Boiler Layup Procedures

Boilers must be stored under carefully controlled conditions during non-operating periods to avoid corrosion damage that can occur in the absence of proper lay-up procedures. Improper lay-up and storage will result in rust formation, pitting-type corrosion and general deterioration of boiler metal surfaces. This damage can occur on both the waterside and fireside of the boiler.

If the waterside of the boiler is exposed to the atmosphere, corrosion will occur at the liquid-to-air interface. Corrosion damage is also possible in the preboiler and afterboiler sections. Once formed, the by-products of corrosion can then be transported to the operating boiler when the system is returned to service. These corrosion products may deposit on critical heat transfer surfaces, increasing the potential for localized corrosion or overheating during system operation.

The two major factors which determine the corrosion rate on boiler metal are moisture and dissolved oxygen. Under completely dry conditions, the corrosion of steel is negligible. In a moist or wet environment, however, the amount of dissolved oxygen in the water determines the severity of the corrosion. Conditions that increase the oxygen concentration in the water, or allow the continued addition of oxygen, will increase the corrosion rate.

The fireside of the boiler is also subject to corrosion damage. Like the waterside, corrosion damage to the fireside will occur if the metal surfaces are wet and exposed to oxygen. Sulfurous and sulfuric acid residues, which are by-products of the condensation of acidic flue gases, will also promote corrosion attack.

For these reasons, proper boiler storage procedures must be followed to protect and prolong the useful life of the steam generating equipment.

Removing the Boiler from Service

Pre-Shutdown Procedures

Preparation for boiler shutdown should begin 10 days prior to the scheduled shutdown date. The following procedures will help remove accumulated boiler sludge prior to draining the boiler.

  • Increase the blowdown rate to maintain the boiler conductivity at the low end of the normal control range.

  • Keep the steaming rate as high as possible during this period to maintain optimum boiler water circulation

  • Maintain the boiler water chemicals at the normal operating levels

Boiler Shutdown Procedures

The boiler should be brought down in rating slowly. Raise the water level as high in the glass gauge as is consistent with safe operating practice while still unloading some steam to the line. After the pressure has dropped, do the following four operations:

  • Shut off the continuous blowdown system

  • Blow down all manual valves including the sidewall and waterwall headers

  • Open the steam drum vent, close the steam non-return valve and the head stop valve.

  • Blow down all manual valves once for every 25 psig decrease in boiler pressure

Draining the Boiler

The last blowdown should be made at 25 psig. Allow the boiler to cool to approximately 120 o F. This will allow for uniform cooling of the boiler tubes and drum. Drain the boiler as follows:

  • Start the draining process by opening all manual blowdown valves.

  • When the water level is below the steam drum manhole, remove the manhole covers and start washing the boiler with a fire hose at high pressure.

  • Wash down the boiler for at least three hours

  • Allow the boiler to drain completely. Revove the manhole covers from the mud drum and all handhole plates from the sidewall or waterwall headers. Wash the headers and the mud drum until all loose material has been removed.

Chemical Cleaning

The waterside surfaces of the boiler and superheater must be free of any deposits, sludge, oils, corrosion by-products, or other debris. Any deposits that exist on the boiler surfaces will promote under-deposit corrosion. This type of corrosion is characterized by pits which form under loose, porous deposits on the metal surface.

A typical chemical cleaning procedure involves the use of strong cleaners such as inhibited hydrochloric acid. The mineral acid dissolves inorganic scales and corrosion by-products. The acid cleaner is then drained, the boiler flushed with water, and then passivated with an alkaline passivating solution.

The decision to chemically clean the boiler prior to storage is a subjective one. If the visible portions of the boiler internals are clean and free of foreign material, then the chemical cleaning step can be eliminated. Boilers that have significant scale deposits, however, should be chemically cleaned prior to storage.

Boiler Storage Options

Two basic boiler lay-up procedures are in use:

  • Dry lay-up

  • Wet lay-up

These two basic methods have several variations, including nitrogen or steam blanketing, which helps insure the complete exclusion of air from the boiler during lay-up and storage.

Dry lay-up is recommended for boilers that will be out of service for 1 month or more. Here the boiler is drained, dried and stored in a moisture-free environment. This includes installing trays of desiccant, such as silica gel, in the boiler drums to maintain a constant low humidity atmosphere. The boiler is then closed to minimize oxygen ingress. If the boiler is to be stored open, the boiler and superheater are thoroughly dried and a positive dry air flow is maintained from bottom to top during the storage period. Superheaters can be stored under a 5 psig nitrogen blanket for added protection.

Wet lay-up is recommended for boilers that must be maintained in an emergency standby mode. This procedure involves filling the boiler, feedwater heaters, and deaerators with demineralized water, treating the water with a chemical oxygen scavenger, and adjusting the pH between 10 and 11. For boilers without superheaters or for boilers with drainable superheaters, sodium sulfite and caustic soda are generally used for this purpose. For high pressure boilers, or boilers with non-drainable superheaters, volatile chemicals must be used such as hydrazine and ammonia. Alternatively, neutralizing amines such as morpholine, cyclohexylamine, or diethylaminoethanol may be substituted for the ammonia.

