SM
TECHNIFAX®
How to protect your boiler during shutdowns TF-23
Part 1 — Waterside The best corrosion-control program for operating boilers can be completely offset by neglect during outages, as boiler metal surfaces are often attacked and damaged by oxygen from the air during shutdowns. Protection can be achieved by (l) excluding all air from the boiler (wet lay-up) or (2) keeping the surfaces completely dry (dry lay-up). Because of wide variations in boiler design, there is no universal boiler lay-up procedure. System shutdown and the mechanical aspects of lay-up are important, so the boiler manufacturer’s recommendations should always be consulted before proceeding. Arrangements must be made to protect the boiler, superheaters, reheaters and economizers during storage. Deaerators and feedwater heaters also require protection. In the dry procedure, nitrogen blanketing and/or chemical desiccants are recommended. Nitrogen blanketing will expel air and prevent oxygen ingress, while trays of moisture-absorbing chemicals will prevent air/moisture contact.
WET LAY-UP Boilers without superheaters or reheaters Boilers without superheaters or reheaters can use a variety of chemicals to achieve a proper wet lay-up. Either a sulfite or ELIMIN-OX® approach can be used.
SULFITE Sulfite: Fireside surfaces should be free of deposits during boiler lay-up.
The choice between wet and dry depends on the length of time a boiler will be out of service. Wet lay-up is recommended for short outages (30 days or less); dry layup is recommended for longer periods. The wet method has the advantage of permitting the boiler to be returned to service on short notice. Dry lay-up is practical only if the unit can be drained hot. A boiler should be drained and inspected prior to any lay-up. When time does not allow for inspection, the boiler may be stored wet without draining if the chemical treatment is injected into the boiler before it comes off line.
NALCO CHEMICAL COMPANY ONE NALCO CENTER
5.7 lb NALCO® 19-P/1000 gal water or 15.2 lb NALCO® 1720/ 1000 gal water (400 ppm as SO3 ) TRANSPORT-PLUS®: 1 lb/l000 gal water (120 ppm) Caustic: 1/3 lb NALCO® 7292 (50% caustic/l000 gal water (37.5 ppm)
ELIMIN-OX ELIMIN-OX: 12.5 lb/l000 gal water (1500 ppm) TRANSPORT-PLUS: 1 lb/l000 gal water (120 ppm) NALCO® 350: 8-9 lb/l000 gal water (1000 ppm)
NAPERVILLE, ILLINOIS 60563-1198
SUBSIDIARIES AND AFFILIATES IN PRINCIPAL LOCATIONS AROUND THE WORLD
Registered Trademarks of Nalco Chemical Company ©1977, 1997 Nalco Chemical Company All Rights Reserved Printed in U.S.A. 4-97
The highest quality water available should be used for lay-up. Steam condensate is preferred due to its lower oxygen content, but demineralized water and softened water are generally acceptable for boilers that don’t have superheaters. The concentration of NALCO 7292 or NALCO 350 should be sufficient to maintain a pH of 10. If the pH level falls below 10, add more product. The TRANSPORT-PLUS product used should be the same as that used for the internal treatment program. 1. After the boiler has been drained and inspected, add the chemicals as the boiler is refilled with water. (When time does not allow for inspection, add the chemicals at least 30 minutes before the boiler comes off line.) 2. Fill the boiler to the water level in the drum. The chemicals must be well mixed in the water. It is desirable to start a light fire and operate the boiler at low load for approximately 30 minutes to circulate the chemicals. Nitrogen at 25 psig should be introduced at this point. Water level can then be increased to the top of the drum. After filling, the boiler must be tightly blanked or closed. Test the boiler water weekly during the outage to make sure proper control levels are being maintained. Control of pH is the most critical factor in overall corrosion protection. A minimum pH of 10 should be
maintained for minimum protection. Tests should include pH, sulfite or ELIMIN-OX, iron and hardness levels. Add more chemical as needed and recirculate.
After the boiler is filled with treated water and brought up to 50 psig, nitrogen is injected and maintained under a positive pressure of 5 psi to prevent air inleakage. All manholes should be closed and all connections tightly blanked to prevent air in-leakage.
