Optimizing ORP measurements for reliable chlorine control in cooling tower applications is essential for maintaining water quality, preventing corrosion, and ensuring work health and safety.
Introduction
The proliferation of microorganisms and the formation of slime are common issues in aqueous systems. Problematic slime-producing microbes include bacteria, fungi, and algae. Slime deposits occur in many municipal and industrial applications ranging from water and wastewater treatment, to refining and power generation.
Cooling tower directs drive cooling in chemical plant.
Microbial growth poses serious problems for water systems across almost all industries. These systems include once-through or recirculating cooling systems, holding ponds, intake pipes, ballast water tanks, and reservoirs that all draw from potentially contaminated water sources. Microbes can cause fouling in piping and heat exchangers, reducing efficiency and accelerating corrosion on metal surfaces. They also contribute to increased downtime, deterioration of cooling tower components, and potential health risks from airborne bacteria such as Legionella.
To combat these issues, companies invest in system maintenance, cleanings, and repairs while addressing worker health and safety concerns. To control biological growth, various biocides and antimicrobials have been developed.
Chlorine is widely used as an oxidizing biocide in industrial cooling towers. Monitoring oxidation-reduction potential (ORP) is essential for assessing water sanitization strength, as ORP millivolt readings correlate with disinfectant concentration and effectiveness.
Challenge
For chlorine (Cl₂) to function effectively, it must exist in the oxidizing forms of hypochlorous acid (HOCl) or hypochlorite ion (OCl⁻). The balance between these species depends on pH and can be monitored using ORP electrodes.
ORP measures a solution's oxidizing or reducing strength. In redox reactions, a reductant donates electrons to an oxidant, leading to oxidation (electron loss) and reduction (electron gain). These reactions always occur together, with electron transfer continuing until equilibrium is reached.
Below a pH of 1.9, chlorine exists as a diatomic molecule (Cl2) in water. As the pH increases above 1.9, chlorine oxidizes-water to produce HOCl, which further dissociates into OCl above a pH of 7.3. Elemental chlorine is not as effective in killing organisms as HOCl and OCl-. Thus, for optimum effectiveness it is desirable to control the pH of the cooling tower between 7-8 pH. It is also necessary to control the quantity of chlorine in the tower to insure there is enough to control the biological growth, but not so much that it causes corrosion of equipment or creates an excessive load on the water treatment facility. An ORP measurement system can be used to indicate both the quantity and activity of the chlorine in cooling water.
Measurement Solution
Since addition of chlorine increases the oxidizing capability of water, measurement of the ORP provides a useful indicator of the quantity and effectiveness of the chlorine present in the water. However, pH levels affect the oxidizing potential of the available chlorine so the ORP will vary with pH changes as well as changes in the chlorine levels. If the total amount of chlorine in the system remains constant, but the pH changes, there will be a corresponding change in the measured ORP reading. To provide a better indication of chlorine level, the ORP measurement must be adjusted for variations in pH.
To solve chlorine measurement challenges, Yokogawa provides the SM21-AG4 Industrial pH electrode to compensate for changes in pH levels. When combined with the Yokogawa FLXA402 or FLXA202 Liquid Analyzer and an SM21-PT9 Platinum ORP sensor, a complete chlorine measurement solution is created. This solution is extremely effective over a narrow range pH 6.5-8.0 but should only be used in simple systems that operate at stable temperatures (The measurement is not compensated for temperature changes). This complete analytical solution from Yokogawa allows better control of chlorine levels, improving chemical usage and reducing corrosion, while maintaining health and safety.
Product Recommendations
- FLXA402 four-wire or FLXA202 two-wire Analyzer
- SM21-PT9 Platinum ORP sensor with FF20-P33 Polypropylene Flow Fitting
- SM21-AG4 pH glass sensor (used as reference)
Industries
-
Chemical
Chemical plants rely on continuous and batch production processes, each posing different requirements for a control system. A continuous process calls for a robust and stable control system that will not fail and cause the shutdown of a production line, whereas the emphasis with a batch process is on having a control system that allows great flexibility in making adjustments to formulas, procedures, and the like. Both kinds of systems need to be managed in available quality history of product, and to be able to execute non-routine operations. With its extensive product portfolio, experienced systems engineers, and global sales and service network, Yokogawa has a solution for every plant process.
-
Industrial Water
Industrial water is essential for production and process water, manufacturing facilities, feed water for boilers and cooling towers, food & beverage processes, and other treatment needs.
-
Water & Wastewater
All water, regardless of its source, has value. However, with water resources being finite, a focus on "One Water" is necessary to maximize every drop within the water system, ensuring a stable supply of clean and safe water. As we consider current and future needs, planning and implementation are critical for long-term resilience and reliability, meeting both community and ecosystem demands. Yokogawa believes water is a human right for all. Thus, contributing to a sustainable water cycle is one of our Sustainable Development Goals (SDGs).
Related Products & Solutions
-
Flow/NPT Fittings FF20/FS20
FF20/FS20 Fittings for pH and ORP
-
Multi Channel 4-Wire Analyzer FLXA402
Available in single or multi-sensor measurement
-
Pure Water pH and ORP Sensors
Achieving accurate and reliable readings using a traditional pH analyzer is challenging, however with the right equipment stable and accurate pure water pH measurements can be accomplished. Yokogawa's specialized bellomatic pH sensor is proven best solution for high purity water applications. For those individuals that do not like the maintenance of refilling sensors, then the FU24 which incorporates the successful patented bellow system in an All-in-one body is the ideal solution.
-
Liquid Analyzers
Increase process optimization and control
-
pH and ORP Analyzers
Optimize field maintenance, calibration, and system configuration
-
pH and ORP Sensors
Ensure fluid process operations
Have Questions?
Contact a Yokogawa Expert to learn how we can help you solve your challenges.