Sulfites for Oxygen Control

The sulfite/oxygen reaction is known to be inhibited by some alcohols, phenols, amines, and thiosulfate. Other contaminants or organic treatment chemical such as corrosion inhibitors, scale inhibitors, and biocides may also slow down reaction time. A slow reaction can present a problem at early phases in a system and require the use of catalysts or feeding techniques that provide maximum time for the reaction to occur. The reaction rate for sulfite appears to be the fastest of all of the scavengers, followed by erythorbic acid and DEHA. Slower rates, in general, have been reported for hydroquinone, carbohydrazide, and hydrazine. View full article →

The Role of Organophosphates in Cooling Water Treatment

Cooling Water is used in large industries to remove unwanted process heat with the help of heat exchangers, condensers. Due to the continuous contact of water with the metallic surface corrosion, scale, deposition & fouling of the heat transfer surfaces occur. These cause equipment damage and operating losses and sometimes result in costly shutdown of the plant. Addition of chemical inhibitors in controlling these problems.

Chromate based inhibitors along with polyphosphates, have been in use for long and they have been reasonably effective against both corrosion and scaling. But they have their own limitation. Polyphosphates are hydrolyzed to orthophosphate at higher operating temperature of the cooling water circuit, which lead to orthophosphate scale. Orthophosphate formed is also a nutrient for bacterial growth chromates are toxic to aquatic life and ecological concern has been the primary reason for the search for new substances.

Among the various substance stadius organophosphates have been found to be the best. Organophosphate based formulations give comparable protection with respect to corrosion, scaling and fouling and they are hydrolytically stable. This stability of organophosphates permits greater flexibility, during operation, as they are stable over a greater range of pH and at higher temperature, thereby retaining their activity for longer periods of time. The control on various parameters is more relaxed and they are non – toxic. Phosphonate are compatible with most other chemicals used in cooling systems like chlorine, non-oxidizing biocides, silt control chemicals etc. slowly organophosphates are finding more acceptance the world over, primarily because of its freedom from environment problem.

Properties of Organophosphonates: Structurally, organophosphates have the carbon atom directly linked to the phosphorus atom. The two most widely used Organophosphonates in cooling systems are:

“Six phosphate based cooling system corrosion inhibitors were studied for their relative corrosion inhibiting capabilities by an accelerated static test. The static test was standardized by controlling pH and temperature and using an oxidizing accelerator, potassium persulphate. Results indicate that orthophosphate aminotrimethylene phosphonate and hexametaphosphate are the prospective corrosion inhibitors.

Six phosphate based inhibitors selected are Sodium Hexametaphosphate (SHMP), Sodium tripolyphosphate (STPP), tetrasodium pyrophosphate (TSPP), Sodium Orthophosphate (O-PO4), Aminotrimethylene Phosphonate (ATMP) and Hydroxyethylidene diphosphonate (HEDP), these are frequently used in non-chromate based cooling water treatment.

The key properties of Organophosphonates are:

1. Threshold effect and crystal distortion.

2. Hydrolytic Stability.

3. Sequestration characteristics.

4. deflocculation.

5. Chlorine stability

Corrosion Control: Corrosion Control of metallic surface can be obtained by inhibiting the cathodic, the anodic or both these reaction. A combination of Organophosphonates and zinc works synergistically to give very good corrosion protection by interfering with the cathodic reaction. Polyphosphates, when used as corrosion inhibitor, give rise to excessive orthophosphate sludges whereas Organophosphonates with Zinc give good corrosion protection without leading to any sludge formation.

Due to the sequestering ability of Phosphonate, the zinc ions are present in a complexed form limiting the rate of reaction of zinc with hydroxyl ions. Therefore, useful concentration of zinc hydroxide allows the formation of a thin hydroxide film at the surface giving the desired corrosion resistance.

Scale and Fouling Control: Organophosphonates are one of the best deposit control agents presently available. The threshold and crystal distortion property of these compounds interferes with the nucleation of the hardness crystals causing much higher levels of hardness to stay in solution. When scales are formed they are so distorted that they are non-adherent and form very soft sludges. Phosphonate also provide excellent

Control of hydrated ferric oxide deposits which are formed as a result of corrosion. They adsorb on the particle surfaces and reduce the attractive forces between individual iron particles. The sequestering ability of phosphonate enables it to control heavy matter (Fe, Cu & Zn) deposits and this control is far superior to other traditional chelants. Phosphonate also help to disperse suspended particles.

Choice of Phosphonate: From the two most commonly used phosphonate for cooling water treatment HEDP is preferred to ATMP for the following reasons:

  1. ATMP is more corrosive to Cu cooling systems involving Cu or Cu alloys, are therefore very sensitive to ATMP. With ATMP one would then have to use Cu corrosion inhibitors like Thiazoles and Trizoles making the treatment more expensive. The corrosivity towards Cu is due to the fact that a very strong complex with Cu is formed, the dissociation constant of the chelate being about 10 –13.

  2. HEDP has better stability to chlorine than the Nitrogen containing ATMP. Nitrogen containing compound have a tendency to form chloramines. Though, when complexed with Zinc. ATMP exhibits stability towards chlorine it should be used with caution in chlorinated cooling water systems especially when continuous chlorination is used.

    The addition of Zinc to ATMP to a certain extent inhibits the dissolution of copper. In the presence of ATMP, however, the powerful oxidizing potential of chlorine promotes the dissolution of copper, when chlorine is used as a biocide.

