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Power plant water issues: Effectively cleaning cooling tower fill

While once-through cooling was a common feature at many power plants in the last century, environmental regulations regarding intake and discharge issues have basically forced a transition to cooling towers, or in some cases air-cooled condensers, for new projects.  Essential to steady heat transfer in cooling towers, and also their physical stability, is proper chemistry control.  But even with diligent chemical feed and monitoring, cooling towers, and especially the tower fill, can accumulate scale and microbiological deposits that inhibit heat exchange, and, in worst case scenarios, may induce partial collapse of the tower.  This article examines methods to clean tower fill before fouling causes irreversible damage.

Background

Cooling tower performance is highly dependent on the efficiency of contact between the hot return water from heat exchangers and the cool air being pulled or blown through the tower.  Heat transfer is enhanced by the use of cooling tower fill, which, over the decades, has evolved into sophisticated designs to maximize air-water contact.  An illustration of high efficiency PVC film fill is shown below.

 

 

Figure 1. High efficiency cross-fluted fill (Photo courtesy of Brentwood Industries)

The transition from early splash fill designs to modern high efficiency types reduced cooling tower capital and operating costs.  However, the generally tortuous path that provides good contact between air and water also makes these fills highly prone to fouling.  Advanced fouling results in a ~10x weight gain, leading to fill collapse into the sump and expensive fill replacement. 

Proper Chemical Treatment

While this article focuses on methods to clean tower fill that has begun to accumulate deposits, it is paramount to understand that proper chemical treatment during normal operation is essential to prevent severe or sudden scaling and fouling problems.  With regard to scale (and corrosion) control in cooling systems, the four-decade long methodology of phosphate/phosphonate treatment is giving way to polymer-based, non-phosphorus chemistry for two primary reasons.  One is that phosphorus discharges to the environment are being increasingly regulated and restricted due to problems with toxic algae blooms that have afflicted numerous bodies of water.  Secondly, the new polymer programs are proving to be more effective for scale and corrosion control in cooling systems.  (Post, R., Kalakodimi, R., and B. Buecker, “An Evolution in Cooling Water Treatment”; PowerPlant Chemistry Journal).

The most serious issues in cooling systems are usually related to microbiological fouling.  Thus, virtually all systems have as primary treatment some form of oxidizing biocide, most commonly bleach but also possibly gaseous chlorine, bleach/sodium bromide, chlorine dioxide, monochloramine, and monobromamine.  A problem at many facilities, and this is particularly true in the power industry, is that the regulations developed for and by the United States Environmental Protection Agency (USEPA) allow no more than 0.2 ppm free available chlorine average residual for 2 hours per day as “Best Available Technology.”  For plants so constrained, treatment is only allowed for less than 9 percent of any day, thus giving microbes a chance to settle and begin forming protective slime layers.

 

Figure 2

 

Options for dealing with fouled fill

Many facilities have suffered from fouled cellular plastic fill.  Replacement of the fill in kind is a potential solution, but potentially sets up repeat situations.  Another option is a switch to low-fouling fill designs that generally feature a more vertical flow pattern, less surface texturing, and sometimes wider spacing between the plates, all at the expense of some cooling efficiency.  Since fill replacement can be expensive in terms of both materials and outage time, others have chosen to clean the fill chemically, or sometimes, mechanically.  The choice of replacement vs. cleaning, as well as the cleaning methodology, requires careful consideration.  The decision depends on the extent of the fouling, the physical and chemical nature of the foulant, the type of fill, and environmental considerations in dealing with cooling tower blowdown.  For example, in heavily fouled film packs, some passages may be completely blocked, preventing the cleaning solution from flowing through, and perhaps acting as a filter for solids removed in other parts of the pack.  The total mass of deposits, if released at once into the recirculating water flow will result in very high suspended solids, and blowdown may have to be diverted or treated prior to discharge.  The type of foulant also varies considerably depending on the nature of the circulating water and the treatment chemistry employed.  Over time, the fouling matrix behaves as a filter media, trapping additional suspended solids in the crevices of the fill pack and impeding air and water flow.  At this point, the efficiency criteria that constituted the driving force for selecting the fill has become null. 

Figure 3.  Fouled film fill that is no longer effective.

The loss in cooling tower capability as a function of weight gain for a fill of offset flute design is trended in Figure 4.

Figure 4.  Tower capability loss vs. fill weight gain for a standard offset flute cellular plastic fill pack. (Monjoie, Michel, Noble, Russell, and Mirsky, Gary R., Research of Fouling Film Fill.  Cooling Technology Institute, TP93-06, New Orleans, LA, 1993.)

Over time, the high efficiency fill becomes increasingly less efficient, may gain as much as 10x its initial weight, begins to extrude around the supporting beams, and ultimately collapses into the sump.  At the point where performance loss becomes obvious to operators or the fill begins to deform, it is too late to consider cleaning as an option; fill replacement is required.  However, if the fouling is detected in its early and moderate stages, several cleaning options are available, depending on the nature of the foulant.

