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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.

HOSPITAL REDUCES WATER USAGE IN COOLING TOWERS WITH AUTOMATION

With the state's initiative to reduce water usage by 20 percent by the year 2020, many plants in California are striving to become more environmentally friendly. One such facility includes a leading California hospital that sought to reduce water treatment costs for its HVAC system. The hospital has three individual cooling tower systems that service three centrifugal chillers, with a combined total of 2,800 tons of capacity.

The water treatment program currently in use at the facility was operating at 2.8 cycles of concentration, resulting in 35.7 percent of the tower water makeup being bled to the sewer by the current treatment provider. Given the water quality in the area, this was the maximum cycles of concentration that could be achieved without employing the use of acid or water softening.

The savings that the hospital sought were realized by reconsidering various ways to optimize the water treatment program. Working closely with the Los Angeles Department of Water and Power (LADWP), it was revealed that by introducing a water conservation program to reduce water use through increased cycles of concentration, the facility would actually save more money than it would spend to alter the program, making the proposed project sustainable.

Through testing and lab analysis, the team was able to conclude that six cycles of concentration could be attained, resulting in only 16.7 percent of the tower makeup water being bled into the sewer treatment system. This could be achieved through the introduction of a safe acid feed system that would minimize scale, corrosion and microbiological fouling to enable the increase in cycles of concentration while also protecting facility staff from coming into contact with the chemicals.

The evaporation of the cooling tower remained the same, but U.S. Water was able to reduce blowdown, cutting water usage by an estimated 3.6 million gallons per year and decreasing water and sewage costs. The plant was able to save over $76,000 (see Fig. 1).

Anytime chemistry in the cooling tower is stressed by adding more cycles, tight control of the chemistry is required to prevent scale formation. This led to the introduction of U.S. Water's advanced automation controls. The advanced automation program included wireless monitoring and alarm notifications to manage the overall program performance, and the equipment monitored conductivity, pH, scale inhibitor levels, tower makeup usage, and tower bleed usage.

At any given time, designated hospital personnel and U.S. Water representatives, using various levels of password-protected security outlined by the facility, can securely access the data for review and online adjustment. If designated parameters fell above or below the specified range, a U.S. Water representative was alerted for quick response (see Fig. 3).

Second to irrigation, cooling towers offer the largest potential for water savings in California. As an added incentive, the state of California has put programs in place to rebate facilities for the cost of automating their systems. LADWP and the Metropolitan Water District (MWD), for example, offer three programs that finance automation for cooling towers due to their ability to increase cycles of concentration, which reduces water use.

This financing allowed U.S. Water to implement the $34,000 advanced automation program to monitor and control the water treatment program for this hospital at no cost to the hospital.

Results to date for the facility include significant reduction in water usage, lower water and sewage bills and more efficient monitoring due to the installed automation software to protect the equipment assets.

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

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

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/

Ozonation of Cooling Towers

The objective of ozone use with cooling towers is to maintain the highest purity of water with the least amount of water waste and chemical use. Chemical use in cooling towers leads to ever-increasing total dissolved solids (TDS), which must be reduced by eliminating water (blow down/bleed off) and then refilling with raw/lower TDS water. This is a vicious circle that will never end unless one of the TDS-increasing culprits (a.k.a. chemicals) is eliminated or reduced.

The Problem

Cooling tower water quality tends to be extremely poor. Cooling tower traditional treatment is based on extreme chemical use only. This means that you, the water treatment professional, have a chance to create an entirely new income base and aid in environmental integrity and responsibility. There are three main problems surrounding cooling towers.

* Water quality control is difficult due to

- Evaporation rate,

- Environmental contaminants, and

- Extreme chemical use.

* Chemical dependence is promoted by an industry that serves and maintains cooling towers. Most cooling tower manufacturers do nothing about recommending or selling treatment equipment along with the towers. In most cases, it is left up to the end users to set up the treatment method. The cost of chemicals is lower on the front end than water treatment equipment, but far higher based upon the ongoing nature of the use.

* The water waste issue. For example, it is not uncommon to see a 3,000-gallon cooling tower constantly draining water, then constantly replenishing raw water just to lower TDS. This ever-increasing TDS is contributed to a great degree by the chemicals that are used for treatment.

Not only is there an extreme amount of water being wasted on a daily basis, but the environmental impact from the chemical-laden wastewater is deplorable. This chemical-laden wastewater eventually will make its way into our lakes, streams, rivers and groundwater. That is why this wastewater is becoming the subject of more stringent U.S. Environmental Protection Agency regulations.

Cooling Towers and Ozone Primary Uses

Ozone is used in cooling tower treatment for

* Bacteria/virus elimination/prevention

* Organic build-up elimination/prevention

* Blow-down reduction/elimination

* Bleed-off reduction/elimination

* Improved clarity

* Scale reduction

* Cooler running temperatures where scale is inhibited or reduced

* Reduction or elimination chemicals needed for algae control

Principle of Operation

Ozone is injected into the water flow created by a separate circulation pump. This pump pulls the water from the tower's sump or basin and sends it to the ozone injector, contact tank and scale removal/filtration system. Lastly, the treated water returns back to the sump or basin. The principle is to treat the water and eliminate/reduce the following contaminants.

* Scale-forming minerals

* Organics

* Algae

* Harmful microbes

The clean water then is used to clean the entire sump, basin, pipes and peripheral equipment.

The ozone treatment system is simple and can be broken down into three easy steps.

