Low and medium pressure boilers must be protected from scale deposition and corrosion to promote optimum energy efficiency and prolong the useful life of the plant equipment.
The definition of low and medium pressure is somewhat discretionary. For the purposes of this discussion, low pressure shall apply to boilers up to 150 psig. These boilers are typically used in space heating applications where the percentage of return condensate is high. Medium pressure boilers fall in the range of 150 to 650 psig. These are typically power generating plants where process steam is required. The makeup demand is greater in medium boiler plants due to steam consumption and loss.
In either case, the water treatment requirements can be met with basic chemicals. However, in the specialty chemical market, the various brand names and proprietary formulations create some confusion as to the best practice for boiler water treatment. This article will clear away some of the clutter by presenting a basic approach to providing effective chemical treatment for low and medium pressure boilers.
The basic boiler water treatment chemicals can be broken down into five (5) groups:
- Oxygen scavengers
- Scale-control agents
- Alkalinity builders
- Sludge dispersants
- Condensate treatment
Residual dissolved oxygen in the boiler promotes pitting-type corrosion that is an insidious, highly localized form of attack. If left unchecked, the propagation of the pit ultimately leads to tube failure. Boiler makeup can contain up to 10 ppm dissolved oxygen depending on the temperature. The first line of defense is removal by mechanical deaeration. This reduces the oxygen concentration to 7 parts per billion. The remaining oxygen is removed by chemical scavengers fed to the feedwater storage section of the deaerator.
Sodium sulfite Na2SO3 is the most commonly used and fastest acting oxygen scavenger. Available as a 90% active powder or less-concentrated liquid, sodium sulfite reacts rapidly with residual dissolved oxygen to form harmless sodium sulfate. Eight (8) parts of sodium sulfite are required to react with one (1) part of dissolved oxygen. An excess residual of 20 to 50 ppm of sodium sulfite are carried in the boiler to protect against oxygen ingress. Sulfite is available in a catalyzed version to enhance its reaction at lower temperatures, but uncatalyzed sulfite is acceptable for use at boiler saturation temperature and pressure.
Although very effective as an oxygen scavenger, sulfite does not readily react with boiler metal to promote the formation of a protective black iron magnetite surface. Black iron magnetite is a more passive (corrosion resistant) form of iron as compared to red iron hematite.
Hydrazine is often used as an oxygen scavenger in higher pressure boilers since it does not add dissolved solids to the boiler water. It also has the advantage of converting red iron oxide (hematite) into black iron oxide (magnetite). One part of hydrazine is required to react with 1 part of dissolved oxygen. An excess residual of 1 to 3 ppm is typically carried in the boiler to protect against oxygen intrusion and maintain the protective magnetite film.
Hydrazine has been classified as a potential carcinogen, hence its use has been in decline. Several hydrazine alternatives have been developed, however, that offer the advantage of being a metal passivator without the health and safety concerns associated with hydrazine.
Hydrazine alternatives fall into two (2) categories: volatile and non-volatile. In addition to reacting with oxygen in the pre-boiler and boiler, volatile scavengers carry with the steam into the condensate system where they further react with dissolved oxygen. Non-volatile scavengers such as sulfite and hydrazine do not.
The class of volatile oxygen scavengers include carbohydrazide, methylethylketoxime (MEKO), hydroquinone and diethylhydroxylamine (DEHA). These chemicals react much slower with dissolved oxygen as compared to sodium sulfite. However, they offer the benefit of promoting a black iron magnetite surface. They also react with dissolved oxygen in the condensate. Because of their volatile nature, these products are not used in boilers where the steam comes into contact with food or pharmaceutical products.
Sodium erythorbate is a non-volatile oxygen scavenger that can be used as an alternative to sodium sulfite and hydrazine. It has the advantage of being a metal passivator like hydrazine. However, since it appears on the Generally Recognized as Safe (GRAS) list of food additives, it does not pose the same health and safety concerns as hydrazine and the other alternatives. The theoretical dosage of erythorbate is 11 ppm per ppm dissolved oxygen.
SCALE CONTROL AGENTS
Hardness (calcium and magnesium) and iron in feedwater can react in the boiler to produce an insulating deposit on heat transfer surfaces. Scale deposits are also a fundamental cause of overheating and stress boiler tube failures.
The first line of defense in preventing unwanted boiler deposits is softening the boiler makeup by ion exchange or hot lime softening. Ion exchange softeners essentially remove all hardness and iron from the boiler makeup. Chemical treatment is required, however, to react with residual hardness and provide a safeguard against hardness leakage.
Various chemicals are used to prevent the formation of scale and baked-on sludge deposits. These include sodium phosphate, chelating agents like EDTA, and synthetic polymers.
Two forms of sodium phosphate find application in low and medium pressure boilers; disodium phosphate (NaHPO4 having a 49% P2O5 content) and sodium metaphosphate (NaPO3 having a 69% P2O5 content). Both react under boiler conditions to produce orthophosphate (o-PO4). PO4 readily reacts with calcium hardness and alkalinity to form an insoluble sludge of hydroxyapatite. The boiler sludge produced by this reaction is effectively removed by routine surface and bottom blowdown. Magnesium hardness reacts with silica and hydroxide alkalinity to yield an insoluble sludge. In general, boiler blowdown is controlled such that the suspended solids produced by these precipitation reactions do not exceed 500 ppm.
As an alternative to precipitating treatment programs, chelating agents are often used to keep calcium and magnesium soluble thereby avoiding the formation of an insoluble sludge. Ethylenediaminetetraacetic acid (EDTA) reacts with calcium, magnesium, iron and copper such that precipitation does not occur. It also reacts to a lesser extent with boiler metal surfaces and may remove some magnetite, hence overfeeding of chelants should be avoided. Four (4) parts of EDTA are required per ppm metal ion. EDTA is available as a powder and a 35% solution.