Cleaning Options For Cellular Plastic Fill

The most appropriate method for cleaning tower fill depends on several factors, including safety concerns, system metallurgy, in-service vs. out-of-service cleaning, potential impact on plant operations, disposal options for the cleaning solution, impact on the environment, and the chemical and physical nature of the foulant.

Mineral Scales

Hard mineral deposits most commonly consist of silica/silicates or calcium carbonate (calcite).  Silica solubility is lowest at low temperature, and deposits often occur near the bottom of the counterflow fill pack where the temperature is lowest, the water is most concentrated, and uneven water/air distribution can lead to dry spots or locally concentrated areas.  Calcite deposits often occur throughout the fill pack, but are generally heaviest toward the bottom.  Higher temperature near the top of the fill pack has the lowest calcite solubility and promotes faster deposition kinetics.  However, as the water passes through the fill, the minerals are concentrated slightly by evaporation, and the pH will rise slightly as excess CO₂ is stripped.

One technique that can be used effectively on either type of hard scale in its early stages is to apply certain types of surfactants that penetrate the hard deposit and induce it to spall from the slightly flexible plastic substrate. The surfactant is typically applied in addition to the normal scale inhibitor program for an extended period of 60-180 days. This program is never 100% effective, but will often result in removal of 70-80% of the fouling minerals.  Prior to implementing the cleaning process, it is imperative to identify and correct the scaling condition.

For large cooling systems, where the predominant scale deposit is calcite, the fill can be cleaned by reducing the operating pH and/or cycles of concentration until the water is undersaturated with respect to calcite at the fill conditions.  Calcite often serves as the binder for the deposit matrix, so dissolving the calcium carbonate in the deposit matrix can be disproportionately effective.  In principle, any degree of undersaturation will be effective over time.  Sulfuric acid is an obvious choice for many plants that already use it for pH control, but very careful planning involving plant personnel, the chemical supplier, and any outside contractors is required before using such a hazardous chemical. Other plants may prefer to use safer, less corrosive acids such as organic acids or inhibited sulfamic acid. [4]  Some organic acids are more effective than mineral acids at intermediate pH, and are synergistic with sulfuric acid.  At pH 5, application of the appropriate organic acid will accelerate the rate of calcite dissolution by 10-20x as compared to sulfuric acid alone.

For predominantly light calcium carbonate scaling, off-line foam acid cleaning has been used very successfully, at least on smaller towers. Strong acid foam is applied by skilled specialists from the top of the fill pack. The nature of the foam allows the acid to contact the scale as it slowly passes downward through the fill.  The relatively low volume of spent and mostly neutralized foam cleaning solution is either collected in the sump and disposed of, or is allowed to mix with other circulating water from neighboring tower cells that may be in service, depending on plant safety and environmental requirements.

Mineral scales can also be mechanically cleaned with some success in situ or ex situ.  Due to its brittle nature relative to the flexible PVC, the scale can be dislodged with some success by mechanically cleaning the fill pack in-situ from below.

Microbiological/Organic Deposit Matrices

Deposits where microbiological growth or organics serve as the binder for the deposit matrix are characterized by a soft, sometimes putty-like consistency.  Unlike mineral scales, deposits of microbiological origin tend to accumulate primarily in the middle of the fill pack.  Water velocities directly under the spray nozzles are generally high enough to discourage microbiological adhesion.  For this reason, microbiologically initiated fouling sometimes goes undetected because it is not visible on inspections from the top looking down beneath the spray headers.  As the water velocity slows down several inches into the fill, microorganisms begin to colonize the surface, acting as a filter for suspended solids passing through the fill.  Fouling tends to be more intense in the middle of the fill than at the bottom because suspended solids are filtered out prior to reaching the bottom layer, and because the last few inches of fill do not physically support a thick, soft deposit mass.  The inability to clearly view microbiological fouling from either top or bottom, combined with the difficulty of inspecting the middle layers of fill, often allows this type of fouling to progress undetected until it has reached an advanced stage.  Plant personnel have attempted to monitor fill fouling during tower operation using sections of fill suspended from load cells, or by cutting an access window into the end of the tower casing to allow a middle section to be removed periodically for inspection using a man lift, or by suspending a section of fill beneath the main fill pack to allow it to be easily inspected and weighed.  All of these methods can work, but none have proven to be totally satisfactory.

Several effective methods exist to remove biological-silt matrix deposits from cooling tower fill.  Hyperhalogenation is a widely attempted method, but its effectiveness is usually disappointing.  Potential corrosion of system components and the need to dechlorinate prior to discharge are important considerations.

