CLEANING BOILER FIRESIDES AND WATERSIDES
Boiler heat transfer surfaces must be kept clean to provide for safe and economical boiler operation. In this section we will describe the methods and procedures involved in fireside and waterside cleaning.
CLEANING BOILER FIRESIDES
Excessive fireside deposits of soot, scale, and slag cause the following conditions: reduced boiler efficiency, corrosion failure of tubes and parts, reduced heat transfer rates and boiler capacity, blocking of gas passages with high draft loss and excessive fan power consumption, and fire hazards.
Methods for cleaning boiler firesides include wire brush and scraper cleaning, hot-water washing. wet-steam lancing, and sweating.
Wire Brush and Scraper Cleaning
When too much soot is deposited and the passages become plugged, hand lancing, scraping, and brushing are generally used. Special tools required for reaching between the lanes of tubes may be made from flat bars, sheet metal strips cut with a saw-toothed edge, rods, and similar equipment. Some boilers have different sizes of tubes, so you need various sizes of brushes and scrapers to clean the boiler tubes. The brushes or scrapers are fastened to a long handle, usually a piece of pipe, inserted and pushed through the tubes.
This method of cleaning is often used to clean superheaters, economizers, and other sections of the steam generator that are difficult or impossible to reach by brushing or scraping. The water may be applied with hand lances and/or boiler soot blowers. Dry out the boiler setting immediately after water washing to reduce damage to the refractory and other parts of the setting.
Safety is always paramount; therefore, always be cautious when washing boiler firesides. Some precautions you should observe are as follows:
- Wet the boiler refractory and insulation as little as possible. Install canvas shields or gutters where possible to reduce wetting of refractories.
- Protect electrical equipment from water damage. Provide all necessary instructions and protectiveequipment for workers.
- Provide a compressed air lance to loosen scale after water washing.
- Provide adequate equipment to heat and pump the hot water. The water should be heated and maintained at a temperature close to about 150°F, because water exceeding this temperature cannot be handled safely and efficiently. However, because cold water does not clean satisfactorily, you have to maintain the water temperature as close as 150 degrees as possible. A water pressure of 200 to 250 psig should be provided at the cleaning lances or soot blowers. The water jets must penetrate the tube banks and strike with enough force to break up the slag accumulations.
- Start the water washing at the top of the unit and work down.
- The unit must be dried out immediately after washing.
The wet-steam lancing method is similar to the hot-water method except that wet steam is used instead of hot water. The steam should be wet and at a pressure of 70 to 150 psig. The unit must be dried out immediately after lancing is completed.
Fireside slag can be removed from the convection superheaters by forming a sweat on the outside of the tubes. Cold water is circulated through the tubes. and moisture from the air condenses on the tubes to produce sweat. The hard slag is changed into mud by the sweat, and the mud can be blown off by an air or a steam lance. A large tank filled with water and ice can be used as the cold-water source. Steam can be blown into the area around the tubes during the cold-water circulating period to provide adequate moisture in the air.
The procedures for cleaning boiler tiresides are as follows:
1. Remove the boiler from service and allow it to cool. Make sure the boiler is cool enough for a person to enter. Someone must be standing by whenever a person is in the boiler. DO NOT force-cool the boiler.
2. Disconnect the fuel line openings. Secure all valves, and chain. lock. and tag all fuel lines to the burner and install pipe caps.
3. Disconnect the electrical wiring. Secure and tag the electrical power to the boiler. Disconnect the burner conduit and wiring. Mark and tag all electrical wiring to ensure proper reinstallation.
4. Open the boiler access doors by loosening all nuts and dogs and swing the door open. Be careful not to damage the refractory door lining.
5. Remove the burner from the boiler openings. Follow the manufacturer’s instructions for specified burners. Wrap this equipment with plastic, rags. or other suitable protective coverings. Remember. soot and loose carbon particles must be kept out of the moving parts of the burner because they can cause the burner to malfunction.
6. Provide all spaces with free-air circulation by opening doors and windows, or provide fresh air by mechanical means. An assistant should be stationed outside the opening and be ready at all times to lend a hand or to be of service in case of a mishap.
7. Cover the floor area around the tube ends with drop cloths to catch soot. Position a vacuum cleaner hose at the end of the tube being cleaned. Keep soot from contacting wet areas because soot and water form carbonic acid.
8. Remove tube baffles where possible and pass a hand lance or rotating power cleaner brush through each tube slowly and carefully so no damage occurs to personnel or equipment.
9. Inspect tube surfaces for satisfactory condition before continuing on to the nest tube. Use a drop cord or flashlight for viewing through the entire length of a tube. Wire brush all tube baffles either by hand or use of power tools.
