How can a cooling tower spread Legionnaires' Disease?

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

Brewing up success!

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

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 →

Chlorine Dioxide in Brewing Process Water

Brewing up Success

UK-based Fuller, Smith and Turner Ltd have been brewing some of Britain’s most popular beers by the Thames since 1845. Producing over 70,000 litres of beer every day comes with significant water management challenges – for any brewery of this size.

Fuller’s, however, goes one step further to ensure its flagship organic product, Honey Dew, stays all natural and chemical free - from the first grain to the last bottle.

The brewing process utilizes water in several different ways, using some for the final product and some for the production processes. The water that is used to make the beer, known as brewing liquor, has been highly purified to remove trace chemicals such as chlorine which are added by water utility companies.

This helps to protect the unique strains of yeast used by Fuller’s to make its wide variety of beers and ales.

A separate stream of water is used for cleaning tanks, powering heat exchangers and rinsing bottles. This water, known as process liquor, is also highly purified, however a disinfectant needs to be added to prevent microbiological contamination.

Rather than using traditional disinfectants which have long-lasting residuals and can form organic disinfection by-products, Fuller’s has implemented a chlorine dioxide dosing system.

Why is ClO2 Better?

Making Sure the Chlorite’s Alright

One of the few by-products formed is the oxidised form of chlorine dioxide, called chlorite (ClO2-). Being able to accurately measure chlorite is essential as the Soil Association set an upper limit of 0.5 ppm for water that could potentially come into contact with an organic product.

In order to ensure the organic approval of its product, Fuller’s has turned to the Palintest ChlordioX Plus, which is the only portable instrument with EPA approval for measuring chlorine dioxide and chlorite. Using Palintest’s unique disposable sensor technology, the ChlordioX Plus utilises chronoamperometry which eliminates the interferences typically associated with colorimetric methods.

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

This allows Fuller’s to accurately monitor the quality of its process liquor across its site, without the need transport samples to a lab. This efficiency means the right dosing decisions can be made at the right time and the brewery can keep producing great tasting organic beer.

Read this article at  Palintest

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

Calculate boiler water blowdown flow

To control the level of total dissolved solids (TDS) within the boiler

As water is boiled within the boiler and steam is produced, then the solids remain in the water and concentrate. Thus, over time the level of total dissolved solids (TDS) increases. Further evaporation causes these dissolved solids to come out of solution, and to produce suspended solids (sludge). As the dissolved solids increase there is a risk of ‘carry over’ of boiler water into the steam. It is therefore extremely important to control the level of Total Dissolved Solids. This is achieved by either continuous or intermittent blow down. Manual bottom blow down through the main bottom blow down valve should still be carried out at regular intervals to remove sludge.

Manual Control of TDS and Blowdown

Time
Boiler Water TDS Maximum Allowable Average

Automatic Control of TDS and Blowdown

Time
Boiler Water TDS Maximum Allowable Average

Boiler Water TDS Boiler Water TDS

How to calculate the required blowdown rate of a boiler

Blowdown rate =

𝐹𝐹 × 𝑆𝑆 Where: 𝐵𝐵−𝐹𝐹F

 

For example:

To keep a steam boiler producing 4000 kg/hr of steam below 3500 ppm TDS when fed from a feed tank having a TDS of 80ppm, it will need to blowdown at least;

F 80 ppm 80 × 4000 S 4000 kg/hr 3500 80
B 3500 ppm

Typical permissible levels of boiler water TDS

= 93.6 kg/hr

Feed tank TDS in ppm

Actual boiler steam production in kg/hr

Maximum TDS allowed in the boiler in ppm

Type of Boiler

TDS level in parts per million (ppm)

Water tube – High Pressure

1,000

Vertical shell

2,500

Modern packaged 3 pass

3,000

Older economic 2 pass

4,500

Water tube – Low Pressure

5,000

Lancashire

10,000

Read more at http://byworth.co.uk/wp-content/uploads/2015/06/Best-Practice-in-Boiler-Water-Treatment-Part-21.pdf

Orthophosphates versus Polyphosphates

The selection of a phosphate water treatment chemical additive can be one of the most difficult chemical treatment decisions that many public water systems will make. This is particularly true because the chemistry of orthophosphate and polyphosphate chemical additives is complex, and phosphate water treatment chemical additives are commercially available in an overwhelming number of chemical blends.