    The other phosphonate used to a much lesser extent are, ethylene diamine tetramethylene phosphoric acid, hexamethylene diamine tetramethylene phosphoric acid and diethylene triamine pentamethylene phosphoric acid.

Check out the complete article at http://www.altret.com/templates/images/editor/role-of-organo-phosphate-in-cwt.pdf

Cooling Product Testing and Control

With the extremely high cost of molybdenum in recent years, its use as a corrosion inhibitor or tracing agent in cooling water products, where product consumption is significant, has become essentially cost prohibitive. Other corrosion inhibitors such as phosphates, zinc, silicates, and organo- phosphorous compounds are now used largely in the absence of molybdates. Also, the use of molybdenum has been restricted in some areas because of environmental concerns, mostly centered around concentration limitations in municipally generated sludges.

Phosphates

Where orthophosphate or polyphosphates are in use, testing for the phosphate is a good and accurate test. There are a number of phosphate procedures, but all tests determine orthophosphate. Other forms of phosphate such as polyphosphate or organo-phosphates must first be converted to orthophosphate to determine their concentrations with a phosphate test procedure.

Control can become more complicated when there is phosphate in the makeup water. The form of the phosphate (orthophosphate, polyphosphate, or both) and the concentration range needs to be known so that it is accounted for in the cycled cooling water.

Example

Makeup water contains 0.5 ppm of orthophosphate and 0.4 ppm of polyphosphate as PO4. The cooling tower is operated at five cycles of concentration and a cooling water product that contains 4 % of orthophosphate is being applied. The desired inhibitor product dosage is 100 ppm.

At five cycles, there will be 2.5 ppm of orthophosphate from the makeup water orthophosphate, and 2.0 ppm of polyphosphate applied from the makeup water, but some of it will have reverted to orthophosphate. You should test for polyphosphate in the tower water initially and then periodically to determine the reversion rate for your system. Typically, we assume about a 50% reversion rate. The actual reversion rate will depend upon pH and retention time, and the specific type of polyphosphate.

If when tested the polyphosphate showed to be 1 ppm in the cycled tower water, then the total orthophosphate from the makeup would be 3.5 ppm. 100 ppm of the inhibitor product would add 4 ppm orthophosphate, so a tested residual of 7.5 ppm or orthophosphate would indicate that 100 ppm of the product was in the system.

Table 1: Phosphate Summary

Phosphate Concentrations

Orthophosphate (ppm)

Polyphosphate (ppm)

Makeup Water

0.5

0.4 as PO4

Tower Water,
5 Cycles Before Reversion

2.5

2.0

Tower Water, 5 Cycles After Reversion

3.5

1.0

Orthophosphate From Product

4.0

Total in Cycled Tower Water

7.5

1.0

 

Phosphonates

Most all cooling tower products contain one or more phosphonates that are used for scale inhibition, corrosion inhibition, or both. Phosphonate testing is not as accurate as phosphate testing, but they can be used for controlling product feed. Phosphonates are subject to oxidation to orthophosphate by chlorine or bromine and are lost to precipitation with cations such as calcium. If the system is chlorinated or brominated, assume a 20 – 30% degradation to phosphate. The actual amount can be determined by testing for residual phosphonates and phosphate.

There are several phosphonates tests that can be used:

Hach UV digestion, then phosphate test.
Boiling with acid and persulfate, followed by phosphate test. Palintest drop test.
Taylor drop test.

UV Digestion

The test procedure is the most accurate and has a reproducibility of about ± 10%. A persulfate reagent is used along with a UV light to decompose the organo-phosphate (phosphonate) to orthophosphate. An orthophosphate test procedure then determines the amount of phosphate contributed by the phosphonates. Any orthophosphate already present before the digestion is subtracted from the total orthophosphate after digestion. This can be done by adding reagent to the tower water that has not had the digestion and use this as the blank, or actually determine orthophosphate in the tower water and subtract it from the total orthophosphate determined after the persulfate digestion.

The amount of phosphorus in each specific phosphonate molecule varies, so there is a specific conversion factor from orthophosphate to phosphonate. Each ppm of orthophosphate created by HEDP digestion = 1.085 ppm HEDP. The phosphorus content of PBTC is much lower. Each ppm of orthophosphate created from the digestion of PBTC = 2.84 ppm of the PBTC molecule.

Phosphonate Test Example:

The cycled tower water has 6 ppm orthophosphate and a cooling water product that contains 2.5% PBTC and 1.8% HEDP is being applied at a desired dosage of 120 ppm.

Assuming all of the phosphonates remain as phosphonates and have not been oxidized in the cooling tower by bromine or chlorine and assuming it has not been lost to precipitation, you should get 3.05 ppm of orthophosphate from the phosphonates after a persulfate / UV digestion.

Table 2: Phosphate Summary

From PBTC: 120 ppm x 2.5% = 3 ppm
3 ppm PBTC
÷ 2.84 ppm PBTC per ppm PO4 =

1.06 ppm orthophosphate

From HEDP: 120 ppm x 1.8% = 2.16 ppm

2.16 HEDP ÷ 1.085 ppm HEDP per ppm PO4 =

1.99 ppm orthophosphate

From orthophosphate in the tower water:

6 ppm

Total orthophosphate in sample after digestion:

9.05 ppm

Orthophosphate from phosphonate digestion:

3.05 ppm

 

Boiling With Acid and Persulfate

A digestion can also be accomplished by adding acid and persulfate, then boiling for about 30 minutes. If just acid were used, only polyphosphate would be hydrolyzed or reverted to orthophosphate. It persulfate is also added, the organo-phosphates and polyphosphates will be digested to orthophosphate. This test would be more applicable for samples that do not have polyphosphates, since the test will not distinguish between orthophosphate developed from phosphonates or polyphosphates.

