NALCO CHEMICAL COMPANY ONE NALCO CENTER NAPERVILLE, ILLINOIS 60563-1198SUBSIDIARIES AND AFFILIATES IN PRINCIPAL LOCATIONS AROUND THE WORLD

Registered Trademarks of Nalco Chemical Company ©1977, 1997 Nalco Chemical CompanyAll Rights Reserved Printed in U.S.A. 4-97

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TECHNIFAX® How to protect your boiler duringshutdowns

Part 1 — WatersideThe best corrosion-controlprogram for operating boilerscan be completely offset byneglect during outages, as boilermetal surfaces are often attackedand damaged by oxygen fromthe air during shutdowns.Protection can be achieved by(l) excluding all air from theboiler (wet lay-up) or (2) keepingthe surfaces completely dry (drylay-up). Because of wide varia-tions in boiler design, there is nouniversal boiler lay-up proce-dure. System shutdown and themechanical aspects of lay-up areimportant, so the boiler manu-facturer’s recommendationsshould always be consultedbefore proceeding. Arrange-ments must be made to protectthe boiler, superheaters, re-heaters and economizers duringstorage. Deaerators and feed-water heaters also requireprotection. In the dry procedure,nitrogen blanketing and/orchemical desiccants are recom-mended. Nitrogen blanketingwill expel air and preventoxygen ingress, while trays ofmoisture-absorbing chemicalswill prevent air/moisturecontact.

Fireside surfaces should be free ofdeposits during boiler lay-up.

The choice between wet and drydepends on the length of time aboiler will be out of service. Wetlay-up is recommended for shortoutages (30 days or less); dry lay-up is recommended for longerperiods. The wet method has theadvantage of permitting the boilerto be returned to service on shortnotice. Dry lay-up is practical onlyif the unit can be drained hot.

A boiler should be drained andinspected prior to any lay-up.When time does not allow forinspection, the boiler may bestored wet without draining if thechemical treatment is injected intothe boiler before it comes off line.

WET LAY-UPBoilers without superheatersor reheaters

Boilers without superheaters orreheaters can use a variety ofchemicals to achieve a properwet lay-up. Either a sulfite orELIMIN-OX® approach can beused.

SULFITESulfite:

5.7 lb NALCO® 19-P/1000 galwater 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-OXELIMIN-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)

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The highest quality water availableshould be used for lay-up. Steamcondensate is preferred due to itslower oxygen content, but de-mineralized water and softenedwater are generally acceptable forboilers that don’t have superheaters.

The concentration of NALCO7292 or NALCO 350 should besufficient to maintain a pH of 10.If the pH level falls below 10, addmore product.

The TRANSPORT-PLUS productused should be the same as thatused for the internal treatmentprogram.

1. After the boiler has been drainedand inspected, add the chemi-cals as the boiler is refilled withwater. (When time does notallow for inspection, add thechemicals at least 30 minutesbefore the boiler comes off line.)

2. Fill the boiler to the water levelin the drum. The chemicalsmust be well mixed in thewater. It is desirable to start alight fire and operate the boilerat low load for approximately30 minutes to circulate thechemicals. Nitrogen at 25 psigshould be introduced at thispoint. Water level can then beincreased to the top of thedrum. After filling, the boilermust be tightly blanked orclosed.

Test the boiler water weeklyduring the outage to make sureproper control levels are beingmaintained. Control of pH isthe most critical factor inoverall corrosion protection. Aminimum pH of 10 should be

maintained for minimum pro-tection. Tests should includepH, sulfite or ELIMIN-OX,iron and hardness levels. Addmore chemical as needed andrecirculate.

Boilers with superheatersand reheaters1. Boilers with superheaters and

reheaters require high puritywater for flooding (chemicallytreated condensate or deminer-alized water). All chemicalsused for lay-up must be vola-tile. Inject the following chemi-cals into the water while back-filling the superheaters fromthe outlet header in order toensure 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 concentrationshould be sufficient to raise thepH to 10.0.

2. All manholes should be closedand all connections tightlyblanked or closed.

Nitrogen blanketingNitrogen blanketing may be usedto exclude air from a boiler. Itseffectiveness is dependent on howwell a boiler can be sealed againstair in-leakage. It will not protectnondrainable superheaters unlessthey have been completely dried.