Orthophosphates and polyphosphates are salts derived from two different forms of phosphoric acid. Orthophosphates are small molecules, formed from the smallest and most basic form of phosphoric acid. Polyphosphates are larger molecules, formed from a longer chain version of phosphoric acid.

Even though the words orthophosphate and polyphosphate contain the word "phosphate," these two chemical compounds serve radically different water treatment purposes. A public utility system's failure to understand the significant differences between these two treatment compounds could result in serious water-quality problems and possible MCL violations. An incorrect selection of phosphate chemical blends by a public utility system could even create serious public health problems.

In public water systems, orthophosphates are used for lead and copper corrosion-control purposes. Orthophosphates chemically react with lead and copper atoms that have leached off of piping and have entered into the surrounding water. This chemical reaction of orthophosphates with lead and copper atoms forms lead and copper phosphate. The lead and copper phosphate is then electrochemically drawn back down onto the piping surface, where it forms a tough, water-resistant coating on the piping. This tough, water-resistant coating helps prevent further leaching off of lead and copper atoms into the surrounding water. Most public utility systems have experienced far greater success with orthophosphate lead corrosion control than they have experienced with orthophosphate copper corrosion control.

Polyphosphates are sequestering agents that are virtually ineffective against lead and copper corrosion. When a jury in a criminal trial is sequestered, that jury is "held in seclusion." A chemical sequestering agent is a chemical agent that surrounds another molecule or atom and holds that other molecule or atom "in seclusion." By surrounding the other molecule or atom and holding it in seclusion, the chemical sequestering agent hides the molecule or atom from sight and prevents it from entering into various chemical reactions. As a sequestering agent, polyphosphates will only sequester soluble "invisible-in-water" metals that have not been oxidized into their insoluble forms. Polyphosphate applied to water before the water is chlorinated will prevent invisible iron and manganese from becoming visible after the water is chlorinated.

As a sequestering agent, water treatment polyphosphate is used to sequester soluble iron atoms that remain in settled water before it is chlorinated or that leach off of iron piping in water distribution systems. By surrounding and sequestering these soluble iron atoms, they are prevented from displaying the typical reddish colors associated with iron oxides and iron hydroxides. Water treatment polyphosphates also interfere with the crystallization of and formation of calcium and magnesium carbonate scales, but not with the cystallization of and formation of magnesium hydroxide scales. If any soluble manganese atoms are still present in water after the floc has settled out, polyphosphates will also serve to sequester these soluble manganese atoms, preventing them from displaying the typical dark manganese dioxide color.

A gross misconception about polyphosphate is the belief that the use of polyphosphate sequestrants to hide iron and manganese is a casual, routine treatment technique for the removal of excess iron and manganese that was not removed during a water treatment plant's sedimentation and filtration processes. In reality, the use of polyphosphate to sequester iron and manganese that a plant failed to remove during the sedimentation and filtration processes is a desperation maneuver. Undesirable quantites of iron and manganese in raw water should be properly oxidized by aeration, permanganate, or ozone and should be deposited in sedimentation basins as part of the floc. Polyphosphates, which sequester iron and manganese for only a limited period of time, are not the ideal or the preferred solution for any water treatment plant's iron and manganese problems. Polyphosphates should only be used to catch the few particles of iron and manganese that were missed during the initial aeration and oxidation process.

Most phosphate treatment compounds used by public water treatment systems are actually blends of polyphosphates and orthophosphates. Polyphosphates are usually added in their sodium or potassium polyphosphate form. Orthophosphates are added in the sodium or potassium orthophosphate form or in an orthophosphate form that is mixed with zinc chloride or zinc sulfate. The zinc in the mixture plays no role in forming the coatings that prevent lead and copper corrosion. Instead, the zinc plays a significant role in protecting galvinized surfaces (galvanized means "zinc-coated") and in preventing asbestos fibers from eroding off of asbestos-cement piping. It should however be noted, some phosphate blends may also contain zinc polyphosphates, but zinc orthophosphate formulations are much more commonly used in public water treatment operations.