Phosphonate Drop Counts

We recommend the Palintest procedure. This procedure is less accurate and subject to interferences. It is best to determine the number of drops on a known product concentration and relate the number of drops to that concentration. It is advisable to also compare these results initially and periodically to the digestion method.

Where PBTC is in use, the Palintest method is preferred. The procedure buffers the pH to around 3.0 and is more effective at detecting the PBTC along with the HEDP and AMP.

On the Palintest method, each 0.7 ppm of HEDP or AMP in the water should require one drop of titrant, and each 2.0 ppm of PBTC should require one drop.

Polyphosphate and some organics will interfere with the test and show up as phosphonates. To account for this, a blank is run on the makeup water. If it takes two drops for the color change on the blank, then those two drops are subtracted from the test results of the treated water. Note that the blank results are not cycled up by the tower cycles. Polyphosphates revert to orthophosphate which does not interfere and experience has shown that cycling the blank should not be done. If the product contains polyphosphate and a residual in the cycled water, it will increase the number of drops required.

If fluorides are in the cycled tested water at > 1.0 ppm, this causes a substantial interference that may disqualify the drop test procedure from being usable. It is advisable to check with the city supplier to see if they add fluorides and at what level. If high fluorides are present, an idea that may work is to first run the drop test procedure on the tower water to get a baseline number. Then take a sample of the cycled tower water and add 100 ppm of product and see how many drops are required. Subtract the number of drops used for the baseline from the drops required for the 100 ppm sample to determine how many drops represent 100 ppm of product as a basis for setting control limits.

The Palintest end point is the drop when the color change from green/gray to blue/purple first occurs.

Palintest Phosphonate Drop Count Example:

The cooling water is treated with 140 ppm of a product that contains 2.5% PBTC and 1.8% HEDP. The product has a specific gravity of 1.16. There is no fluoride in the water.

First, determine the interferences in the makeup water by running the test procedure on an untreated sample. On this example assume it took two drops.

Next, make a 100 ppm solution. To do this add 1 gram or 0.86 mL (1mL/1.16 gram/mL) of the chemical product to 99 grams (99 mL) of makeup water. Mix this up well, then add 1 gram (1 mL) of this 1% solution to 99 grams (99 mLs) of makeup water. This is now a 0.01% solution or 100 ppm of the product. This would place 1.8 ppm of HEDP and 2.5 ppm of PBTC in the solution. Run the phosphonates test on this solution, and for this example it required the theoretical number of drops of about 6.

Table 3: Theoretical Phosphonate Titrant Usage

From HEDP: 1.8 ppm ÷ 0.7 ppm HEDP / Drop

2.5 Drops

From PBTC: 2.5 ppm ÷ 2.0 ppm PBTC / Drop

1.25 Drops

From Blank:

2 Drops

Total Drops:

5.75 drops, which will require 6 drops to see the color change.

140 ppm of product would be about (140 ÷ 100) x 4 drops = 5.6 drops or 5-6 drops + 2 drops for the blank = 8 drops. This can be confirmed by making a 140 ppm solution and testing it.

Azole, Zinc, or Silica Tests

The Hach test procedures for azole, zinc, or silica can be used to check product dosage if the specific ingredient is in the applied product. Remember, as with phosphonates, the applied concentrations and actual residuals can be different. Azole residuals decrease as they film with copper. Zinc is lost as it precipitates at the cathode or in the bulk water. Silica is lost as it films metal surfaces. In establishing control ranges and dosages, take into account some of this loss. For example, we may apply azole at 2 ppm, but have a desired residual in the water of only 1 ppm.

Mass Balance

Chemical dosages should be confirmed by mass balances and compared to chemical testing. Mechanisms should be set up on each system to conveniently determine water makeup, cycles, water loss, and chemical consumption. The concentration in the recirculating water should be calculated from the actual product usage and blowdown or water loss.

Mass Balance Example:

The cooling tower is operating at five cycles of concentration. The makeup meter shows 120,000 gpd makeup. At five cycles, this is a water loss of 24,000 gpd. The product being fed contains 1.8% HEDP, 2.5% PBTC, 1.5% BZT, and 1% zinc; and the desired dosage is 100 ppm.

Daily product use determined by drum level and confirmed with drawdown cylinder testing is 28 lbs per day. This is a calculated applied dosage of 140 ppm of product in the cycled cooling tower water (140/120 x 24,000/1000 = 28 lbs).

Chemical testing showed 4 drops of phosphonates (6 drops from the test – 2 drops for the blank), which was previously determined to represent 100 ppm product. Testing also revealed 1.5 ppm BZT and 0.8 ppm of zinc residuals in the water. All of the chemical tests show that some portion of the active component has been consumed or residuals would have been higher at 140 ppm of applied product.

