7 Steps to Boiler Chemical Cleaning and Treatment Selection in Thermal Power Plant
Boiler Chemical Cleaning and Treatment Selection involves the use of inhibited acid to dissolve the scales.
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 removes oil and hydrocarbons that might interfere with the dissolution of the scale by an 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 well-passivated 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 boiler 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 criterion 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 removed by chemical cleaning treatment. This selection depends on the degree of contamination. Use hot alkaline treatment for boiler chemical cleaning when organic deposits interfere with solvent cleaning.
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.
Solvent/Acid Cleaning Selection – Citric Acid
Citric Acid solvent is compatible with alloy steels and requires 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 removed copper from high copper-content scales.
- This solvent can reduce cleaning time by eliminating the need to drain, flush and refill the boiler.
Ethylene-diamine-teraacetic-acid is also known by several other names and is generally expensive in comparison with other solvents 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 a single solution.
EDTA has a potential to eliminate temporary circulating pumps, connections, and pipework.
Sulfuric acid is an effective solvent for removal of iron oxide and iron sulfides and lowers 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 the formation of insoluble calcium, It is not recommended where scales contain significant calcium.
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 the high cost.
High-Pressure Water Jetting (HPWJ):
High-Pressure Water Jetting is mostly used to manage risk in the blocked surface of a boiler and can be effectively remove loosened deposits. The used of HPWJ solvent is recommended after acid cleaning and neutralization. After applying HPWJ on a boiler, always remove corrode surface, flash rust and passivate the boiler before startup.
Neutralization and Passivation:
The neutralization of boiler equipment is essential as a 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 a 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 a boiler, if the ambient temperature is can be achieved.
To achieve a high degree of passivation, ammonia and sodium nitrite are added later.
Steam Blowing is one of the processes of removing any foreign material and prevent the damage of the turbine in power plant.
This is achieved by following Procedure:
Three phases: main steam, hot reheater, and cold reheater
The blowing flow: equal to a normal operation flow
The blowing start-pressure: 600~800 psig (40~54 ?/?)
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
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:
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