pumps, connections, and pipe work.

Control Parameters of EDTA Cleaning Stage Chemical Concentration 1. Iron Removal Phase EDTA 3 to 10 % by weight Inhibitor 0.2 to 0.3 % by volume Ammonia To pH 9.2 Corrosion Rates < 200 mpy Circulation Rate Natural Circulation Temperature Limit 121 °C to 149 °C

Chemical Concentration 2. Copper Removal and Passivation Phase Sodium nitrite 0.5 % by weight Residence Time 12 to 18 hours Corrosion Rates Circulation Rate Temperature Limit

< 200 mpy Natural Circulation 60 °C to 71 °C

Sulfuric Acid: Sulfuric acid is an effective solvent for removal of iron oxide and iron sulfides and lower in cost than hydrochloric acid. It is more compatible with stainless steels and considerably more dangerous to handle. The disadvantage of sulfuric acid is, Due to formation of insoluble calcium, It is not recommended where scales contain significant calcium.

Control Parameters of Sulfuric Acid Chemical Sulfuric acid Inhibitor Surfactant Temperature Circulation Rate Residence Time Corrosion Rates Total dissolved Iron

Concentration 4 to 8 % by weight 0.2 to 0.3 % by volume 0.1 to 0.2 % by volume 60 °C to 82°C 1200 to 4500 liters/minute 4 to 12 hours < 600 mpy 10,000 mg/L maximum

Sulfamic Acid Sulfamic acid has the advantage of being a crystalline solid, which is relatively stable, easy to store, handle, and non-volatile in nature. It is also known as Amidosulfonic acid and compatible with stainless steels. It is mostly used on low volume equipment due to high cost.

Control Parameters of Sulfamic Acid Cleaning Chemical Sulfamic acid Inhibitor Surfactant Temperature Circulation Rate Residence Time Corrosion Rates Total dissolved Iron

Concentration 5 to 10 % by weight 0.1 to 0.2 % by volume 0.1 to 0.2 % by volume 55 °C to 65°C 1200 to 4500 liters/minute 4 to 12 hours < 600 mpy 10,000 mg/L maximum

High Pressure Water Jetting (HPWJ): High Pressure Water Jetting is mostly used manage risk in blocked surface of boiler and can be effectively remove loosened deposits. The used of HPWJ solvent is recommended after acid cleaning and neutralization. After applying HPWJ on boiler, always remove corrode surface, flash rust and passivate the boiler before start up.

Control Parameters of High Pressure Water Jetting Equipment Specification Pump Capacities 750 kW, 68.94 MPa (10,000 psig) 1500 kW, 137.88 MPa (20,000 psig) Water Volume 30 to 50 liters/minute Nozzle orifice diameter 0.8 to 2.4 mm Number of Nozzles & 4 each, facing 45 ° rearwards and 90 ° Orientation to the direction of lance advancement Maximum distance between nozzle tip and 25 mm (between nozzle tip and surface to be surface cleaned) Flexible Hose diameter 19 mm minimum Water for jetting Cold steam condensate Polymer (Additive) 0.3 % by volume Surfactant (Additive) 0.1 to 0.2 % by volume

Neutralization and Passivation: The neutralization of boiler equipment is essential as boiler is critical for maintaining both the life and efficiency of boiler operation. This is achieved by,  

Usually with 0.5 percent sodium carbonate, or During the passivation treatment, where pH is > 7.

If citric acid or EDTA processes have been used for chemical cleaning of boiler, the used of these solvents are normally extended to effect neutralization and passivation by adding suitable pH adjustment and the oxidizing agent.

The process of neutralization is accomplished with citric acid, nitric, and ammonia.

The nitrite/phosphate treatment will provide protection to the metal surface of boiler, if ambient temperature is can be achieved.

To achieve a high degree of passivation, ammonia and sodium nitrite are added later.

Control Parameters of Carbonate Neutralization Chemical Sodium carbonate Sodium nitrite Temperature Circulation Rate Residence Time Corrosion Rates

Concentration 0.5 to 1.5 % by weight 0.5 % by volume 88 °C to 93°C 1200 to 4500 liters/minute 8 to 12 hours < 2 mpy

Control Parameters of Phosphates and Nitrite Neutralization Chemical Concentration Sodium nitrite 0.5 % by volume Monosodium phosphate 0.25 % by volume Disodium phosphate 0.25 % by volume Sodium hydroxide Adjust pH to 7 Temperature 50 °C to 65°C Circulation Rate 1200 to 4500 liters/minute Residence Time 8 to 12 hours Corrosion Rates < 2 mpy Control Parameters of Citric Acid, Ammonia, Sodium Nitrite Neutralization Chemical Concentration Citric acid 2.5 % by weight Ammonia Adjust pH to 4.0 during flash rust removal stage Adjust pH to 9.5 during passivation stage Sodium nitrite 0.5 % by volume Inhibitor 0.2 to 0.3 % by volume Temperature Flush Rust Removal Stage 65 °C to 90°C Passivation Stage 45 °C to 50°C Circulation Rate 1200 to 4500 liters/minute Residence Time 8 to 18 hours Corrosion Rates Flush Rust Removal Stage < 600 mpy Passivation Stage < 2 mpy

Summary of Typical Iron Oxide Removal Solvents

Deposit Removal Characteristics of Typical Iron Oxide Removal Solvents

Steam Blowing: Steam Blowing is one of process of removing any foreign material and prevent the damage of the turbine in power plant.

This is achieved by following Procedure: 1.

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Three phases : main steam, hot reheater and cold reheater The blowing flow : equal to normal operation flow The blowing start-pressure : 600~800 psig (40~54 ă&#x17D;?/ă&#x17D; ) Blow in sections, each section being immediately :To prevent foreign material being transported from one section and deposited in the next Start unit in the normal manner, following the cold start-up procedures Pressure up Fully open the temporary valve or main-steam valve When the drum pressure drops to about 200 psig, close the blowoff valve Repeat the cycle of blowing and stopping Inspect the impact specimen

Chemical Cleaning Evaluation:

Evaluating Effectiveness of Chemical Cleaning Deposit Present, (mg/cm2) 1.0 or less Between 1 – 2 Between 2 – 3 Between 3 – 5 >5 Passivation Stage

Performance Guideline Best Better Good Acceptable Flash rust removal 65 °C to 90 °C Not Acceptable

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