Dosing Agents for Steam Boiler Operation

handling Ι phosphate Ι sulphite Ι ammonia Ι caustic soda Ι hydrazine Ι DEHA Ι MEKO Ι POC

In addition to external water treatment by chemical and physical processes, various conditioners are dosed to boiler feed water, in order to scavenge residual hardness and oxygen, or to increase the basicity.

According to our experience, the most economical dosing agents are often trisodium phosphate, sodium sulphite and ammonia, assuming a fully functional external water treatment and continuous boiler operation. For boilers in salty or low-salt operation mode with an operation pressure up to 30 bar, the dosing of phosphate and sulphite directly into the feed water tank is common practice. In case of branched out condensate networks, or a high condensate return, ammonia is dosed in addition to that, in order to alkalise the condensate. In case of salt-free operation mode, the usual practice is dosing of phosphate or in some cases caustic soda for alkalisation of the boiler water, and ammonia for alkalisation of the condensate.

For the foodstuffs industry, only non-volatile dosing agents are permitted to be used in larger amount, like for example phosphate and sulphite.

In case of oxygen intrusion caused by regular boilter shut-down and restart, a common practice is the dosing of phosphate, in addition to a volatile oxygen scavenger and passivating agent like hydrazine or DEHA for conservation of the condensate system.

In case of hardness intrusion into the condensate for reasons inherent to the system, a common practice is the dosing of POC as a  hardness disperser.

Furthermore, there is a wide variety of dosing chemicals available on the market, often combined products. The cost-effectiveness of such dosing programs needs to be analysed thoroughly for each particular case.

Handling (General)

The dosing agents are usually injected either into the feed water tank, or into the suction line of the feed water pump. In case of high pressure steam generators, non-volatile alkalising agents are injected either into the water space of the steam generator, or downstream of the attemperation water source.

The dosing pumps are often parallel-controlled to the feed water pumps or to the make-up water inlet control, in order to achieve a quasi quantity-based dosing. The dosing feed is adjusted according to the values measured in the boiler water, in order to maintain a concentration of dosing agents within the guideline values. An operation within a range of 30 ... 100% of the dosing pump feed capacity is expedient. Should this not suffice, the concentration of the dosing dilution needs to be adjusted. For mixing of the dosing dilutions, soft water and demineralized water or permeate respectively are to be used.

The exact values for concentration of dosing agents in the boiler water are to be taken from the guidelines of the respective boiler manufacturer. Also see Guideline and Limit Values for Feed and Boiler Water of Steam Generators.

For storing and handling of dosing chemicals, the respective safety regulations are to be adhered to, as well as the information on the respective safety data sheets.

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Phosphate (Trisodium Phosphate)

Application: Trisodium phosphate (Na3PO4) is dosed to boiler feed water for binding of residual hardness and for alkalisation.

Effect: The residual hardness components are bound to non-dilutable phosphate sludge, for example calcium phosphate (Ca3(PO4)2), and removed with the boiler blow-down. The basicity of the boiler water is also raised by reception of protons. Phosphate is non-volatile in steam, and remains within the boiler water, where it accumulatess depending upon the degree of boiler water concentration.

Limitations: Phosphate increases the salt content (TDS) of the boiler water. Accordingly, it is only dosed in low concentrations in case of salt-free boiler operation mode or high operation pressures.

Form of Delivery: As solid matter (white powder), or as 13% Na3PO4 dilution.

Dosing: The usual concentration of the dosing dilution is 1 ... 10% Na3PO4. At 20 °C, 132 g phosphate can be diluted in 1 l water. The preparation of the phosphate dilution can be made easier by adding warm water, for example condensate.

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Sulphite (Sodium Sulphite)

Application: Sodium sulphite (Na2SO3) is an alkaline oxygen scavenger.

Effect: The residual oxygen is bound to sodium sulphate (Na2SO4). The thus produced sodium sulphate also increases the basicity of the boiler water. Sulphite is non-volatile in steam, and remains within the boiler water, where it accumulatess depending upon the degree of boiler water concentration.

Limitations: In case of higher pressures and temperatures, sulphite can react to sulphurous acid, which will cause corrosion in the boiler. That is why the use of sulphite is limited to boilers with an operation pressure up to 35 bar. Sodium sulphite may not be dosed together with other reducing agents, because those may also reduce the sodium sulphite to sulphurous acid.

Form of Delivery: As solid matter (white powder), or as 20% Na2SO3 dilution.

Dosing: The usual concentration of the dosing dilution is 1 ... 10% Na2SO3. At 20 °C, 260 g sulphite can be diluted in 1 l water. The preparation of the sulphite dilution can be made easier by adding warm water, for example condensate.

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Ammonia

Application: Ammonia (NH3), or as dilution ammonium hydroxide (NH4OH) respectively, serves for alkalisation as well as for binding of free carbonic acid in boiler water and condensate.