Microbiological matrices often have high water content and will shrink and detach from surfaces when thoroughly dried.  However, effectively drying out cooling tower fill can prove problematic even with the help of fans, even if the tower is located in a low humidity climate.  Chlorine dioxide has also been used as a cleaner for cooling tower biofilms with some success.  However, the most widely practiced and effective cleaning method for deposits with microbiological or organic binders is hydrogen peroxide (H₂O₂) due to its oxidizing strength and the physical action of the oxygen micro-bubbles produced as the chemical reacts with organic deposits.  The positive environmental profile of hydrogen peroxide involving rapid breakdown to water and oxygen, and its ease of application are additional factors favoring peroxide as a tower fill cleaner.  Typical dosages are in the range of 500-3,000 ppm active H₂O₂.  As with most cleaning operations, the addition of low levels of surfactants will help loosen deposits.  Polymeric dispersants are generally added to assist in keeping the removed solids in suspension until they can be blown down.

Much of the biomass consists of extracellular and intracellular water and organics that will dissolve with peroxide cleaning.  A substantial portion of the deposit typically contains much mud and silt that will be released into the water.  Figures 5 and 6 illustrate the appearance of a slime-clay matrix on moderately fouled high efficiency cooling tower fill before and after cleaning. 

In cases where the deposit contains a high percentage of inorganics, the circulating water can be expected to become highly turbid.  The potential for high suspended solids in the cooling tower blowdown should be anticipated when cleaning a severely fouled system and taken into account in the job planning scope.

Summary

• Every effort should be made to prevent deposition from occurring in the first place.  A fill type should be specified that is compatible with reasonable expectations for the system, considering influent water quality, microbiological control, presence or absence of pre-treatment equipment, and the possibility for external foulants that might enter the tower through airborne contamination or process fluid leaks. 
• The microbiological and deposit control program should be diligently monitored to ensure that it is within expectations and delivering the required results. 
• The performance of any pretreatment and sidestream solids removal equipment should be reviewed to ensure such equipment is delivering and maintaining suspended solids within specifications.
• Plant personnel should be proactive with inspection and monitoring.  There are more options, and less expensive ones, if the fouling is detected at an early stage.
• Periodic, light, preventative maintenance tower fill cleanings should be considered.  Most high efficiency fills tend to gain weight slowly over time. Annual preventative maintenance cleanings can stabilize or reverse that trend.
• If cleaning is indicated, safety and environmental considerations must be adequately addressed.  Cleanings require careful planning and coordination between plant personnel and the chemical supplier/consultant.  Personnel safety is critical.  Also, cleanings can release many suspended solids that if not carefully controlled can foul equipment or present disposal problems without proactive planning.
• If fouling occurs, all remediation options should be considered, but, generally, the least costly and least aggressive methods applicable to the nature and quantity of the deposit should be the starting point.  Identifying and correcting the fouling conditions at an early stage is least expensive, and preferable to aggressive cleaning or ultimate fill replacement if the fouling conditions are allowed to persist.
• All systems are different, and careful consultation with the cleaning vendor and water treatment experts should be a priority.

 You can find the original article @ https://www.power-eng.com/2019/08/21/power-plant-water-issues-effectively-cleaning-cooling-tower-fill/#gref

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How can a cooling tower spread Legionnaires' Disease?

It really should be one of the happiest, most carefree destinations on the planet; however perennial family favourite, Disneyland, found itself at the epicentre of an outbreak of Legionnaire’s disease just before Christmas, as you might have read elsewhere. As well as being an obvious PR disaster for the globally-renown resort behemoth, any possible risk presented by the presence of the potentially deadly respiratory infection could spell more far-reaching bad news for those directly affected by an episode.  View full article →

Brewing up success!

Amperometric methods are traditionally the domain of large laboratory instrumentation which require high levels of user care and maintenance of the electrodes, both of which have been overcome with the ChlordioX Plus, in an instrument a fraction of the size and a fraction of the capital investment. View full article →

Chlorine vs. Chlorine Dioxide different features against biofilm growth

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Industrial Water Analysis

Read more: https://www.lenntech.com/products/boiler-water-analysis.htm#ixzz4x5ZX3UHF

Methods for determining chlorine dioxide and its oxychlorine by-products in water

The analysis of chlorine dioxide and its oxychlorine by products in water is a difficult topic due to the volatility of chlorine dioxide and to the interferences from other species with standard test methods. In the real samples, this is further complicated as chlorine dioxide is often used in a system which is dynamic and therefore sampling is also important.

USEPA Regulations

The USEPA require that both chlorine dioxide (ClO₂) and chlorite (ClO₂^-) are monitored daily at the start of a distribution network and that chlorite is measured less frequently at various points throughout a network. Chlorate is not required, although is included under the Information Collection Rule.

The following are methods that have been approved by the USEPA for ClO₂ monitoring:

• Amperometric titration (Standard Method 4500 - ClO₂ E)
• Colorimetric DPD (Standard Method 4500 - ClO₂ D)
• Colorimetric Lissamine Green (USEPA Method 327.0 V1.1)
• ChlordioX Plus Sensor System

As DPD has been removed as a standard method for determining ClO₂ by the AWWA Standard Methods committee, the EPA will also probably remove it sooner rather than later.