* Ozone injection. Ozone is injected into the side stream flow. Oxidation starts to take place immediately on microbes, organics, bacteria and viruses.

* Contact/mixing. A contact tank helps to further the ozone's ability to oxidize particles allowing them time to react prior to returning to the system. As water flows down the off-gas tank, ozonated water rises and strips any gas in the incoming water. (The off gas tank is the same design as what was discussed in my column "Ozone Installation," February 2003, and "Well-Ozone Again," December 2002, Water Quality Products.)

* Filtration, scale control, particle removal. Possibly the most important aspect of any water treatment is the removal of the particles that have been oxidized. Without this step, all you have done with the ozone is change the structure of the particles by making them larger, insoluble and/or heavier. This step is necessary for systems that require scale control and particulate removal.

It is very important not to construct an ozone unit too large to handle the bacteria, scale and algae. The problem encountered at this point could be corrosion. If you carry an ozone residual too high to de-scale downline you stand a chance of creating a corrosive situation in the sump and its adjacent equipment. For this reason, it's important to utilize existing water treatment technology and equipment in conjunction with ozonation. The result is a system that works without high maintenance, dangerous chemicals, extreme water waste and costly corrosion.

Read this article at https://www.wqpmag.com/o-zone-todays-lesson-ozonation-cooling-towers

ALTERNATIVAS DE CLORO EN LA PRODUCCIÓN PRODUCTO FRESCO

El agua de lavado utilizada en la producción de productos frescos siempre contiene un desinfectante residual utilizado para reducir la carga microbiana. Hay una tendencia creciente hacia desinfectantes alternativos tales como dióxido de cloro en lugar de los agentes oxidantes tradicionales tales como el cloro. Una cuidadosa elección del desinfectante y el tiempo de contacto de control puede conducir a una vida útil más larga para los productos que generan una ventaja significativa para los fabricantes.

Introducción

La cantidad de productos frescos que se producen en todo el mundo está aumentando debido a los objetivos nacionales para promover la alimentación saludable y el papel central que las frutas y hortalizas frescas en la consecución de esos objetivos. El aumento de la demanda está hermanada con las crecientes limitaciones en el uso del agua por los fabricantes. La escasez de agua ha llevado a un renovado interés por la industria de productos frescos en usar el agua de manera más eficaz y revisar su elección de desinfectante de agua de lavado. La reutilización de agua puede conducir a una acumulación de material orgánico en el agua potencialmente resultando en niveles más altos de subproductos de desinfección (DBPs).

Zero Liquid Discharge (ZLD) es cada vez más un objetivo de cualquier fabricante usando agua en su proceso. La reutilización de agua para lograr ZLD por los fabricantes es una tendencia creciente en la industria de productos frescos que cambia el énfasis en el tratamiento de agua, tanto durante el proceso de lavado y una vez que el agua ha sido utilizada. Directrices1 ahora se están produciendo para dar asesoramiento a los fabricantes de productos frescos que pueden ayudar a los SPD de control producida en el lavado de los productos frescos.

El análisis del agua de lavado usada en el proceso de lavado es crucial para controlar la microflora que se pueden encontrar en el producto final y comprender el potencial de la PAD crucial en la comprensión de su resultado de la prueba.

Microbiología

Como productos frescos generalmente se cultiva al aire libre que siempre contendrá algunos microflora, (definido como bacterias y algas microscópicas y hongos, especialmente los que viven en un sitio particular o hábitat).

Ninguna cantidad de LAVADO SE QUITE siempre totalmente todos los agentes patógenos que pueden estar presentes EN EL PRODUCTO

Una demanda de productos frescos, independientemente de si es en temporada conduce localmente a problemas en la obtención de los productos en bruto. El reto es causada por la microbiología natural del medio ambiente y / o normas de higiene siendo significativamente diferente de donde el producto final es finalmente consumida en crecimiento. Hidroponía como un medio de crecimiento pueden circunnavegar algunos de los problemas asociados a donde se cultiva el producto, pero esto no resuelve la cuestión de la calidad de la higiene durante la cosecha.

Lavar el Producto

lavar el producto es el único método para reducir la carga microbiana y ninguna cantidad de lavado será nunca eliminar completamente todos los agentes patógenos que pueden estar presentes en el producto.

La variabilidad de las reducciones de registro está relacionado con el tipo de producto a que se lava, el tiempo de contacto y el desinfectante utilizado. Típicamente, cuando un producto o químico se prueba para la eficacia en matar los gérmenes, bacterias, virus, etc. se utiliza la reducción logarítmica plazo.

En términos simples, log reducción proporciona una medida cuantitativa que describe qué porcentaje de los contaminantes que estaban presentes cuando la prueba comenzó murieron durante la prueba.

Como un ejemplo, si partimos de una carga microbiana de 1.000.000 de células, una reducción logarítmica de 3 = 1.000.000 x 0,10 x. 0,10 x 0,10 = 1,000 células permanecen (0,1%); una tasa de muertes 99,9%. La siguiente tabla muestra los valores Ct de que la desactivación de los virus por varios desinfectantes *:

 

Desinfectante

Unidades

 

La inactivación

 

 

 

2 log

3 log

4 log

Chlorine1

mg - min / L

3

4

6

Chloramine2

mg - min / L

643

1067

1491

El cloro Dioxide3

mg - min / L

4.2

12.8

25.1

Ozono

mg -min / L

0.5

0.8

1.0

UV

mW - s / cm2

21

36

N / A

 

* valores de CT de la AWWA, 1991

1 - valores basados ​​en la temperatura de 10 ° C, rango de pH de 6 - 9 y un residual de cloro libre de 0,2 - 0,5 mg / L

2- valores basado en la temperatura de 10 ° C, pH de 8 3- valores basado en la temperatura de 10 ° C, intervalo de pH de 6 - 9

Si desinfectante adecuada está presente, todas las células muertas se eliminan a través de la oxidación y el desinfectante deben administrar las células restantes hasta que se lleva a cabo la siguiente purga programada. Sin embargo, si las células se unen para formar una biopelícula, incluso con un buen nivel de desinfección, el nuevo crecimiento del biofilm es probable que ocurra rápidamente. En particular, si no se han eliminado las células muertas. El dióxido de cloro es especialmente eficaz en la lucha contra las biopelículas.

