Control of the three major microbiological classes, bacteria, algae and fungi, and macroorganisms including zebra mussels, is essential for healthy cooling tower operation. For many years, chlorine was the treatment chemical of choice. However, chlorine use is declining due to environmental, safety and performance-related issues. This article examines the positive and negative aspects of chlorine and of replacement oxidizing biocides.
Microbiocides generally belong to one of two groups: oxidizers and non-oxidizers. The former attack cells by oxidizing (an electron transfer reaction) microorganism cell components. Non-oxidizers react with cell components via different chemical processes. Oxidizing biocides are still the most common biological control agents, and even though chlorine use is declining, it continues to be an important player in the cooling water treatment industry.
When chlorine is injected into a cooling water stream, it disproportionates into hypochlorous and hydrochloric acid as follows:
Cl2 + H2O Æ HOCl + HCl
HOCl is the oxidant that attacks cell structures. An increase in pH increases the dissociation of HOCl into the hypochlorite ion (OCl-):
HOCl H+ + OCl-
Although both HOCl and OCl- are oxidants, OCl- is a much weaker disinfectant, possibly because the charged OCl- ion has a more difficult time penetrating the cell wall. Chlorine`s biocidal efficiency greatly decreases as the pH rises above neutral. (See sidebar.)[Native Advertisement]
For years, the most popular cooling water treatment program at many facilities was low-level sulfuric acid feed to control calcium carbonate formation, with supplemental feed of chromate and zinc for corrosion inhibition. This suited chlorine as a microbiocide because the mildly acidic pH maintained the chlorine residual predominantly as HOCl. However, chromate discharges to the environment have been banned due to the potential release of toxic hexavalent chromium. Modern cooling tower treatment programs operate in the less corrosive alkaline range of pH 8.0 to 9.0, in which advanced calcium carbonate scale inhibitors have replaced sulfuric acid. Such programs do not favor chlorine as a microbiocide. This problem has been exacerbated by the development of more efficient cooling tower fill (Figure 1), whose close spacing makes the material susceptible to pluggage.
Safety issues are another factor in chlorine`s reduced popularity. Chlorine gas is quite hazardous, and regulations governing its storage and leak detection are becoming increasingly stringent. Rather than deal with the safety requirements for gaseous chlorine, many plant managers are opting for alternatives.
The potential for formation of chlorinated organics has also become an important issue. Many halogenated organics are known or suspected carcinogens, and tighter restrictions are being placed on the amount of allowable halogenated organics. In 1979 the Environmental Protection Agency (EPA) set an interim maximum contaminant level (MCL) of 0.100 ppm for total trihalomethanes (TTHM`s). The agency has proposed to reduce the MCL to 0.080 ppm, and may lower the standards even further in the future.
In 1982, the power industry was required to meet optimized technology-based standards for chlorine use. The requirements limited the maximum chlorine discharge from cooling towers to 0.5 ppm, with an average discharge of 0.2 ppm for no more than two hours per day. In 1985 the EPA announced more stringent ambient water quality criteria, which applied to all industries. These guidelines limited fresh-water chlorine concentrations at the boundary of a calculated effluent mixing zone to 0.011 ppm over a four-day average, or 0.019 ppm average for one hour. Restrictions for salt water are even more stringent at 0.0075 ppm and 0.013 ppm, respectively.
The regulations have made it particularly difficult to use chlorine to control cooling water biological fouling. The situation has been further complicated by the spread of macrofouling species, the most notable of which are zebra mussels and Asiatic clams. Continuous or semi-continuous chlorination is necessary to control the growth of macrofoulers, especially adults, but continuous chlorination is expensive, particularly when bleach is the biocide. Continuous chlorination can also harm non-target organisms in once-through cooling systems. Many facilities must dechlorinate their cooling water prior to discharge in order to comply with water quality standards. This process typically requires the feed of a reducing agent, such as sodium bisulfite, into the effluent.
A popular substitute for chlorine is bromine (Br2). Like chlorine, bromine reacts with water to produce a hypohalous acid, in this case HOBr. Bromine has nearly the same oxidizing power as chlorine, but it offers several advantages over chlorine in certain conditions. First, the dissociation of HOBr occurs at a higher pH than HOCl (Figure 2), which makes it more effective in alkaline environments. Second, bromine does not react irreversibly with ammonia as does chlorine. Chloramines are much less effective disinfectants than free chlorine, which makes chlorination of ammoniated waters problematic. Third, bromine is less corrosive than chlorine to copper alloys.