Product Component

Expected Residuals with No Loss When Applied at 140 ppm

Calculated Dosage Based on Actual Residual

Product Loss to System Reactions

Phosphonate

8 drops

6 drops

= 100 ppm Product

40 ppm Product

BZT

2.1 ppm

1.5 ppm BZT

= 100 ppm Product

40 ppm product

Zinc

1.4 ppm

0.8 ppm Zinc

= 80 ppm Product

60 ppm Product

 

Conclusion

Mass balance is the most accurate way to determine applied dosage. If the product dosage was projected to be effective at 100 ppm, it is likely that this product is being overfed by 40%. Chemical testing suggests that there is more than sufficient residual of active components even after some loss to the system, so product dosage can be lowered and results monitored to confirm that desired results are maintained. There is expected to be some loss of active components as they react with the materials in the system and the impurities in the water.

Where molybdate is used or has been used as a monitoring method for product control and consumption, generally its loss to the system is minimal. That means that if the product shown above contained 1% molybdate as Mo,

it is likely that the test results would have been very close to 1.4 ppm Mo and the product dosage would have been decreased to 100 ppm to lower Mo to 1.0 ppm. Molybdate used as a tracer, then, would commonly yield a lower product usage rate because the other active components would not ordinarily be used to control the dosage.

Reduccion del uso de agua en torres de enfriamiento con automatización

Con la iniciativa del Estado para reducir el consumo de agua en un 20 por ciento para el año 2020, muchas plantas en California están tratando de ser más respetuosos con el medio ambiente. Una de esas instalaciones incluye un hospital líder en California, que trató de reducir los costos de tratamiento de agua para su sistema de climatización. El hospital cuenta con tres sistemas de torres de enfriamiento individuales que dan servicio a tres enfriadores centrífugos, con un total combinado de 2.800 toneladas de capacidad.

El programa de tratamiento de agua actualmente en uso en la instalación estaba operando a 2,8 ciclos de concentración, resultando en 35,7 por ciento de la composición agua de la torre se sangró a la alcantarilla por el proveedor de tratamiento actual. Teniendo en cuenta la calidad del agua en la zona, estos eran los ciclos máximos de concentración que podrían lograrse sin emplear el uso de reblandecimiento ácido o agua. El ahorro que el hospital solicitó se realizaron mediante la revisión de varias formas de optimizar el programa de tratamiento de agua. Trabajando en estrecha colaboración con el Departamento de Agua y Energía (LADWP) Los Ángeles, se reveló que mediante la introducción de un programa de conservación de agua para reducir el uso del agua a través de mayores ciclos de concentración, la instalación realidad ahorraría más dinero que se gastaría para alterar el programa , por lo que el proyecto propuesto sostenible.

A través de pruebas y análisis de laboratorio, el equipo fue capaz de concluir que seis ciclos de concentración podrían alcanzarse, resultando en sólo el 16,7 por ciento del agua de maquillaje torre siendo desangrado en el sistema de tratamiento de alcantarillado. Esto se podría lograr mediante la introducción de un sistema de alimentación de ácido seguro que minimizaría escala, la corrosión y el ensuciamiento microbiológico para permitir el aumento de ciclos de concentración al mismo tiempo proteger personal de la instalación entre en contacto con los productos químicos.

La evaporación de la torre de refrigeración sigue siendo el mismo, pero el agua de Estados Unidos fue capaz de reducir la purga, cortando el consumo de agua en un estimado de 3.6 millones de galones por año y la disminución de los costos de agua y alcantarillado. La planta fue capaz de ahorrar más de $ 76.000 (ver Fig. 1).

química en cualquier momento en la torre de refrigeración está estresado por la adición de más ciclos, se requiere un control estricto de la química para evitar la formación de incrustaciones. Esto llevó a la introducción de controles de automatización avanzada de agua de Estados Unidos. El programa de automatización avanzada incluye notificaciones de vigilancia y alarmas inalámbricas para gestionar el rendimiento general del programa, y ​​el equipo de conductividad, pH, los niveles de inhibidor de incrustaciones, el uso de la torre de maquillaje, y la utilización de la torre de purga monitoreado.

En un momento dado, el personal del hospital y los representantes designados de agua de Estados Unidos, utilizando varios niveles de seguridad de la contraseña-protegida indicado por la instalación, se puede acceder de forma segura los datos para la revisión y ajuste en línea. Si los parámetros designados cayeron encima o por debajo del intervalo especificado, un representante de aguas US fue alertado para una respuesta rápida (véase Fig. 3).

Segundo para riego, torres de enfriamiento ofrecen el mayor potencial de ahorro de agua en California. Como un incentivo adicional, el estado de California ha puesto en marcha programas para rebajar la instalaciones para el coste de la automatización de sus sistemas. LADWP y el Distrito Metropolitano de Agua (MWD), por ejemplo, ofrecen tres programas que financian la automatización de las torres de enfriamiento debido a su capacidad para aumentar ciclos de concentración, lo que reduce el consumo de agua.

 

Esta permitido la financiación de agua de Estados Unidos para implementar el programa de automatización avanzada $ 34.000 a monitorear y controlar el programa de tratamiento de agua para este hospital sin costo alguno para el hospital.

Los resultados hasta la fecha para la instalación incluyen la reducción significativa en el consumo de agua, el agua baja y las facturas de aguas residuales y un control más eficiente debido a la automatización de software instalado para proteger los bienes de equipo.

You will find this article here: http://www.waterworld.com/articles/iww/print/volume-14/issue-5/columns/case-study/hospital-reduces-water-usage-in-cooling-towers-with-automation.html

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
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.

Ammonia, Conductivity, and pH

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.