Phosphate water treatment mixtures are available in dozens of different blends. There is no perfect blend that is universally usable in all situations. If a water treatment system uses a blend with more polyphosphate than their system needs, the coatings laid down by the orthophosphate can be stripped away. If a water treatment system uses a phosphate blend with more orthophosphate than their system needs, iron can be stripped away from iron piping. Water treatment plants need to regularly assess the lead, copper, iron, manganese, and asbestos levels in their water and consult with a professional phosphate specialist whenever a change in phosphate blends seems to be warranted.

COMMON POLYPHOSPHATE SALTS: Sodium acid pyrophosphate, Tetrasodium pyrophosphate, Tetrapotassium pyrophosphate, Sodium tripolyphosphate, Potassium tripolyphosphate, Sodium trimetaphosphate, Sodium hexametaphosphate (glassy)

COMMON ORTHOPHOSPHATE SALTS: Monosodium orthophosphate, Monopotassium orthophosphate, Disodium orthophosphate, Dipotassium orthophosphate, Trisodium orthophosphate, Tripotassium orthophosphate, Zinc orthophosphate

Original post by

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.

Cooling Tower Molybdate Disapearence

A customer had a cooling tower system with pH control, a molybdate traced scale/corrosion inhibitor, and bleach-based biocide program. The operators contacted their water management specialist with concerns about why their tested molybdate levels were so low. Their results did not correspond with the amount of scale/corrosion inhibitor being feeding into the system.

What factors could affect the tested molybdate levels in a cooling tower system? Take a few moments to consider the system diagram below and think of what could cause this.

City Makeup

Cooling Tower Blowdown Process

3005NSS Feed

Citric Acid Feed

Bleach Feed

Cooling Towers 279

 

Problems such as this can be very perplexing and can have several causes. Sometimes, you have to go the extra mile to get the answer. This is exactly what the Associate in charge of this account did.

Applied Chemistry

The chemistry applied to the cooling tower system was:

  • 3005NSS - a molybdate-containing scale and corrosion inhibitor
  • Citric acid - pH control
  • Bleach - an oxidizing biocide used for microbiological control

 

Possible Causes of Low Molybdate Readings

  • Low Cooling Tower Cycles: Running the cooling tower are lower cycles of concentration will result in lower molybdate levels even though the 3005NSS feedrate was the same. This was not the case in this situation.
  • 3005NSS Underfeed: Underfeeding the 3005NSS would directly affect the tested molybdate residuals; however, the theoretical feed rate and actual feed rate compared closely to one another.
  • Chemical Line Leak: A leak in the 3005NSS chemical line would obviously reduce the amount of chemical fed to the cooling tower, but no leak was found.
  • Improper Product Blend: It is rare, but not impossible to get a product that has been blended improperly. Being able to do a dilution to test product concentrations is a skill all professional water treatment experts should possess. A sample of 3005NSS was diluted and tested for molybdate concentration. The tested molybdate level was very close to the theoretical level expected.
  • Loss of Chemical Component in System: Some chemistries are naturally lost in the system due to evaporation or consumption.

Molybdate is nonvolatile and consumption to form a protective passivated layer on the metal surfaces should be minimal once the chemistry has been established. Molybdate was not being lost in the system.

  • Test interference: An expert water treater should be aware of possible interferences to the tests he runs. The analysis procedures manuals for the colorimeters and spectrophotometers do a great job listing these interferences. An initial review of these interferences showed nothing in the system that should be interfering with the molybdate testing. Both the bleach feed and citric acid feed were Cooling Towers 280 new additions to the treatment program and corresponded to the approximate time that the operators started having problems with their molybdate testing. The analysis interference chart showed that it would take a chlorine reading of 7.5 ppm to cause interference. The free chlorine levels had never been anywhere near that high in the system. The analysis interference chart did not specifically list citric acid; however just because something isn't listed does not mean it is not an interference.

