How to Select a pH Sensor for Harsh Process Environments

How to Select a pH Sensor for Harsh Process Environments

This guest post is authored by Vickie Olson, product marketing manager at Honeywell Process Solutions.

For today’s process plants, pH is an important parameter to measure in a host of demanding applications. A good example is flue gas desulfurization (FGD) control with systems using wet scrubbers in lime or limestone slurries. Proper pH can maximize sulfur oxide (SOx) removal and minimize the 160826412build-up of scale. Selection of the best sensor type will also enable longer life and more reliable control.

Choosing a pH sensor for use in harsh process environments requires careful consideration of numerous key factors. Extreme temperature at higher pH ranges reduces pH life due to consumption of hydrogen-sensitive ions on the glass membranes. An electrolyte (typically potassium chloride in gel or liquid form) tends to diffuse faster with temperature and high flow. In addition, abrasion removes the membrane surface over time.

Many process industry users specify a pH sensor of the combination style, meaning the device has a measuring electrode — either glass or ion-sensitive field effect transistor (ISFET) — or a reference electrode, which is usually based on silver/silver chloride. These pH sensors are considered rugged or robust compared to general-purpose sensors, since they are better able to withstand abrasive and alkaline conditions such as in FGD slurry. General-purpose pH sensors may not last a single day in sulfuric, abrasive and high-temperature environments. Depending on the style of ruggedized pH sensor and its maintenance frequency, this device can last from a few weeks to many months in operation.

Various types of reference protection on the market allow electrolyte diffusion, but reduce the infiltration of contaminants from the process fluid that can plug the junction or cause fouling of the reference. The porous junctions at the tip areas may be composed of double or multiple sections designed to slow contamination, which can result in poisoning of the silver reference material. Typically, the junction material in rugged pH sensors is composed of solid polytetrafluoroethylene (PTFE), ceramic or fibrous polyvinylidene fluoride (PVDF).

Several pH sensor designs employ wood or acrylic material containing electrolytes to slow the spread of contaminants while maintaining the required electrical connection of the reference with the measuring electrodes. An additional method to delay poisoning with solid reference designs is to locate the reference wire at the back of the sensor body, rather than hang it from the back, approaching the front of the device.

In terms of ruggedized glass measuring electrodes, the tip may have thicker glass and more hydrogen-sensitive material on the membrane. Flat glass is sometimes substituted for hemispherical or round glass on the tip to avoid breakage due to hard materials. This is less important in FGD applications, although the design is successfully used in the pulp & paper industry with heavy pulp slurry. In lime slurry, the flat tip does not have as large a measuring surface — leading to faster wear than rounded-type tips.

Most recently, ISFET measuring electrode technology was been paired with rugged reference technologies to provide a durable pH measurement solution. The advantages of ISFET include robustness, stability and precision. Sensors of this type utilize an integral automatic temperature compensator in one-piece construction, making them well suited for varying pH and temperature ranges.

The more demanding the application, the more critical it is to consider process operating conditions and expectations for a pH sensor. This is particularly important when harsh environments require frequent sensor replacement. Users employing the right device can realize savings from extended sensor service life, reduced replacement and maintenance costs, and ultimately, accurate and reliable pH measurement.

About the Author
Vickie OlsonVickie Olson is an analytical product specialist for Honeywell Process Solutions based in Atlanta, Ga. She has been involved in process instrumentation and analysis for industrial and municipal applications for more than 25 years as a chemist, product specialist and sales manager for Honeywell, Hach, and other companies. Vickie has spoken on a variety of topics related to water analysis and control at ISA and numerous other symposiums. She earned a bachelor’s degree in textile chemistry from the Georgia Institute of Technology and a master’s degree in business administration from Georgia State University.
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How to Determine the Health of a Glass Electrode

How to Determine the Health of a Glass Electrode

The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #86.

There is not a whole lot of current information on the response time of pH electrodes. Tests done in the 1960s for a clean, healthy pH glass electrode showed that the time constant was smaller for a positive pH change, a higher velocity, and a higher degree of buffering. For a buffered solution at 4 fps fluid velocity, the time constants were 0.75 and 1.5 sec for a positive and negative pH change, respectively. At 0.5 fps the time constants became 3 and 12 seconds for the same electrode and solution.Paper pH - 0041 - White

A case history published in 1990 showed that the time to reach 98 percent of the final response (the “response time”) deteriorated from 10 seconds to 7 minutes for a 1 mm slime coating. The time to reach 98 percent of the final response is four time constants plus the deadtime. The use of a 98 percent response time can be misleading and difficult because pH electrodes typically have a much slower approach to a final value above 90 percent than a first order classical response. Waiting to 98 percent means that the test time is greatly extended and the inferred time constant may be grossly overestimated. Also, a small amount of noise can lead to inconsistent results.

