25 May 2005
Start-up of New Cooling Water Tower Systems
Initial Cleaning. A new cooling water tower system will contain dirt, oils,
greasy films, and rust located on the metal surfaces as a result of system fabrication. A
pre-operational alkaline cleaning process is required to remove the dirt, oil, grease, and
rust, and to prepare metal surfaces for initial corrosion control. When galvanized steel is
involved, special care must be taken so that the pH does not exceed 8.5 during the
cleaning and passivation process to avoid conditions that promote white rust. The
cleaning and passivating procedure for a new cooling tower system is usually part of a
new system construction project and, as such, is not in the direct control of the cooling
system operations personnel. Verification of the completion of these important steps is
obtained from the contracting officer involved with the project. A cleaning and
passivation procedure for cooling tower systems with galvanized steel is provided
below. The procedure is similar for systems that do not contain galvanized steel, except
that the pH restriction does not apply.
Cleaning and Passivation Procedures. In the startup of new cooling
tower systems containing complex metallurgies that include galvanized steel, follow the
cleaning and passivation procedures for cooling tower systems containing galvanized
steel, stainless steel, mild steel, or copper. These procedures should not be used for
cleaning and passivating aluminum. Conventional chemical cleaners and typical
heating, ventilation, and air conditioning (HVAC) water treatment programs use highly
alkaline, high-pH water, which can initiate white rust formation on galvanized surfaces
and copper tubes. Galvanized steel can simply be passivated by exposure to highly
aerated water and a phosphate or polysilicate treatment. This treatment forms a zinc
corrosion inhibition layer, which is a natural, dense, adherent, and protective corrosion
product film. High pH (above 8.5) produces unwanted white rust that is fragile and not
protective against corrosion. The following procedures are recommended.
Hydrostatic Testing. Perform hydrostatic testing:
a) Inspect and remove all debris.
b) Fill the system with treated water and circulate. This should include
orthophosphate or hexametaphosphate (as PO4) at 40 to 60 ppm, and
tolyltriazole (active) at 10 to 20 ppm.
Pre-operational Cleaning Procedure. If the system passes hydrostatic
testing, proceed immediately to this pre-operational cleaning and passivation procedure.
Do not allow untreated water to be added to the system.
a) Drain and flush any debris from hydrostatic testing.
b) Dose and circulate (for 24 hours) a sufficient amount of a cleaning formula
based on the volume of the system. A formulation is provided below.
c) Drain and flush the system, and clean all strainers.