Bromine may be introduced into a cooling water system by several different methods. Most common is to react liquid sodium bromide (NaBr) with chlorine or sodium hypochlorite in a sidestream loop of the cooling water makeup. Chlorine activates the bromide salt to hypobromous acid as follows:
NaBr + HOCl Æ HOBr + NaCl
Sodium bromide, being the bromine analog of common table salt (NaCl), is stable and may be stored in a simple bulk tank. NaBr is usually supplied as an aqueous solution of approximately 40 percent concentration. The sodium bromide and chlorine or sodium hypochlorite should be fed separately into the slipstream to obtain at least a 100:1 dilution. This prevents the formation of undesirable bromate byproducts.
Like chlorine, bromine is toxic to non-target organisms and it can form halogenated organics. For these reasons plant cooling water discharges containing bromine are regulated similarly to chlorine, although some states or EPA regions have established more restrictive standards for bromine residuals.
Even though the cost of sodium bromide adds to the total delivered cost of the oxidants, users often find it possible to reduce the overall quantity of oxidant required to achieve the equivalent performance. Frequently, the reduction in chlorine consumption more than offsets the cost of the sodium bromide, especially where liquid sodium hypochorite is the chlorine source.
For smaller cooling water systems, solid bromine donors may be a cost-effective alternative to the arrangement mentioned above. One of the most common solid biocides is bromo- chloro-dimethyl-hydantoin, or BCDMH, which releases bromine as it dissolves in water. Several of the major water treatment vendors supply solid bromine or chlorine donors in granules, pellets, or tablets along with a feed system. As water passes through the dissolving vessel, the BCDMH dissolves at a controlled rate to release HOBr and HOCl:
C5H6O2N2BrCl + 2H2O Æ C5H8O2N2 + HOBr + HOCl
Solid donor systems are also available for strictly chlorine-releasing products. Some of the most common chlorine-based solid donors include dichloro-dimethyl-hydantoin, calcium hypochlorite [Ca(OCl2)] and chlorinated isocyanurates. The latter two compounds are widely used as swimming pool chemicals, but will also work well in some cooling water applications. Specific feed systems are available for each halogen donor. All of these products are strong oxidizers and must be handled and stored properly; it can be hazardous to use one compound in a feeder designed for another.
Prominent advantages of the solid halogen donors are:
Handling--no potential for liquid spills;
Stability--stable compared to bleach;
Effectiveness--strong oxidizers that work well at alkaline pH and in the presence of ammonia;
Water Chemistry--less corrosive to system materials; do not significantly alter cooling water pH; and
Environmental and Safety Factors--no chance of toxic gas or liquid release.
The solid halogen donors are best for systems with a low or moderate chlorine demand, and where simple operation is desirable. Solid donors are generally more expensive than chlorine and even bleach, and can be slow to dissolve at water temperatures below 60 F.
Sodium hypochlorite (NaOCl) may be a suitable non-gaseous alternative. It comes in bulk solution, at a concentration ranging from 10 percent to 12.5 percent as NaOCl. The hypochlorite can be metered directly into the cooling system. An important point to remember about NaOCl is its effect on cooling water pH. Gaseous chlorine lowers the pH due to the production of both HOCl and HCl. Sodium hypochlorite tends to raise the pH.
Sodium hypochlorite will degrade over time to form sodium chloride, oxygen, and sodium chlorate (NaClO3). Temperature and impurities greatly affect decomposition. (See sidebar for specifications and storage guidelines for sodium hypochlorite.)
The improved safety and convenience of sodium hypochlorite versus gaseous chlorine has a tradeoff in cost. Chlorine gas in ton cylinders costs $0.15 to $0.20 per pound delivered. By contrast, 12.5 percent sodium hypochlorite costs around $0.60 per gallon ($0.059 per pound) delivered in bulk. This corresponds to $0.50 per pound on an equivalent Cl2 basis, or approximately three times the cost of gaseous chlorine.