10. Apply a light coat of mineral oil to all cleaned surfaces. To do this, fix an oil soaked rag to the end of a brush or rod long enough to extend through the tubes and thoroughly swab each surface, including baffles. Mineral oil is the only lubricant that prevents rusting and also burns off freely without leaving a carbon deposit.
11. Clean all flat surfaces by brushing with the hand or power tools. Make sure that powered equipment is grounded.
12. Use an industrial vacuum cleaner to remove loose soot.
CLEANING BOILER WATERSIDES
Any waterside deposit interferes with heat transfer and thus causes overheating of the boiler metal. Where waterside deposit exists, the metal tube cannot transfer the heat as rapidly as it receives it. What happens? The metal becomes overheated so that it becomes plastic and blows out, under boiler pressure, into a bubble or blister.
The term waterside deposits include sludge. oil, scale. corrosion deposits. and high-temperature oxide. Except for oil. these deposits are not usually soluble enough to be removed by washing or boiling out the boiler.
The term waterside corrosion is used to include both localized pitting and general corrosion. Most, if not all, is probably electrochemical. There are always some slight variations (both chemical and physical) in the surface of boiler metal. These variations in the metal surface cause slight differences in the electric potential between one area of a tube and another area. Some areas are ANODES (positive terminals).
Iron from the boiler tube tends to go into solution more rapidly in the anode areas than at other points on the boiler tube. This electrolytic action cannot be completely prevented in any boiler. However, it can be reduced by maintaining the boiler water at the proper alkalinity and by keeping the dissolved oxygen content of the boiler water as low as possible.
The watersides of naval boilers may be cleaned in two ways—mechanically, by thorough wire brushing of all drums, headers, and tubes; and chemically. by circulating chemical cleaning solutions through the boiler.
Before mechanical cleaning of watersides is begun, the internal fittings must be removed from the steam drum. The fittings (particularly the steam separators and apron plates) must be marked or otherwise identified as to position in the steam drum to ensure their correct reinstallation. All internal fittings must be wire brushed and cleaned before they are reinstalled.
Cleaning the watersides of the generating tubes requires a special tube cleaner. There are several types available, but perhaps one of the most common is the pneumatic turbine-driven tube cleaner shown in figure 1-31. This type of cleaner consists of a flexible hose, an air-driven motor, a flexible brush holder, and an expanding wire bristle brush. The turbine-driven motor consists of a set of turbine blades made to revolve when compressed air is admitted through the hose. The turbine-driven motor, in turn, drives the wire brush. There are several sizes of brushes available (figs. 1-32 and 1-33). Figure 1-34 shows a brush refill for the type of brush shown in figure 1-32.
Figure 1-31.—Boiler tube cleaner (pneumatic turbine-driven type).
Figure 1-32.—Wire bristle brush for cleaning generating tubes.
Figure 1-33.—Wire bristle brush for cleaning large tubes.
Figure 1-34.—Brush refill.
Before you start cleaning tubes, be sure that adequate ventilation and lighting have been arranged. Someone should also be stationed outside the drum to act as tender and to assist whomever is working in the drum. Keep a written checkoff list of all tools and equipment taken into the watersides and be sure that the same tools and equipment are removed.
With the air shut off, insert the tube cleaner in the tube until the brush is about even with the far end of the tube. Wrap friction tape, a rag, or some other marking material around the hose to show how far the tube cleaner can be inserted without having the brush protrude beyond the far end of the tube. Then remove the cleaner from the tube. Remember that the tubes in each row are the same length; however, the tube lengths vary from row to row. Therefore, separate markings have to be made on the hose for each row of tubes.
After the hose has been marked, insert the brush in the tube and turn on the air to start the brush rotating. Pass the brush slowly along the length of the tube until the identifying mark has been reached. Then slowly draw the brush back. withdrawing the cleaner from the tube. You do not have to shut off the air to the tube cleaner each time the cleaner is withdrawn from the tube. However, be sure to steady the brush assembly with your hand to keep the cleaner from whipping. Allowing the brush to whip at either end of the tube is the most common cause of broken tubes.
Establish a new mark for the next row and proceed with the cleaning. Make as many passes as necessary through each tube to ensure adequate cleaning. Be careful not to stop the tube cleaner in any one place in the tube, as the continued rotation of the brush in one place might damage the tube. Be careful, also, to see that the brush and the flexible shaft do not protrude from the other end of the tube, as this may result in a broken shaft.
The tube is most easily cleaned from the steam drum. However, some rows of tubes are not accessible from the steam drum and must be cleaned from the water drum or header. The lower ends of ALL tubes must be cleaned from the water drum or header. You may also find tubes bent so that brushes cannot be forced around the bend without breaking the tube cleaner. These tubes must be cleaned from both ends. Tube cleaners must be kept in good operating condition. The rotor and blades of the air motor should be kept clean and well lubricated. The hose connections should be kept tight and free from leaks. The flexible shafts should be inspected frequently and renewed when they show signs of wear or damage. When the brushes become too worn to work efficiently, a new set of brush refills should be inserted into the brush body. Store tube cleaners in a clean, dry container.