After the boiler is filled withtreated water and brought up to50 psig, nitrogen is injected andmaintained under a positivepressure of 5 psi to prevent air in-leakage. All manholes should beclosed and all connections tightlyblanked to prevent air in-leakage.

DRY LAY-UPThe objective of dry lay-up isto keep metal surfaces free ofmoisture which could containdissolved oxygen. The two recom-mended procedures are nitrogenblanketing and the use ofdesiccants.

Nitrogen blanketingDrain the boiler before the pres-sure falls to zero and pressurizewith 5 psig nitrogen while drain-ing. Maintain this nitrogen pres-sure throughout draining andstorage. Pressurize all circuits toexclude air in-leakage.

Use of desiccants1. The boiler should be free of

moisture-retaining depositsand thoroughly dried out.

2. Drying can be aided by blowinghot dry air through boiler circuits.

Note: Special attention must begiven to the superheaters. It isimportant to dry them thoroughlysince they are particularly suscep-tible to corrosion.

Fireside lay-up procedures aredesigned to keep metal surfacesdry. Moisture and oxygen pro-duce corrosion by forming acidsthat attack steel. Precautions takenduring lay-up inhibit metal degra-dation and prolong boiler life.

Corrosion is a threat to a coldboiler regardless of fuel type. Thecommon factor in all fuels is thelevel of impurities. While theboiler is still firing, the impuritiesmay be thermally decomposed asthey pass through the flame zone.The elements of the impuritiescombine into an assortment ofdifferent compounds. As thecompounds stabilize, they mayform either solid deposits on thetube surfaces, or acidic liquidsthat corrode metal.

Lay-up is a critical period forboiler maintenance and repair.It is important to clean firesidesurfaces to maximize heat trans-fer. Consider that a 0.1" scaledeposit on the waterside mayincrease fuel consumption asmuch as 16%. Fireside depositsmay be several inches thick andsignificantly alter the efficiency ofheat transfer.

The deposits themselves mayinitiate three types of problems.First, as deposits form, they mayproduce corrosion at the interfaceof the metal and deposit surface.Second, the deposit may trap flyash, which adds to the bulk of thedeposit surface. Third, fly ashconstituents such as iron, vana-dium, and sodium may react withsulfur compounds to form highlycorrosive, low pH deposits.

Part 2 — Fireside

Higher SO3 and H2O concentrationsfavor formation of corrosive liquid acidat higher metal temperatures.

Clean fireside boiler tubes willhelp reduce fuel costs and main-tain a low stack temperature whileproviding maximum superheattemperatures. Fireside clean-up isusually done by water washingthe tube surfaces with lances orhigh-pressure hoses. Tenaciousdeposits may have to be sand-blasted from the tube surface.

The problem associated withclean metal during lay-up is thevulnerability of tube surfaces torust and corrosion. The waterused to clean the fireside reactswith sulfur compounds in the ashdeposits to form sulfuric andsulfurous acids. Iron or vanadiumwill catalyze this reaction andintensify 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

3. Place commercial grade silicagel in trays and distribute inboiler drums. The trays shouldbe placed so that air maycirculate underneath. Use 5 lbof silica gel/30 ft3 of boilervolume.

4. Seal the boiler carefully, blankingoff all openings to prevent airin-leakage.

5. Inspect every two months forevidence of active corrosion.Check the desiccant and replacewhen necessary. Reseal andrestore to proper conditions.

RETURNING IDLE BOILERSTO SERVICE

Wet lay-up — Vent the nitrogen,if used. Reduce the chemicalconcentrations in the lay-up waterto normal operating levels. Thiscan be accomplished by partiallydraining the boiler and filling tooperating level with feedwater. Ifsuperheaters are stored wet, theyshould be drained. Careful firingis then required to completely dryout the superheaters beforenormal operation.

Dry lay-up — The normal start-up procedure for the boiler can befollowed. Thoroughly purge theequipment of nitrogen with dryair. Remove all desiccant, includ-ing any spillage from trays.

Warning: Do not enter a boilerthat has been nitrogen cappeduntil oxygen levels are checked.Entering a nitrogen-filled boilercan be fatal.