Históricamente, superchlorination de agua de lavado fue el método predominante de tratamiento de productos frescos y puede conducir a una reducción de la carga microbiana por 10 a 100 veces más largo que el tiempo de contacto es suficiente y la forma de cloro presente en el agua de lavado se controla a través ordinario pruebas. La agitación y la inmersión del producto durante el lavado es una parte esencial de asegurar la máxima eficacia del desinfectante. En los últimos años se ha producido un cambio hacia formas alternativas de desinfectantes debido a preocupaciones sobre la cloración de producción por productos cuando superchlorinating.

Aunque la evidencia está limitada hasta el momento, las lecciones aprendidas de la industria del agua potable (donde las pruebas para la cloración de los productos es un requisito legal) han impulsado a los fabricantes a mirar a los desinfectantes alternativos. Esto es especialmente cierto en el creciente mercado de productos orgánicos productos frescos y en ciertos mercados donde se restringe superchlorination de productos frescos (por ejemplo, Dinamarca). El dióxido de cloro supera algunas de las desventajas del uso de cloro para desinfectar, ya que no depende de un control cuidadoso del pH del agua de lavado. Como es volátil por lo general se requiere para ser generada en el sitio pero las ventajas más de cloro son ahora cada vez más evidente.

Kits de prueba de calidad del agua

On-line controladores se utilizan con frecuencia para controlar el nivel de desinfectante en el agua de lavado. Aunque eficaz en la vigilancia de los cambios en el nivel de desinfectante en el agua de lavado, ya que se basan a menudo en ORP (potencial de oxidación-reducción) de medición, que carecen de selectividad lo que significa que no se puede confiar únicamente en la hora de garantizar una desinfección eficaz está teniendo lugar. Un método de prueba secundaria casi siempre se requiere con el fin de calibrar el dispositivo de en línea y proporcionar un método de ensayo secundario para si el mal funcionamiento en el controlador de línea. Controles in situ de en-línea de eficacia controlador se lleva a cabo generalmente usando un método portátil tal como un colorímetro.

Algunos de la reluctancia en el cambio a formas alternativas de desinfectante se basa en dificultades asociadas con estos métodos secundarios de las pruebas de agua. métodos de ensayo tradicionales involucran el uso de métodos colorimétricos portátiles para determinar los niveles de desinfectante en el agua de lavado. Sin embargo los inconvenientes de este método son conocidos por la industria de los productos frescos.

Ellos incluyen una falta de especificidad (por ejemplo, no ser capaz de determinar fácilmente el cloro libre en oposición a cloro combinado, específicamente en los niveles Superchlorination), la complejidad de la prueba y el uso de artículos de vidrio y químicas reactivos que no es apropiado en entorno de producción de alimentos.

Cronoamperométrica desechables SENSOR MÉTODOS están cambiando la forma en que PRUEBA portátil es REALIZADA

Los avances en los métodos de prueba portátiles, tales como métodos de sensores desechables cronoamperométrica están cambiando la manera en la que las pruebas portátil se lleva a cabo dentro de la industria de productos frescos. La superación de muchos de los inconvenientes de los métodos colorimétricos, la simplicidad y la facilidad de uso de los sensores es la fuerza impulsora clave detrás de su adopción. También son mucho más altamente selectivo cuando varios oxidantes están presentes en la muestra.

Conclusión

A medida que la industria de productos frescos crece, hay un aumento de la motivación para que los fabricantes ambos consideran formas alternativas de desinfectante como el dióxido de cloro y para centrarse en la reutilización del agua de lavado. Al hacerlo, la industria está adoptando las mejores prácticas aprendidas de la industria del agua potable.

Con respecto a las pruebas de agua, la comprensión de la capacidad del método de ensayo que se utilizan pueden ayudar a manejar los fabricantes de procesos de producción y de proceso de alimentos necesitan para construir relaciones más estrechas con los fabricantes de equipos de análisis de agua con el fin de asegurarse de que tienen los mejores métodos de análisis para su línea de producción . Esto es especialmente importante cuando se considera la desinfección potencial by-productos3 y cuando hay varios oxidantes presentes en cualquier muestra, como se representa en la siguiente tabla:

 