As for chlorite, there are a few methods (such as ion chromatography) approved for chlorite monitoring. None of them are truly portable as the ChlordioX Plus is making it the only portable USEPA approved method for determining chlorite.

Methods in detail

• Iodometric titration (Standard Method 4500 - ClO2 Method B) - Iodometric titration of ClO₂, Cl₂, ClO₂^- and ClO₃^- is possible with although it is a very difficult and time consuming procedure to separate all the oxychlorine species. It is a better method for standardising chlorine dioxide solutions.

• Amperometric methods

Standard Method (4500 -ClO₂ Method C) - Amperometric titration is an electrochemical method that measures current flow when a fixed voltage is applied to an electrode. By measuring the current whilst carrying out a titration with phenylarseine oxide, each oxychlorine species can be separated out and quantified. However, Method C has now been removed as an EPA approved method for measuring chlorine dioxide in drinking water due to inherent weaknesses in the test method.

Amperometric Method (4500 - ClO₂ Method E) - Principle is as per Method C and the procedure is also similar but the inherent test weaknesses are avoided. This method is the standard method which all the ChlordioX Plus evaluations were compared to and is the industry standard method. In theory, as well as chlorine dioxide, chlorine and chlorite, chlorate can also be carried out using this method but it is complicated and therefore usually carried out via ion chromatography.

• DPD method Colorimetry (4500 - ClO₂ Method D - Reserved) - The industry standard for portable testing of chlorine dioxide (and to a degree chlorite) but according to recent research is not selective enough in the presence of both chlorine dioxide and chlorite and also suffers from other interferences such as chromate and oxidised manganese. It is no longer a standard method (AWWA) but is still an EPA approved method.

• Lissamine (LGB) Green - Not a standard method but an EPA approved method for measuring chlorine dioxide (and chlorite in the presence of horseradish peroxidase). It is temperature dependent as it removes colour from the lissamine green indicator and, therefore, is not easy to use in the field and at its best in a laboratory environment.

• Ion Chromatography (4110 Determination of Oxyhalides using Ion Chromatography) - The standard method for chlorite and chlorate determination and a USEPA requirement although obviously not a field test.

• Spectrophotometry - Chlorine dioxide can be measured photometrically at 360nm using a standard spectrophotometer although the detection limit is relatively high and solutions containing chlorine dioxide and chlorite can be susceptible to interference (especially at longer wavelengths) so again it is best used as a tool for standardisation of solutions. Some field test kits also use this method but at wavelengths in the visible region.

Other colorimetric methods

Other colorimetric methods are available however none are approved for compliance testing.

This summary of methods is based on White’s Handbook of Chlorination and Alternative Disinfectants by the Black and Veatch Corporation, published by Wiley in 2010.

See here for a list of EPA methods approved for chlorine dioxide and chlorite monitoring under the alternative test methods program

Diseno de succion y descarga de bombas de dosificacion de quimicos

Sistema de tubería de succión de la bomba

 La tubería debe ser distribuido de modo no hay puntos altos ocurren donde pueden formar vapor bolsillos. bolsas de vapor reducen el área de flujo efectiva de la tubería y por lo tanto hacen cebado y funcionamiento de la bomba difícil. Vent cualquiera de los puntos altos inevitables y proporcionar calibre y drenar las conexiones adyacentes a la bomba.

 Tamaño de línea

Muchos de los problemas de la bomba de resultado de una línea de succión que es demasiado pequeño en el diámetro, o demasiado largo. La tubería de succión debe ser del siguiente modo para proporcionar una transición suave del flujo de fluido y dar lugar a pérdidas de tuberías de fricción reducidos:

Ser corta y directa

• Ser de una a dos veces más grande que la bomba conexión de aspiración. Utilice reductores de tubería tipo excéntricas en la bomba con el lado plano hacia arriba para evitar una bolsa de vapor posible.
• Contener un número mínimo de vueltas. Cumplir con las vueltas necesarias con los codos de radio largo o laterales.

 

 Sistema de bomba Tubería de Descarga

 A continuación se enumeran los requisitos fundamentales para un sistema de tuberías de descarga.

La tubería no debe ser menor que la conexión de descarga de la bomba, y debe:

• Ser lo más corta y directa posible.
• Sea uno a dos tamaños más grandes que la conexión de descarga de la bomba con multiplicadores utilizados en la bomba.

Contener un número mínimo de vueltas. Llevar a cabo cualquier vueltas necesarias con los codos de radio largo o laterales.

• Estar provisto de calibre y de drenaje conexiones adyacentes a la bomba.