Magnesium-based products canbe used to reduce the effects ofcorrosion during lay-up periods.The products use two anti-corro-sion mechanisms:

1. Formation of a physical barrierthat inhibits exposure of cleanmetal surfaces to oxygen andwater vapor.

2. The magnesium-based productabsorbs acids such as H2SO4and chemically neutralizes them.

There are two common methodsof fireside lay-up: “hot” and“cold.” Cold lay-up is better forextended outages or many typesof boiler repairs because it doesnot use energy. For minor repairs,a short outage or when keepingthe boiler on idle, a hot lay-upmay be preferred.

With either method of lay-up, ifsignificant deposits are present,the fireside should be cleaned. Ifwater washing is unnecessary, anoil dispersible magnesium-basedadditive such as NALCO® 8253should be used continuously fortwo weeks prior to shutdown ata dosage range of 7-8 pints per1000 gallons of oil. Sootblowerswould remove the additive, sothey should not be activated whenthe boiler is taken out of service.The boiler should be inspectedmonthly during lay-up to checkfor trouble spots and activecorrosion sites.

HOT LAY-UP/BOILER IDLEFor hot lay-up, metal surfacesshould be kept at 170°F or higher

to reduce moisture in the system.The temperature can be controlledby using an auxiliary heat source.

If natural gas or electric air heat-ers are used to maintain boilertemperatures, a dry magnesiumproduct such as NALCO® 156Ccan be introduced at the forceddraft (FD) fan inlet.

COLD LAY-UPThe potential for corrosion is muchgreater with cold lay-up becauseof the lower temperatures. Theimportance of metal skin tempera-ture cannot be over-emphasizedin corrosion problems. A metalsurface below 300°F with 10 ppmSO3 present will be cool enough tocause condensation of sulfuricacid, opening the door to corro-sion. During cold lay-up, the metaltemperatures are usually far be-low 300°F, and the formation ofSO3 will be the preferred chemicalreaction.

When the unit is off line and thewater temperature has droppedbelow 140°F, wash down theboiler, economizer, air heater andID fan. Washing removes the ashand impurities that can contributeto corrosion. Drain all wash waterfrom the boiler.

To protectively coat the fireside,add NALCO 156C at the FD faninlet. The fan must be on whilethe product is being added. Thedosage will range from 25 to500 lb of 156C per boiler. Use1 pound of 156C per 1000 lb/hrof boiler capacity.

Example:Average Steam Capacity =75,000 lb/hr

75,000 lb/hr 1000 lb/hr

= 75 pounds of 156C

Important: Do not activatesootblower after the product hasbeen fed to the system.

An alternate procedure is to applyNALCO® 8263 PLUS to the fire-side surface with a paint spraygun, or in severe problem areas, apaint brush. The spray gun musthave a nozzle orifice of at least0.028" to accommodate the sus-pension. Assuming a dry thick-ness of 1 mil (0.001"), one gallonof 8263 PLUS should cover about500 square feet.

Brush application generally pro-vides a heavier layer, so coveragewould be reduced. The productshould be applied full strength.

Abrasion of the paint sprayequipment should not be a prob-lem as long as the applicationpressures are less than 15 psi.Stainless steel equipment is pre-ferred, but most plastics, brass,and copper are also suitable. Thematerial cannot be sprayed usingaluminum, natural rubber, neo-prene or carbon steel materials.Metal surfaces should be kept dryby using heat lamps, desiccants,or dry, warm air circulation. Sealthe furnace to prevent moist air orrain from entering.

Hard hats, safety glasses, andsafety shoes should be worn whenentering a furnace. Falling slag,tools, etc., are hazards that mustbe recognized. Preventive mainte-nance procedures used with theboiler and safety rules observedaround the boiler are the key ele-ments of a successful boiler lay-up.

PatentsThe purchaser of this product is granted a license under U.S. Patent 4,457,847 to use the productin boilers. The selling price includes a prepaid royalty. If the product is being used so as to notinfringe the above listed patent, please advise Nalco. Licenses are available from Nalco at thesame royalty rate under this patent irrespective of the source of the product. Other patents havebeen granted to Nalco on related inventions whlch are available for license.