Liquid analysis in steam power generation

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.

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/

Porque debe instalar un controlador de la purga de una caldera

Antecedentes

Para reducir los niveles de sólidos disueltos totales y suspendidos en una caldera, agua periódicamente es descargada o purgada. Altos contenido de sólidos disueltos pueden conducir a la formación de espuma y arrastre de agua de la caldera en el vapor. Esto podría generar golpes de ariete, que pueden dañar tuberías, trampas de vapor, o equipos de proceso. La purga de superficie elimina sólidos disueltos que se acumulan cerca de la superficie del líquido de la caldera y, a menudo, es un proceso continuo.

Sólidos suspendidos y disueltos también pueden formar lodo. Lodos deben eliminarse ya que reducen la capacidad de transferencia de calor de la caldera, dando por resultado baja eficiencia de conversión de combustible a vapor y danos a contenedores presurizados. Los lodos se elimina por purga de lodos o fondo.

Durante el proceso de purga de superficie, una cantidad controlada de agua de la caldera, que contiene concentraciones altas de sólidos disueltos, se descarga en el alcantarillado. Además de desperdiciar agua y productos químicos, el proceso de purga desperdicia energía de calor, porque el líquido de purga esta a la misma temperatura que el vapor producido, aproximadamente 366 ° F para 150-pounds-per-square-inch-gauge (psig) vapor saturado, y los sistemas de recuperación de calor de purga, si están disponibles, no son 100% eficientes. (Residuos de calor puedes ser recuperado a través de un intercambiador de calor de purga o un tanque flash en combinación con un sistema de recuperación de calor.

Ventajas de los Sistemas de Control Automático

Con control manual de purga de superficie, no hay ninguna manera de determinar la concentración de sólidos disueltos en el agua de la caldera, ni la tasa de purga óptima. Los operadores no saben cuándo purgar ni por cuanto tiempo. Además, utilizando una tasa fija de purga no se toman en cuenta los cambios del agua de alimentación ni las condiciones del agua de alimentación, o las variaciones en la demanda de vapor o condensado de retorno.

Un sistema de control de purga automática optimiza las tasas de purga de superficie regulando el volumen de agua descargada de la caldera en función de la concentración de sólidos disueltos presentes. Los sistemas de control automático de purga de superficie mantienen la química del agua dentro de límites aceptables, mientras se minimiza la purga y la reducción de las pérdidas de energía. Ahorros de costo provienen de la reducción significativa en el consumo, disposición, tratamiento y calentamiento de agua.

Cómo funciona

Con un sistema de control de purga automática, sondas de alta o baja presión se utilizan para medir la conductividad. Las sondas de conductividad retroalimentan a un controlador de purga que compara la conductividad medida con un valor de consigna y luego transmite una señal de salida que llega a una válvula de purga.

La conductividad es una medida de la corriente eléctrica llevada por iones positivos y negativos cuando se aplica un voltaje a través de electrodos en una muestra de agua. Conductividad aumenta cuando aumentan las concentraciones de iones disueltos.

La corriente medida es directamente proporcional a la conductividad específica del fluido. Sólidos disueltos totales, sílice, concentraciones de cloruro, o alcalinidad, contribuyen a las medidas de conductividad. Estas especies químicas son indicadores fiables de sales y otros contaminantes en el agua de la caldera.

Aplicaciones

Calderas sin un sistema de recuperación de calor de purga y con tasas de purga alta ofrecen el mayor potencial de ahorro de energía. La tasa de purga óptima es determinada por una serie de factores, incluyendo el tipo de caldera, presión de trabajo, tratamiento de agua y calidad del agua de reposición. El ahorro también depende de la cantidad de condensado a la caldera. Con un bajo porcentaje de retorno de condensado, se necesita más de reposición de agua y se requiere purga adicional. Las tasas de purga de calderas a menudo van desde 1% al 8% de la tasa de flujo de agua de alimentación, pero pueden ser tan altas como 20% para mantener los límites de sílice y de alcalinidad cuando el agua de reposición tiene un contenido alto de sólidos.

Ejemplo de rendimiento y precio

Para una caldera de 100.000 libras por hora (lb / hr) de vapor, disminuyendo la tasa de purga requerida de 8% a 6% de la tasa de flujo de agua de alimentación, se logran reducir los requerimientos de agua de reposición por aproximadamente 2.300 libras/hr. El ahorro anual de energía, agua y productos químicos debido a las reducciones de tasa de purga por un sistema de muestra se resume en la tabla a continuación. En muchos casos, estos ahorros pueden proporcionar un período de recuperación simple de 1 a 3 años de la inversión en un sistema de control de purga automática.

Ahorros a través de la instalación del sistema de Control de purga automática

Reducción de purga, lb/hr

Ahorro anual de $

Combustible

Agua y productos químicos

Total

1.000

27.200

4.200

31.400

2.000

54.400

8.400

62.800

4.000

108.800

16.800

125.600

 

La compra e instalación de un sistema de control de purga automática puede costar entre $2.500 y $6.000 (Paquete de Control de caldera). El sistema completo consiste en una sonda de presión baja y alta conductividad, compensación de temperatura, controlador y una válvula de purga. Algunos sistemas están diseñados para controlar la conductividad de agua de alimentación y purga de múltiple calderas. El costo total del sistema de purga automático es dependiente de la presión de trabajo del sistema y las opciones de diseño y las prestaciones especificadas.

