What is the procedure for boiler cleaning?

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What safety rules a person must keep in mind before starting boiler tube cleaning

Before starting boiler tube cleaning, it is important to follow all safety rules and procedures. This will help to prevent injuries and accidents. Here are some of the most important safety rules to keep in mind:

• Isolate the boiler. This means shutting off all steam and water lines to the boiler. It is also important to lock out the boiler so that it cannot be accidentally started.

• Depressurize the boiler. This should be done slowly and carefully to avoid any sudden changes in pressure.

• Drain the boiler. This should be done completely to remove all water and steam.

• Ventilate the boiler area. This will help to remove any harmful gases or fumes that may be present.

• Wear appropriate personal protective equipment (PPE). This includes gloves, goggles, a respirator, and protective clothing.

• Follow the manufacturer&#;s instructions for cleaning the boiler tubes. This may vary depending on the type of boiler and the cleaning method being used.

• Be careful not to damage the boiler tubes. If you are using a mechanical cleaning method, such as a brush or lance, be careful not to over-brush or lance the tubes, as this could cause damage.

• Inspect the boiler tubes after cleaning. This will help to ensure that all of the scale and debris has been removed.

Why boiler tube cleaning is important 

Boiler tube cleaning is important because it keeps boilers working well and safe. Imagine a boiler as a big pot that makes steam to power machines or heat buildings. Inside this pot, there are tubes where hot gases pass through to heat water. Over time, these tubes can get dirty with stuff like soot and ash.

When tubes get dirty, they can&#;t transfer heat efficiently, like when you have a dusty window that&#;s hard to see through. This makes the boiler less effective at making steam and can even make it dangerous because it might overheat.

Cleaning these tubes is like cleaning the dusty window. It helps the boiler work better, use less energy, and last longer. Plus, it keeps things safe by preventing accidents like explosions. So, boiler tube cleaning is crucial to keep things running smoothly and safely.

In this article I will start the first article about Boilers and my idea was begin with the boiler cleaning procedures and understand a little bit more "why we need cleaning the internal boiler surface and how should be the procedures to do it properly?".

It is for information and guidance and always depends the situation and may be recommended by a Chemical Service Engineer and Boiler makers with all further information. I'm doing a briefing list with the mainly information considering a recently experience with this matter.

Let's get started with: Why cleaning a Boiler?

Internal Chemical Cleaning of Steam Boilers

During recent years the Chemical method of internal cleaning of boilers has been used in many cases to either supplement or replace the mechanical method of cleaning. The main purpose of internal cleaning of boilers is to produce a clean metal surface on all parts of the boiler which are in contact with steam or water during operation. The materials which may be present on the metal surface may be scale, corrosion products, oils, greases, mill scale, paint, etc. In many instances it is necessary to use some chemical method of cleaning to remove the undesirable materials present. The chemical method to be used should be one which will remove the undesirable material present without injury to the boiler (like metal surfaces, gaskets, joints, seals, etc.) or cause a contamination of the steam after the boiler goes back in service.

Type of Chemical Cleaning

The reagents normally used for internal cleaning of boilers may be classed under the two following headings: (a) alkaline or (b) acid. The alkaline reagents are usually used to remove oils, greases, and paints; however, in some instances theses reagents are used to remove the soften scale. The acid reagents are normally used when it is necessary to remove boiler scale, mill scale, corrosion products and adhering sludge. In some instances an alkaline reagent is used to be followed by an acid reagent. In practically all cases when an acid is used it is followed by the use of an alkaline reagent.

Alkaline Cleaning

Before a new boiler is placed in service it is normally cleanned in order to remove grease, oils, paints, welding flux, etc. This usually involves a boiling out with an alkaline reagent. The reagent to be used is dissolved in water and added to the boiler boiled out at low pressures, usually 3 to 6 bar (50 to 100 psi), pressure for sufficient time to allow the cleaning to take place. The boiler is then cooled and the water removed as rapidly as possible so as to carry out the major amount of the suspended material out of the system. If the first boil-out does not should be thoroughly washed out. Before placing the boiler in service the boiler water should be analyzed so as to determine if any undesirable portion of the cleaning be washed out of the boiler before it is placed in service. The alkalinity does not result from the residue of the cleanning reagent remaining in the boiler.