Generally, boilers should not be stored in the wet lay-up mode for more than 6 months. For extended periods beyond 6 months, the dry lay-up procedure provides better corrosion protection.

Nitrogen blanketing is used to provide an inert, corrosion-free environment. Nitrogen is an odorless, colorless gas with an extremely low dew point. It is used routinely to purge oxygen from enclosed vessels. Corrosion can not occur in an inert nitrogen environment. Under wet lay-up conditions, nitrogen may be connected to the steam vent to provide a low pressure nitrogen blanket to prevent oxygen ingress. Alternatively, nitrogen may be used during dry lay-up conditions to provide a positive nitrogen pressure (5 psig) in the closed boiler vessel to prevent oxygen and moisture intrusion. It may also be used to inert superheaters, and provide a nitrogen blanket in deaerators and feedwater heaters.

Steam blanketing is used to store boilers and auxiliary equipment under a positive pressure to prevent the ingress of dissolved oxygen. The water temperature keeps the metal surfaces above the dew point, which helps protect the fireside from corrosion. Deaerators and feedwater heaters may also be stored under a steam blanket. Because of the high energy costs associated with steam blanketing, however, it is not recommended for storage periods of more than 6 months.

Dry Lay-up Method

The objective of dry lay-up is to maintain the boiler metal surfaces in a moisture-free condition. Corrosion cannot occur in a dry, oxygen –free environment.

Storing Under a Nitrogen Blanket

  • Drain the boiler as described previously

  • Clean the boiler, if necessary

  • Completely dry all circuits of the boiler with a positive air flow from the bottom to the top

  • If nitrogen blanketing is to be used for dry storage, close up the boiler, purge all of the air from the boiler with nitrogen, and then store under a 5 psig nitrogen pressure.

Storing Dry with Chemical Desiccant

  • Dry the boiler out by blowing hot dry air through the boiler, or apply low auxiliary heat to dry the metal surfaces

  • Place bags of desiccant in wooden trays in the steam and mud drums

Amount of Desiccant Required

Desiccant

Pounds per 30 ft3

Telltale silica gel

5.0

Quicklime

2.0

Note: Silica gel is easier to use than quicklime and can be dried and re-used.

  • Close all manholes and blank or close all connections on the boiler as completely as possible to prevent the intrusion of humid air

  • Inspect the waterside of the boiler every 2 to 3 months for active corrosion

  • Inspect the condition of the desiccant and replace, if necessary

Wet Lay-up Method

Wet lay-up is the preferred method for storing boilers that must be maintained in an emergency standby mode. However, boilers should not be stored for periods longer than 6 months in the wet lay-up condition. During this period the boiler should be fired twice per month to bring the boiler water to 160 o F. This circulates the boiler water and improves corrosion protection.

Two methods for chemical treatment of the boiler water during wet lay-up are recognized. For boilers with drainable superheaters, an inorganic treatment program is used consisting of sodium sulfite for oxygen scavenging and sodium hydroxide (caustic soda) for pH adjustment. For boilers with non-drainable superheaters, a volatile treatment program is required to avoid the formation of inorganic deposits in the superheater at the time of startup. In this case, hydrazine is used for oxygen scavenging and ammonia or a volatile neutralizing amine, such as morpholine or DEAE, for pH adjustment.

Alternatively, the non-drainable superheater must be blanked off to prevent the entrance of treated water and then blanketed with nitrogen.

Boiler with Non-drainable Superheaters

  • Drain the boiler as described previously

  • Clean the boiler, if necessary

  • Backfill the boiler through the superheater with demineralized, deaerated and chemically treated water. Disconnect the high water alarms and fill the boiler completely to the top. The economizer and feedwater heaters may also be filled and treated in a similar manner.

Chemical Dosage

Chemical

Dosage

Hydrazine (35%

4.8 lbs per 1000 gallons

Ammonia

0.10 lbs per 1000 gallons

(or) DEAE

0.5 lbs per 1000 gallons

  • After the boiler and superheater are filled with treated water, close or blank all connections. Install a 55 gallon tank with a tight-fitting cover and sight glass at a location above the steam drum. Connect the tank to the vent line on the boiler to create a hydrostatic head. A net loss or gain of water from the boiler will be indicated by the water level in the drum.

  • As an alternative to installing a 55 gallon expansion tank, connect low pressure (5 psig) nitrogen to the boiler vent to pressurize the system.

Boilers with Drainable Superheaters

  • Drain the boiler as indicated above.

  • Clean the boiler, if necessary.

  • Backfill the boiler through the superheater with demineralized, deaerated feedwater. Since the superheater is drainable, the water may be treated with sodium sulfite and caustic soda.

Chemical Dosages

Chemical

Dosage

Sodium sulfite

1.5 lbs/1000 gallons

Sodium hydroxide

3.0 lbs/1000 gallons

  • Fill the boiler all the way to the top and cap with a 55 gallon expansion tank, or pressurize with 5 psig nitrogen.

