A wet lay-up is done after a thorough cleaning of both firesides and watersides. The feedwater used to fill the boiler is deaerated as much as possible. While the boiler is being filled, add caustic soda in sufficient quantities to maintain a pH reading of 9.5 to 11. Additionally, add approximately 0.03-0.06 pounds of sodium sulfite per 1,000 gallons of boiler holding capacity to maintain 30-60 ppm. When equipment is installed in a plant and used in acid treatment of feedwater, it should never be used to fill a boiler for idle standby; this results in a low pH in the boiler, as concentration by boiling is taking place. To ensure the boiler is filled completely, you should add water until it overflows at the top of the boiler through any convenient outlet, and then close the outlet. When there is a superheater on the boiler, add water to fill it completely. If appreciable air is dissolved in the water, you should boil the water to vent out any air after the boiler is nearly filled.
When the chemical feeding system installed is not suitable for continuous feeding and it is necessary to slug feed the chemical while the boiler is being filled, the boiler water must be mixed to obtain uniform distribution of the chemical throughout the boiler. This can be achieved by using a circulating pump to pump water from one section of the boiler to another. When such a pump is not available, mixing can be accomplished by heating the boiler just enough under low fire to set up natural circulation.
After a boiler has been filled for standby, it must be kept filled as long as it is idle with no water flowing in or out. Leakage out, as through a leaky blowdown valve, can admit air and form a waterline in the boiler. A method sometimes used for keeping a boiler completely full consists of using a small tank placed above the boiler with a line connected to any outlet of the boiler or the superheater header. This method also shows when any leakage occurs into or out of the boiler. The small tank is provided with a vent and a water column. When the boiler is filled, water is added up into the tank. Then, if water leaks out of the boiler, water from the tank flows in, keeping the boiler full. When the level in the tank rises, it shows that water is leaking into the boiler, either through the feed line or the steam line.
Water in an idle boiler should be sampled and analyzed weekly. When the causticity or the concentration of sulfite drops considerably, you should ensure additional chemical is fed and the boiler water circulated to distribute the chemical uniformly.
One disadvantage of using the wet method is that when the temperature of the water in the boiler is lower than the outside temperature, condensation or moisture occurs on the outside of a metal boiler, causing corrosion. Some engineers coat the outside of a metal boiler with light oil to help protect it from corrosion.
Dry lay-up should be used when a boiler is scheduled to be out of service for a long period of time or when a boiler is in danger of freezing. The first step is to clean both firesides and watersides of the boiler thoroughly. After cleaning the boiler, the watersides must be completely dried, because any moisture remaining on the surface will cause corrosion. Take precautions to preclude entry of moisture in any form from steam lines, feed lines, or surrounding air.
Place a moisture-absorbing material, such as quicklime, in the boiler at a rate of 2 pounds, or silica gel at the rate of 5 pounds, for 30 cubic feet of boiler volume. Place the chemical-absorbing material in trays and insert it in the drums or manholes. Air carries moisture; ensure that you close all of the manholes and handholes. This method requires checking the moisture-absorbing material every 3 months.
One method of dry lay-up for a large utility type of boiler is to simply feed nitrogen through the boiler vents while draining the boiler. With this method, maintain nitrogen pressure at 5 psig during the storage period.Continue Reading