Emissions, Nuclear

Tracers Track Down Water Problems

Issue 9 and Volume 103.

Power plant cooling water control systems are advancing from simplistic manual adjustment methods to sophisticated automated tracer-based monitoring networks.

Given today’s economic and regulatory climate, many factors are causing power plants to re-think cooling water chemistry control strategies in order to remain competitive. In the past, heavy metal treatments (such as zinc/chromate) were effective and economical for controlling corrosion, and the need for scale inhibitors or dispersants was minimal due to pH suppression of the cooling water. But today’s environmental standards limit the treatment choices and place tough restrictions on how they can be used. As a result, power plants have moved into new cooling water treatment strategies. The effectiveness and economics of these types of programs are dependent on precise control of cooling water chemistries to effectively control corrosion and scale in an environmentally acceptable manner. Underfeed of treatment chemicals can cause corrosion and scaling, while overfeed can result in fouling of heat exchangers and adversely affect treatment economics.

Stairstep


Figure 1
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Many levels of automation-from simple to complex systems-are currently employed by power plants to monitor and control cooling water chemistry and its associated costs. These levels can be visualized in the “stairstep” analogy pictured in Figure 1. Each step up represents an advance in sophistication of the measurement and control opportunities, and commensurate increases in operational efficiencies and cost and performance improvements while maintaining cooling system integrity. Advanced diagnostic programs, as denoted in Figure 1 and described below, can be used to move up the stairs.

Step 1: Little or no automation

Based on manual chemical testing, chemical inhibitors are either slug fed or intermittently pumped into cooling water. While this directly involves the operator in the process, it does not provide for any level of automation in the system and often results in wide variations in treatment dosages and water chemistry, which adversely affect cost and performance.

Step 2: Indirect measurement and automation

Treatment chemical is added based on assumptions, indirect relationships or calculations. The actual treatment chemical concentration in the system is not measured by the control equipment.

Diagnostics of the System: The diagnostic capabilities of inert tracers can be used to characterize operating conditions such as volume, holding time index, blowdown rates, system flow patterns and heat exchanger performance. This information can help the user advance to inert tracer-based dosage systems, which directly and continuously measure treatment dosages.

Step 3: Inert-based tracer systems 1

Automation systems based on inert tracers (see Fluorescent Tracing sidebar) allow for superior control of cooling water treatment dosages, cost and performance improvements, and enhanced operator troubleshooting capabilities.

Diagnostics of Chemistry: In addition to measuring inert tracer levels, fluorescent techniques can be used to directly measure levels of active chemical components commonly found in cooling water systems. This information can be used to measure how actives are consumed as a result of various stresses occurring in the system (see System Stresses sidebar).

Step 4: Tracer and actives-based feed and control 2-7

This step is based on direct measurement of chemical actives concentration, in combination with the use of an inert tracer. In these systems, special detection strategies are designed that allow compounds which are not naturally fluorescent to be detected by fluorometric analyzers. Control is still maintained by controlling the inert concentration. However, the operator benefits from also knowing on a continuous basis the active treatment chemical concentration.

Diagnostics of Performance: Relates the consumption of treatment actives to system performance. System performance can be corroborated by fill fouling, unit throughput, corrosion coupons, corrosion monitor, fouling monitor, heat-exchanger results, etc.

Step 5: Consumption-based feed and control using tracer technology2-7

An additional level of treatment program control and system optimization can occur by measuring inert tracers, treatment actives and consumption of treatment actives. This provides a “chemical balance sheet” based on treatment added to the system to enhance performance. Proper control of this chemical balance sheet provides unprecedented opportunities for treatment dosage, control, and cost and performance optimization.

An important benefit of fluorescent tracer technology is the minimal capital investment necessary to put such a system in place. The primary “hardware” is simply a fluorometer and its associated microprocessor and control circuitry, both of which (due to recent advances) are contained in a 6-inch by 9-inch enclosure. The unit is highly reliable and contains no moving parts, yet can measure tracer concentrations as low as parts per trillion. The fluorometer can mount virtually anywhere in the plant where it can be easily plumbed into the cooling water system. The control unit is designed for easy connection and configuration with automated feed pump controls and other plant data and control systems. The inert fluorescent tracer is pre-mixed with the treatment chemicals, so no special handling or mixing is necessary at plant level.