Prácticas recomendadas

La sociedad americana de ingenieros mecánicos (ASME) ha desarrollado un consenso sobre las prácticas operativas para la purga de la caldera. Secciones VI y VII de la caldera de ASME y código del recipiente de presión describen las prácticas recomendadas. La caldera de ASME y código del recipiente de presión pueden solicitarse a través de la Página Web de ASME www.asme.org .

Puede descargar el documento original del Departamento de energía de los Estados Unidos

Fundamentos de la Medicion de la Conductividad de Calderas

Mientras una caldera genera vapor, impurezas que están en el agua de alimentación de la caldera y, que no son arrastradas por el vapor generado, se concentrarán en el agua de la caldera.

Dentro de la caldera el calor genera burbujas de vapor dentro del agua. Estas burbujas flotan y se rompen al llegar a la superficie del agua, liberando el vapor. Como los sólidos disueltos están cada vez más concentrados, las burbujas de vapor tienden a ser más estables, pudiendo explotar cuando llegan a la superficie del agua de la caldera. Llega un punto (dependiendo de la presión de la caldera, el tamaño y la carga de vapor) donde una parte sustancial del espacio de vapor en la caldera se llena de burbujas y espuma es arrastrada por el vapor.

Esto no es deseable no sólo porque el vapor es demasiado húmedo cuando sale de la caldera, sino que contiene agua de la caldera con un altos nivele de sólidos disueltos y tal vez suspendidos. Estos sólidos contaminarán las válvulas de control, intercambiadores de calor y trampas de vapor, afectando el funcionamiento y operación del sistema.

La generación de espuma puede tener varias causas; altos niveles de sólidos en suspensión, alta alcalinidad o contaminación por aceites y grasas, pero la causa más común de contaminación cruzada (siempre y cuando estos factores se controlan adecuadamente) es un alto nivel de sólidos disueltos totales (TDS). Un control cuidadoso del nivel de TDS del agua de la caldera, junto con la atención a otros factores, debería reducir al mínimo el riesgo de formación de espuma y arrastre.

El TDS se puede expresar de diversas unidades y tabla siguiente da algunas conversiones aproximadas de TDS en ppm a otras unidades.

Grados Baumé y grados Twaddle (también deletreado Twaddell) son escalas alternativas al hidrómetro.

Muestreo del agua de la caldera

La medición de TDS puede ser medido ya sea por; Tomar una muestra y determinar el TDS fuera a la caldera, o por un sensor dentro de la caldera que proporciona una señal a un monitor externo.

Toma de muestras para análisis externo

Cuando se toma una muestra de agua de la caldera es importante asegurarse de que es representante. No se recomienda tomar la muestra del medidor de nivel de vidrio o cámaras de control externo; el agua aquí es relativamente puro condensado formado por la continua condensación de vapor en el cristal externo / cámara. Asimismo, muestras de cerca de la conexión de entrada de agua de alimentación de caldera son capaces de dar una lectura falsa.

Hoy en día, la mayoría de los fabricantes instalan una conexión de purga de TDS, y es generalmente posible obtener una muestra representativa de este lugar.

Si simplemente se extrae agua de la caldera, una proporción del agua se flashea ya que la presión de vapor se reduce rápidamente. No sólo es potencialmente muy peligroso para el operador, sino cualquier análisis posterior será también incorrecto, debido a la pérdida del vapor flasheado, la muestra se concentra.

Puesto que una muestra fresca es necesaria para el análisis, un enfriador de muestra ahorra tiempo y favorece pruebas más frecuentes.

Un enfriador de muestra es un intercambiador pequeño de calor que utiliza agua fría para enfriar la muestra de agua de purga.

 

Método de conductividad

La conductividad eléctrica del agua también depende del tipo y cantidad de sólidos disueltos que contiene. Puesto que la acidez y la alcalinidad tienen un gran efecto sobre la conductividad eléctrica, es necesario neutralizar la muestra de agua de la caldera antes de medir su conductividad. El procedimiento es el siguiente:

  • Añadir unas gotas de solución indicadora de fenolftaleína a la muestra refrigerada (< 25° C).
  • Si la muestra es alcalina, se obtiene un color morado fuerte.
  • Añadir ácido acético (típicamente 5%) gota a gota para neutralizar la muestra, mezclar hasta que el color desaparezca.  

Ejemplo

Conductividad de una muestra neutralizada a 25 ° C = 5.000 Μ S/cm

                                                                            TDS = 5, 000x0.7 Μ S/cm

                                                                           TDS = 3.500 ppm

Alternativamente, el medidor de conductividad compensada por temperatura que se muestra a continuación es conveniente para el uso hasta una temperatura de 45 ° C. Es necesario medir la conductividad del agua de caldera dentro de la caldera o en la línea de purga. Obviamente, las condiciones son muy diferentes a las de la muestra obtenida a través de la muestra más fresca que será enfriada y posteriormente neutralizada (pH = 7). Los aspectos principales son la diferencia de la gran temperatura y pH alto. Un aumento de temperatura resulta en un aumento en la conductividad eléctrica. Para el agua de la caldera, la conductividad aumenta a un ritmo de aproximadamente 2% (del valor a 25° C) para cada aumento de 1° C de temperatura. Esto puede escribirse como:

               Σ T = Σ 25 [1 + α (T-25)] Σ T = conductividad a la temperatura T (μs / cm)

                Α = Coeficiente de temperatura, por ° C (típicamente 0.02 / ° C o 2% ° C)

               T = temperatura (° C)

Una muestra de agua de la caldera tiene una conductividad no neutralizada de 5 000 μS / cm a 25° C. ¿Cual es la conductividad del agua de caldera a 10 bar g?