As far as the controller is concerned, what happens at the beginning of the response is the most important. I prefer to use an 86 percent response time for a faster test and a more accurate time constant that is less sensitive to noise. The 86 percent response time is used for control valve response testing in the ISA-75.25.01 standard for some of the same reasons.

Test results for a glass electrode prematurely aged by exposure to high temperature showed an increase in the 86 percent response time from 10 seconds to 50 minutes. Dehydration, abrasion, and chemical attack can also cause a large increase in response time. An increase in the sensor response time (lag time) slows down the response of the loop to upsets. In the study cited, a 1 mm of slime coating increased the amplitude of oscillations by a factor of 10 and the period doubled.

Aging of a glass electrode also shows up as a decrease in efficiency; that is, span. However, this effect is not as detrimental to loop performance. The loss in span causes a decrease in process gain but there are much larger influences on the process gain, such as the shape of the titration curve. For a setpoint at 7 pH (the zero millivolt point) there is no effect on operating point. The actual pH is 7 when the measurement is at setpoint. As the pH setpoint gets further away from 7, the effect becomes larger. However, standardization by the analysis of a grab sample will compensate for the difference between the actual pH and the setpoint, reducing the effect efficiency back to a process gain. Losses in efficiency are less problematic than offsets from the reference electrode (Tip #87).

Concept: A thin coating on the glass and/or aging of the glass will show up as a huge increase in the sensor time constant. (The 86 percent response time is the electrode deadtime plus two time constants.) A large sensor lag causes deterioration of loop dynamics. A measurement of the response time by making a setpoint change or by putting the electrode in different buffer solutions provides a sensitive indicator of the health of a glass electrode.

101 Tips for a Successful Automation CareerDetails: When buffers are used to calibrate an electrode, estimate the time to 86 percent of the final response. The electrode deadtime is usually negligible. Use the data historian and trend chart in the DCS to estimate the response. If you have a wireless gateway, use a wireless pH transmitter instead of a lab pH meter to get the calibration data into the historian. Errors of less than 10 seconds in the estimate are not important because glass electrode measurement problems show up as large changes in the measurement time constant. Use setpoint changes to measure the response time of a glass electrode. If you have multiple electrodes, the increase in electrode time constant will show up as a nearly constant time shift between the response curves. A decrease in electrode efficiency shows up as an increasing time shift. Remove and clean or rejuvenate, if necessary, the slowest responding electrode(s). Use a dilute 5 percent hydrochloric acid (HCl) solution to remove alkaline deposits and strip away the outer, aged layer of glass to rejuvenate an electrode. Use a dilute 1 percent sodium hydroxide (NaOH) solution to remove acidic deposits. Use a detergent solution to remove organic deposits (oil and grease). The household rules of cleaning solutions to remove stains may be applicable to cleaning electrodes. More tenacious deposits may require a solvent. Be careful to avoid solvent attack on the sensor’s o-rings and seals. Limit exposure time to prevent contamination of the reference junction and chemical attack on the glass by the cleaning solution. To minimize coating while in service, ensure that the fluid velocity past the electrode is greater than 5 fps and that the protective shroud provides exposure of the glass surface to the flow stream unless the fluid is abrasive. Use high temperature glass to prevent premature aging from exposure to temperatures above 40 °C.

Watch-outs: The equilibration of reference potentials may make finding the final response difficult, particularly as the measurement electrode response gets slower. The extremely long equilibration time of some solid-state references will make measurement of the 98 percent response time thoroughly inconsistent. The measurement may appear to drift or never reach the buffer solution pH despite repeated calibration adjustments for both offset and span. The percent of final response is often not given in response time statements. Sometimes people mistakenly use response time when they mean time constant. Because the number of time constants is dramatically different between 63 percent and 98 percent (one versus four time constants), the source should be questioned as to the percent of final response.

Exceptions: For loops subjected to frequent changes in setpoint that are not corrected by an upper level loop, a loss in efficiency (span) may become more important than an increase in sensor lag. Electrodes that have an extreme loss in efficiency are in danger of becoming dead electrodes and must be replaced even if they are fast-responding.