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The selection of the proper alkaline cleaning reagents and their concentrations should be determined in accordance with the materials to be removed. For normal first boiling out of new boilers the reagents used are soda ash, caustic soda, phosphates, complex phosphate and silicates. Usually a mixture of the vaious chemicals is used. The proper chemicals to be used should be determined by one conversant with this method of cleaning.

Acid Cleaning

Boilers may be acid cleaned to remove original mill scale remining in a new boiler or two remove scale sludge, or corrosion products in a boiler that has been in service. Acid may react with the steel in the boiler to cause three main types of difficulties. These are: hydrogen embrittlement, hydrogen gas explosion and corrosion.

Hydrogen Embrittlement

Acid reacts with steel to form hydrogen as one of the products. During the formation of the hydrogen the hydrogen tends to penetrate and reduce the normal ductility of the steel. This loss of ductility is commonly referred to as hydrogen embrittlement. If the metal is allowed to stand for a long period of time at normal temperatures the normal ductility returns. However, if the metal is heated the ductility returns very rapidly. In acid cleaning of power plant equipment the possibility of hydrogen embrittlement being present should be recognized. This means that no mechanical working of the metal should done until the metal has been reheated to restore the normal ductility. In the regular procedure of acid cleaning the final alkaline boil out will normally remove the danger of hydrogen embrittlement.

Figure 1: Typical failure of water tube resulting from hydrogen embrittlement.

Hydrogen Gas Explosion

Since hydrogen is also envolved as a gas during the acid cleaning of steel precautions must be taken to prevent an explosive mixture of hydrogen and air from forming in a area where sparks or flames may ignite it and cause an explosion. During the acid cleaning the equipment should be closed and properly vented so that the hydrogen gases are removed to an area free from contact with flame or sparks. After the acid washing the equipment should be thoroughly purged of all remining hydrogen before the workmen enter the equipment or do any torch cutting or welding on connecting pipes. When hydrogen is recognized as a potential hazard and proper precautions taken there is very little possibility of any difficulty occurring.

Corrosion

Acids react with steel and dissolve the steel. If steel is left in contact with acids for any lenght of time serious corrosion may occur. In order to make use of acid as an agent to remove the scale , sludge, mill scale and corrosion products without at the same time having additional corrosion of steel, inhibitors are added to the acid. The inhibitors tend to reduce the action of the steel with the acid but stilll allow the acid to react with the material which is to be removed. The inhibitors thus bring about a condition which allows the acid to react with the mill scale, corrosion products, scale sludge, etc, but retards its action on steel. An inhibitor does not entirely stop the reaction of the acid with steel but will under proper conditions reduce it to a fraction of one per cent of the reaction with uninhibited acid. Thus in the normal acid cleaning of a boiler the amount of iron dissiolved from the boiler metal is an extremely small amount.

The inhibitor appears to protect steel from acid attack but does not prevent the acid from reacting with iron oxide or various types of slag inclusions often found in steel. Thus if an oxide or slag inclusion is present in steel the inhibited acid will cause what might be considered corrosion in this areas.

The protective action of the inhibitor may be reduced or destroyed by changes taking place during the cleaning of the equipment. Temperature has a marked effect on the protective action of inhibitors. Each inhibitor has temperature limits above which its protective action is no longer effective. In general it has been found 65ºC (150ºF) is the maximum temperature at which most inhibitors may be used. Chemical reactions may take place during the cleaning which tend to destroy the inhibiting action of the inhibitor. If the metal has been cold worked such as at tube ends, etc., it tends to react with the acid faster than in areas where it has not been cold worked. The inhibitor should be of a type which will reduce this type of attack to a minimum value . There are many different inhibitors available; consequently , it is not possible done; consequently, the selection of the inhibitor should be made by one thoroughly familiar with the action of acids and inhibitors on power plant equipment.

Figure 2: Corrosion observed during internal inspection of water tubes, may caused by a bad practice of cleaning and wrong water treatment procedures.

