July 2008

Process Automation

Boiler water for powerful plant

Practical approaches to measuring pH of condensate and feed water in the power industry

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  • pH monitors and controls the balance of corrosion control chemicals.
  • Plant chemistry, scaling, and corrosion control, and preventing unscheduled outages.
  • See practical, cost effective approaches to selection, installation, and maintenance of pH equipment.
 
By Dan Livermore, James Yarbrough, and John Connelly

Today's competitively run power plants have fewer individuals responsible for a greater array of tasks. Yet the requirements for consistent water quality remain.

While plant automation spreads, the analytical measurements remain, at times, islands unto themselves.

The reliability of the pH measurement in low ionic composition streams, such as pure water, depends largely upon the technician understanding the measurement and the factors that affect the process of obtaining an accurate measurement.

The demands and workload placed on the power plant technicians not only affect their understanding of the measurement, but the patience required to deal with associated problems of the application.

Understanding the basics of this measurement will provide the technician with confidence in dealing with any problems.

The process of measuring clean pure water under controlled conditions still remains a difficult problem. The initial cause of the problem is the water is just too clean.

Any validation with lab measurements only seems, at times, to compound the issue. We must address all factors contributing to the success and failure of this measurement from installation to temperature and flow effects of pure water.

The basics of measuring pH

What is pH?

The simplest definition is "it's the activity of the hydrogen ion in solution, which is an indication of the acidity or alkalinity of a solution." A solution is the dispersion of one or more chemical substances in another, usually water, so as to form a homogeneous mixture.

When water molecules dissociate they form equal amounts of hydrogen ion (H+) and hydroxyl ion. This is a process that happens naturally in water, and when there are equal amounts of hydrogen ion and hydroxyl ion, the water is a neutral pH or pH 7.

When chemicals add to water, they can and usually do influence the pH. Acids such as hydrochloric acid when added to water will increase the amount of hydrogen ion and activity and drive the pH down.

Conversely, bases like sodium hydroxide will increase the hydroxyl ion concentration and activity and drive the pH up. The pH of process water can be manipulated for desired effects by adding acids or bases to adjust the pH to the desired point.

In power industry applications, we control the pH of water to enhance the effectiveness of corrosion inhibitors.

The pH sensor

A basic pH sensor has two independent electrodes or half-cells. The measuring half-cell generates an electrical potential that reflects hydrogen ion activity. The other half-cell, the reference cell, completes the electrical circuit and acts as the ground lead in typical voltage measurement. Ideally, the reference half-cell remains stable, and the measured voltage between the two half cells is due solely to the pH of the sample.

The Nernst Equation relates the galvanic potential developed at an electrode with the activity of an ion, in this case the hydrogen ion.

E=E°+RT/nF•ln[H+]

Where:

E = the galvanic potential of the electrode in equilibrium with the solution

E° = the standard potential of the electrode at pH 7

R = the gas constant

T = temperature in degrees Kelvin

n = charge number of the ion

F = Faraday's constant

H+ = the hydrogen ion activity

The measuring half-cell is a silver/silver chloride system. The silver wire lead terminates with a silver chloride coating and dips into internal filling solution that contains constant hydrogen ion activity.

Specially formulated, pH sensitive glass surrounds the filling solution and electrode. The characteristics of the pH sensitive glass mem¬brane allow an electrical potential to be developed across the glass when the hydrogen ion activity on the inside surface is different from on the outside surface.

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The reference half-cell contains a silver/silver chloride electrode, similar to the measuring electrode, immersed in electrolyte and restrained-flow porous junctions. Typically, reference junctions are made of ceramic, wood, or perfluoro-polymer.

The double junction design creates greater stability of the chemical environment surrounding the reference electrode and thus greater stability of the potential on the electrode. The primary function of the reference electrode half-cell is to complete the measurement circuit while contributing little or no voltage to the measurement. Electrolyte formulations vary but usually contain potassium chloride in a liquid or gel matrix.