Insight: The sensor response time is a sensitive indicator of the effects of coatings and aging on glass pH electrodes.

Rule of Thumb: Use the 86 percent response time to determine when to clean, rejuvenate, and/or replace the glass electrode.

How Optimal Measurement Location Maximizes Sensor Sensitivity and Signal-To-Noise Ratio

How Optimal Measurement Location Maximizes Sensor Sensitivity and Signal-To-Noise Ratio

The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #67.

 

I became sensitized to the importance of measurement location when I found the easiest way to keep a pH electrode from fouling was to install it in a pipe with a flow velocity of 5 to 7 fps, preventing the usual 100X deterioration in the speed of response that resulted from just a few millimeters of coating. The higher velocity also made the electrode much faster-responding when clean. The conventional wisdom of putting an electrode into a vessel was proven wrong on several counts. The velocity in even the most highly agitated vessels is only 1 fps, resulting in a slow response and the need to remove the electrode more frequently. Furthermore, removing an electrode from a vessel in service is more problematic than removing it from a recirculation line that can be isolated.

I also found that pH electrodes installed too close to the outlet of a static mixer were too noisy. Moving the electrodes downstream 25 pipe diameters made a world of difference. The increase in loop deadtime was only 1.5 sec from the additional transportation delay (9 feet of 4-inch pipe at 6 fps). The decrease in noise allowed me to use a smaller filter time so that the actual total loop deadtime was less.

The same principle applies to thermowells, although the effect is less dramatic. Higher velocities decrease the fouling rate and decrease the measurement lag, the result of an increase in the heat transfer coefficient. (The annular clearance (air gap) between the sensor and the inside diameter of the thermowell has a bigger effect than velocity.) The thermowell should also be about 25 pipe diameters downstream of a heat exchanger to allow for mixing of the flows from the tubes.

Bubbles in liquid streams and droplets in gas streams cause noise when they hit a sensor. Bubbles from air, oxygen, and carbon dioxide spargers in bioreactors and chemical reactors can cause dissolved oxygen and pH signals to become noisy. Droplets of water at a desuperheater outlet cause a noisy temperature measurement. Ammonia bubbles at a static mixer outlet cause a noisy pH measurement.

The tip of an electrode or thermowell should be near the pipe centerline because the temperature and composition vary over the cross section of the pipe. For highly viscous fluids, the error is pronounced. I found that the temperature measurement in extruder outlets and the pH measurement in static mixer outlets with a sulfuric acid reagent are particularly sensitive to the depth of insertion of the sensor tip due to the effects of the high viscosity of polymers and of 98% sulfuric acid.

Differential head meters and vortex meters should be located where the velocity profile is uniform, the flow is turbulent, and there is a single phase – or wherever the piping designer tells you (just kidding).

Concept: The sensor location should provide sufficient residence time and mixing to ensure a single phase and a uniform mixture. The location should minimize the volume between the point of injection and the sensor to minimize delay. For differential head and vortex meters, a consistent velocity profile is required. Most importantly, the location must be sensitive to changes in both directions of the process.

Details: Maximize the detection of changes in the process from disturbances and setpoint changes. For composition, pH, and temperature choose the location that shows the largest change in both directions for a positive and negative change in the ratio of the manipulated flow to the feed flow, realizing that there are cross-sectional and longitudinal temperature and concentration profiles in pipes and equipment. For distillation columns, the best location for the thermowell is the tray with the largest change in temperature for an increase or decrease in the reflux to distillate ratio or steam to distillate ratio. A temperature or pH sensor and an analyzer sample tip should be near the center of a pipe and should extend well past equipment walls. A series of temperature sensors across a fluidized bed at several longitudinal distances is often necessary, with averaging and signal selection to get a representative measurement and prevent hot spots. The insertion length of a thermowell should be more than five times the diameter of the thermowell to minimize thermal conduction-induced errors from heat conduction along the thermowell wall between the tip and the process connection. To prevent vibration failure from wake frequencies in pipes, calculations should be run with the program supplied by the manufacturer on the allowable maximum length. A location with good mixing and a single phase will minimize fluctuations in measured temperature and concentration and the disruption caused by bubbles or solids in liquids and liquid droplets in gases hitting temperature or pH sensors or getting into sample lines for analyzers or into impulse lines for pressure and level measurements. Pressure probes in high-velocity gas streams and furnaces must be designed to minimize momentum and vacuum effects. Sensors and sample probe tips should not be installed on pump suctions and should be downstream of strainers. Minimize sensor deadtime and lag by reducing transportation delays and increasing velocities.