Recirculating the Boiler Water

During the wet storage period the boiler should be fired twice per month to bring the water temperature to 160 oF and then allowed to cool to ambient temperature. This insures good circulation of the treatment chemicals.

An alternate method is to install a small recirculating pump on a bottom connection to the boiler mud drum such as a T-connection off the bottom blowdown line. The boiler water can be circulated from the mud drum back to the steam drum at a rate sufficient to give one turnover of boiler water every 8 hours. The pump can also be operated to mix the boiler lay-up chemicals or circulate the lay-up solution through the system.

Testing the Boiler Water

The boiler lay-up water should be tested once per week. This will confirm that adequate corrosion protection is being maintained. The control limits for these chemical levels are indicated below:

Chemical Control Limits

Volatile Program for Non-drainable Superheaters

Chemical

Control Range

Hydrazine

200 to 400 ppm

Amine

pH 10 to 11

Inorganic Program for Drainable Superheaters

Chemical

Control Range

Sodium sulfite

100 to 200 ppm

Caustic soda

OH alk = 400 ppm

If the chemical residuals drop below the indicated minimums, drain some water from the boiler, add additional lay-up chemical, fire the boiler to 160 oF to circulate and retest.

Startup After Wet Lay-up

The following guidelines should be followed when returning the boiler to service after wet lay-up.

  • Purge the nitrogen from the system, or disconnect the hydrostatic tank from the boiler vent connection.

  • Drain the boiler water to normal operating levels. Drain the deaerator and feedwater heater, if stored in a similar manner.

  • Reset the high water alarm.

  • Open all valves and disconnect the recirculating pump, if used.

  • The boiler can be fired with the lay-up chemicals. If a non-drainable superheater is stored wet, waste steam to the atmosphere to purge the treated water from the superheater. For drainable superheaters, completely drain it prior to startup and blow with dry compressed air prior to firing the unit.

  • Re-start the normal chemical treatment program and water quality control practice.

Startup After Dry Lay-up

  • If stored under a nitrogen blanket, purge all nitrogen from the system. Do not enter the boiler. Nitrogen will not support life, and will cause death by suffocation.

  • If stored with trays of desiccant, remove them from the steam and mud drums.

  • Refill the boiler and return to service in a normal manner.

Fireside Lay-up Procedure

Provisions should also be made for protecting the boiler fireside from corrosion during the boiler lay-up period. To minimize the corrosion caused by acidic flue gases, the fireside of the boiler, air heater, and ID fan should be washed down with a 5% solution of sodium carbonate. Drain all wash water from the boiler. Dry the fireside and keep it dry by continuous circulation of dry air through the furnace. Seal the furnace as completely as possible to minimize the entry of humid air.

The fireside should be inspected once per month. Any signs of active corrosion should be noted and corrective action taken to eliminate the cause of the problem.

Safety Considerations

As with all chemical products, the boiler lay-up chemicals should be stored and handled according to recommended procedures as stated in the Material Safety Data Sheets.

Nitrogen is an inert gas that does not support life. If nitrogen is used to inert boilers and auxiliary equipment, do not enter the equipment until all nitrogen is purged from the system and tests show that sufficient air is present to support life.

Hydrazine is toxic. It has been classified as a suspected carcinogen. Skin or eye contact can cause permanent injury or dermal sensitization. Inhalation of hydrazine can be irritating and can cause health problems. Do not enter equipment until the concentration of hydrazine in the air is below 1 mg/l.

Wear safety equipment such as rubber gloves, eye goggles, aprons and boots when handling lay-up chemicals. Follow all other precautions as stated in the Material Safety Data Sheets.

A Summary of Boiler Lay-up Procedures

Boilers must be stored under carefully control conditions to prevent rust formation, pitting-type corrosion and general deterioration of boiler metal. Two methods for boiler lay-up are recognized as effective: (1) dry lay-up and (2) wet lay-up.

Dry lay-up is recommended for boilers that will be out of service for 6 months or longer. With this procedure the boiler is drained, dried and inerted with a nitrogen blanket. As an alternative to nitrogen blanketing, trays of telltale silica gel are installed in the steam and mud drums, and the boiler is closed up to minimize the entry of humid air.

Wet lay-up is intended for boiler that must be stored under emergency standby conditions. Here the boiler is backfilled through the superheater. The boiler is filled completely to overflowing and then pressurized with a nitrogen blanket to prevent air ingress. As an alternative to nitrogen capping, the steam vent can be connected to a 55-gallon drum of treated water to provide a hydrostatic head. Or the boiler can be pressurized under a steam blanket. If the boiler has drainable superheaters, sodium sulfite and caustic soda are used to protect the system. If the superheaters are not drainable, an all volatile treatment program is required using hydrazine and ammonia (or alternatively a neutralizing amine) for corrosion protection.

These basic lay-up procedures can be adapted to meet individual plant requirements. Overall, however, the goal is to maintain an oxygen-free, non-corrosive environment to protect the boiler and auxiliary equipment while in short or long-term storage.

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