Metal process

Solve problems with pH sensors in high purity water treatment operations

Process industries depend on water quality monitoring and control to protect their equipment from excessive wear and tear in high pressure and high temperature environments. The necessary exposure to moisture will eventually break down critical assets like boilers and steam turbines. To extend the life of this expensive equipment, water quality is carefully regulated and tightly monitored. If the pH of the water is too low in these systems, corrosion occurs. If the pH is too high, scale becomes a problem. Operators aim to keep their water at a neutral pH to avoid problems, reduce the need for maintenance and save money on equipment replacement in the long run.

To maintain high purity water with a neutral pH, operators use pH sensors to let them know when water treatment is needed and ensure the right amount of chemicals are applied. Unfortunately, ultra-pure water is particularly difficult for pH sensors to measure. These difficulties dramatically shorten the life of each pH sensor, increase maintenance and replacement costs, increase the likelihood of unplanned downtime, and increase safety risks.

Fortunately, there is a low flow solution that solves these pH sensor problems in high purity water applications. By controlling the flow through the sensor inside the device, operators can mitigate problems that lead to decreased sensor life. When combined with an advanced pH sensor, this robust system is extremely reliable in a wide range of aqueous solutions and process applications.

Critical applications depend on high purity water

While many manufacturing industries could benefit from using high purity, low conductivity water in their systems, the energy industry is the largest employer of ultrapure water. Power plants are particularly concerned with reducing unplanned downtime, as unplanned outages can result in significant regulatory fines. Their facilities must use the cleanest systems possible to keep power grids operational at all times. The initial capital investment in these high purity systems is helpful in proactively mitigating corrosion and scaling as well as system failures, downtime and environmental issues.

Electrical applications depend on boilers and high pressure turbines which require slightly alkaline water with a pH between 7 and 8.5. Maintaining this pH level causes a protective oxide film to form on the surfaces of the boiler tubes, which helps keep corrosion at bay on the base metal and allows film breaks to heal effectively . Operators control pH by carefully administering sodium hydroxide and sodium phosphate salts to feed. If they accidentally add too much or too little, corrosion or scaling will develop quickly; therefore, it is vital that operators continuously monitor the pH of their ultrapure water.

Most pH instruments cannot tolerate the high temperatures and pressures of industrial boilers. Instead of being placed inside these harsh environments, these devices should be installed in a side-flow sample and the sample cooler placed upstream of the pressure-reducing valve to prevent flashing. General purpose pH sensors work well in low pressure, high conductivity boilers, but struggle and fail in high purity, low conductivity applications.

Why High Purity Water Shortens pH Sensor Life

pH is a measure of the concentration of hydrogen ions in a fluid. Typically, pH is measured by potentiometric glass pH sensors. They work by producing an electrical voltage proportional to the concentration of hydrogen ions in the fluid. The device has two main components: a pH sensitive electrode and a reference electrode. The reference electrode is immersed in an electrolyte solution which maintains a constant pH. The glass pH electrode is exposed to the solution under test. The electrical voltage across the glass bulb changes in response to the hydrogen ion concentration of the process fluid in which it is immersed. If the hydrogen ion concentration is higher outside the bulb, the voltage is positive, indicating an acidic process fluid. If the concentration is lower outside the bulb than inside, the voltage becomes negative, indicating that the process fluid is basic. A balance of hydrogen ions on either side of the glass suggests a neutral pH of seven. The pH of the process fluid is then calculated from the voltage difference between the reference electrode and the pH sensitive electrode.

A reference junction is the electrical connection between the reference electrolyte solution and the solution under test. Over time, the electrolyte suspension is depleted as ions flow from the reference junction into the process fluid to complete the electrical circuit with the pH sensitive electrode. In high purity water, these ions flow much faster through the process fluid, draining the reference electrolyte solution and disrupting the electrical connection between the reference electrode and the pH sensitive electrode. When this happens, either the electrolyte solution must be replenished or the pH sensor must be completely replaced.

Full release of the reference solution can occur in just a few months, making maintenance incredibly difficult and expensive at each measurement point. Another challenge arises when a pH sensor is calibrated in the low conductivity environment of high purity water. The electrical potential develops in the buffers of the reference junction and is part of the calibration of the pH sensor. If the electrical potential of the junction is different from that of the sample, this introduces an error in the calculation of the pH. The error is minor when the process fluid has a higher conductivity, typically greater than 20 microsiemens per centimeter (μS/cm). However, ultrapure water contains very few ionic particles to make it conductive. This lower conductivity potentially introduces an error as high as 0.5 pH. This is an unacceptably high margin of error for critical applications such as high purity water used to maintain critical assets in power plants.

A simple, low-flow solution to the conductivity problem

In the past, pH measurement and monitoring of high purity water applications involved an additional device designed to continuously replace the electrolyte solution in the pH sensor. This reservoir would attach to the boiler above the pH sensor and feed the solution into the instrument. Unfortunately, this system does not really eliminate the disadvantages of additional maintenance and cost. It simply moves it to managing additional consumables, ordering and storing the electrolyte reservoir, and gives operators yet another thing to track and maintain.

A high performance general purpose pH sensor that can handle high purity water provides a different solution. By pairing a low-flow controller with an advanced pH sensor, operators can save time and money and reduce water consumption by more than two-thirds compared to comparable side-flow solutions. The system works by creating a separate flow of process fluid directly through the pH sensor, thereby circumventing the problem of rapid depletion of the electrolyte solution. The flow rate is kept constant at less than 3 gph. Its water level is supported by an interior drain that maintains a constant water column, head pressure and flow rate. The device materials are transparent, so technicians can quickly and easily verify the continuous flow of the process sample through the flow path. This approach works in an easy-to-use system that eliminates the need to manage additional consumables.

Emerson’s Rosemount 3900 general-purpose pH/ORP sensor, combined with a low-flow controller, for example, is able to respond to pH changes at a minimum conductivity of 0.1 μS/cm. Plug-and-play features make installation and maintenance easy. This technology also minimizes drift to provide an isolated and stable pH measurement. Sensor life with this advanced system is four times longer and has allowed operators to spend 60% less time in the field.

The future of pH measurement in high purity applications

Compared to high cost specialty pH sensors for high purity water applications, the combination of a low flow controller and general purpose pH sensor saves operators a lot of money in terms of initial purchase and ongoing costs of ownership. Using general purpose pH sensors also means that the devices can be repurposed for other applications such as cooling water and wastewater. It is no longer necessary to purchase separate specialized devices for boiler chemical control programs.

Adding a flow control element to pH sensors provides a better way to measure high purity boiler water for power plants and process industries as a whole. The technology facilitates the introduction of high purity systems into manufacturing processes to help protect critical assets, reduce maintenance and lower replacement costs. Upgrading a general purpose pH sensor in this way allows for more accurate and precise measurements in ultra-pure environments and more effective monitoring and control of water quality.

Jacalyn Saint Rudolph is global product manager for Emerson’s liquids analysis business and supports the company’s industrial portfolio with a focus on high purity water applications and the development and commercialization of novel liquid sensors for the biopharmaceutical manufacturing industry. Saint Rudolph joined the company in 2013 and holds a bachelor’s degree in mechanical engineering from the University of St. Thomas.