         A 10 bar g, la temperatura de saturación = 184 ° C (a partir de tablas de vapor)

                                          Σ T = 5.000 [1+0.02(184-25)]

Esto significa que los efectos de la temperatura que se corregirán en el controlador de purga, compensación automática de temperatura, o suponiendo que la presión de la caldera (y por lo tanto temperatura) es constante. Las pequeñas variaciones en la presión de la caldera durante las variaciones de la carga tienen un efecto relativamente pequeño, pero si se precisan lecturas de TDS en calderas que funcionan a presiones variadas, entonces compensación automática de temperatura es esencial.

Constante de la célda

Donde: K = constante de celda (cm)

En el ejemplo anterior la conductividad del agua de caldera fue 20 900 μs / cm. Una constante de celda de 0.3, ¿cuál es la resistencia medida por el regulador?

Resistencia = 14,4 Ohm  

Por lo tanto es necesario utilizar un voltaje de corriente alterna para medir la resistencia de la sonda, el cual es el método preferido en los controladores de purga. Una frecuencia relativamente alta (por ejemplo 1 000 Hz) es necesaria para evitar la polarización por alta conductividad del agua de la caldera.

Mientras que la conductividad del agua de caldera se convierte en una resistencia a través de la sonda, no puede medirse usando un medidor de resistencia de Corriente Continua simple. Si se aplica una tensión a la sonda, burbujas de oxígeno e hidrogeno se forman en la superficie debido a la electrólisis del agua. Este efecto, llamado polarización electrolítica, provoca resistencia mayor a medir.

                                       Σ = Conductividad (S/cm)

                                       R = resistencia (ohmios)

  Conductividad y la resistencia están relacionadas por la constante de celda, como se ve en la siguiente ecuación:

Mientras más se aleje la punta de la sonda de cualquier parte de la caldera, mayor será la constante de la célda. Diferencias en la constante de la célda se toman en consideración cuando se 'calibra' el controlador.

Una sonda para medir la conductividad de un líquido tiene una constante de la célda. El valor de esta constante depende de la disposición física de la sonda y la ruta eléctrica a través del líquido.

                                                      Σ T = 20.900 μs/cm

                                         Σ 25 = conductividad a 25 ° C (μs / cm)

Donde:

Medición de la conductividad en la caldera

      TDS = 3.500 ppm

Conductividad de una muestra neutralizada en 25 ° C = 5.000 Μ S/cm

Nota: Esta relación sólo es válida para una muestra neutra a 25 ° C.

TDS (ppm) = (conductividad en μS/cm) x 0,7

El TDS en ppm es aproximadamente como se muestra en la ecuación:

Donde:

R = resistencia (ohmios)

K = constante de celda (cm)

                                               Σ = Conductividad (S/cm)

En el ejemplo anterior la conductividad del agua de caldera fue 20 900 μs / cm. Una constante de celda de 0.3, ¿cuál es la resistencia medida por el regulador?

Resistencia = 14,4 Ohm

Why installing a automatic boiler blowdown system?

Background

To reduce the levels of suspended and total dissolved solids in a boiler, water is periodically discharged or blown down. High dissolved solids concentrations can lead to foaming and carryover of boiler water into the steam. This could lead to water hammer, which may damage piping, steam traps, or process equipment. Surface blowdown removes dissolved solids that accumulate near the boiler liquid surface and is often a continuous process.

Suspended and dissolved solids can also form sludge. Sludge must be removed because it reduces the heat-transfer capabilities of the boiler, resulting in poor fuel-to-steam efficiency and possible pressure vessel damage. Sludge is removed by mud or bottom blowdown.

During the surface blowdown process, a controlled amount of boiler water containing high dissolved solids concentrations is discharged into the sewer. In addition to wasting water and chemicals, the blowdown process wastes heat energy, because the blowdown liquid is at the same temperature as the steam produced—approximately 366°F for 150-pounds-per-square-inch-gauge (psig) saturated steam—and blowdown heat recovery systems, if available, are not 100% efficient. (Waste heat may be recovered through the use of a blowdown heat exchanger or a flash tank in conjunction with a heat recovery system. For more information, see Steam Tip Sheet #10, Recover Heat from Boiler Blowdown.)

 

Advantages of Automatic Control Systems

With manual control of surface blowdown, there is no way to determine the concentration of dissolved solids in the boiler water, nor the optimal blowdown rate. Operators do not know when to blow down the boiler, or for how long. Likewise, using a fixed rate of blowdown does not take into account changes in makeup and feedwater conditions, or variations in steam demand or condensate return.

An automatic blowdown-control system optimizes surface-blowdown rates by regulating the volume of water discharged from the boiler in relation to the concentration of dissolved solids present. Automatic surface-blowdown control systems maintain water chemistry within acceptable limits, while minimizing blowdown and reducing energy losses. Cost savings come from the significant reduction in the consumption, disposal, treatment, and heating of water

 How it Works

With an automatic blowdown-control system, high- or low-pressure probes are used to measure conductivity. The conductivity probes provide feedback to a blowdown controller that compares the measured conductivity with a set-point value, and then transmits an output signal that drives a modulating blowdown release valve.