 

Todo alternativo de desplazamiento positivo bombas suministran fluido y construir presión hasta que se tomen medidas para controlar y estabilizar el trabajo de la bomba o se produce un fallo. Para proteger la bomba, tuberías, y personal de los peligros asociados con el funcionamiento de una bomba de “desplazamiento positivo” contra una “cabeza muerta” una válvula de alivio de seguridad siempre debe ser proporcionado entre la bomba y la válvula de descarga.

 

La válvula de seguridad debe estar dimensionado para pasar toda la capacidad de la bomba y la presión de apertura debe ser fijado en el 10 por ciento sobre la presión de descarga de trabajo especificado y tienen una presión de acumulación no superior a 110 por ciento de agrietamiento presión.

La conexión de salida de la válvula de alivio de seguridad ideal sería que se canaliza de vuelta al recipiente de suministro de aspiración. Tubería de nuevo a la tubería de succión puede causar discontinuidades en el flujo de la tubería de succión que puede resultar en un mal funcionamiento de la bomba y daños. En caso de que sea necesario tubería de la válvula de seguridad de nuevo a la tubería de succión, la conexión en el tubo de succión debe tener un mínimo de diámetros de tubería 10 de aspiración de longitud de vuelta hacia el recipiente de suministro de aspiración alejado de la conexión de aspiración de la bomba. Esto permitirá que cualquier discontinuidad de flujo creado por el flujo de la válvula de alivio en la succión

tubería para ser suavizadas por el tiempo y efecto viscoso.

 

Una línea de derivación de descarga de la tubería de descarga de la bomba de vuelta al recipiente de suministro de aspiración permite la lubricación para llegar a las partes críticas de la bomba y duro durante el inicio, sin someterlos a cargas altas y permite que todas las cámaras de bombeo cilindro de fluido a convertirse totalmente cebadas.

Una línea de derivación con una válvula de cierre se debe instalar en la tubería de descarga entre la bomba y la válvula de retención de vuelta a la fuente de suministro de aspiración, no en la línea de succión de la bomba para evitar la discontinuidad del flujo.

Instalar una válvula de retención de descarga allá de la conexión de derivación para proteger la bomba de presión del sistema de descarga durante los periodos de inactividad de la bomba y el arranque de la bomba.

Las descargas piping “puntos muertos” deben ser evitados o provisto de dispositivo de amortiguación. Este tipo de característica puede ser responsable de los armónicos de tuberías indeseables y puede contribuir a niveles elevados de vibración y ruido.

Para algunos servicios las fluctuaciones de presión de la bomba o de flujo naturales pueden no ser apropiados. En estos casos, es prudente utilizar un amortiguador de pulsaciones de la instalación. Para una eficacia máxima del amortiguador debe ser montado adyacente al cilindro de fluido de la bomba. Recomendaciones para el tamaño de amortiguador y el tipo pueden obtenerse de fabricantes mojadores basado en detalles de tipo de bomba y tamaño, las condiciones de servicio, y el sistema de tuberías.

Instalar bridas o uniones como cerca de la bomba como sea práctico para permitir la eliminación cilindro de fluido durante el mantenimiento.

se requieren válvulas de cierre en ambas líneas de succión y descarga para aislar la bomba cuando se requiere mantenimiento. Ellos deben ser de diseño apertura completa, tal como una válvula de compuerta. Cuando la conexión de dos o más bombas a una aspiración común

y / o línea de descarga de cuidado ejercicio para prevenir una onda de presión mutuallly de refuerzo que se produzcan durante el funcionamiento. Esto se puede lograr mediante la adición de las capacidades de todas las bombas que operarán simultáneamente para determinar las velocidades de línea para el dimensionamiento de la tubería y el cálculo de la cabeza de aceleración. La mejor manera de evitar una onda de presión de refuerzo mutuo es instalar las líneas de succión y de descarga independientes para cada bomba.

La Figura 31 proporciona un ejemplo de las recomendaciones esbozadas en la sección anterior para un sistema de tubería de la bomba apropiada, mientras que la Figura 32 proporciona un ejemplo de una configuración no apropiado sistema de tubería de la bomba.

 

Metering pump suction and discharge design

Pump Suction Piping System

 

Piping should be laid out so no high points occur where vapor pockets may form. Vapor pockets reduce the effective flow area of the pipe and consequently make pump priming and operation difficult. Vent any unavoidable high points and provide gauge and drain connections adjacent to pump.

 

Line Size

 

Many pump problems result from a suction line that is too small in diameter, or too long. Suction piping should be as follows to provide a smooth transition of fluid flow and result in reduced piping friction losses:

 

Be short and direct

• Be one to two sizes larger than pump suction connection. Use eccentric type pipe reducers at pump with flat side up to avoid a possible vapor pocket.
• Contain a minimum number of turns. Accomplish necessary turns with long radius elbows or laterals.

 

 

Pump Discharge Piping System

 

Listed below are the fundamental requirements for a discharge piping system.

