Validating Water Cycle pH Requirements Using Conductivity and Calculated pH

In thermal power plants, pH measurement and control in the water steam cycle are critical to minimizing corrosion. Cycle chemistry guidelines specifically target narrow pH ranges.

However, in high purity water and steam, there are not enough ions in the water to generate a pH response, causing the pH reading to be unstable. Fortunately, conductivity has a direct relationship with pH when the sample contains only pure water with the typical alkaline treatments, such as ammonia and amines. In fact, cycle chemistry in many plants relies primarily on the related specific conductivity values. However, correction for minor contaminants by measuring the cation conductivity of the sample is needed to provide accurate results. The specific and the cation conductivity values are used in an industry accepted calculation that provides the highest accurate conversion to pH. Under normal operating conditions, this calculation provides more accurate and reliable results than pH electrodes can provide, for two reasons:

1. Conductivity is linear, with concentration which provides much higher resolution than pH sensors all of which give nonlinear response to concentration.
2. Conductivity sensors can be directly calibrated with minimal introduction of error. 

All pH reference electrodes have a variable junction or diaphragm potential that can change between buffer calibration and process measurement, introducing some uncertainty to subsequent measurements.

This makes calculated pH based on conductivity significantly more accurate. However, with major contamination, such as when a condensate exchange polisher's resin is exhausted, and an acid leak occurs, the pH would drop below 7, which is outside the ideal range for using calculated pH and would generate errors. Thus, to avoid contamination, it is recommended that both traditional pH measurement, and calculated pH based on conductivity, be used for optimal pH control.

METTLER TOLEDO Thornton's M800 multi-parameter transmitter can provide traditional pH measurement, calculated pH, as well as specific and cation conductivity to ensure accurate and reliable pH control. The M800 even has a built-in alarm to alert you if the pH measurement deviates from the calculated pH measurement for extra security and peace of mind.

For more infomration about METTLER TOLEDO Process Analytics products, contact Alliance Technical Sales by calling 630-321-9646 or by visiting https://alliancets.com.