After all tubes, drums, and headers have been cleaned and after all tools and equipment have been removed from the watersides, blow through the tubes with air; then wash out the drums, tubes, and headers with fresh water. Ensure all dirt is removed from the handhold seats. Then examine the seats for scars, pits, or other defects that might cause leakage. All bottom blow, header blow, and test cock valves should be inspected and repaired under the manufacturer’s instructions during each waterside cleaning.
After washing, thoroughly dry out the boiler watersides. Inspect the watersides to determine the condition of the metal to see if the cleaning was satisfactory. Also, inspect the boiler to be sure that all the parts are tight. Be sure that all openings between drums and gauge glasses, blow valves, and safety valves are clean and free of foreign matter. These openings are sometimes overlooked.
In most cases. mechanical cleaning is the preferred method for cleaning watersides. Chemical (acid) cleaning requires special authorization. since it requires elaborate and costly equipment and rather extensive safety precautions. However, you may have to use the chemical method, so a limited discussion on it is given here.
Inhibited acid cleaning is used to remove mill scale from the watersides of new or recently serviced boilers. When compared with mechanical cleaning, acid cleaning of boilers has the following advantages:
- Less outage time is required.
- Less dismantling of the unit. Lower cost and labor.
- A more thorough job is accomplished because the acid reaches areas inaccessible to mechanical cleaners.
- Because the cleaning is more complete, it is possible to examine the unit thoroughly for defects, such as cracks and corrosion pitting.
Acids for Cleaning
The following acids are used to clean boilers: hydrocholoric acid, phosphoric acid, sulfamic acid, citric acid, and sulfuric acid.
HYDROCHLORIC ACID is most frequently used for boiler cleaning because it has a relatively low cost and satisfactory inhibitors are available. Also, the chemical reactions of the hydrochloric acid with the boiler deposits usually result in soluble chlorides.
PHOSPHORIC ACID can remove mill scale from new boilers. With this acid, the boiler can be fired directly without producing noxious or corrosive fumes. Direct firing produces good circulation and distribution of the cleaning solution. Another advantage of phosphoric acid cleaning is that the metal surfaces resist corrosion after cleaning. When cleaned with phosphoric acid, you must protect metal surfaces from surface corrosion during draining and before neutralization.
SULFAMIC ACID is available in powder that must be placed in solution. The powdered acid is easier and safer to handle than liquid acids in carbons. It does not produce noxious fumes as it dissolves and it is less corrosive than hydrochloric acid, especially at higher concentrations and temperatures
CITRIC ACID AND SULFURIC ACID are used for removing boiler waterside deposits. Sulfuric acid is economical and easily inhibited. However, a danger is that the sulfuric acid can form insoluble salts, such as calcium sulfate.
Inhibitors. Without inhibitors, acid solutions attack the boiler metal as readily as they attack the deposits. With the addition of suitable inhibitors, the reaction with the boiler metal is greatly reduced. Inhibitors used include arsenic compounds, barium salts, starch, quinolin, and pyridin. Commercial inhibitors are sold under trade names by various chemical concerns. Other inhibitors are manufactured by companies that furnish complete acid cleaning services.
Safety Precautions. When acid cleaning a boiler installation, you must observe safety precautions as follows:
- Before acid cleaning, replace all brass or bronze parts temporarily with steel or steel alloy parts.
- Provide adequate venting for safe release of acid vapors.
- Close all valves connecting the boiler with other piping or equipment.
- Provide competent chemical supervision for the cleaning process.
- Do not exceed the specified acid and inhibitor allowable temperature. The inhibiting effect decreases with the temperature rise and the probability of acid attack of the boiler metal increases.
- After acid cleaning, be sure to thoroughly flush out all of the tubes that are horizontal or slightly sloping. Obstructions in these tubes can cause poor circulation, overheating, and failure of tubes when the unit is placed in service.
- Use goggles, rubber gloves, and rubber aprons when handling acids.
- Slowly pour the acid into water when mixing the solutions.
- Never pour water into acid
- Always pour acid into water
- Do not chemically clean boilers with riveted joints.
During acid cleaning, hydrogen gas can develop through the reaction of the acid on the boiler metal. Some of the generated gas becomes part of the atmosphere inside the boiler, and the remainder is absorbed by the boiler metal, then liberated gradually. Because hydrogen air mixtures are potentially explosive, be careful when opening a unit for inspection after acid cleaning. Until the atmosphere within the boiler pressure parts has been definitely cleared of explosive gases, do NOT use open flames. flashlights, lighting equipment. or anything that might produce a spark near the openings to the pressure parts. Do NOT enter the boiler. The unit can be cleared of explosive gases by thoroughly flushing the unit with warm water with a positive overflow from the highest vent openings. The water temperature should be as near to 212° F as possible to accelerate the liberation of hydrogen absorbed in the metal. After opening the unit, place air blowers at the open drum manholes to circulate air through the unit. Use a reliable combustible gas indicator to test the boiler atmosphere for explosive mixtures.