Piping should not be smaller than pump discharge connection, and should:

• Be as short and direct as possible.
• Be one to two sizes larger than pump discharge connection with increasers used at pump.

Contain a minimum number of turns. Accomplish any necessary turns with long radius elbows or laterals.

• Be provided with gauge and drain connections adjacent to pump.

 

All positive displacement reciprocating pumps deliver fluid and build pressure until action is taken to control and stabilize the pump’s work or a failure occurs. To protect pump, piping, and personnel from hazards associated with operating a “positive displacement” pump against a “dead head” a safety relief valve should always be provided between the pump and discharge valve.

 

The safety relief valve should be sized to pass the entire pump capacity and the cracking pressure should be set at 10 percent over the specified working discharge pressure and have an accumulation pressure not exceeding 110 percent of cracking pressure.

The safety relief valve outlet connection should ideally be piped back to the suction supply vessel. Piping back to the suction pipe can cause discontinuities in the suction pipe flow that can result in poor pump operation and damage. Should it become necessary to pipe the safety relief valve back to the suction piping, the connection into the suction pipe should be a minimum of 10 suction pipe diameters in length back toward the suction supply vessel away from the pump suction connection. This will allow any flow discontinuity created by the relief valve flow into the suction

pipe to be smoothed out by time and viscous effect.

 

A discharge bypass line from pump discharge piping back to the suction supply vessel permits lubrication to reach critical pump and drive parts during startup without subjecting them to high loads and allows all fluid cylinder pumping chambers to become fully primed.

A bypass line with a shut-off valve should be installed in discharge piping between pump and check valve back to suction supply source, not into the pump suction line to prevent flow discontinuity.

Install a discharge check valve beyond the bypass connection to protect pump from discharge system pressure during pump idle periods and pump startup.

Discharges piping “dead ends” are to be avoided or provided with dampening device. This type of feature can be responsible for undesirable piping harmonics and can contribute to elevated levels of vibration and noise.

For some services the natural pump pressure or flow fluctuations may not be appropriate. In these cases it is prudent to use a pulsation dampener for the installation. For maximum effectiveness the dampener should be mounted adjacent to the pump fluid cylinder. Recommendations for dampener size and type can be obtained from dampener manufacturers based on details of pump type and size, service conditions, and piping system.

Install flanges or unions as close to the pump as practical to allow for fluid cylinder removal during maintenance.

Shut-off valves are required in both suction and discharge lines to isolate pump when maintenance is required. They should be of full opening design, such as a gate valve. When connecting two or more pumps to a common suction

and/or discharge line exercise care to prevent a mutuallly reinforcing pressure wave from occurring during operation. This can be achieved by adding the capacities of all pumps that will operate simultaneously to determine line velocities for sizing pipe and calculating the acceleration head. The best way to avoid a mutually reinforcing pressure wave is to install independent suction and discharge lines to each pump.

Figure 31 gives an example of the recommendations outlined in the previous section for an appropriate pump piping system, while Figure 32 provides an example an inappropriate pump piping system configuration.

Fundamentals of corrosion control in water systems

Liquid analysis systems and sensors are cost effective tools against corrosion.

Water plus metal equals corrosion. This reality attacks the bottom line of every steam driven power generation plant in the world.

In a steam power plant, high purity water is heated and boiled to make steam, which energizes and powers a turbine to produce electricity.

Water and steam are in constant contact with metal surfaces threatening the integrity of plant equipment like condensers, heaters, pumps, piping, boilers, and turbines.

Fortunately, water purification and chemical treatment greatly reduce and control the corrosion in the plant. Ensuring good cycle chemistry to prevent corrosion, however, requires accurate and continuous analytical measurements in the demineralization train, cooling water, condensate, and boiler feed-water and steam systems.

While the guidelines given below address the needs of a steam driven power generation facility, they can also be useful in other manufacturing facilities where water plays an important role.

Corrosion occurs when metal ions transfer from a base metal to water and combine with oxygen to become hydroxides and solid metal hydroxides. Resultant particles often travel to other parts of the system and are deposited.

Rust reaction

Deposit is a poor conductor

Once a deposit forms, it attracts more suspended solids and the deposit grows. Deposits frequently accumulate on heat exchange surfaces, boiler tubes, and heaters.

The deposit is a poorer conductor of heat than metal and therefore interferes with heat transfer across the tube. This lowers the overall cycle efficiency and can cause local tube overheating failures. Deposits can also significantly lower the efficiency of the turbines and, in turn, become corrosion sites when dissolved solids trapped in the deposit concentrate as the liquid boils away. Eventually, the concentration reaches highly corrosive levels and severe under-deposit corrosion occurs.

A tough oxide film that protects the base metal is the best way to defend iron and copper from corrosion. For iron and carbon steel, the protective film is magnetite.

For copper and copper alloys, the protective film is cuprous oxide. This film works only in the presence of properly controlled water chemistry.