Desinfectante

desinfección organohalogenic subproductos

desinfección inorgánico subproductos de desinfección no halogénico subproductos

desinfección no halogénico subproductos

El cloro (Cl2 / ácido hipocloroso [HOCl])

trihalometanos, ácidos halógenos acético, haloacetonnitrils, hidratos de cloro, cloropicrina, clorofenoles, N-cloraminas, halofuranones, bromhidrinas

clorato (particuarly la aplicación de hipoclorito)

aldehídos, ácidos alcanos, benceno, ácidos carboxílicos

El dióxido de cloro (ClO2)

clorito, clorato

clorito, clorato

desconocido

Las cloraminas (NH2Cl etc)

haloacetonnitrils, cloro ciano, cloraminas orgánicas, ácidos chloramino, clorhidratos, haloketons

nitrito, nitrato, clorato, hidracina

aldehídos, cetonas

El ozono (O3)

bromoformo, ácido acético monobromine, acetona dibromuro, bromo ciano

clorato, yodato, bromato, peróxido de hidrógeno, ácido hypobromic, epoxi, ozonates

aldehídos, cetonas, cetoácidos, ácidos carboxílicos


referencias

1- Directriz no. 70 - Directrices para la reutilización de agua potable para las operaciones de procesamiento de alimentos. Preparado por el Grupo de Trabajo de reutilización del agua del Panel de Microbiología. Editado por el Dr. John Holah 2012

2- EPA Directriz para desinfectantes alternativos, Sección 4.8.1, ventajas y desventajas de dióxido de cloro Use (1999)

3- tabla que muestra desinfectantes y su desinfección común subproductos, Palintest

Chlorine alternatives in Fresh Produce production

Wash water used in the production of fresh produce always contains a residual disinfectant used to reduce microbial load. There is a growing trend towards alternative disinfectants such as chlorine dioxide rather than traditional oxidising agents such as chlorine. Careful choice of disinfectant and controlling contact time can lead to an extended shelf life for products generating a significant advantage for manufacturers.

Introduction

The amount of fresh produce being produced across the globe is increasing due to the national targets for promoting healthy eating and the core role that fresh fruit and vegetables play in achieving those targets. Increasing demand is twinned with growing constraints on the use of water by manufacturers. Water scarcity has led to a renewed focus by the fresh produce industry on using water more effectively and reviewing their choice of wash water disinfectant. Re-use of water can lead to a build up of organic material in the water potentially resulting in higher levels of disinfection by-products (DBPs). 

“disinfection

Zero Liquid Discharge (ZLD) is increasingly an aim of any manufacturer using water in their process. Reuse of water to achieve ZLD by manufacturers is a growing trend within the fresh produce industry which shifts emphasis onto the treatment of water both during the washing process and once the water has been used. Guidelines1 are now being produced to give advice to fresh produce manufacturers which may help control DBPs produced in washing fresh produce. 

Testing the wash water used in the washing process is crucial to controlling the microflora that can be found in the final product and understanding the DBP potential crucial in understanding your test result. 

Microbiology

As fresh produce is generally grown outdoors it will always contain some microflora, (defined as bacteria and microscopic algae and fungi, especially those living in a particular site or habitat). 

caption: no amount of washing will ever completely remove all the pathogens that may be present in the productNO AMOUNT OF WASHING WILL EVER COMPLETELY REMOVE ALL THE PATHOGENS THAT MAY BE PRESENT IN THE PRODUCT

A demand for fresh produce regardless of whether it is in season locally leads to issues in sourcing the raw produce. The challenge is caused by the natural microbiology of the growing environment and/or standards of hygiene being significantly different from where the final product is finally consumed. Hydroponics as a growth medium can circumnavigate some of the issues associated with where the product is grown but this doesn’t resolve the issue of standard of hygiene during harvesting. 

Produce washing

Produce washing is the only method of reducing microbial load and no amount of washing will ever completely remove all the pathogens that may be present in the product. 

The variability of log reductions is related to the type of produce being washed, the contact time and the disinfectant used. Typically, when a product or chemical is tested for effectiveness in killing germs, bacteria, virus, etc. the term log reduction is used. 

In simple terms, log reduction provides a quantitative measurement describing what percentage of the contaminants which were present when the test began were killed during the test. 

As an example, if we start with a microbial load of 1,000,000 cells, a log reduction of 3 = 1,000,000 x 0.10 x. 0.10 x 0.10 = 1,000 cells remain (0.1%); a 99.9% kill rate. The table below shows CT-values for the deactivation of viruses by various disinfectants*: 

 

Disinfectant  Units  Inactivation 
2 log  3 log  4 log 
Chlorine1 mg - min/L 
Chloramine2 mg - min/L  643  1067  1491 
Chlorine Dioxide3 mg - min/L  4.2  12.8  25.1 
Ozone  mg -min/L  0.5  0.8  1.0 
UV  mW - s/cm2 21  36  N/A 

 

*CT values from the AWWA, 1991
1 - values based on temperature of 10°C, pH range of 6 – 9 and a free chlorine residual of 0.2 – 0.5mg/L
2- values based on temperature of 10°C, pH of 8
3- values based on temperature of 10°C, pH range of 6 – 9

If adequate disinfectant is present, all dead cells are removed via oxidation and the sanitizer should manage the remaining cells until the next scheduled purge is conducted. However, if the cells come together to form a biofilm, even with a good sanitizer level, biofilm regrowth is likely to occur quickly. Particularly, if the dead cells have not been removed. Chlorine dioxide is especially effective in tackling biofilms. 

Historically, superchlorination of wash water was the predominant method of treating fresh produce and can lead to a reduction in microbial load by 10 to 100 times as long as the contact time is sufficient and the form of chlorine present in the wash water is controlled through regular testing. Agitation and submersion of the produce during washing is an essential part of ensuring the maximum efficacy of the disinfectant. In recent years there has been a shift to alternative forms of disinfectants due to concerns over the production chlorination by products when superchlorinating. 

