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How to Properly Shut Down a Boiler

Here at ChemREADY, we know how important it is to properly maintain boiler systems and keep your boiler operating at peak efficiency. Boilers are often damaged by improper shut down. When drained hot, the residual heat in the boiler furnace can cause warpage of tubes or tube sheets, causing leaks and requiring tube rolling. The residual heat can also bake soft sludge into scale. If cold water is used for cooling, warpage, strain and leaks can result.

Boilers are often damaged by corrosion during out-of-service periods. Idle boilers are very vulnerable to attack when air (21% oxygen) contacts moist metal surfaces. To prevent corrosion, the boiler metal must be protected by either keeping the surfaces completely dry or excluding all air from the boiler by completely filling it with properly treated water. Because of variations in boiler design, there is no single, detailed procedure that covers all steps in boiler lay-up. (The mechanical aspects of lay-up procedure are important and they vary). The basic principles in protecting boilers against corrosion, however, will be covered below.

Shut Down Procedure

When the time comes to turn off your boiler, or layup the boiler for a season, there are some basic procedures that should be followed.

1. For scheduled shut down, double the normal blowdown rate for two weeks preceding shut down and increase the sludge conditioner dosage to 1 1/2 to 2 times normal.
2. After the boiler has been taken out of service, blowdown several times from all blowdown outlets, including headers, making up with hot deaerated feedwater. Follow the boiler manufacturer’s instructions for taking the boiler out of service. Never use cold water to cool the boiler as leaks in rolled joints will develop. When the boiler is cool enough to get into the furnace, drain the boiler.
3. To prevent suspended matter (sludge) in the boiler water from baking on the tubes, never drain water from a hot boiler. Never drain the water from a boiler and let it dry out without thoroughly washing it. Start washing down as soon as the water has drained away from the tubes. Flush out all feed and control lines to remove any sludge accumulations.
4. Flush the water-side surfaces thoroughly with a high-pressure hose and inspect to determine whether further cleaning is needed before lay-up. All soot, fly ash, and slag should be cleaned off the fireside surfaces at the first opportunity. These deposits can foster severe corrosion on the fireside. For corrosion protection, the fireside can be coated with light oil after cleaning.

Storage Options

There are two basic ways of laying up boilers: wet and dry.  In storing a boiler dry, trays of moisture-absorbing chemicals are distributed in the boiler drum (or drums), and the boiler is sealed. This removes the moisture and prevents premature wear on the boiler tubes that may cause future pitting and leaks. The alternate method, wet storage, involves forcing air out of the boiler by completely filling it (to overflowing) with water that has been specially treated.  Nitrogen gas under a slight pressure can also be used to displace air and “blanket” the boiler open surfaces.  Special consideration should be given to protecting superheaters during lay-up, particularly the non-drainable type, see your operating manual is this is your case.

The choice between the wet and dry methods of lay-up depends to a great extent on how long the boiler is to be out of service. Dry lay-up is preferable for long outages.  The wet method has the advantage of permitting the boiler to be returned to service on reasonably short notice.  It is preferable that a boiler be drained, flushed and inspected prior to any lay-up.  When time does not permit this, however, the boiler may be stored wet without first draining it.  In this case the chemical treatment for lay-up is injected into the boiler just before it comes offline.

Dry Lay Up Procedures

Closed and Dry

The following steps should be taken in laying up a boiler closed and dry.

1. After the boiler has cooled, drain the unit completely.

2. Flush thoroughly and inspect. If cleaning is needed, this should be done before the boiler is laid up.

3. Dry thoroughly, giving special attention to drying out superheaters. Circulated warm air may be used in drying the metal. All valves must be tightly closed to ensure that no water or steam enters the boiler.

4. Place quicklime (not hydrated lime) or commercial grade silica get in trays and distribute in the boiler drums. The trays should be placed so that air may circulate underneath. For fire tube boiler, place a single tray on top of the tubes or in the bottom of the shell. (Use at least 7 lbs. or lime for each 1000 lb./hour steaming capacity).

5. Seal the boiler very carefully, blanking off all openings through which air or steam might enter.

6. After every two months of lay-up, open the boiler, inspect the moisture absorbing chemical (replacing it when necessary), and reseal.

Open and Dry

The following steps should be taken in laying up a boiler open and dry.

1. After the boiler has cooled, drain the unit completely.

2. Flush thoroughly and inspect. If cleaning is needed, this should be done before the boiler is laid up.

3. Dry thoroughly, giving special attention to drying out superheaters. Circulated warm air may be used in drying the metal. All valves must be tightly closed to ensure that no water or steam enters the boiler.

4. Open all openings to allow a free flow of air.

5. The boiler should be inspected for condensation weekly.

6. Small heaters, heat lamps, or pilot flames can be used to maintain the boiler above the dew point.

Wet Lay-Up Procedure

The following procedure covers the storage of a boiler using a wet lay-up procedure.

  • At least 30 minutes before the boiler comes offline, add the following chemicals. When the boiler is to be drained and inspected first, the chemicals should be added, in solution, as it is being refilled with water. It is most important that the chemicals be well distributed in the boiler water. When the boiler is refilled, start a light fire to provide circulation of the chemicals.
    • Sodium Sulfite or appropriate treatment compound 5 lb/1000 gallons of water
      (400 ppm as SO3)
    • Caustic Soda or appropriate treatment compound 10 lb/1000 gallons of water (1000 ppm total alkalinity as CaCO3)
  • Fill the boiler to the top of the drum, blanking off outlets as necessary to avoid flooding the steam line or the superheater. A simple device that can be used to ensure that the boiler is completely full is shown in the image.
  • Alternatively, maintain a positive pressure of 5 psig of nitrogen using a cylinder and a two-stage regulator.
  • Test the boiler water each week or two to make sure the proper levels of sulfite and caustic are being maintained. Add additional chemicals as needed.

Returning Idle Boilers to Service

With boiler stored dry the normal start-up procedure can be followed. One precaution, however, is to make sure all of the moisture-absorbing chemicals are removed. If there has been any spillage, it should be cleaned up.

With wet storage, the water is merely partially drained and made up to the operating level with feedwater. This dilutes the chemical residuals. How much feedwater dilution is needed depends on the level of dissolved solids that a particular boiler can tolerate during operation.

When superheaters are stored wet, they should be drained (or blown out with air) as much as is possible before the start-up. Careful firing (as soon as possible after air blowing) is then required to completely dry out the superheater before normal operation.

The ChemREADY Advantage

Along with supplying the industry’s best products that help reduce corrosion and scale buildup in boiler systems, ChemREADY offers an expertise that’s invaluable at keeping your systems operating at peak efficiency. We offer a variety of services, including:

  • Site surveys
  • Total Service Management
  • Supervisory programs with or without chemicals
  • Commercial and industrial water treatment

If you want to learn more about how to improve your boiler system management, click here to learn more about ChemREADY’s Total-Service-Management service.