Chlorine dioxide (ClO2) is a powerful oxidizer with excellent biocidal properties. It offers several potential advantages compared to chlorine. Chlorine dioxide:
does not form halogenated organics,
is less corrosive to copper alloys than chlorine,
does not react with ammonia and primary amines,
efficiently destroys phenols and sulfides,
is not affected by pH like chlorine and works well in alkaline waters, and
is more effective against mollusks.
These advantages come at a price, however. Chlorine dioxide is a very reactive compound that is hazardous to transport at practical concentrations. It must be generated on-site from other reactive chemicals that also present some handling risks. Chlorine dioxide does not react with water or ionize in solution, thus it remains as a dissolved gas that is easily stripped across a cooling tower. The principal reaction product, chlorite, affects some forms of aquatic life at low levels. Chlorite can be difficult to neutralize with reducing agents.
Chemical costs for sodium chlorite are typically in the $0.50 to $1.00 per pound range for a 25 percent aqueous solution. This corresponds to $2.00 to $4.00 per pound of active ClO2. The costs of bleach or acid for reaction, along with the 80 percent to 95 percent efficiency of the ClO2 generation reactions, can push the price above $3.50 per pound, or over seven times the cost of chlorine. Still, there are some situations where the effectiveness of chlorine dioxide may offset the added cost.
Ozone (O3) is a powerful, short-lived oxidant that is generated by passing air through an electric discharge. The air stream is then bubbled into the cooling water through a diffuser.
Ozone is too reactive to transport or store on-site, so it must be generated at the point of use. This requirement has so far limited ozone`s application to relatively small cooling systems. Ozone is the most powerful oxidant of all, but its oxidizing strength can be a mixed blessing. Ozone rapidly destroys biofilms and produces sparkling clear water, but it also breaks down most scale and corrosion inhibitors, and it slowly degrades plastic, rubber and gasket materials. Similar to chlorine dioxide, ozone does not ionize in water but remains a dissolved gas, which can be stripped during passage through the cooling tower. Ozone offers environmental advantages in that its residuals are short-lived, it does not produce halogenated reaction products, and it breaks down into oxygen, which can be beneficial. However, ozone can produce secondary oxidants if bromide is present, and it can also produce aldehydes and ketones via reaction with organics.
One of the major advantages of ozone is that no hazardous chemicals are transported, stored or handled on-site. For small cooling systems in sensitive locations, ozone can be an attractive alternative to chlorine. Researchers continue to investigate methods of ozone treatment for larger systems. p
Figure 1. Cooling tower film fill.
At a pH of 6.5 tests have shown that a 1.0 ppm chlorine solution will kill 99 percent of all microorganisms within 30 seconds. At this pH, a large percentage of the dissolved chlorine exists as HOCl. The percentage rapidly decreases as pH rises.
Thus, in an alkaline cooling water treatment program with a pH between 8 and 9, the available hypochlorous acid is well below 50 percent (Figure 2).
Sodium hypochlorite will decompose into oxygen, sodium chloride, and sodium chlorate. The decomposition rate is affected by temperature and by the catalytic action of some metals, most notably iron and copper. A specification for bulk quantities of sodium hypochlorite should contain the following:
Iron concentration less than 0.5 ppm;
Copper concentration less than 1.0 ppm; and
pH range from 11.0 to 11.2.
Temperature can have a dramatic impact on hypochlorite solutions. For example, the half-life of a hypochlorite solution is reported to be 800 days at a temperature of 59 F. At 77 F the half-life drops to 220 days, and at 140 F, the half-life is only 3 days. Bulk storage tanks of sodium hypochlorite should be kept as cool as possible by sun-shading, painting them white, or both.
Fiberglass-reinforced-plastic is the recommended construction material for bulk storage tanks. Poly ethylene was once the material of choice, but cracking problems have been reported after exposure to hypochlorite.
Read More: http://www.power-eng.com/articles/print/volume-102/issue-7/features/choose-the-right-cooling-tower-chemicals.html
If you supervise a limited-attendance or unattended boiler then Lenntech can assist you with your water testing requirements.
Boiler water tests available
The specific method of chemical treatment used varies with the type of boiler and the specific properties of the water from which the boiler feed is derived. This is very site specific but Lenntech has the testing capability to cover all your requirements.