The transportation delay in a pipe or sample line is the volume divided by the flow rate or the distance divided by the velocity. The lag time of temperature and pH sensors decreases with velocity by an increase in the heat transfer and mass transfer coefficient, respectively. Fouling also decreases with velocity.

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Watch-outs: Material volumes behind baffles or near the surface or bottom of an agitated vessel or at the outlet of inline equipment may not be well-mixed. Packed and fluidized bed equipment may have uneven composition and temperature distribution from flow channeling. Programs for vibration analysis may only be looking at thermowell failure and will not predict RTD failure. The use of calcium hydroxide (lime) or magnesium hydroxide as a reagent may seem cost-effective until you consider the cost of poor control and solids going downstream.

Exceptions: The best location may not be accessible or maintainable due to height or obstructions.

Insight: The best measurement location maximizes the sensor sensitivity and maximizes the signal-to-noise ratio and minimizes deadtime.

Rule of Thumb: Find a location that is sensitive to changes in the process, where the fluid has a uniform mixture and a single phase, and where sensor lag and transportation delay are minimized.

Industrial Wireless Sensor Networks: Trends and Developments

Industrial Wireless Sensor Networks: Trends and Developments

This is an excerpt from the September/October 2012 InTech Web exclusive feature by Mareca Hatler. For the entire article, please see the link at the bottom of this post.

ON World’s 2012 survey shows continued growth and new opportunities for wireless sensors

Despite a challenging economy, the industrial Wireless Sensor Network (WSN) market has doubled over the past two years. A recently completed ON World survey of 216 industrial automation professionals, in collaboration with ISA, HART Communication Foundation (HCF), and the Wireless Industrial Networking Alliance (WINA), points to increasing WSN adoption and expanding markets.

When ON World started researching industrial wireless sensing 10 years ago, deployments of more than 20 nodes were rare. Today, network densities are increasing, and several sites have deployments of more than 3,000 nodes. What is responsible for much of this growth? The 2012 survey indicates this is a result of increased education, reliability of today’s WSN systems, maturing wireless mesh solutions, and a rapid migration to industry standards, such as WirelessHART and ISA100.11a.

In 2016, there will be 24 million wireless-enabled sensing points. At this time, 39% will be new applications, uniquely enabled by WSN.

Within the next five years, installed wireless industrial field devices will increase by 553% when there will be nearly 24 million wireless-enabled sensors and actuators, or sensing points, deployed worldwide. By 2016, 39% of deployed nodes will be used for new applications that are uniquely enabled by WSN technology. WSN is impacting industrial automation by disrupting wired automation, extending wired sensor networks, and driving new sensing and control solutions. …

To read Mareca Hatler’s full article, click here.

ISA Technical Report Presents Wireless User Requirements for Factory Automation

The ISA100 committee has completed a new technical report that presents user and market-related requirements for the design, operation and maintenance of wireless systems throughout their life cycles in factory automation. Factory automation applications involve specific technology, control and information systems that are characterized by discrete operations with possible extensions into batch process control. Examples include automotive manufacturing, packaging machinery, machining and robotics.

ISA-TR100.00.03-2011, “Wireless User Requirements for Factory Automation,” provides use case descriptions and comparisons, descriptions of factory automation topologies, and recommendations for attributes and values for existing, emerging and conceptual solutions for wireless communications as applied to factory automation applications.

The technical report was development by ISA100 Working Group 16 on Factory Automation, which has focused on wireless technology and system architectures for the link between sensors/actuators and automation systems. While wired links between conventional sensors and actuators can often be used, many repetitive motion applications such as machine tools and robotics make wire replacement a major source of work flow interruption and high maintenance cost—driving the desire to find ways to replace wiring with fast, low-cost wireless connections.

The ISA100 standards committee is developing a family of standards for wireless communications in industrial automation applications. The new technical report represents the extension of ISA100 to the factory automation sector, notes Cliff Whitehead of Rockwell Automation, who coordinated the collaborative development of the document. “We plan to further extend this reach with an industry forum on Wireless in Factory Automation in the March-April timeframe of 2012,” he stated.

For more information on the technical report, the ISA100 committee and the 2012 Industry Forum on Wireless in Factory Automation, contact Linda Wolffe of ISA Standards, lwolffe@isa.org.

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