Although evidence is limited thus far, lessons learned from the drinking water industry (where testing for chlorination by products is a legal requirement) have driven manufacturers to look at alternative disinfectants. This is especially true in the increasing organic fresh produce market and in certain markets where superchlorination of fresh produce is restricted (e.g. Denmark). Chlorine dioxide overcomes some of the disadvantages of using chlorine to disinfect as it not reliant on careful control of the pH of the wash water. As it is volatile it is generally required to be generated on site but the advantages over chlorine are now becoming clear. 

Water quality test kits

On-line controllers are frequently used to monitor the level of disinfectant within wash water. Although effective at monitoring changes in disinfectant level within the wash water, as they are often based on ORP (oxidation-reduction potential) measurement, they lack selectivity meaning they cannot be solely relied upon in ensuring effective disinfection is taking place. A secondary testing method is almost always required in order to calibrate the on-line controller and provide a secondary test method for if the on line controller malfunctions. Spot checks on on-line controller efficacy is usually carried out using a portable method such as a colorimeter. 

Some of the reluctance in switching to alternative forms of disinfectant is based around difficulties associated with these secondary methods of water testing. Traditional testing methods involve using portable colorimetric methods for determining the levels of disinfectant in the wash water. However the drawbacks of this method are known to the fresh produce industry. 

They include a lack of specificity (e.g. not being able to easily determine free chlorine as opposed to combined chlorine, specifically at superchlorination levels), the complexity of the test and the use of glassware and chemical reagents which is not appropriate in food production environment. 

caption: chronoamperometric disposable sensor methods are changing the way in which portable testing is carried outCHRONOAMPEROMETRIC DISPOSABLE SENSOR METHODS ARE CHANGING THE WAY IN WHICH PORTABLE TESTING IS CARRIED OUT

Developments in portable testing methods such as chronoamperometric disposable sensor methods are changing the way in which portable testing is carried out within the fresh produce industry. Overcoming many of the drawbacks of colorimetric methods, the simplicity and ease of use of the sensors is the key driving force behind their adoption. They are also much more highly selective when multiple oxidants are present in the sample. 

Conclusion

As the fresh produce industry grows, there is increased motivation for manufacturers to both consider alternative forms of disinfectant such as chlorine dioxide and to focus on the reuse of wash water. In doing so the industry is adopting best practice learned from the drinking water industry. 

With regards to water testing, understanding the capability of the test method being used can help manage the production process and food process manufacturers need to build closer ties with water testing equipment manufacturers in order to ensure they have the best methods of analysis for their production line. This is especially important when considering the potential disinfection by-products3 and when there are multiple oxidants present in any one sample, as represented in the table below: 

 

Disinfectant  organohalogenic disinfection by-products  inorganic disinfection by-products non-halogenic disinfection by-products  non-halogenic disinfection by-products 
Chlorine (Cl2/ hypochlorous acid [HOCl]) trihalomethanes, halogenic acetic acids, haloacetonnitrils, chlorine hydrates, chloropicrin, chlorophenols, N-chloramines, halofuranones, bromohydrins  chlorate (particuarly the application of hypochlorite)  aldehydes, alkanic acids, benzene, carboxylic acids 
Chlorine dioxide (ClO2) chlorite, chlorate  chlorite, chlorate  unknown 
Chloramines (NH2Cl etc) haloacetonnitrils, cyano chlorine, organic chloramines, chloramino acids, chlorohydrates, haloketons  nitrite, nitrate, chlorate, hydrazine  aldehydes, ketones 
Ozone (O3) bromoform, monobromine acetic acid, dibromine aceton, cyano bromine  chlorate, iodate, bromate, hydrogen peroxide, hypobromic acid, epoxy, ozonates  aldehydes, ketones, ketoacids, carboxylic acids 


References

1- Guideline no. 70 - Guidelines on the reuse of potable water for food processing operations. Prepared by the Water Reuse Working Party of the Microbiology Panel. Edited by Dr. John Holah, 2012

2- EPA Guideline to Alternative Disinfectants, Section 4.8.1, Advantages and Disadvantages of Chlorine Dioxide Use (1999)

3- Table showing disinfectants and their common disinfection by-products, Palintest

Utilize valvulas de control para optimizar su eficiencia

El gasto de tratamiento de agua para el uso en torres de refrigeración, calderas, y otras aplicaciones de la planta está aumentando rápidamente. Además de los altos costes, las plantas se enfrentan a menudo la necesidad de cumplir con los efluentes de las regulaciones que rigen, incluyendo el agua de enfriamiento, enviada a través de las instalaciones de tratamiento antes de su descarga.

funcionamiento óptimo de los sistemas de agua de refrigeración significa un uso mínimo de agua mientras se mantiene la temperatura adecuada para limitar el crecimiento de algas y enfriar todo el equipo correctamente. Una forma de ayudar a lograr estos objetivos al tiempo que reduce significativamente el consumo de energía es la instalación de válvulas de control de agua (CW) de refrigeración.

Un sistema bien equilibrado

Funcionamiento de un sistema de agua de refrigeración de manera eficiente requiere equilibrio. Un sistema bien equilibrado es una de la que se elimina los cortocircuitos. El cortocircuito se produce cuando el agua de refrigeración excesiva fluye a través de un enfriador causando flujo insuficiente a través de los otros. Esta morir de hambre a menudo se produce al final de un sistema o en unidades en las elevaciones más altas.

