The Monitor

Production Process Insights

Maximize pH Measurement with Direct and Calculated pH

Posted by AJ Percival on 5/10/21 8:00 AM


Effectively monitoring cycle chemistry in a power plant relies on analyzer data's availability, accuracy, and trustworthiness. If your plant isn't utilizing smart chemistry alarms, you may be missing out on easily accessible information.

A SWAS (Steam and Water Analysis System) conditions the process sample and supplies a representative amount of the process stream to the online analyzers. This transportation is done safely and without changing the parameters of interest.

However, working with these analyzers can be challenging due to: 

  • Sample temperature and flow issues
  • Sensor stability in ultra-pure water
  • Inaccurate measurements during startups

Manual calculations support accurate Analysis

To help ensure analyzers and measurements are accurate and trustworthy, operators can perform manual calculations that confirm analyzer readings.

Inferring pH from conductivity and cation conductivity

Inferred pH is a method of continuously calculating pH from specific (straight) and cation conductivity.

The correlation of pH and conductivity of ammonia has been used for many years to compare cycle chemistry measurements. For a given ammonia concentration in water, a pH and conductivity value is calculated from dissociation and conductance data.

Although pH sensors can be accurate even at minimum flows, these devices are fragile and need regular maintenance and recalibration. The calculated pH can be more precise and reliable than the high purity pH measured with a conventional glass electrode.

pH meters are calibrated against buffer solutions with higher conductivity than feedwater. However, modern instruments address this problem by using reference electrodes designed to suppress the ingress of sample waters into the electrode junction. With careful attention and regular recalibration, it is possible to reduce errors in pH measurement of feedwater and boiler water to less than 0.05.

Conductivity measurements are easy to make with inexpensive, rugged, and pre-calibrated sensors. If the cation conductivity is low, the pH of the condensate and feedwater can be estimated from the specific conductivity.

Calculating inferred pH from straight and cation conductivity involves four steps:

  1. Convert the straight and cation conductivity to values at 25°C.
  2. Convert cation conductivity to sodium chloride concentration.
  3. Subtract the conductivity of sodium and chloride ions from the straight conductivity. The result is the conductivity of the ammonia or sodium hydroxide solution.
  4. Convert conductivity to pH.

The pH of feedwater, boiler water and steam are temperature dependent. Errors arising from these effects are minimized if the sample temperature is adjusted close to the reference temperature.

Calculated carbon dioxide (CO2) and anions

Inferring CO2 concentration from cation conductivity and degassed cation conductivity is another manual method plant operators can use.

Carbon dioxide in steam and condensate can come from the decomposition of organic materials in makeup water treatment. The presence of carbon dioxide can also be an indication of condenser leakage or inadequate deaeration.

Degassing the sample is the best means of monitoring condensate. Degassing determines how much of the cation conductivity value is due to contamination and how much is due to CO2.

With appropriate instrumentation, operators can use those values and known CO2 conductivity data to derive the CO2 concentration online. With the CO2 removed, the remaining degassed cation conductivity can be interpreted as total non-volatile anions with readout as ppb chlorides or sulfates.

Plants with significant amounts of carbon dioxide (and therefore high cation conductivity values) need to distinguish bicarbonate from more corrosive anions.

Ion chromatography is the most accurate way to do this, but it's too costly and operator-intensive to be practical for most fossil fuel plants. Degassed cation conductivity can provide the information operators need at a lower cost.

Mitigate manual calculations with smart alarms

Smart chemistry alarms monitor the signals generated by analyzers to provide real-time intelligence about what's happening and what steps resolve the chemistry event or analyzer issue.

  • Reduce nuisance alarms
  • Indicate when critical instrumentation is reading incorrectly
  • Differentiate between an analyzer issue and a critical chemistry event
  • Identify chemistry events
  • Provide plant-specific troubleshooting guidance
  • Ensure analyzers are correct on startup

To ensure accurate, reliable results, our technicians can help verify the accuracy of each analyzer and perform calibration to meet factory specifications and certification requirements.

With your analyzers performing reliably, you can make the best decisions about the condition of your plant and conduct regulatory compliance reporting with confidence.

Contact us to learn more about how Sentry Equipment can support your plant operations.

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Topics: Power, Steam & Water

Written by AJ Percival

Picture of AJ Percival
A.J. Percival spent 12 years in the Sentry service group as a technician, training manager, and team manager before transitioning to the Regional Sales Manger role for the Western US in early 2020. He uses his field experience to guide customers to real world solutions for steam and water systems specific to their plants.