A temperature measurement element is a part of the sensor to compensate for the affects of
temperature. The Nernst equation contains temperature as a term, which alters the output of the electrode.

By measuring the temperature nearby the half-cells, the pH meter can then use the actual temperature of the electrode in the Nernst equation to accurately convert the measured potential to pH.

Temperature affects the response of the electrode to changes in pH. Most modern pH sensors and analyzers incorporate a form of automatic temperature compensation.

The actual design of the complete pH sensor varies with manufacturer and intended application for the pH sensor. There are three functional elements of the sensor: the measuring half-cell, the reference half-cell, and the temperature element.

Some manufacturers offer a sensor assembly with each element separately wired and installed in a housing. Other designs incorporate these functional elements sealed in a single, rugged, probe body.

The pH analyzer is a high impedance voltmeter, with the additional function of turning the voltage produced by the sensor into a visual display of pH with a usable process signal (4-20mA). Temperature may also display and transmit as a process variable in most analyzers.

Many modern pH analyzers offer features such as sensor diagnostics and digital communication protocols. Sensor diagnostics provide indication of the well-being of the sensor and the validity of the measurement. Digital communications allow all the diagnostic information to transmit along with the primary measurement.

Analyzers frequently include programmable relays, which can configure to indicate high or low measurements or one or more sensor diagnostic conditions.

High purity boiler water

Power generation requires a substantial investment in capital equipment, and facility managers attach great importance to proper maintenance of the powerhouse facilities. Unfortunately, high purity water is corrosive to boiler and condensate systems. In addition, leaks in the system can add unwanted chemicals that increase corrosion potential. To protect equipment and combat corrosion, trace level corrosion inhibiting chemicals and oxygen scavengers add in and are part of the boiler feed water. Effective chemical treatment is critical.

Poor boiler feed water quality can cause such problems as scaling, caustic embrittlement, pitting, and corrosion of the internal parts of the boiler and condensate systems. Very low levels of chemical additions are made to high purity boiler feed water to reduce the threat of corrosion. Monitoring the conductivity of the water provides indication of change in the chemical nature of the feed water. Conductivity alone does not indicate the amount or direction of a pH upset in water chemistry of the feed water. By on-line measurement of pH, in addition to conductivity, one can minimize corrosion problems.

Because of the nature of pure water and the design of most conventional pH probes, potential problems exist. pH sensors consist of a reference electrode and a glass measurement electrode. The measurement of pH depends upon the sensitivity of the glass electrode to the changes in pH. At the same time, the reference electrode must remain stable. The difference measured in mV between the two electrodes is proportional to the pH.

Since pure water is a low ionic strength solution, it makes the measurement of pH difficult for three reasons:

  1. The pure water causes the pH sensitive glass to become less sensitive.
  2. The reference junction develops interfering electrical potentials.
  3. The sensor surfaces develop static charges, producing a noisy signal.

Standard solution calibrate

All pH sensors should be calibrated when they are first installed and regularly throughout the service life of the sensor. Standard solutions of known pH value, called pH buffers, are available and widely used for calibration. The most widely used pH buffers have values of 4, 7, and 10 pH. One should use two buffers, 4 and 7 or 7 and 10, to perform a two-point calibration. Two-point calibration allows the pH meter to properly set the sensitivity for voltage to pH conversion. One successively dips the sensor into each of the two buffers, and after a short stabilization period, the analyzer is set to display the value of the chosen buffer. Older or dirty sensors require longer stabilization times.

Some published standard methods recommend two-point calibration only at the initial installation of the sensor, however most suppliers recommend occasional two-point calibrations. The benefit of the two-point calibration is it tests the sensitivity and speed of response of the sensor. As sensors age, they loose sensitivity and speed of response; the two-point calibration is a valuable indicator when it is time to replace an aging sensor.