Boilers with superheaters and reheaters 1. Boilers with superheaters and reheaters require high purity water for flooding (chemically treated condensate or demineralized water). All chemicals used for lay-up must be volatile. Inject the following chemicals into the water while backfilling the superheaters from the outlet header in order to ensure adequate mixing. NALCO 350: 8-9 lb/l 000 gal water (1000 ppm) ELIMIN-OX: 12.5 lb/l000 gal water (1500 ppm) The NALCO 350 concentration should be sufficient to raise the pH to 10.0. 2. All manholes should be closed and all connections tightly blanked or closed.
Nitrogen blanketing Nitrogen blanketing may be used to exclude air from a boiler. Its effectiveness is dependent on how well a boiler can be sealed against air in-leakage. It will not protect nondrainable superheaters unless they have been completely dried.
DRY LAY-UP The objective of dry lay-up is to keep metal surfaces free of moisture which could contain dissolved oxygen. The two recommended procedures are nitrogen blanketing and the use of desiccants.
Nitrogen blanketing Drain the boiler before the pressure falls to zero and pressurize with 5 psig nitrogen while draining. Maintain this nitrogen pressure throughout draining and storage. Pressurize all circuits to exclude air in-leakage.
Use of desiccants 1. The boiler should be free of moisture-retaining deposits and thoroughly dried out. 2. Drying can be aided by blowing hot dry air through boiler circuits. Note: Special attention must be given to the superheaters. It is important to dry them thoroughly since they are particularly susceptible to corrosion.
3. Place commercial grade silica gel in trays and distribute in boiler drums. The trays should be placed so that air may circulate underneath. Use 5 lb of silica gel/30 ft3 of boiler volume. 4. Seal the boiler carefully, blanking off all openings to prevent air in-leakage. 5. Inspect every two months for evidence of active corrosion. Check the desiccant and replace when necessary. Reseal and restore to proper conditions.
RETURNING IDLE BOILERS TO SERVICE Wet lay-up — Vent the nitrogen, if used. Reduce the chemical concentrations in the lay-up water to normal operating levels. This can be accomplished by partially draining the boiler and filling to operating level with feedwater. If superheaters are stored wet, they should be drained. Careful firing is then required to completely dry out the superheaters before normal operation. Dry lay-up — The normal startup procedure for the boiler can be followed. Thoroughly purge the equipment of nitrogen with dry air. Remove all desiccant, including any spillage from trays. Warning: Do not enter a boiler that has been nitrogen capped until oxygen levels are checked. Entering a nitrogen-filled boiler can be fatal.
Part 2 — Fireside Fireside lay-up procedures are designed to keep metal surfaces dry. Moisture and oxygen produce corrosion by forming acids that attack steel. Precautions taken during lay-up inhibit metal degradation and prolong boiler life. Corrosion is a threat to a cold boiler regardless of fuel type. The common factor in all fuels is the level of impurities. While the boiler is still firing, the impurities may be thermally decomposed as they pass through the flame zone. The elements of the impurities combine into an assortment of different compounds. As the compounds stabilize, they may form either solid deposits on the tube surfaces, or acidic liquids that corrode metal. Lay-up is a critical period for boiler maintenance and repair. It is important to clean fireside surfaces to maximize heat transfer. Consider that a 0.1" scale deposit on the waterside may increase fuel consumption as much as 16%. Fireside deposits may be several inches thick and significantly alter the efficiency of heat transfer. The deposits themselves may initiate three types of problems. First, as deposits form, they may produce corrosion at the interface of the metal and deposit surface. Second, the deposit may trap fly ash, which adds to the bulk of the deposit surface. Third, fly ash constituents such as iron, vanadium, and sodium may react with sulfur compounds to form highly corrosive, low pH deposits.
Higher SO3 and H2O concentrations favor formation of corrosive liquid acid at higher metal temperatures.