El logro de un sistema equilibrado es un proceso detallado y complicado. Las caídas de presión deben ser figurado para cada pieza de equipo y las tuberías correspondientes, y para cada rama del circuito de agua de refrigeración. Incluso si estos cálculos se realizan cuando una planta es nueva, las condiciones cambian con el tiempo. Depósitos se acumulan en las superficies que alteran el coeficiente de transferencia de calor y resistencia al flujo (caída de presión). Adición o eliminación de equipo del sistema también cambia el equilibrio y puede conducir a cortocircuitos.

equilibrio manual de un sistema de agua de refrigeración usando placas de orificios es difícil y consume tiempo. Las preocupaciones de seguridad a menudo dictan que un orificio estar dimensionado para la máxima demanda. Como resultado, la bomba de agua de refrigeración debe ser dimensionado para, y a menudo debe operar a, altas velocidades de flujo en exceso.

A veces se hacen intentos para equilibrar un sistema mediante el uso de una válvula de globo y estrangular manualmente el flujo. Por desgracia, este enfoque conduce a menudo a un operador de abrir la válvula completamente cuando se necesita flujo máximo, entonces no es el reajuste. De nuevo el resultado es alto flujo cuando el sistema requiere un flujo promedio o mínimo.

El exceso de flujo se indica por una temperatura de enfriamiento de salida de agua sólo unos pocos grados por encima de la entrada. Esta condición de flujo desequilibrado conduce a un mayor consumo de energía de bombeo y zonas distantes o elevadas que a menudo se ven privadas para el agua.

Superior de retorno de agua de refrigeración (salida) las temperaturas dan como resultado un menor consumo de agua de refrigeración. En estas condiciones, la temperatura del agua de refrigeración se debe aumentar hasta el máximo permitido por el proceso. Este incremento se logra reduciendo al mínimo el flujo. Pero antes de tomar esta acción, otras condiciones deben ser evaluados.

Las temperaturas más altas (por encima de 120 F) pueden hacer que el calcio para precipitar fuera del agua a una velocidad alta, resultando en escala y conduce a una mayor caída de presión y la transferencia de calor reducida. El aumento de las temperaturas también promueven el crecimiento de algas. La tasa varía con la calidad del agua y tipo de tratamiento.

La distribución de agua de refrigeración a lo largo de un sistema requiere controladores adecuados que mantienen las temperaturas de salida dentro de un rango especificado, incluso durante el enfriamiento parcial. Si las temperaturas de salida no se pueden aumentar, los controladores todavía pueden reducir el flujo cuando las necesidades de agua caen.

configuraciones de válvula de control

Las válvulas de control se aplican con éxito en una variedad de sistemas de refrigeración de agua. En la mayoría de sistemas, una válvula de control CW proporcional se puede instalar en la línea de retorno (Fig. 1). La válvula, que controla el caudal de agua en proporción directa a la temperatura de salida, debe estar situado tan cerca del enfriador como sea posible.

Cuando el agua de refrigeración está frío, la válvula reduce el caudal a un ligero sangrado. A medida que la temperatura de salida se eleva, la válvula se abre y se regula el flujo para mantener una temperatura de descarga constante. La válvula CW debe estar diseñado para mantener un flujo de purga constante. Sin algo de flujo, el elemento sensor de la válvula no puede decir lo que está pasando.

El uso de válvulas de control CW asegura equilibrado automático del sistema de agua de refrigeración, debido a que la válvula sólo utiliza tanta agua como el enfriador requiere. uso reducido de agua asegura se proporciona un suministro adecuado de agua de refrigeración, incluso a zonas alejadas de la nevera o en elevaciones más altas.

El mantenimiento de una temperatura de proceso en un valor preciso requiere un esquema de control diferente. Un sensor de temperatura (termopar), el controlador y la válvula de control accionada neumática o eléctricamente puede ser utilizado. Otra opción es una válvula de control de auto Cualquier disposición controla temperaturas de la corriente de proceso con diferentes grados de precisión. En muchos casos, la válvula de acción automática ofrece una precisión razonable a un menor coste de instalación.

pautas de aplicación

En aplicaciones que tienen una descarga abierta a un desagüe, la línea de descarga de la válvula CW debe estar siempre lleno. Esta condición puede garantizarse con un sello de bucle en la tubería de salida a una altura por encima de la válvula que luego va a grado (Fig. 3). Sin un sello líquido, líneas pueden vaciar cuando el equipo está apagado. El líquido o elementos de sellado termostáticas lleno de cera se pueden secar y fallar prematuramente.

Un colador puede ser instalado aguas arriba de la válvula de CW si las condiciones de calidad del agua requieren. Suciedad y los residuos afectan cierre adecuado y caricias de la válvula. Si existe este problema, una anulación de neumático (Fig. 4) puede ser utilizado para purgar la válvula de suciedad.

La mejor manera de controlar el crecimiento de algas y la acumulación es mantener la alta calidad del agua. Otros factores que contribuyen al crecimiento de algas incluyen la temperatura y la velocidad. Manteniendo la temperatura por debajo de 120 F es deseable. Dimensionamiento de la velocidad de flujo para lograr una mayor velocidad también tiende a obstaculizar el crecimiento de algas. Las algas limo ha sido conocido para formar en líneas con velocidades que pueden alcanzar tan alto como 10 pies / seg.

El lado del agua de refrigeración de un proceso a menudo se pasa por alto como incontrolable. Sin embargo, el enfriamiento válvulas de control de agua puede promover el ahorro al reducir el uso de agua, bomba de las necesidades de energía, y los costos de tratamiento de agua. En la nueva construcción, pequeños tamaños de tuberías y bombas pueden reducir los costes de equipamiento. válvulas CW también proporcionan mejor control del proceso mediante el mantenimiento de una diferencia de temperatura fijo a través de la entrada de agua refrigerante y la salida. En la mayoría de los casos, los análisis justificar la instalación de dichos controles.