Commonwealth Corrosion

Commonwealth Edison’s Byron nuclear power station provides an example of how a utility advanced from Step 2 (indirect measurement and automation) to Step 3 (inert tracer-based feed and control of treatment dosage).5 The Byron station consists of two pressurized water reactors, cooled by means of two natural draft hyperbolic cooling towers. The makeup water source is the nearby Rock River, which is highly variable in nature. Byron’s recirculating cooling water system is comprised of two interrelated loops: main condenser (MC) and non-essential service water (NESW). The essential service water (ESW) system is separate.

Corrosion problems in the ESW system began to receive greater attention one year after Unit 1 became operational. Average corrosion rates of mild steel and copper alloys were much higher than desired. Chemical treatment in the early years of operation focused on scale inhibition and dispersancy only. No corrosion inhibitor was used in either the NESW or ESW. However, early warning signs indicated a developing problem.

Commonwealth Edison embarked on a three-phase program to reduce corrosion effects in the ESW system. While the ESW system was being successfully optimized, similar corrosion problems were becoming apparent in the MC and NESW systems. The operating characteristics of the NESW and MC systems, however, made it very difficult to manually control water treatment chemical dosage. Manual control could only maintain target treatment chemical concentrations 43 percent of the time, and overfeeding was necessary to prevent concentrations from falling below lower specification limits. Overfeeding treatment chemicals adversely affected cost and performance results and risked violation of effluent water discharge permit. To solve this multi-dimensional challenge, Commonwealth Edison turned to fluorescent tracer technology.

Installing a fluorescent tracer system yielded immediate positive results, enabling Byron to continuously monitor scale inhibitor and dispersant dosage. Variability of scale inhibitor and dispersant dosage quickly fell from 6.0 ppm to 0.5 ppm when automatic fluorescent tracer control replaced manual control. These actions improved dosage control and resulted in overall cost savings of $150,000 in the first year.


Figure 2
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After six months of operation, the new control system allowed the upper and lower specification limits of chemical dosage to be significantly tightened, while still maintaining greater than 90 percent conformance to the significantly tightened specifications (Figure 2). Based on this success, Commonwealth Edison placed the alkaline zinc corrosion inhibitor under similar fluorescent tracer control, and has experienced 85 percent conformance to its aggressive dosage control specifications as well as complete conformance to its NPDES discharge limit of 1 ppm for zinc.

Texas TT

Texas TT

An example of the move from Step 3 (inert-based tracer) to Step 4 (tracer and actives-based feed and control) is the West Texas Utilities’ Rio Pecos gas-fired generating facility.6-7 The facility has two similar operating units (Units 5 and 6) and cooling water systems. Microbiological control was based on daily slug-feeds of hypochlorite bleach and maintaining the residual chlorine at less than 0.2 ppm (as Cl2) during the bleach slug-feed. Tolyltriazole (TT) dosage was manually controlled based on colorimetric analysis of TT concentration in cooling water grab samples. Although operating and chemical treatment conditions were similar, Unit 6 experienced severe corrosion problems that required the condenser to be replaced two times, while Unit 5 did not have corrosion problems of the same severity.

To address this problem, Rio Pecos installed a fluorescent analyzer for TT on the Unit 6 cooling water system to continuously monitor and control TT concentration. Readings quickly established that TT concentration varied significantly on Unit 6, and dosage dropped to virtually zero when hypochlorite bleach was added to the cooling system. The results indicated that the Unit 6 cooling water system had a very large, highly variable TT consumption. Since implementing fluorescence analysis and control, Rio Pecos has been able to substantially reduce the corrosion problems and increase the Unit 6 condenser lifetime, while significantly improving treatment dosage control and cost and performance of the system.