Clean fireside boiler tubes will help reduce fuel costs and maintain a low stack temperature while providing maximum superheat temperatures. Fireside clean-up is usually done by water washing the tube surfaces with lances or high-pressure hoses. Tenacious deposits may have to be sandblasted from the tube surface. The problem associated with clean metal during lay-up is the vulnerability of tube surfaces to rust and corrosion. The water used to clean the fireside reacts with sulfur compounds in the ash deposits to form sulfuric and sulfurous acids. Iron or vanadium will catalyze this reaction and intensify the corrosion. The typical corrosion reactions are: Sulfuric Acid Corrosion 2Fe + 4H2O + SO3 → H2SO4 + Fe2O3 + 3H2↑ Carbonic Acid Corrosion Fe + 2H2CO3 →Fe+2 + H2↑ + 2HCO3– Oxygen Corrosion 4Fe + 6H2O + 3O2 → 4Fe(OH)3
Magnesium-based products can be used to reduce the effects of corrosion during lay-up periods. The products use two anti-corrosion mechanisms: 1. Formation of a physical barrier that inhibits exposure of clean metal surfaces to oxygen and water vapor. 2. The magnesium-based product absorbs acids such as H2SO4 and chemically neutralizes them. There are two common methods of fireside lay-up: “hot” and “cold.” Cold lay-up is better for extended outages or many types of boiler repairs because it does not use energy. For minor repairs, a short outage or when keeping the boiler on idle, a hot lay-up may be preferred. With either method of lay-up, if significant deposits are present, the fireside should be cleaned. If water washing is unnecessary, an oil dispersible magnesium-based additive such as NALCO® 8253 should be used continuously for two weeks prior to shutdown at a dosage range of 7-8 pints per 1000 gallons of oil. Sootblowers would remove the additive, so they should not be activated when the boiler is taken out of service. The boiler should be inspected monthly during lay-up to check for trouble spots and active corrosion sites.
HOT LAY-UP/BOILER IDLE For hot lay-up, metal surfaces should be kept at 170°F or higher
to reduce moisture in the system. The temperature can be controlled by using an auxiliary heat source.
Example: Average Steam Capacity = 75,000 lb/hr
If natural gas or electric air heaters are used to maintain boiler temperatures, a dry magnesium product such as NALCO® 156C can be introduced at the forced draft (FD) fan inlet.
75,000 lb/hr = 75 pounds of 156C 1000 lb/hr
COLD LAY-UP The potential for corrosion is much greater with cold lay-up because of the lower temperatures. The importance of metal skin temperature cannot be over-emphasized in corrosion problems. A metal surface below 300°F with 10 ppm SO3 present will be cool enough to cause condensation of sulfuric acid, opening the door to corrosion. During cold lay-up, the metal temperatures are usually far below 300°F, and the formation of SO3 will be the preferred chemical reaction. When the unit is off line and the water temperature has dropped below 140°F, wash down the boiler, economizer, air heater and ID fan. Washing removes the ash and impurities that can contribute to corrosion. Drain all wash water from the boiler. To protectively coat the fireside, add NALCO 156C at the FD fan inlet. The fan must be on while the product is being added. The dosage will range from 25 to 500 lb of 156C per boiler. Use 1 pound of 156C per 1000 lb/hr of boiler capacity.
Patents The purchaser of this product is granted a license under U.S. Patent 4,457,847 to use the product in boilers. The selling price includes a prepaid royalty. If the product is being used so as to not infringe the above listed patent, please advise Nalco. Licenses are available from Nalco at the same royalty rate under this patent irrespective of the source of the product. Other patents have been granted to Nalco on related inventions whlch are available for license.
Important: Do not activate sootblower after the product has been fed to the system. An alternate procedure is to apply NALCO® 8263 PLUS to the fireside surface with a paint spray gun, or in severe problem areas, a paint brush. The spray gun must have a nozzle orifice of at least 0.028" to accommodate the suspension. Assuming a dry thickness of 1 mil (0.001"), one gallon of 8263 PLUS should cover about 500 square feet. Brush application generally provides a heavier layer, so coverage would be reduced. The product should be applied full strength. Abrasion of the paint spray equipment should not be a problem as long as the application pressures are less than 15 psi. Stainless steel equipment is preferred, but most plastics, brass, and copper are also suitable. The material cannot be sprayed using aluminum, natural rubber, neoprene or carbon steel materials. Metal surfaces should be kept dry by using heat lamps, desiccants, or dry, warm air circulation. Seal the furnace to prevent moist air or rain from entering. Hard hats, safety glasses, and safety shoes should be worn when entering a furnace. Falling slag, tools, etc., are hazards that must be recognized. Preventive maintenance procedures used with the boiler and safety rules observed around the boiler are the key elements of a successful boiler lay-up.