A boiler requires testing of three different water types as shown below:
Boiler feedwater is sourced from many different places. Some supplies come from industry owned bores and treatment plants, while others come directly from a council supply, however all feedwater should be analysed in order to correctly determine dose rates of treatment chemicals.
Water quality can change as it passes through a delivery or reticulation system, so it is important to check for various parameters at point of use - ie where it enters the boiler or pre-treatment system.
Boiler feedwater is usually a combination of returned condensate plus pre-treated makeup water from a softener, reverse osmosis, or other purification system. Typical tests used for boiler feedwater include:
Not all water supplies will require all the tests shown here, and if the supply is constant the tests will not need to be repeated very often.
The boiler water itself must be dosed in order for the boiler to run efficiently and safely. A chemical imbalance can lead to corrosion and damage to the system and this damage can ultimately lead to boiler failure and injury.
Boiler water analyses are basically aimed at keeping the parameters within established limits.
Good condensate is the best quality, least expensive water most systems can generate. You do not want to lose it, or contaminate it unnecessarily.
Steam condensate analysis should include
|Make-up, Raw Water||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Sulphur, Chloride, Ortho-Phosphate, Total Inorganic Phosphate|
|Clarifier, Softener, Filter-Alum||pH, P/M-Alkalinity, Conductivity, Total Aluminium, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Sulphur, Chloride|
|Clarifier, Softener, Filter-Lime||pH, P/M-Alkalinity, Conductivity, Total Hardness, Filtered Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Sulphur, Chloride, Total Inorganic Phosphate|
|Sodium Zeolite, Dealkalizer, Desilicizer, Softened Make-up||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Sulphur, Chloride|
|Hydrogen Zeolite, Strong Acid Cation||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Sulphur, Chloride|
|Mixed Bed Exchanger, Degasifier, Anion Exchanger, Demineralizer||Conductivity, Filtered Hardness, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Reactive Silicate, Sulphur, Chloride|
|Deaerating Heater, Feedwater, Condensate Polisher||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Reactive Silicate, Sulphur, Chloride, Total Phosphate.|
|Blowdown – Expected Conductance >300 µS/cm||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Sulphur, Chloride, Nitrate, Ortho-Phosphate|
|Blowdown – Expected Conductance >300 µS/cm||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Silica Reactive, Sulphur, Chloride, Nitrate, Ortho-Phosphate|
|Steam Condensate||Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Silicate, Reactive Silica, Sulphur, Chloride|
Cooling tower is a heat removal devices used to eliminate waste heat of air released to atmosphere. This process allows airborne contaminants, organic matters and particles to become deposited into the cooling water. This, combined with the contaminants in the feed water, creates an environment for microorganism growth, solid deposits and scaling.
Improper treated cooling tower water could be an amplifier of biological hazardous agent. The warm and moist environment of a cooling tower favors the growth of Legionella bacteria which causes the outbreak of the deathly Legionnaires' disease. Thus, cooling tower water quality must be monitored in a regular basis to prevent spreading of diseases to users.
|Make-up, Raw Water||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Total Manganese, Sodium, Total Silica, Sulphur, Chloride, Ortho-Phosphate, Total Inorganic Phosphate, Total Zinc|
|Cooling Tower, Air Washer||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Total Manganese, Sodium, Total Silica, Sulphur, Chloride, Ortho-Phosphate, Total Zinc|
|Sea water/Brine||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Manganese, Total Iron, Total Copper, Total Silica, Sulphur, Ortho-Phosphate, Total Zinc|
|High Cycle Tower, Jacket, Brine||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Manganese, Total Iron, Total Copper, Total Silica, Sulphur, Ortho-Phosphate, Total Zinc|
|Closed System, Glycol||pH, Specific Gravity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Total Silica, Sulphur, Chloride|
|Closed System, Non-Glycol||pH, P/M-Alkalinity, Conductivity, Total Hardness, Total Calcium, Total Magnesium, Total Iron, Total Copper, Sodium, Total Silica, Sulphur, Chloride|
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
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.
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:
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.
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
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
Automatic Control of TDS and Blowdown
Boiler Water TDS Boiler Water TDS
How to calculate the required blowdown rate of a boiler
Blowdown rate =
𝐹𝐹 × 𝑆𝑆 Where: 𝐵𝐵−𝐹𝐹F
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
Modern packaged 3 pass
Older economic 2 pass
Water tube – Low Pressure