JPCL • May 2003 • PCE4 www.paintsquare.com

News

n the past few years, mate-rials and gear for contain-ment of painting work have

advanced significantly. This is espe-cially true for cold weather paintingof water tanks. Significantly, withcontainment and heating, paintingprojects on water storage tanks cannow proceed later in the year thanpreviously. This change is importantbecause summer is generally theworst time to take water tanks out ofservice since demand is highest. Thisarticle summarizes the challenges ofpainting in cold weather, describesthe importance of ambient tempera-ture and the role of containment inmaintaining that temperature, andgives tips for success.

Challenges of the Cold In Michigan and in most northernstates, the coating season has fol-lowed the pattern of a very busyspring, a slow summer, a very busyfall, and no work in winter. Unless they are blessed withmultiple tanks, owners of towers cannot remove a tankfrom the system during the peak demands of summer.

Cold weather painting requires modifying coatings to al-low for application. However, when a coating is modifiedto improve one property, a different property is often sac-rificed. In the past, every cold weather project required ananalysis of whether the benefits of winter painting out-weighed the disadvantage of a shorter-lived system. Manycold weather systems could be applied at 35 F (2 C) andcould be recoated the next day, but they still required 28days of cure before immersion. This extensive cure time,even during a period of low water consumption, is toolong if the water system does not have multiple tanks. Thekey still is to raise the temperature to speed the cure.

Importance of Ambient TemperaturesPainting can be successful only with the proper control ofambient conditions. In the winter, the ambient conditionmost in need of modifying is temperature: the temperatureof the substrate, the temperature of the surrounding air,and the temperature of the painter. Regardless of advances

in low-temperature cure coatings, a cold,unhappy painter will complete a lessthan optimal project. Moreover, the ma-chinery does not work as well in coldweather. For example, diesel engines andall battery-started machines are harderto start; water gets into gas lines; andmoisture in sand pots freezes the sandbecause the moisture separator is frozenat the exhaust and is then bypassed. Thepainter’s time is split between coatingapplication and equipment repairs.

The colder it is outside, the more heatis needed to warm an interior space. Thedifference in temperature between theinside air mass and the outside air massis the thermal gradient. The larger thethermal gradient is at the tank’s surface,the greater the heat loss will be as bothair masses try to minimize the tempera-ture difference. Considering that themass of the heated air of the tank is ex-tremely low in relation to the outside airmass, heat rapidly dissipates from thetank. Heat dissipation is assisted by

wind that blows away the heat lost to the outside. This phe-nomenon immediately intensifies the gradient between interi-or and exterior air, allowing more heat to escape.

A good reference for understanding winter conditions ischapter 10 of the American Water Works Association (AWWA)Manual 42 Steel Storage Tank—Cold Weather Operations. Aheat loss chart in Chapter 10, M-42 (originally from the Na-tional Bureau of Fire Underwriters), shows that with an interi-or temperature of 42 F (6 C) and an outside temperature of 0 F(-18 C), a 250,000-gallon (950,000-liter) tank will lose 779,000BTU in a 12 mph wind. (This relatively mild wind is measuredat the steel interface, which is over 100 ft [33 m] in the air.)

Preventing Heat LossThe only thing to prevent heat loss is the barrier between theconfined air and the outside air. A steel barrier (i.e., the tankwall) is a good conductor of heat and a poor insulator. A con-crete tank will at least offer some insulating value. If work isbeing completed on the inside of the tank, then thermal blan-kets or even a temporary insulated composite roof should beconsidered. If the project scope includes the exterior, then a

Painting Water Storage Tanks in WinterBy William Dixon, Dixon Engineering Inc., Lake Odessa, Michigan

MAINTENANCE TIP

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Painting in cold weather is now feasible, given the proper gear, materials, and conditions.

Courtesy of Niles, Michigan

JPCL • May 2003 • PCE 7www.paintsquare.com

NewsSSPC Guide 6 (Guide for ContainingDebris During Paint Removal Opera-tions), Class 1A or 2A rigid contain-ment or even a flexible containmentsystem offers a small insulating value.