Conductivity is a measure of the electrical current carried by positive and negative ions when a voltage is applied across electrodes in a water sample. Conductivity increases when the dissolved ion concentrations increase.

The measured current is directly proportional to the specific conductivity of the fluid. Total dissolved solids, silica, chloride concentrations, and/ or alkalinity contribute to conductivity measurements. These chemical species are reliable indicators of salts and other contaminants in the boiler water.

Applications

Boilers without a blowdown heat-recovery system and with high blowdown rates offer the greatest energy-savings potential. The optimum blowdown rate is determined by a number of factors, including boiler type, operating pressure, water treatment, and makeup-water quality. Savings also depend upon the quantity of condensate returned to the boiler. With a low percentage of condensate return, more makeup water is required and additional blowdown must occur. Boiler blowdown rates often range from 1% to 8% of the feedwater flow rate, but they can be as high as 20% to maintain silica and alkalinity limits when the makeup water has a high solids content.

Price and Performance Example

For a 100,000 pound-per-hour (lb/ hr) steam boiler, decreasing the required blowdown rate from 8% to 6% of the feedwater flow rate will reduce makeup water requirements by approximately 2,300 lb/hr. (See Steam Tip Sheet #9, Minimize Boiler Blowdown.) Annual energy, water, and chemicals savings due to blowdown rate reductions for a sample system are summarized in the table below. In many cases, these savings can provide a 1- to 3-year simple payback on the investment in an automatic blowdown-control system.

Savings Through Installation of Automatic Blowdown-Control System

Blowdown Reduction, lb/hr

Annual Savings, $

Fuel

Water and Chemicals

Total

1,000

27,200

4,200

31,400

2,000

54,400

8,400

62,800

4,000

108,800

16,800

125,600

 

Purchasing and installing an automatic blowdown-control system can cost from $2,500 to $6,000 (Boiler Control Package). The complete system consists of a low- or high-pressure conductivity probe, temperature compensation and signal conditioning equipment, and a blowdown-modulating valve. Some systems are designed to monitor both feedwater and blowdown conductivity from multiple boilers. A continuous conductivity recording capability might also be desired. The total cost of the automatic blowdown system is dependent upon the system operating pressure and the design and performance options specified.

Recommended Practices

The American Society of Mechanical Engineers (ASME) has developed a consensus on operating practices for boiler blowdown. Sections VI and VII of the ASME Boiler and Pressure Vessel Code describe recommended practices. The ASME Boiler and Pressure Vessel Code can be ordered through the ASME website at www.asme.org.

You can download the original paper from  U.S Department of Energy

Cooling Water Treatment 101 - Conductivity Sensor Principle

Conductivity is the primary measurement of mineral contamination in water and has been in use for this purpose for well over a century. As an on-line measurement it provides inexpensive, reliable monitoring of water quality through the various stages of treatment. The technology for conductivity measurement has improved through many generations of analog and microprocessor-controlled measuring circuits and has greatly improved accuracy and temperature compensation.

When a voltage is applied to the two electrodes of a conductivity sensor immersed in a solution, ions between the electrodes are attracted to the oppositely charged electrode and move toward it as the current flow in the solution. (Current flow in the wiring is carried by electrons.) This current flow is used for the conductivity measurement. However, if the ion migration and electrochemical reaction at the electrode surfaces are significant, these effects will interfere with the conductivity measurement. These effects are often referred to as polarization.

To minimize polarization, AC voltage is always used for conductivity measurements. This changes the polarity of the electrodes frequently enough that ions don't move or react significantly. In addition, in state-of-the-art instrumentation, the voltage and/or frequency of the applied AC are selected by internal auto-ranging to achieve the optimum compromise between obtaining adequate signal and minimizing polarization and other interferences. This auto-ranging is invisible to the user.

The sensor cell constant, determined by the geometry of the electrodes, is another factor critical to the measurement. Lower conductivity measurements generally require a lower cell constant, with relatively large cross-sectional areas of solution and closer electrode spacing in order to obtain a good signal without having to measure too high a resistance.

Higher conductivity measurement generally requires sensors with higher cell constant. This means smaller cross-sectional areas of solution with higher current density and wider electrode spacing to prevent having to measure too low a resistance. The exact cell constant requirements for a particular range depend also on the measuring circuit, the cable and the surface condition of the electrodes. An integrated measuring system is required to assure achieving specified performance across the entire measurement range.

Most process conductivity sensors in the low to moderate conductivity range now use the coaxial cell design with inner and outer electrodes which provides the same effective ratio as the easier to visualize parallel plate electrode design.

Previous work has detailed the determination of the precise cell constant with traceability to ASTM and NIST standards, how this has been incorporated into factory calibration facilities, and many other aspects of conductivity measurement.

In real installations, the vulnerability to interference from both bubbles and particulate contamination is greatly reduced if the spacing between sensor electrodes can be made wider. In addition, wider spacing gives less flow restriction and faster response which can be important in deionizer rinse-down processes. Therefore there is an advantage to providing a measuring circuit that is capable of measuring pure water accurately with a higher cell constant.

Integrated sensors described here use a nominal 0.1 cm-1 cell constant for the full range, including very accurate ultrapure water measurements. Other instrumentation must use lower cell constants with more electrode surface area spaced more closely together. In some applications, the reliability of measurement can be improved significantly by using the higher cell constant but the measuring circuit must be able to accommodate it. Range specifications should be reviewed carefully.