Proper water chemistry also ensures that the film won't wear away and, if a break occurs, the film quickly repairs itself.

Controlling water chemistry requires maintaining high purity water, controlling pH, monitoring for trace quantities of dissolved oxygen, and, if necessary, controlling the feed of a scavenging agent like hydrazine.

Demineralization train

The first line of defense against corrosion in a steam power plant is the use of high purity water. Producing that water is the function of the demineralization train, which converts raw water containing between 100 and 1,500 ppm dissolved solids into water that contains no more than 10 to 20 ppb dissolved solids. Treatment steps may include filtration, softening, chlorine removal, reverse osmosis, degasification, and ion exchange.

Efficient reverse osmosis (RO), in which water forces through a semi-permeable membrane, can remove approximately 98% of the dissolved salts and silica in raw water and nearly all large organic molecules. Contacting conductivity sensors placed in the feed water and the permeate of the RO let plant operators monitor the water quality and overall efficiency of the RO system.

Conductivity measurements in RO permeate and high purity water are not simple, however. Calibration of sensors is complex and must take place by comparing the sensor against a National Institute of Standards and Technology (NIST) traceable calibrated cell of a known cell constant or by calibrating the sensor in a certified solution. However, upon exposure to the atmosphere, high purity conductivity standards and water foul through the absorption of carbon dioxide from the surrounding air and any residue in the sample container. To prevent contamination, it may be desirable to use sensors pre-calibrated to NIST standards. Conductivity validation instruments are available that connect to the process via tubing, eliminating the effects of the atmosphere on the measurement.

Typically, feed-water to an RO system will undergo treatment and will already contain chemicals to ensure optimum operation. These chemicals, however, require careful monitoring, or they may attack the RO membranes. This is particularly true if the feed-water is outside the desired acidic range. Plant operators require general-purpose pH sensors to maintain mild acidity in the feed-water. Chlorine may be in the feed water in some plants as a biocide or need removal in others by means of a carbon bed because it attacks the RO membranes. However, carbon beds reach saturation over time, therefore, chlorine monitors detect breakthrough of chlorine.

Reverse osmosis alone can rarely produce water of sufficient purity for make-up. The RO permeate is usually polished using an Ion Exchanger (IX). These systems consist of tanks containing resin beads selectively treated to adsorb either cations or anions. A cation bed exchanges positively charged ions (such as calcium, magnesium, and sodium) for hydrogen, and the anion bed exchanges negatively charged ions (such as chloride, sulfate, and bicarbonate) for hydroxyl. The displaced hydrogen and hydroxyl combine to form pure water. After a certain amount of use, these systems become exhausted and must be regenerated using sulfuric or hydrochloric acid for cation resin and sodium hydroxide for anion. The monitoring of the concentration of both of these substances must happen continuously with conductivity sensors measuring the regenerant as it enters the tank. During rinse, toroidal conductivity measurements made on the bed effluent determine how well rinsed the regenerants are.

Variations in cooling tower design

In the condenser, recirculating cooling water converts turbine exhaust steam into condensate. Cooling water usually contains high levels of dissolved solids, and leakage of cooling water into the steam cycle is a major source of contamination.

Leaks introduce ions that raise the conductivity and increase the corrosiveness of the feed-water, boiler-water, and steam. To give early indication of leakage and to monitor the overall condenser performance, the cation conductivity of the condensate pump discharge registers on a flow-through conductivity sensor.

In addition, monitoring condensate and feed-water purity requires measuring cation conductivity. After the condensate passes through the cation column, the conductance of the contaminating salt increases as it converts to a significantly more conductive acid.

There is an increased emphasis in the industry on the re-use of cooling water using cooling towers. The cooling effect comes by the evaporation of a small fraction of water and heat exchange with the air passing through the cooling tower. As the water evaporates, however, the dissolved solids concentrate, ultimately causing scale and corrosion in the heat exchange equipment. While there are many variations in cooling tower design, a common feature is the control of water quality with the use of continuous pH and conductivity measurements to maintain a given set of conditions. A contacting conductivity sensor measures the relative concentration of the impurities in the water. The analyzer for that sensor initiates the opening of a blowdown valve when the conductivity becomes too high. Higher purity make-up water is then introduced which lowers the conductivity.

Since most impurities in cooling water are alkaline, a small quantity of sulfuric acid adds in to the circulating water to lower the pH and thus prevent the formation of scale. Measuring this sulfuric acid concentration and keeping the pH below seven, where scaling is less likely to occur (as indicated by the Langelier Index), is best accomplished by a general-purpose pH sensor. Cooling water that contains a high level of suspended solids, however, requires the use of more specialized pH sensors more resistant to fouling.