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Grab-sample or single-point calibration is another type of calibration and can be effective as a quick and easy means to insure pH accuracy. In this case, a sample of process water goes to a laboratory for analysis by an independently calibrated pH meter. Then the process analyzer is set to agree with the grab sample value. The advantages of this method are speed and convenience. The process sensor does not need removal from the process mounting, and only a single point procedure is necessary on the process analyzer. The disadvantages of this method are the lack of information about the sensitivity and speed of response, and there is significant potential for calibration errors with the grab sample method. Precautions are important.

Another contributing factor is sample contamination. Grab samples are commonly used to validate the on-line analysis instruments. In the case of pH, the grab sample does not represent real time data. The technician must withdraw the sample, hopefully into a clean sample container, and transport it to the lab for analysis. If there is a change in temperature on the way to the lab, then the pH has also changed. If the sample is not covered, then it is susceptible to a pH change because of the presence of carbon dioxide in the air. This is because of the un-buffered nature of pure water. Once it reaches the lab, the lab pH system can be subject to error.

The solutions

The measurement electrode portion of the sensor should be of low pH impedance glass. The reference electrode should be a double junction design with self-pressurizing electrolyte. The reference surface area of the outer junction should be large and surround the electrode, with the solution grounding in close proximity to the measuring surface. This sensor construction helps eliminate the problems that would have arisen with conventional pH sensors. The recommendation for this application also includes the use of a mounting adaptor and low flow chamber for ease of installation. The flow through the cell should be less than 125 mL/min. The reason to control the flow of the sample is to keep the velocity low and constant, thereby minimizing fluctuations in voltage across the reference junction. This type of sensor is set for low maintenance and ease of use. The self-pressurized gel electrolyte eliminates the need in many applications for electrolyte reservoirs.

Sometimes, overcoming junction potential problems and associated recovery issues with a probe can only happen by using a flowing reference pH sensor. The system consists of a flow cell, a separate pH electrode, reference, temperature compensator, and a KCL electrolyte container, for refilling the reference. These systems are more costly than the flow cell approach and require maintaining a pressurized reservoir to replenish the fill lost through the junction in the sample. A high junction flow can also throw off measurements in beakers and contaminate a process.

The challenges of measuring pH in pure water are surmountable. Selecting a pH sensor specifically designed for low ionic strength solutions is vital. Equally important is the installation of the sensor and understanding the effects of calibration techniques, grounding, and flow rate.

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ABOUT THE AUTHORS

Dan Livermore (danlivermore@control-techinc.com)is the president of Control Technologies, Inc., a systems integrator headquartered in Illinois. James Yarbrough (James.yarbrough@ips.invensys.com) is analytical product consultant in the Measurement and Instrumentation division at Foxboro Analytical in Nashville, Tenn. John Connelly (John.connelly@ips.invensys.com) is pH, ORP, and ISE manager at the Measurement and Instrumentation division of Foxboro Analytical in Massachusetts.

Terms and concepts

pH: The negative log of the hydrogen ion-hydronium, H3O-concentration. A measure of the acidity or alkalinity of a solution

Ion is an atom or group of atoms, normally electrically neutral, that has lost or gained one or more electrons and thus now has a charge. The simplest ions are the proton (a hydrogen ion, H+, positive charge) and alpha particle, which is a helium ion, He2+, consisting of two protons and two neutrons

NaOH is sodium hydroxide, a strong base, highly alkaline, and highly corrosive.

Corrosion is deterioration of essential properties in a material due to reactions with its surroundings. In the most common use of the word, this means a loss of an electron of metals reacting with water or oxygen.

Boiler is a closed vessel in which water heats under pressure. The fluid then circulates out of the boiler for use in various processes or heating applications.

Electrolyte is a substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions.

ORP is oxidation-reduction potential that measures in millivolts and indicates the potential of the active sanitizer in the water to oxidize organic matter.

ISE: Ion selective electrode

 

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