Effect: Ammonia is volatile, and thus enters the condensate system with the steam. There, it increases the basicity of the condensate, and binds free carbonic acid. This results in forming of ammonium bicarbonate (NH4HCO3) and ammonium carbonate ((NH4)2CO3). In order to not already expel the ammonia in the thermal deaerator, it is usually dosed either below the water level of the feed water tank, or into the suction line of the feed water pump.

Limitations: Since condensate with ammonia and pH-values of 9.5 or higher can dilute copper, the ammonia concentration in the condensate should not exceed 3 ppm.

Form of Delivery: As 25% NH3 dilution (ammonia water).

Dosing: The usual concentration of the dosing dilution is 0.5 ... 2% NH3. The filling and diluting needs to be conducted in gas-tight plants.

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Caustic Soda

Application: In case of salt-free steam boiler operation, caustic soda (NaOH) is often used for alkalisation instead of phosphate. A binding of residual hardness is no longer neccessary in case of salt-free boiler feed water.

Effect: Caustic soda reacts alkaline. Caustic soda is non-volatile, and remains in the boiler water.

Limitations: NaOH should not be dosed to new plants, since it can accumulate locally inside fissures. This can cause lye fissure brittleness of the boiler material. In case of older systems, those fissures are closed by corrosion cover layers. In those cases, caustic soda is often used as a low-priced alkalisation agent, usually together with ammonia..

Form of Delivery: Caustic soda is available as 20% , 33% or 45% NaOH dilution.

Dosing: The usual concentration of the dosing dilution is 2 ... 5% NaOH. The correct amount of dosing dilution is monitored by the pH-value of the boiler water.

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Hydrazine

Application: Hydrazine (N2H4) is a volatile oxygen scavenger and passivating agent.

Effect: Hydrazine enters the steam and condensate system, and acts as oxygen scavenger and passivator. About 1.0 ... 3.5 ppm hydrazine are required in order to bind 1 ppm Oxygen.

Limitations: Hydrazine is a carcinogenic substance, and is thus only rarely these days, and under the respective health and safety regulations.

Form of Delivery: As ready-to-use 15%, 35% or 65% N2H4 dilution.

Dosing: Depending upon the boiler pressure and operation mode, an excess concentration of 0.1 ... 1.0 ppm N2H4 in the boiler water is suggested. For dosing and filling, gas-tight, design-approved plants are to be used.

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DEHA (Diethylhydroxylamine)

Application: DEHA (diethylhydroxylamine, (C2H5)2NOH)) is an oxygen scavenger and passivator, and is especially suited for protection of the the steam and condensate system.

Effect: DEHA is volatile, and acts as oxygen scavenger as well as passivating. About 1.24 ... 3.0 ppm DEHA are required in order to scavenge 1 ppm Oxygen.

Limitations: Among the reaction products are weak organic acids. That is why DEHA should be used together with an alkaliser like ammonia, and a sufficient solid basicity should exist in the boiler water respectively. DEHA can usually be used for boiler operation pressures up to 85 bar.

Form of Delivery: As 85% DEHA dilution, or as combined product.

Dosing: The usual dosing concentration is 15 ... 30% DEHA. An excess concentration of about 0.1 ... 0.25 ppm DEHA in the feed water is suggested.

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MEKO (Methylethylketoxime)

Application: MEKO (Methylethylketoxime) is used as oxygen scavenger, especially for protection of the steam- and condensate system.

Effect: MEKO is a highly volatile oxygen scavenger. About 5 ... 6 ppm MEKO are required in order to scavenge 1 ppm oxygen.

Limitations: MEKO is difficult to dilute in water. In the boiler water area, the oxygen scavenging process is only partially effective. MEKO acts only slightly passivating, and is not very suited for conservation of the steam boiler plant in case of shut down. MEKO can usually be used for boiler operation pressures up to 85 bar.

Form of Delivery: As ready-to-use concentrate.

Dosing: Dosing is conducted with either non-diluted concentrate, or an up to 10% MEKO dilution. An excess concentration of about 0.75 ... 1.25 ppm MEKO in the boiler water is suggested.

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POC (Polyacrylates / Polyacrilic Acids)

Application: POC (polyacrylates / polyacrilic acids) serve for dispersing of residual hardness and ferric oxides in boiler water and condensate.

Effect: POC is a hardness stabilizer. Unlike phosphate, POC does not react to hardness-containing sludge. The residual hardness remains diluted within the boiler water. An accumulation of sludge within the boiler is thus prevented.

Limitations: The residual hardness remains in the boiler water. That makes it difficult to prove the effectiveness of POC in individual application cases.

Form of Delivery: As ready-to-use dilution.

Dosing: The following excess concentrations are suggested:
feed water: 1 ppm POC
boiler water: 20 ... 50 ppm POC

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