Condensate feed-water

The cooling tower turns steam into water after leaving the turbine. Make-up water from the demineralization train adds to this water to become feed-water, which pumps through a series of heaters to the boiler. Controlling corrosion in the condensate and feed-water system is usually accomplished in one of two ways-all volatile treatment (AVT) and oxygenated treatment (OT). AVT uses ammonia to control pH and hydrazine to provide a reducing environment for protection of copper alloys. AVT requires measurement of ammonia, dissolved oxygen, and hydrazine. Ammonia measurement can happen either directly or indirectly from pH and conductivity. The indirect method is useful because ammonia reacts in water to produce hydroxide ion. Both conductivity, which is a measurement of ions in solutions, and pH, which is an indirect measurement of hydroxide ion, can combine to yield the ammonia concentration.

OT uses ammonia to control pH and trace oxygen to provide a slightly oxidizing environment that promotes formation of a tough modified oxide film. Water quality for OT is more stringent than for AVT, requiring cation conductivity of less than 0.15 micro Siemens/centimeter. It is necessary to measure dissolved oxygen, pH, and cation conductivity in feed-water systems using the OT method. pH measurement can be difficult in low conductivity water and requires the use of flowing reference technology. A pH measurement requires electrical continuity between the reference and glass electrodes and a path to the solution ground. High purity water does not provide enough conductivity to reliably complete these paths and causes junction potential that registers as erratic drift and offset in the pH measurement. A flowing reference eliminates this effect by stabilizing the junction potential. This measurement takes place in a bypass line in order to preserve the quality of the feed-water and preferably in a stainless steel measurement chamber to dissipate the electrostatic current generated by the high purity water. Since high purity pH is flow sensitive, flow rates should be very low and constant.

Boiler water steam treatment

The boiler is the final collection point for all the corrosive and scale-producing contaminants generated upstream. Solid corrosion lands on the boiler tube surfaces and grows by collecting more suspended matter. Eventually, overheating and tube failure occur. Maintenance of a protective oxide film is the optimum way to limit water corrosion, and this more readily happens when maintaining a low concentration of dissolved solids in a slightly alkaline pH environment. To accomplish this, continuous measurement of both pH and conductivity needs to happen. Conductivity measures the concentration of dissolved solids and a long-life conductivity sensor is required. To maintain the alkaline environment required, power plants commonly buffer the boiler water with sodium hydroxide and sodium phosphate salts. Overfeeding or underfeeding of these chemicals can be damaging, however, and therefore accurate pH and phosphate measurements are critical.

Boiler water also undergoes treatment in order to produce high purity steam. Impurities enter this boiler water from the boiler drum and from vaporous carryover, which deposits on the turbine and causes erosion damage. Silica is the most notorious contaminant, and it is necessary to measure it in the boiler water and steam. Salts such as sodium hydroxide and ammonia salts also vaporize in the steam and flow into the turbine where they precipitate, concentrate, and become highly corrosive. To control contamination in the steam, the conductivity measurement of the boiler water must happen, which indirectly measures the dissolved solids. Then, blowdown controls the amount of contamination.

So, to avoid the uncontrolled corrosion that costs the power industry billions of dollars every year, monitor water quality rigorously and control that quality continuously.

Liquid analysis systems and sensors are hard working, easy-to-use, cost effective tools when measured against the impact of corrosion on plant costs and operations.

While every plant is different, generally an array of pH and conductivity sensing instruments is required for virtually every step of the steam-power generation process.

Beyond that, individual plants will require dissolved oxygen, ozone, chlorine, and other more specialized measurements.

Many plants are opting for centralized digital control systems to continuously monitor the output of analyzers and automate many control functions. This reduces impact on staff and allows corrosion control management to run like a well-oiled machine.

Most important, the key to successful corrosion control is the continuity of measurement.

Grab samples and other periodic measurement techniques are inadequate to the task. Only continuous, real-time analysis offers the assurance of water quality that corrosion control requires.

Sensing pH a venerated pursuit In the sixteenth century, alchemist Leonard Thurneysser discovered that the hue of violet sap changed with the addition of either sulfurous or sulfuric acids. This early indicator was widely used through the subsequent centuries to detect acids. With Svante Arrhenius's introduction of ionic theory in the 1880s, the first theories concerning disassociation of acids and bases were developed. Johannes Bronsted, who postulated that acids and bases are substances capable of either donating or accepting hydrogen ions, further refined these initial theories. By 1904, Hans Friedenthal had successfully established the first scale for classifying acids by determining the dissociation constants for weak acids, according to conductivity and correlating color changes corresponding to different hydrogen ion concentrations using 14 indicating dyes. The hydrogen ion concentration numbers from Friedenthal's calculations were small and awkward to manipulate. Thus, Lauritz Sorensen suggested using the negative logarithm of these numbers, which he dubbed the "hydrogen exponent" or "pondus Hydrogennii." This led to the development of the term pH and the creation of the modern pH scale. The modern pH scale

 

 

Originated published at: https://www.isa.org/standards-and-publications/isa-publications/intech-magazine/2005/may/sensing-ph-controlling-ph/