Acid Cleaning Procedures. Boiler units can be acid cleaned by either the "circulation" or "fill and soak" method. The circulation method (fig. 1-35) can be used to clean units with positive liquid flow paths, such as forced circulation boilers. The inhibited acid solution is circulated through the unit at the correct temperature until test analyses of samples from the return line indicate that the acid strength has reached a balance and no further reaction with the deposits is taking place. Because the strength of the acid solution can be determined frequently during the cleaning process, this method can be more accurately controlled and can use lower strength solutions than the fill-and-soak method.
Figure 1-35.—Acid cleaning by circulation method.
The fill-and-soak method (fig. 1-36) is used for cleaning units with natural circulation. The boiler unit is filled with the inhibited acid solution at the correct temperature and allowed to soak for the estimated time. It is not possible to obtain accurate representative samples of the cleaning solution during the soaking period.
Figure 1-36.—Acid cleaning by fill-and-soak method.
FLUSHING AND NEUTRALIZING—After acid cleaning, drain and then flush the unit with clean, warm water until the flushing water effluent is free of acid and soluble iron salts.
Next, a neutralizing solution is circulated through the unit until the effluent shows a definite alkaline reaction. The types of neutralizing solutions used are as follows: soda ash, trisodium phosphate. sodium tripolyphosphate, or other nontoxic chemicals. After circulation of the neutralizing solution, the water level can be dropped to the normal level and the boiler fired at 50 psig with open vents to permit the escape of liberated gases. Finally. the boiler is again drained and flushed with clean, warm water.
New boilers. or boilers that have been fouled with grease or scale, should be boiled out with a solution of boiler compound. New boilers must be washed out thoroughly. The steps required for one method of boiling out are as follows:
- Dissolve 5 pounds of caustic soda and 1 1/2 pounds of sodium nitrate or 10 pounds of trisodium phosphate for each 1,000 gallons of water the boiler holds at steaming level. Put the mixture into the boiler as a solution. In multiple-drum boilers, divide the charge and put equal amounts in each of the lower drums.
- Fill the boiler with hot feedwater to the level of the bottom of the steam drum. Turn the steam into the boiler through the usual boiling out connections, or bottom blow, and allow the boiler to fill gradually to the top of the gauge glass.
- Steam pressure in the boiler should be kept between 5 and 10 pounds. The boiling out should continue for 48 hours. Immediately after boiling out, give a series of bottom blows to remove the bulk of the sludge. The boiler should be cooled, washed out immediately, and given the usual mechanical cleaning.
You may not always want to use the above method for boiling out. The steps for a second satisfactory method for boiling out are as follows:
- Clean out all loose scale and any scale adhering to the boiler that can be removed manually.
- Place about 15 pounds of caustic soda or soda ash and 10 pounds of metaphosphate for each 100-boiler horsepower (hp) of the boiler.
- Seal the boiler openings but OPEN ALL VENTS. Fill the boiler about three-quarters full with water.
- Start the burner and raise the temperature of the water in the boiler to about 200°F. Maintain this temperature for about 24 to 48 hours. Add makeup water as required during this period to fill the boiler to the base of the safety valve.
- Analyze the boiler water during the boiling out period and add enough caustic soda and metaphosphate to maintain the following concentrations:
- Causticity as ppm OH 300 to 500
- Phosphate as ppm PO3 100 to 150
- Open the boiler at the end of the boiling-out period and clean out the sludge and loose scale. Pay particular attention to removing scale and sludge from water legs in fire-tube boilers.
- Flush the boiler thoroughly.
- If a lot of corrosion is exposed when the scale is removed, notify your superior so a boiler inspection can be made.
When the boiler is operated, any residual scale may cause faulty operation. The boiler should be taken out of service at frequent intervals to remove sludge formed from disintegrated scale. As soon as personnel can work in the boilers, wire brush the drums and ends of all tubes.
Then clean the interior of all tubes, using the approved style of boiler tube cleaning brushes.
You should operate all cleaners in the same way. After cleaning all the tubes, follow up by blowing them out thoroughly with a strong air jet. Then inspect to see if replacement of any of the tubes is necessary.
Q27. What are the four methods used to clean boiler firesides?
Q28. What pressure range should steam be for effective wet-steam lancing?
Q29. What conditions are considered waterside corrosion?
Q30. What acids are used for cleaning boiler watersides?
Q31. What are the two methods of acid cleaning?