For oxidising biocide feed monitoring and management, it is crucial to accurately measure ORP (Oxidation-Reduction Potential), and comprehend its limitations in order to reduce the risk of increased corrosion rates, excessive chemical feeds, and bio-fouling.
Let’s understand the concept of ORP and how it is used to treat cooling water.
What is ORP or Oxidation-reduction potential?
An indicator of a system's capacity to oxidise or decrease a material in water is its ORP value. In layman's terms, water in an oxidising environment has an ORP greater than 0 mV, whereas water in a reducing environment has an ORP lower than 0 mV. An assessment of the residuals and activity of an oxidising biocide can be made using ORP.
Maintaining proper ORP Values
It's not as easy as aiming for a specific ORP value or increase over the background operating levels, to use ORP to limit oxidizer additions. The goal range for oxidising biocide feeds should be the equivalent ORP level when the free, or total chlorine readings are in the desired range. Whatever the ORP value or growth in ORP, as long as the appropriate chlorine residuals are maintained, it doesn't matter.
How are ORP values used to treat cooling water?
Firstly, it can be difficult to monitor and manage oxidising biocide additions based on ORP values. Chlorine is an oxidising agent that increases ORP, whereas sulphite is a reducing agent that decreases ORP. The readings will also be impacted by a number of additional factors. For instance, ORP and temperature have an inverse connection, meaning that the greater the temperature, the lower the ORP.
Secondly, a reduced ORP response to oxidizer feeding is observed at higher pH levels. These considerations lead to drastically varied oxidizer residuals and ORP values, when the same dosage of an oxidising biocide is given to two distinct water systems. Particularly crucial in the treatment of cooling water, is having a fundamental understanding of the link between pH and ORP.
Thirdly, due to a higher concentration of the less active hypochlorite/hypobromite to hypochlorous/hypo bromic acid, the ORP response decreases as pH increases. Remember that only the ORP reaction varies with pH; the free and total chlorine residuals do not.
The ORP set-point necessary to maintain a specific oxidizer residual also varies, when the pH in a system changes as a result of changes in makeup water quality, inconsistent cycles, prolonged pre-bleed, bleed lockout times, or high pH chemical additions. The oxidizer residual associated with a given ORP set-point can even change within the same system.
Lastly, additional oxidizer demand caused by stabiliser in some oxidising biocides, excess ammonia in makeup water treated with chloramine, and the presence of organics in a dirty tower, can also considerably lower the ORP reaction in response to a given free/total chlorine value.
It is advised that you regularly check the ratio of free to total chlorine readings, during oxidizer feeds as a sign of potential problems caused by high demand. For successful microbiological management, the system may need to be cleaned, flushed, or disinfected if there is a significant difference, in the values. Where makeup water treated with chloramine is utilized, you can also discover that high oxidizer dosages are necessary to achieve a free chlorine residual, and that it is difficult to achieve.
For ORP controls to be effective, they must be correlated with the target free and total halogen residuals, on each individual system. As long as you are aware that residuals, not ORP readings, are what matter, using ORP to manage halogen input can be helpful in preserving target residuals, as well as treating water.
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