But the real insulating benefit ofcontainment is the buffer zone added.It reduces the thermal gradient be-tween the inside air mass and the out-side air mass. The wind effect is nowmoved to the outside of the contain-ment and is not as severe because thedifference in temperature and extentof heat loss are not as great. Heat canbe added to the containment area toreduce the amount of heat needed in-side the tank. (However, all exhaustsmust be vented outside.)

Remember, proper ambient condi-tions for painting and, most impor-tantly, the actual steel temperature arecritical. It is possible to heat a tankand not achieve the necessary steeltemperature if the outside temperatureis low enough. This condition is morecommon on the lower shell sectionsbecause the heat tends to rise. (Even ifan insulating blanket is used on theroof, where heat loss is greatest, thesidewalls will still rapidly transferheat.)

Sample Project One project in Michigan in the ex-treme northwest Upper Peninsula,completed in December and January,involved the construction of a newtank. Because of the nature of the fa-cility, the tank had to be in service assoon as possible—winter delay wasimpossible. The contractor had topaint the wet interior (the portion ofthe tank in direct contact with water)before the tank was placed in service.The exterior and dry interior could bepainted during the summer. The base-bell and stem were constructed in theair, but the sphere portion was builton the ground. The paint subcontrac-tor then built a rigid containment sys-tem using scaffolding and provided in-direct heat during painting.

Because of the cost of fuel, the con-

tractor tended not to fill the heaters atnight and wanted to remove theheaters as soon as the last drop ofpaint was applied. He was required tomaintain the heaters day and night un-til the coating cured (10 days). He alsolost workdays when the heaters wentoff at night. Following the coating op-eration, the erection contractor lifted

the sphere and welded it in place. Inthe late spring, when the outside waspainted, the interior was checked andfound to be in excellent condition.Turning off the heaters at night (untilcorrected) delayed the cure, but sincethe coating was a low-temperaturecuring material and the weather was

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JPCL • May 2003 • PCE8 www.paintsquare.com

Newsmild, the coating was not damaged. If the weather hadbeen as cold as normal, this could have led to failure. Thisproject was successful because the heat was maintained ina containment area. It was too cold to have worked with-out the air buffer provided by the containment.

Tips for Specifiers and ContractorsWinter projects require specifications that are very thor-

ough and that step a little over the line from performancespecifications to prescriptive specifications. Performancespecifications give the most flexibility to the contractorwhile prescriptive specifications give the bidders a morelevel playing field and more guidance. For example, we liketo add equipment to our performance specifications. Wespecify the type of dehumidifier and the minimum size, butof course, based on the contractor’s work layout (pipe runs[size and distance], point-of-entry into tank), the dehu-midifier size may need to be changed.

Specifications must be governed by ambient require-ments, and the contractor must thoroughly investigate thesite. (All this equipment must fit somewhere.) He must con-sider design of his ventilation, dehumidification, and heat-ing systems as well as fuel and manpower needs. If he has

to heat for 24 hours, he might as well work for 24 hours.The engineer is not done once the specification is writ-

ten. He must review the bids but not on a low bid basis.Heating, ventilation, and containment plans should be sub-mitted with the bid, and the award should be made to themost “responsible and responsive” bidder. Award of thecontract must consider not only cost but also how the con-tractor intends to meet all ambient requirements.

The specification should allow for weather disruptions,define a weather day, and specify how many weather daysare built into the project. Large snowfalls are better forecastnow. Just as in the summer with severe weather, there willbe some days when work should not attempted.

The contractor should minimize the potential for lostwork through careful scheduling—for example, by plan-ning for temporary interruptions. And the contractorshould partition large tanks internally to reduce thechance of lost work as well as the heat and dehumidifi-cation requirements.

Finally, winters are usually windier than other seasons,so repair material for the containment should be available.Fabric for covering the roof should be able to hold two orthree inches of snow and should be sloped to drain.