The City of Houston has selected planning, engineering and program management firm Lockwood, Andrews & Newnam Inc. (LAN) to develop...
At Work on Water Storage
Tank painting projects can cost many thousands of dollars,
but often the most costly aspect of tank rehabilitation is having your tank out
of service. What can be done to get your tank back in service sooner? One
approach is to control the environment and minimize delays due to poor weather.
Another is to use coatings that tolerate a wide range of climatic conditions
and cure quickly at ambient temperature. By combining these methods, down-time
can be greatly reduced.
Controlling the environment generally refers to an interior
coating application but also can be accomplished for exterior surfaces by using
a containment system. In either
case, controlling humidity (dew point) is the most critical factor. Heating or
cooling also can be used along with ventilation during the blasting and coating
operations. Where dehumidification, heating, cooling and/or dust collection are
being used, ventilation is the basic component of total air treatment.
Ventilation. The primary purposes of ventilation during
blasting and painting operations in enclosed areas are for worker health,
safety and visibility. Proper ventilation also is required to reduce airborne
contamination of the freshly blasted surfaces and to reduce and remove solvents
during coating operations. Ventilation is measured in terms of the volume of
air movement over time, expressed as cubic feet per minute (cfm). A general
guideline is to provide one complete air change every three minutes during
Dehumidification. Determining the dehumidification (DH)
requirements involves knowing the type of coatings that will be used.
Moisture-cured coatings such as inorganic zincs, moisture-cure urethanes and
some epoxies will require moisture to cure. This must be taken into consideration
when sizing the DH unit for a particular job.
Heating/cooling. One last item to consider is additional
heating requirements. On warm days with cool nights, the change in the surface
temperature may cause moisture to form unless you have a very low dew point. To
help overcome this problem, providing additional heat will help maintain the
surface temperature and dew point spread and also decrease the relative
humidity. Heat also may be needed to maximize proper cure of catalyzed
coatings. In very warm conditions, cooling may be used in addition to
dehumidification to maintain worker comfort.
Coating selection is critical when trying to minimize
out-of-service time whether dehumidification is used or not. For tank
interiors, there are several options.
Epoxy, the most widely used coating for the interiors of
water tanks, can be accelerated for faster recoating and cure. New high-solids,
rapid-cure formulas also are available for quick turnarounds.
Two-component, quick-setting, 100 percent solids
polyurethanes also have been used successfully. At 70° F, initial set is in
just three minutes and ultimate cure is only 48 hours.
Exterior systems, typically epoxies with polyurethane topcoat, also are
available in accelerated and quick-cure formulas. They provide faster recoating
and quicker moisture resistance. Another type of exterior system is a
moisture-cured polyurethane. These coatings can be applied at up to 100 percent
humidity without compromising their appearance or performance.
Whether your concern is maintaining adequate water pressure
for your water customers’ needs, the additional costs for temporary
piping and pumps, or even mandated limitations on out-of-service time, there
are coating systems and engineered systems that can make it possible for you to
properly maintain your water tank and minimize out-of-service time.