Figure 3
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In addition to the use of inert tracers and actives (e.g., corrosion inhibitor) for cooling water system control, the consumption of treatment actives can also be used for more effective usage of chemical treatment. At Rio Pecos, TT consumption has been used to optimize usage of the corrosion-inhibiting treatment program. The halogen-based biocide in use at the plant controlled microbiological growth, but also degraded some of the corrosion inhibitor. By implementing a stabilized-halogen biocide program (instead of the original bleach and bromide program), the consumption of TT was significantly reduced due to the greater specificity of the biocide program (Figure 3). p

Fluorescent Tracing

Fluorescent tracing is based on a very simple measurement concept. Inert chemical “tracers” are mixed, at a known specified level, with the water treatment chemicals. A sidestream of recirculating cooling water is then piped through a flow cell containing a microprocessor-based fluorometer. Light of a specific wavelength is passed through the flow cell, exciting the tracer compound and allowing its emissions to be read by a detector. The resultant signal is then sent to the microprocessor, which instantly determines tracer concentration in the cooling water. Since the ratio of tracer to chemical inhibitor is known, this also provides a direct measure of inhibitor concentration added to the system. A chemical pump that feeds cooling water inhibitor is automatically controlled by the system when the chemical level is below a user-defined set-point.

System Stresses

There are two basic classifications of stresses in a recirculating cooling water system: hydraulic stresses and reactive stresses. Hydraulic stresses are water losses (blowdown, drift, leakage, etc.) from the system that cause the removal of both active ingredients and inerts. Reactive stresses (scaling, fouling, corrosion, degradation, etc.) cause the loss of active ingredients, but not the inert tracer. The ability to measure variations due to these stresses is a key difference between traced and non-traced systems.

Fluorescent tracer systems not only help to identify hydraulic stresses, but to control them as well. Since a signal from an accurate, instantaneous and continuous direct measurement of tracer concentration is available, the feed system can quickly and automatically replace inhibitor chemicals in real time. The fast reaction of this automatic system results in consistent concentrations over time, instead of the swings of overconcentration and underconcentration experienced with other monitoring and control systems.

This leads to another important advantage of inert tracer technologies. They allow the direct measurement of actives consumption when combined with measurements for residual actives. By “neutralizing” variability due to hydraulic stresses, one can directly compare inert tracer concentration (automatically monitored by the system) and actives concentration (in a number of cases by fluorescence-based methods). The difference between the two represents consumption.

References:

1. U.S. Patent 4,783,314 (assigned to Nalco Chemical Co.), J. E. Hoots, B. E. Hunt, 11/8/88.

2. U.S. Patent 5,171,450 (assigned to Nalco Chemical Co.), J. E. Hoots, 12/15/92.

3. U.S. Patent 5,278,072 (assigned to Nalco Chemical Co.), N. M. Rao, F-Y Lu, J. E. Hoots, 1/11/94.

4. U.S. Patent 5,435,969 (assigned to Nalco Chemical Co.), J. E. Hoots, M. R. Godfrey, 7/25/95.

5. R. Gossman, et. al., “Implementation and Evaluation of a Mild Steel Corrosion Control Program at Commonwealth Edison, Byron Nuclear Station,” EPRI Corrosion in Electric Power Plant Service Water Systems Workshop, April 22-24, 1992.

6. J. E. Hoots, et. al., “Actives-Based Chemical Control for Recirculating Cooling Water Systems,” International Water Conference, Oct. 30 – Nov. 1, 1995.

7. J. E. Hoots and S. J. Armitage, “Problem-Solving Tools for Improved Performance of Treatment Programs in Cooling Water Systems,” International Water Conference, October 21-23, 1996.

Authors-

Eric Hale is a Market Development Manager for the Power Industry Group at Nalco Chemical Company. He has 5 years’ experience at Nalco and holds a bachelor’s degree in nuclear engineering from the University of Lowell.

John Hoots is a Research Associate at Nalco Chemical Company. He has been employed at Nalco for 15 years. Hoots received his Ph.D. in inorganic chemistry from the University of Illinois (Champaign-Urbana).

Steve Nicolich is a Product Manager at Nalco Chemical Company. He has 10 years’ experience in application engineering, sales and marketing. Nicolich holds a bachelor’s degree in mechanical engineering from Valparaiso University.