- Editado por Jeanine Katzel, Editor Senior, 847-390-2701, j.katzel@cahners.com

Más información

El autor responderá a las preguntas técnicas sobre este artículo. Se le puede contactar por teléfono al 201

Conceptos clave

funcionamiento óptimo de los sistemas de agua de enfriamiento ayuda a limitar el crecimiento de algas y equipo fresco adecuadamente mientras se mantiene la temperatura adecuada.

Enfriamiento válvulas de control de agua reducen el uso del agua, las necesidades de energía de la bomba, y los costes de tratamiento de agua.

ahorro de agua y energía suelen proporcionar un rápido retorno de la inversión sistema de válvulas.

La justificación de los costes

Los ahorros en agua y la energía de refrigeración típicamente proporcionan una recuperación de la inversión rápida de la inversión en el sistema de la válvula. Enfriamiento válvulas de control de agua también reducen los costos de capital de una nueva instalación al permitir el uso de bombas y filtros más pequeños, y, en algunos casos, la reducción de tamaño de las tuberías.

Un ejemplo de ahorros obtenidos en un sistema retrofit se muestra a continuación.

Condiciones existentes:

Q = calor tasa de eliminación del enfriador, 700.000 Btu / hr

T (sub i) = entrada de temperatura del agua refrigerante, 50 F

T (sub o) = temperatura de salida del agua de refrigeración sin control, 59 F

C (sub p) = calor específico, 1 Btu / lb / ° F

m = tasa de flujo de masa, lb / hr

v = velocidad de flujo volumétrico, gpm

m = Q / C (sub p) (T (sub o) - T (sub i)) = 700.000 / 1 (59 - 50) = 77.777 lb / hr

v = 155.5 gpm

(Para v, para convertir lb / hr en gpm divide por el factor de conversión de 500, que se obtiene multiplicando 8,33 lb / gal. De agua por 60 min / hr.)

Después de que se instala una válvula de CW, la temperatura de descarga se puede ajustar a 82 F. Inserción de la nueva T (sub o) en los rendimientos ecuación:

m = Q / C (sub p) (T (sub o) - T (sub i)) = 700.000 / 1 (82 - 50) = 21.875 lb / hr

v (nueva tasa de flujo volumétrico) = 43,7 gpm

En un sistema en el que no se recircula agua, el uso gotas de agua 72%. En un sistema de circuito cerrado, una cierta cantidad de agua se pierde por evaporación en la torre de enfriamiento y durante la purga. los costes de tratamiento del agua también deben tenerse en cuenta en el análisis.

Además de un ahorro de agua, la energía se conserva porque se necesita menos energía para bombear menos agua. La figura ahorro de energía de la bomba muestra tres gráficos de la cabeza de descarga. Eficiencia y consumo de energía de una bomba típica centrífuga se representan frente a desplazamiento de volumen. Tenga en cuenta que incluso con una reducción en la eficiencia, el consumo de energía es de 6,5 kW antes de instalar la válvula de control y 3,5 kW después, una reducción de energía del 46%.

A un costo de agua tratada de $ 0.50 / 1000 gal. y el costo de energía de $ 0.05 / kWh, el ahorro total anual de $ 7190. La figura supone una pérdida de agua 10% de la evaporación de purga. El ahorro anual para un sistema abierto de descarga son más de $ 60.000.

 

Using Control Valves To Optimize Cooling Water System Efficiency

The expense of treating water for use in cooling towers, boilers, and other plant applications is rapidly increasing. In addition to high costs, plants often face the need to comply with regulations governing effluents, including cooling water, sent through treatment facilities before being discharged.

Optimum operation of cooling water systems means minimum use of water while maintaining proper temperatures to limit algae growth and cool all equipment properly. One way to help achieve these goals while significantly reducing energy consumption is to install cooling water (CW) control valves.

A well-balanced system

Operating a cooling water system efficiently requires balance. A well-balanced system is one from which short-circuiting is eliminated. Short-circuiting occurs when excessive cooling water flows through one cooler causing insufficient flow through the others. This starving often occurs at the end of a system or in units at higher elevations.

Achieving a balanced system is a detailed and complicated process. Pressure drops must be figured for each piece of equipment and its associated piping, and for each branch of the cooling water circuit. Even if these calculations are done when a plant is new, conditions change over time. Deposits build up on surfaces altering the heat transfer coefficient and resistance to flow (pressure drop). Adding or removing equipment from the system also changes the balance and can lead to short circuiting.

Manually balancing a cooling water system using orifice plates is difficult and time consuming. Safety concerns often dictate that an orifice be sized for maximum demand. As a result, the cooling water pump must be sized for, and often must operate at, excessively high flow rates.

Sometimes attempts are made to balance a system by using a globe valve and manually throttling the flow. Unfortunately, this approach often leads to an operator opening the valve fully when maximum flow is needed, then never readjusting it. Again the result is high flow when the system requires average or minimum flow.

Too much flow is indicated by a cooling water outlet temperature only a few degrees above the inlet. This unbalanced flow condition leads to higher pump energy consumption and distant or elevated areas that are often starved for water.

Higher cooling water return (outlet) temperatures result in lower cooling water consumption. Under these conditions, cooling water temperatures should be increased to the maximum permitted by the process. This increase is accomplished by minimizing the flow. But before this action is taken, other conditions must be evaluated.

