August 22, 2017
A customer had a cooling tower system with pH control, a molybdate traced scale/corrosion inhibitor, and bleach-based biocide program. The operators contacted their water management specialist with concerns about why their tested molybdate levels were so low. Their results did not correspond with the amount of scale/corrosion inhibitor being feeding into the system.
What factors could affect the tested molybdate levels in a cooling tower system? Take a few moments to consider the system diagram below and think of what could cause this.
Cooling Tower Blowdown Process
Citric Acid Feed
Cooling Towers 279
Problems such as this can be very perplexing and can have several causes. Sometimes, you have to go the extra mile to get the answer. This is exactly what the Associate in charge of this account did.
The chemistry applied to the cooling tower system was:
Possible Causes of Low Molybdate Readings
Molybdate is nonvolatile and consumption to form a protective passivated layer on the metal surfaces should be minimal once the chemistry has been established. Molybdate was not being lost in the system.
Checking for Citric Acid Interference
The water management associate telephoned the water analysis manyfacturer to ask if citric acid interfered with molybdate testing. They were unsure but recommended a test be conducted to determine if it did.
A dilution of 3005NSS was prepared. This dilution was divided into several containers and the pH was adjusted to various levels using citric acid and sulfuric acid (as a control). The results were as follows:
Table 1 - Citric Acid Interference Determination
Acid Used Sample pH Molybdate (ppm)
Citric 6.7 0.7
Citric 6.3 0.3
Citric 4.9 0.0
Sulfuric 7.0 1.6
Sulfuric 6.2 1.5
As Table 1 shows, the citric acid was indeed an interference.
Through the detective work of the water management associate, citric acid was determined to be the interference with the low range molybdate testing.
The next steps required were to:
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