Checking for Citric Acid Interference

The water management associate telephoned the water analysis manyfacturer to ask if citric acid interfered with molybdate testing. They were unsure but recommended a test be conducted to determine if it did.

A dilution of 3005NSS was prepared. This dilution was divided into several containers and the pH was adjusted to various levels using citric acid and sulfuric acid (as a control). The results were as follows:

Table 1 - Citric Acid Interference Determination

Acid Used                   Sample pH                  Molybdate (ppm)

Citric                           6.7                              0.7

Citric                           6.3                              0.3

Citric                           4.9                              0.0

Sulfuric                       7.0                              1.6

Sulfuric                       6.2                               1.5

 

As Table 1 shows, the citric acid was indeed an interference.

Conclusion

Through the detective work of the water management associate, citric acid was determined to be the interference with the low range molybdate testing.

The next steps required were to:

  • Ensure the theoretical 3005NSS dosage was being fed on a daily basis while citric acid was still being fed to the cooling tower.
  • Discuss the pros and cons of sulfuric acid feed with the customer so the inhibitor levels can be properly measured

Boiler Carryover – Cause, Effect and Prevention

Mechanisms

carryover or primingCarryover also known as priming is any solid, liquid or vaporous contaminant that leaves a boiler with the steam. In low/medium pressure boilers (<100 bar) entrained boiler water is the most common cause of steam contamination.

Both mechanical factors such as boiler design, high water levels, load characteristics and chemical factors such as high solids concentration, excessive alkalinity, presence of contaminants contribute to the creation of carryover.

Two of the most common mechanical causes of carryover are operation in excess of design load and sudden increases in load.

Foaming is one of the mechanisms of chemical carryover. Foaming tendencies are increased with increases in alkalinity and solids content. Stable foam bubbles contain boiler solids and are carried forward with the steam giving rise to carryover.

Oil and other organic contaminants can react with boiler water alkalinity to give crude surface active materials which cause foaming and carryover.

Effects

Boiler water solids carried over with steam will form deposits in non-return and other control valves. Process streams can be contaminated by carryover affecting product quality.

Deposition in superheaters can lead to failure due to overheating and corrosion.

Steam turbines are potentially prone to damage by carryover as deposits on turbine blades creates imbalance reducing efficiency and capacity. Solid particles in steam can lead to erosion and corrosion in both turbines and other equipment.

 

Prevention of Carryover

The prime means of preventing carryover is to have good mechanical steam separation devices. For low/medium pressure fire tube boilers where steam purity is not stringent, gravity separation is normally satisfactory. (At least 14 bar and saturation conditions the density of water is 115 times greater than that of steam). As steam pressure rises the difference in density reduces (at 69 bar water is only 20 times more dense than steam) making gravity separation less effective. Steam separators are then used to improve purity and are usually installed in the steam drum of water tube boilers.

Primary separators utilise the difference in density as the means of separation bypassing steam through a series of baffles which reduces turbulence or centrifugal (cyclone) separators.

Secondary separators, where steam is directed in a frequently reversing pattern through a large contact surface. A mist of boiler water collects on the surface and is drained from the unit.

Control of boiler water chemistry is essential to minimise carryover and allow mechanical separation to work effectively. The parameters that must be controlled are:

  • Total dissolved solids
  • Alkalinity
  • Silica
  • Organic contamination.

These should be maintained within the boiler manufacturer guidelines or those of BS 2486.

Whenever carryover is being caused by excessive boiler water concentrations an increase in boiler blowdown rate is normally the simplest and most expedient solution. If carryover is still occurring and increasing blowdown is uneconomic then the addition of antifoam agents can economically reduce carryover. Use of an antifoam may allow the boiler to operate at higher water concentrations, Feedwater offer a product called Defoamer C which is suitable for this job, for more information visit the product page for product usage guidance.

Read more at https://feedwater.co.uk/boiler-carryover-cause-effect-prevention/