Higher temperatures (above 120 F) can cause calcium to precipitate out of water at a high rate, resulting in scaling and leading to increased pressure drop and reduced heat transfer. Increased temperatures also promote algae growth. The rate varies with quality of water and type of treatment.

Distributing cooling water throughout a system requires proper controllers that maintain outlet temperatures within a specified range, even during partial cooling. If outlet temperatures cannot be increased, controllers can still reduce the flow when water requirements drop.

Control valve configurations

Control valves are successfully applied in a variety of cooling water systems. In most systems, a proportional CW control valve can be installed in the return line (Fig. 1). The valve, which controls the water flow rate in direct proportion to the outlet temperature, should be located as close to the cooler as possible.

When the cooling water is cold, the valve reduces the flow rate to a slight bleed. As the outlet temperature rises, the valve opens and regulates the flow to maintain a constant discharge temperature. The CW valve should be designed to maintain a constant bleed flow. Without some flow, the valve sensing element cannot tell what is going on.

Use of CW control valves ensures automatic balancing of the cooling water system, because the valve uses only as much water as the cooler requires. Reduced water use ensures an adequate supply of cooling water is provided, even to areas far from the cooler or at higher elevations.

Maintaining a process temperature at a precise value requires a different control scheme. A temperature sensor (thermocouple), controller, and pneumatically or electrically actuated control valve can be used. Another option is a self-acting control valve with a capillary tube (Fig. 2) inserted in the process stream. Either arrangement controls process stream temperatures with varying degrees of accuracy. In many cases, the self-acting valve offers reasonable accuracy at a lower installed cost.

Application guidelines

In applications that have an open discharge to a drain, the CW valve discharge line should always be full. This condition can be ensured with a loop seal at the outlet piping at an elevation above the valve that then goes to grade (Fig. 3). Without a liquid seal, lines may empty when equipment is shut down. Liquid or wax-filled thermostatic seal elements can dry out and fail prematurely.

A strainer may be installed upstream of the CW valve if water quality conditions require. Dirt and debris affect proper closing and stroking of the valve. If this problem exists, a pneumatic override (Fig. 4) can be used to purge the valve of dirt.

The best way to control algae growth and buildup is to maintain high water quality. Other factors contributing to algae growth include temperature and velocity. Keeping the temperature below 120 F is desirable. Sizing the flow rate to achieve a higher velocity also tends to hinder algae growth. Algae slime has been known to form in lines with velocities that can reach as high as 10 ft/sec.

The cooling water side of a process is often overlooked as uncontrollable. However, cooling water control valves can promote savings by reducing the use of water, pump energy requirements, and water treatment costs. In new construction, smaller pipe and pump sizes can lower capital equipment costs. CW valves also provide better process control by maintaining a fixed temperature difference across the cooling water inlet and outlet. In most cases, analyses justify the installation of such controls.

-- Edited by Jeanine Katzel, Senior Editor, 847-390-2701, j.katzel@cahners.com

More info

The author will answer technical questions about this article. He may be reached by phone at 201-403-1556 or by mail in care of his company, 10 York Ave., West Caldwell, NJ 07006.

Key concepts

Optimal operation of cooling water systems helps limit algae growth and cool equipment properly while maintaining proper temperatures.

Cooling water control valves reduce water use, pump energy requirements, and water treatment costs.

Water and energy savings typically provide rapid payback on the valve system investment.

Justifying the costs

Savings in cooling water and energy typically provide a rapid payback on the valve system investment. Cooling water control valves also reduce capital costs of a new installation by allowing use of smaller pumps and filters, and, in some cases, reduced pipe sizes.

An example of savings achieved in a retrofit system is shown below.

Existing conditions:

Q = heat removal rate of cooler, 700,000 Btu/hr

T(sub i) = inlet cooling water temperature, 50 F

T(sub o) = outlet cooling water temperature without control, 59 F

C(sub p) = specific heat, 1 Btu/lb/deg F

m = mass flow rate, lb/hr

v = volumetric flow rate, gpm

m = Q/C(sub p) (T(sub o) - T(sub i) ) = 700,000/1(59 - 50) = 77,777 lb/hr

v = 155.5 gpm

(For v, to convert lb/hr to gpm divide by the conversion factor of 500, which is arrived at by multiplying 8.33 lb/gal. of water by 60 min/hr.)

After a CW valve is installed, the discharge temperature can be set to 82 F. Inserting the new T(sub o) into the equation yields:

m = Q/C(sub p) (T(sub o) - T(sub i) ) = 700,000/1(82 - 50) = 21,875 lb/hr

v (new volumetric flow rate) = 43.7 gpm

In a system in which water is not recirculated, water use drops 72%. In a closed loop system, a certain amount of water is lost to evaporation in the cooling tower and during blowdown. Water treatment costs also must be taken into account in the analysis.

In addition to water savings, energy is conserved because less power is needed to pump less water. The pump energy savings figure shows three discharge head charts. Efficiency and power consumption of a typical centrifugal pump are plotted against volume displacement. Note that even with a reduction in efficiency, power consumption is 6.5 kW before the control valve is installed and 3.5 kW afterward, a 46% energy reduction.

At a treated water cost of $0.50/1000 gal. and energy cost of $0.05/kWh, annual savings total $7190. The figure assumes a 10% water loss from blowdown evaporation. Annual savings for an open discharge system are more than $60,000.

 

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