In all kinds of water steam cycles, the correct pH measurement in power plants play a key factor in corrosion risk surveillance. A measurement with a glass pH electrode is possible but requires a pH probe designed for low conductivity, high temperatures and pressure. An accurate pH value can also be obtained by measuring the conductivity before and after a strong acid ion exchanger. These measurements are used to calculate the pH. This is a widely accepted methodology, which eliminates the need for high temperature and high pressure pH probes. This type of measurement, which is commonly used for feed water and boiler water, is very popular in Europe. It is also recommended by the VGB organization.
Power Plant Water Quality Requirements
In power plant processes, the pH of water used is critical. As water with a pH outside the recommended range can cause corrosion of equipment and infrastructure. Unexpected maintenance or downtime due to corrosion can be quite costly for power plant companies. Monitoring pH and conductivity of the water used in a power plant allows the control of these parameters, reducing maintenance requirements.
Measuring pH directly with a standard glass pH electrode presents several challenges. The most critical challenge is obtaining an accurate measurement of the pure and ultra-pure water used in power plant operations. Using a standard electrode in pure or ultra-pure conditions may result in unstable and incorrect readings. Additionally, operating conditions within a power plant may create disruptive electrical potentials, which skews the measurements. Ongoing measurements of pH in ultra-pure water also necessitates frequent electrode maintenance. Standard pH electrodes are fragile and require routine recalibration. A more accurate and reliable alternative is the pH calculation using differential conductivity measurements.
Dissolved Oxygen Measurement for Corrosion Protection
Several organizations, including VGB (Germany) and EPRI (USA), recommend an additional corrosion test based on dissolved oxygen concentration. Cleaner water typically has a higher dissolved oxygen content, which leads to better corrosion protection for steel pipes. If high water purity is not attainable, corrosion protection depends more heavily on the pH value. Therefore, a correct measurement of the pH value is extremely important.
pH Calculation by Differential Conductivity Measurement in Pure and Ultra-Pure Water
Setup for pH Calculation by Differential Electrical Conductivity (EC):
Two EC probes are necessary simultaneous measurement before and after a strong acid cation exchanger. The setup measures two different EC values. The probe before the cation exchanger measures the specific conductivity, while the probe after the cation exchanger measures the cation conductivity.
pH Calculation Equation
The VGB Standard VGB-S-006-00-2012-09-EN uses the following equation to calculate the pH of pure/ultrapure water in the range of pH 7.5 to pH 10.5:
pHB = log [CondSC – (CondCC/ 3)/ CB] + 11
- CondSC defines the specific conductivity
- CondCC defines the cation conductivity (or acid conductivity)
- CB is a factor which depends on the alkalizing reagent
The table below lists CB values for common alkalizing reagents:
Alternative for Ammonia Reagent Model:
Another equation can also be used for the Ammonia reagent model:
pH = log [CondSC – (CondCC/3)] + 8.6
The following specifications must be satisfied to successfully calculate a valid pH value:
- 7.5 < pH < 10.5 (NH3: 7 < pH < 10; NaOH: 7 < pH < 10.7)
- Phosphate concentration below 0.5 mg/L
- The reagent must be Ammonia or Sodium Hydroxide
- Below a pH value of 8, the contamination of the sample with other agents has to be very low compared to the concentration of the alkaline reagent
The conductivity reading of the first probe is converted to a 25°C reference temperature based on a user selected conversion model. For example, in a classic all-volatile treatment (AVT), the temperature conversion model for ammonia is set. There are other compensation models available. Typically, the conversion model used is based on the dominant chemical species.
The exchange resin should exchange positively charged ions for protons. Therefore, the alkalizing reagent is replaced with water, and the neutral salt, sodium chloride (NaCl), is converted to hydrochloric acid (HCl). Due to these reactions, the temperature compensation model for the second conductivity reading needs to be set to a strong acid.
A cross check with a glass pH electrode would not be suitable under these conditions due to the very low conductivity and associated risk of mismeasurements.
Advantages of Calculated pH Measurements
pH calculation based off differential conductivity measurement is a reliable and low maintenance alternative to the conventional pH measurement with glass electrodes. This technique is possible even in samples containing mixtures of alkaline reagents. Advantages include:
- The calculated pH values are more accurate without the typical restrictions of glass pH electrodes in samples with low conductivity
- EC sensors are stable over a long period of time with minimal maintenance compared to glass pH electrodes
- The algorithm yields reliable results
- Higher sensitivity due to linear relationship between concentration and conductivity (compared to logarithmic relationship between concentration and pH)
Sensorex Products for Conductivity Monitoring in Ultra-Pure Water
Conductivity measurements for the differential pH calculation can be collected with stainless steel conductivity sensors with cell constant k=0.1. These sensors operate well in typical power plant monitoring conditions of temperature, pressure, and conductivity.