Steam Boiler Layup Procedures

Boiler Layup Procedures

Weekend Shutdown and Wet Storage

Boiler Wet Storage

The function of chemicals used in wet layup is to control corrosion by keeping the water oxygen free, to passivate metal surfaces, and to adjust the pH of the water.

The chemicals used for this purpose may be nonvolatile or volatile.

Only volatile chemicals should be used for laying up units with superheaters and reheaters to avoid formation of objectionable deposits in these locations when operation is resumed.

The specific chemicals usually used and the concentrations at which they should be maintained in the layup water are shown as follows:

Chemical (nonvolatile)	Control Limits
BL 1285	200 ppm minimum as
BL 1513	50 ppm or as needed to adjust pH 10.0 minimum

Note : If BL 117+, use 200 ppm, maintain min. 1 ppm N2H4

If B 120+, use 150 ppm, maintain min. 75 ppm SO3

There are differences of opinion on whether catalyzed oxygen scavengers have advantages over the uncatalyzed type. Regardless of whether an uncatalyzed or catalyzed oxygen scavenger is used, the recommended concentration should be maintained in the boiler water. The oxygen scavenger used for boiler water treatment is suitable for boiler layup.

Recently there has been considerable concern over the use of hydrazine as a water-treating chemical because of its being a suspected carcinogen. OSHA regulations specify that an operator should not be exposed to air containing more than 1 ppm of hydrazine as N₂H₄. Adequate safety precautions must be observed in the handling of hydrazine in order to prevent body contact. Tests have been run for hydrazine in air in operator handling areas and results have shown hydrazine concentrations far below the mg/l (ppm) maximum specified by OSHA. Nevertheless, adequate handling precautions must still be observed.

Closed feeding equipment is available for handling hydrazine so that operator handling and exposure are practically eliminated. Before a boiler or auxiliary equipment laid up with hydrazine-treated water is entered by personnel after draining, the boiler must be flushed with the best quality water available to remove any residual hydrazine. This procedure should reduce the concentration of hydrazine in the air well below the 1 ppm concentration specified by OSHA. Volatile nonhydrazine chemicals have been proposed as substitutes, but field experience with these chemicals is not as extensive as with hydrazine.

For wet layup, an empty boiler is completely filled with water containing the recommended concentrations of layup chemicals. Only volatile chemicals should be mixed with water used to lay up superheaters and reheaters. After a boiler is filled and sealed, a slight pressure should be maintained to prevent air inleakage. This can be accomplished by pressurizing to 5 psig (34 kPa) with nitrogen, or by connecting a head tank filled with treated layup water to the highest vent on the boiler.

An alternate wet layup procedure is to connect the continuous blowdown line from an operating boiler to a convenient bottom location of a laid up boiler, and to pipe the overflowing water from a steam drum vent to an adequate disposal location. When this procedure is used, the blowdown water from the operating boiler must be essentially free of suspended matter.

Regardless of the wet layup procedure used, the fire/gas side of a boiler must be free of deposits, dry, and kept dry during the layup in order to prevent corrosion damage.

¨      One hour prior to scheduled shutdown, raise the pH of the condensed, saturated steam to 9.5 by increasing the feed rate of BL-1513 (5% maximum).

¨      Follow normal takedown procedures by closing the nonreturn valves and drains. Close the continuous and bottom blowdown valves and open the drum vents.

¨      While the boiler is cooling and when pressures drop below 50 psig, open superheater drains to remove condensate buildup. Repeat every four hours until all condensate is removed.

¨      Install a nitrogen bottle with a pressure regulator/indicator to each boiler steam drum vent and pressurize the boiler to 2 psig.

¨      When all boilers are below 2 psig and the nitrogen blanket of 2 psig has been applied, open the superheater vents for a period of one hour to insure air purge. Close all vents after one hour.

¨      Begin circulating pumps (if available) and injecting BL-1285.

¨      After the BL-1285 has been added to all recirculating boiler waters, wait two hours before testing these waters. DEHA residuals should be 100 ppb minimum, with pH at 10.5 minimum.

If the HRSG is to be returned to service without draining and volatile chemicals have been used in the high-pressure boiler, DEHA should be 500 ppb minimum, with pH at 10.0 minimum.

Boiler Dry Storage

Boilers that are out of service are vulnerable to corrosion on the internal surfaces. The procedure that follows is designed to allow boilers to be kept off line for any period of time without damage. Regardless of the method employed, the boiler should be thoroughly cleaned and inspected prior to storage.

Dry layup is generally used in boilers that will be out of service for long periods of time or that are subject to freezing.

¨      The boiler should be thoroughly dried by circulating warm or hot air. This can be accomplished by stoves or, in some units, a small wood fire. Special attention is required for nondrainable superheaters.

¨      A chemical desiccant is placed in each drum so that air circulation below the container is possible. The more commonly used materials are quick lime (8–10 pounds per 1,000 pounds/hour rated capacity) or silica gel (10–12 pounds per 1,000 gallons of boiler holding capacity, or 8–10 pounds per 100 ft³ of air space).

¨      As soon as the desiccant is in place, close all boiler openings and tightly blank all connections.

¨      An inspection of the desiccant and an evaluation of its continued ability to absorb moisture should be made every two to three months.