Oxidation reduction potential (ORP) measurement continues to be an essential tool in power plant water chemistry to monitor chemical dosing and impurities which can initiate corrosion. Advances in sensor technology make ORP measurement easier than ever before.
The significance of ORP measurement
Among the many water quality analytical measurements that can be conducted in power plants, ORP (redox) is one of the most important. As a measure of the degree to which a chemical species is capable of oxidizing or reducing another chemical species, it is indicative of corrosive conditions being present in water.
An ORP sensor, very similar to one for pH, reports the ORP value in millivolts (mV). A positive voltage means that the system is capable of attracting electrons, thus indicative of an oxidizing condition while a negative voltage means a reducing condition exists. As oxygen is a powerful oxidizing agent, water with a high dissolved oxygen concentration will show a positive ORP value.
Control hydrazine levels
In power plants and boiler systems, one practical use of ORP measurement is to monitor hydrazine and carbohydrazide in the steam-water cycle to minimize corrosion.
Hydrazine is a chemical compound that acts as a reducing agent in water systems. It prevents corrosion by reacting with oxygen to form nitrogen gas and water. Therefore, instead of oxidizing the steel of process tanks and pipelines in the steam cycle, the excess concentration of dissolved oxygen is consumed in the presence of hydrazine.
Similar to hydrazine, carbohydrazide reacts with dissolved oxygen to form nitrogen gas, water and carbon dioxide at room temperature.
Since both hydrazine and carbohydrazide consume oxygen, these chemicals are termed “oxygen scavengers”. Just like reducing agents, the presence of oxygen scavengers decreases the ORP of water.
Measurement and monitoring of hydrazine/carbohydrazine and dissolved oxygen in power plants is essential for efficient, cost-effective running.
Prevent overuse of expensive chemical agents
In make-up water treatment, ORP monitoring is critical on the filtered water to ensure that oxidizing chemicals (like chlorine) have been removed before it enters the reverse osmosis (RO) unit. The presence of oxidizing agents in an RO unit can destroy the osmosis membrane allowing dissolved salts to enter the system and lowering the quality of water.
ORP measurement after the condenser polisher and before the deaerator is important to monitor oxygen ingress. De-aerator lowers the concentration of dissolve oxygen before it enters the boiler, hence, its outlet should also have ORP sensors to check the efficiency of the process. More importantly, feedwater systems are usually equipped with ORP sensors to monitor chemical dosage of reducing agents like hydrazine and carbohydrazide to check if their dose is enough to reduce oxygen to an acceptable level.
Feeding excess amount of these chemicals is very costly, while insufficient use can result in corrosion. Aside from these applications, cooling towers must also be equipped with ORP sensors to control oxidizing biocides, sulfites and the build-up of algae that leads to severe plugging and damage to the systems they colonize.
Aside from monitoring oxidation and reduction activities in the steam-water cycle, ORP may also indicate leaks coming from measurement positions (e.g. fittings, casings, glands, etc.).
ORP sensors for low conductivity water
METTLER TOLEDO Thornton offers combined pH / ORP sensors designed specifically for water treatment. The inclusion of Intelligent Sensor Management (ISM®) technology provides Plug and Measure start-up, easier maintenance and advanced diagnostics for predictive maintenance. When connected to a METTLER TOLEDO transmitter such as the M800, both ORP and pH can be measured simultaneously from a single probe. This is useful in measurement points such as after the cooling tower where water treatment decisions need to be made.
The power industry is continually looking for ways to improve water chemistry control and monitoring. ORP measurement is seeing renewed interest because of its simplicity, ease of maintenance and advances in combination with pH measurement. These features assure its position as a vital tool for inhibiting corrosion.
METTLER TOLEDO Thornton’s pHure Sensor™ series provides very accurate pH measurements for ensuring the purity of power plant and microelectronics facility waters.
The standard pHure Sensor features a low-resistance pH glass membrane, shielded flow chamber, and a continuous outflow of reference electrolyte for stable measurement. The special, internally-pressurized gel electrolyte reference electrode produces similar results to a flowing junction reference but offers more convenient installation and maintenance.
The pHure Sensor LE uses free-flowing liquid electrolyte to provide the most accurate pH measurement available for pure waters. The accompanying stainless steel housing provides a controlled flow path with minimum volume to encourage corrosion particles to flush through.
pHure Sensors with Intelligent Sensor Management (ISM) maintain calibration and other data in internal memory, enabling convenient pre-calibration in a lab or workshop. In addition, their digital output signals are far more robust than analog signals.
Learn more about ISM software: https://www.mt.com/ism
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