Figure 3: Water and smoke tubes in good condition, cleaned and without corrosion.

Acids used in cleaning

Various acids have been used for cleaning purposes but the acid most universally used is hydrochloric acid. The concentration of acid used varies from 2 to 10% depending on the condition of the equipment being cleaned. In most instances the dilute acid is added to the boiler and allowed to stay in contact with the boiler for a given period of time.

Preparation for cleaning

In preparing for acid cleaning, the equipment should be isolated from equipment not being cleaned. In many instances this may involve the removal of brass or bronze parts which may come in contact with the acid, blanking off interconnected lines and valves.

Proper vents should be installed at points where vapors might trapped and at the high point in the system. These vents should be of sufficient size to allow all vapors to be readily removed and at the same time to allow sufficient air acess to and from the equipment during draining as well as when filling. The vents should be connected so that the vapors are removed to an area where there is no chance of ignition by flames or sparks.

The boiler to be cleaned should be removed from service for a sufficient period of time to allow all parts of the boiler to be a temperature not more than 65ºC (150ºF). If sufficient time is available it advisable to drain the boiler and thoroughly inspect for any indications of corrosion scale and sludge in the equipment. All losse scale and sludge should be removed prior to cleaning. The type of scale, sludge, and corrosion products in the boiler will serve to indicate the type of acid, inhibitor, and accelerator to be used as well as the acid concentration, temperature and duration time of treatment. In many instances materials are added to accelerate or speed up the reaction rate of the acid with the corrosion products or scale.

Acid cleaning procedure

After preliminary cleaning the boiler should be filled and the unit heated sufficient to have the metal temperature around 65ºC. The boiler is then drained and the hot dilute inhibited acid (not above 65ºC) pumped into the system. The acid may be recirculated or allowed to soak without recirculation. The time of contact will vary from 2 to 6 hours. Samples of cleaning solution should be taken for analyses throughout the cleaning cycle in order to determine if the cleaning is progressing properly.

At the expiration of the cleaning period the boiler is drained as rapidly as possible so as to remove any suspended solids in the solution. The boiler is then filled with hot neutral water, preferably condensate and drained. This is continued until the drains are no longer acid. This rinsing removes the soluble iron as well as the excess acid. If the alkalimne rinse is started before the iron salts are removed a heavy gelatinous precipitate of iron hydroxide may remain in the boiler.

Alkaline Rinse

After the acid and iron salts have been rinsed out of the boiler and alkaline solutions is pumped into the boiler until is completely filled. The water level is then lowered to normal and the unit fired at the pressure between 3 and 6 bar (50 and 100 psi) for at least several hours. The alkaline solution often used is around one pound of soda ash per thousand pounds of water. Some plants have used trisodium phosphate in place of soda ash.

After the alkaline boil out, the boiler is cooled, drained, flushed with clean water, drained, and inspected.

Inspection

The inspection should not be made until after the alkaline boil out and subsequent rinse. This normally removes the hazards of the hydrogen explosion and also prevents the operators from coming in contact with the acid in the equipment. This inspection will reveal the effectiveness of the acid cleaning in removing the scale, sludge and corrosion products. In most instances where there is much iron oxide in the boiler the acid cleaning will loose material may lay in areas near tube bends and on the bottom of slightly inclined tubes, and if not removed may be the cause of tubes failures when the boiler is returned to operation. This may necessitate the flushing of tubes where such material may be present during the inspection. When the boiler has been properly cleaned it is ready to be returned to service. It is advisable to clean the boiler at a time so that when it has been cleaned it may be returned to service as soon as possible. This is due to the fact that the fresh iron surfaces exposed by the acid cleaning may be readily corroded if left damp and exposed to the atmosphere for any period of time.

Conclusions

The chemical cleaning of boilers has proved to be rapid and efficient method of removing deposits from the water side of the boiler. In many boilers this is the only method available today to clean some areas of the equipment. From a corrosion viewpoint, this method can be made a satisfactory procedure when all the factors involved are properly understood and controlled; however, caution should be the password at all times.

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