Water covers more than 70 percent of the earth’s surface and has varying degrees of purity in its natural form, ranging from crystal clear pure mountain spring water to highly saturated brine sea water. In the power generation industry, ultrapure water is used as a source to make steam to drive turbines and for other uses. Ultrapure water does not cause corrosion or lead to stress cracking in equipment such as turbine blades, stainless-steel lines, steam circuits, and cooling systems. Power companies use a great deal of ultrapure water, upward of 500,000 gallons per day for large plants. The ultrapure water can be processed from city water, a nearby river, or even seawater.
In most cases, the systems for producing ultrapure water are supplied by specialists in electrodionization, membrane, reverse osmosis, and other techniques for purifying water, and all require pH monitoring. Not only is pH monitoring required when the water is purified, it is also required as the water is used to maintain correct pH.
The bottom line in power plants is that improperly conditioned water—based on a number of parameters, of which the main two are conductivity and pH—leads to corrosion and scale, which leads to inefficient operation and damage to vital parts.
In the boiler, deposits cause heat transfer problems, reducing steam production capability. Corrosion from these deposits weakens the metal, leading to tube leaks that negatively impact the production of steam. Large boilers, condensers, economizers, and superheater tube leaks can cause adjacent tubes to fail and are actually the largest cause of forced boiler shutdowns.
The steam that passes through the turbine can cause deposits and corrosion from minerals, organics, and detergents that are present in plant water sources. Deposits on the turbines can cause pressure drops and unbalance the turbine, reducing speed and generation capacity.
Unfortunately, conductivity measurement by itself does not provide enough water quality information for ultrapure water chemistry; therefore, pH must also be incorporated. Unless controlled, the effects of improper water treatment parameters will cause boiler tube failures and loss of efficiency due to coating of the tubes. This makes energy costs and ultimately operational costs higher. Boiler manufacturers have tight specifications on the minimum and maximum water quality parameters, pH being one of them.
Water in its pure state is one of the most aggressive solvents. Known as the “universal solvent,” water, to one degree or another, will dissolve virtually everything to which it is exposed. Because pure water has a deficiency of ions, it looks for equilibrium with the ions it comes in contact with, and so it wants to strip these ions away from its host.
For the purposes of this article, pure water is defined as having a conductivity between 0.055 to 10 microSiemens per centimeter (µS/cm), or 18.2 to 0.1 megohms-cm. Common manufacturer specifications for pH sensors can indicate a conductivity range of 10 µS/cm or greater.
Herein lies the first hurdle to best measurement practices: to find pH sensors that are specifically designed to measure water with conductivity less than 10 µS/cm. Fortunately, some pH sensors are able to measure down to 0.1 µS/cm, but these are specialized instruments and must be specified, installed, and maintained accordingly.
There is a deficiency of ions in pure water, and pH sensors have the reputation of being noisy when measuring pH in these low ionic strength solutions. In simple terms, the signal is noisy because the sensor is looking for ions to capture and measure and has a hard time finding them, causing the measured value to meander up and down the pH scale.
Using two or more brand new pH sensors from the same manufacturer—even right after being freshly calibrated in 7 and 4 pH buffers—the sensors may show differing values due to static charges and reference junction potential errors. Pure water is a poor conductor of electricity, and so static charges are an issue as water flows through the piping systems, requiring extra care in proper grounding for signal stability and noise rejection.
Also, extraneous electromagnetic interference (EMI) and radio frequency interference (RFI) can disturb the sensor’s electrical circuitry, especially in a power plant with high-voltage equipment. Walkie-talkie transmissions and electric motors or valves being cycled on and off can also create electrical noise. These interferences cause signal spikes that push the pH signal high or low for brief moments or freeze the signal in place.
The pH sensors use a two-electrode scheme as the measurement apparatus—an active or measuring electrode and a reference electrode. The active electrode can have an input impedance of 100 megohms in high ionic strength solutions such as a pH 7 buffer. So in the best of circumstances, pH measurement has at least a 100 megohm obstacle to overcome. If that same impedance is added to the very low ionic strength of ultrapure water, it adds measurement complexity. There is now a larger resistance for the signal to traverse through the low ionic solution.
The reference junction serves as the return or ground path for the pH measurement. Any shifting of the electrical resistance in the reference path will change the overall resistance of the measurement and cause a shift in pH reading. This equates to a noisy signal. A charge buildup at the reference junction can change as the process changes (e.g., when valves or pumps are cycled) or remain at a constant state and attenuate the pH signal.
If any air is introduced into the piping system of the pH sensor, it will add CO2 into the solution, which acidifies the actual pH value. Therefore, closed loop systems are needed for a constant and uniform measurement.
Consider the practice of taking a grab sample from a closed loop system for pH analysis. When one walks the sample back to the chemistry lab for analysis, what happens to the sample as it is exposed to the atmosphere? It likely changes, sometimes substantially, leading to a preference for in situ sensors.
Changes in the flow rate past a sensor can also lead to changes in the pH measurement. These changes are referred to as streaming current potentials. Changes in process flows cause changes in the reference junction potential and lessen the ability of the glass electrode to maintain its hydrated outer gel layer.
Problems can occur in pH sensor cable connections, terminal strips, and plugs. Connectors can become loose or corroded or have moisture accumulate across the connections. These situations change the resistance of the pH measurement and degrade the signal.
Long runs of cables without the aid of preamplification or signal conversion from analog to digital can lead to changes in the capacitance and resistance of the cable, which can affect the pH readings. Signal cables are also a means by which EMI and RFI can gain access to the transmitter circuitry, also causing measurement errors. Here are best practices for installing and maintaining pH sensors:
- Make the pH measurement in a sealed piping system
- Maintain a slow continuous flow rate past the pH sensor, about 100 mL/min
- Use conductive piping and fittings; 316 SS is common practice
- Keep cable runs as short as possible
- Maintain tight, dry, and corrosion-free electrical sensor connections
- Store unused pH sensors in a solution to maintain hydration: 4 or 7 pH buffer
- Use digital pH sensors instead of analog
Fred Kohlmann is Midwest business manager for analytical products with Endress+Hauser. Since 1976, he has been involved in engineering, design service, marketing, and sales of online analytical water quality and process control instrumentation.
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