Various long chain synthetic organic polymers are beneficial in the treatment of boiler feedwater for the prevention of scale deposits. The polymers are best described as “weak” or “modified” chelants in that they chemically tie up impurities in the feedwater to prevent their deposition in the boiler. In these applications, the polymers replace phosphate and EDTA as the primary scale control agent.
Maintenance of sufficient boiler alkalinity is required to enhance the precipitation reactions with calcium and magnesium hardness plus help in the formation of a passive metal surface. With a phosphate treatment program, hydroxide (OH) alkalinity is required to insure that calcium reacts to form calcium phosphate hydroxide (hydroxyapatite) and that magnesium reacts to form magnesium hydroxide (brucite, aka milk of magnesia).
The most common alkalinity builder is sodium hydroxide (NaOH, aka caustic soda). This is available as a 98% flake form having a 76% Na2O content. More commonly, it is obtained as a 50% active liquid or in more dilute liquid versions depending on the source of supply.
Alternatively, sodium carbonate (Na2CO3, aka soda ash) may be used. This product is available as a 99% active powder having a 58% Na2O content. Soda ash reacts under boiler temperature and pressure to produce sodium hydroxide and free carbon dioxide. This has the disadvantage of increasing the carbon dioxide content of the steam, which, in turn, results in the formation of corrosive carbonic acid in the condensate. For this reason, most low and medium pressure boiler applications favor the use of liquid sodium (or potassium) hydroxide as the alkalinity builder.
No definitive, universally-agreed-upon range for excess caustic (OH) alkalinity exists. Excessively high OH alkalinity should be avoided as this has been shown to adversely affect steam purity and increase the tendency for caustic attack of boiler metal in the form of embrittlement and gouging. In general, experience suggests that a range of 85 to 300 ppm OH alkalinity is typical for boilers operating in the low and medium pressure ranges.
Various boiler additives have been used to reduce the tendency of boiler precipitants to settle out or bake on to generating tubes. Early on, boiler operators applied natural substances like potato starch and tannin/lignin extracts from tree bark. These natural additives are still used today with some success. However, with the development of more chemically refined synthetic polymers, the use of natural organic dispersants continues to decline.
Synthetic polymers are often used in conjunction with phosphate and chelant programs to react with boiler sludge in order to keep it fluid, dispersed and easily removed by surface and bottom blowdown. The first boiler polymers were long chain polyacrylic acid (AA) and polymethacrylate (PMA) molecules. Many other polymeric boiler additives have since been developed including maleic acid, sodium glucoheptonate ,
2-acrylamido-2-methyl propane sulfonic acid (AMPS), and various co-polymers and ter-polymers of AA/AMPS. Not all polymers are suitable for use in food and dairy plants, however. Those that are permissible for such use are listed in the Code of Federal Regulations, 21CFR173.310.
Polymer dosages vary depending on the type of chemical used. In general, the dosage of active polymer falls within the 10 to 20 ppm range. Over-dosing polymers should be avoided as this adversely affects their dispersant performance.
STEAM CONDENSATE TREATMENT
Carbon dioxide that volatilizes with the steam due to the thermal decomposition of carbonate alkalinity in the feedwater dissolves into the condensate to form carbonic acid, which is corrosive. Volatile neutralizing amines are frequently required to neutralize this acid to cause an upward adjustment in pH.
The first step in minimizing problems caused by volatile gases like carbon dioxide is to reduce the bicarbonate and carbonate alkalinity in the boiler makeup. This can be done by ion exchange dealkalization or demineralization. Although dealkalization of boiler makeup is a common practice in high pressure boiler installations, many low and medium pressure boilers operate on soft water makeup. The natural bicarbonate alkalinity is not removed in a sodium exchange water softener. And, as mentioned previously, chemical additives such as sodium carbonate (soda ash), which may be used as an alkalinity builder in the boiler, add to the carbon dioxide potential of the steam.
Neutralizing amines are used alone or in combination to adjust and maintain the condensate pH to within an alkaline range of 7.5 to 8.5. Generally, pH does not adversely affect the corrosion rate of steel above a pH of 6.0. Using neutralizing amines to maintain the pH above 6.0 provides additional protection from pH excursions.
The most common neutralizing amines are morpholine, diethylaminoethanol (DEAE), and cyclohexylamine. Each has a different distribution ratio between the steam/condensate phases. Some trial and error is required to determine the optimum type and amount of neutralizing amine required. Amines are best applied by injection with a quill into the main steam header, but they can also be applied directly to the boiler where they distill off into the steam phase.
In some applications, such as in food and dairy plants, the use of steam condensate treatment is restricted or prohibited. Sorbitol anhydride esters have been recently approved by the FDA for use in treating steam condensate in food plants up to a dosage of 15 ppm in the steam. Other restrictions on amine usage may apply where clean steam is required for pharmaceutical, humidification, sterilization or other manufacturing processes.
Operating low and medium pressure boiler plants at maximum efficiency requires careful control over the boiler water chemistry. This includes protecting the boiler from oxygen-pitting type corrosion, preventing scale deposits on heat transfer surfaces and protecting the steam condensate system from corrosion.
These goals are best achieved by the application of basic water treatment chemicals.
- Sodium sulfite for oxygen scavenging
- Sodium phosphate for reaction with calcium hardness
- Sodium EDTA if a non-precipitating chemical program is desired
- Sodium hydroxide for reaction with magnesium hardness and to adjust boiler alkalinity
- Polymeric dispersants such as polyacrylate, polymethacrylate and various co- and ter- polymers
- Neutralizing amines including morpholine, DEAE, and cyclohexylamine.