Steam Boiler: Chemical Treatment

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Chlorination System in Power Plant

The chlorination system in power plant or dosing liquid chemicals is installed to treat the water (scale inhibition and bio-fouling protection) to use in main cooling water system.

Chlorination system shall be supplied to provide continuous and intermittent shock chlorine dosing to the seawater intake to fulfill the need of the chlorine for the whole Power Plantpurposes.

The chlorination system shall have sufficient capacity to allow intermittent shock dosing of hypochlorite selection into the intake CW pump which can then be directed into either or the circulating water systems by appropriate sectional isolation of the CW discharge header.

It is the intent of this specification that a proven and reliable system shall be provided. The chemicals and dosage rate shall be established to control of corrosion, bio fouling and scale formation efficiently and also maintaining residual chlorine in the cooling water discharge from the main condensers considering the Environmental Regulation, plant performance and life, water quality variations, economies of chemical, etc.

The chlorination system shall consist of the following dosing systems:

Sodium Hypochlorite dosing system with dosing pumps
Valves and piping
Chlorination dosing package, this package has dosing pumps. A test kit for dosing pump is provided for measuring the bacterial content. For the common drives like the filing pump & the chemical drain pump a receptacle is provided for each of the pumps.

The chlorination system shall be arranged and designed to provide complete flexibility of operation in terms of solution strength dosing quantity and treatment timings. The solution shall be injected into the sumps by injection pump from the chemical dosing tank, however.

The chlorination system should be equipped with all of the system in a complete set including piping, valves, pumps, control and instrumentation, equipment and accessories as required for normal operation. The system shall be including to the following component but not limited to:

NaOCl dosing pumps
Bromine dosing pumps
Scale Inhibitor dosing pumps
NaOCI dosing tank
Bromine dosing tank
Scale Inhibition dosing tank
Chemical drain pumps
Residual Chlorine Analyzer skid comprising of Y-type Strainer, Self Priming Pump, and Residual Chlorine Analyzer
Chemical agitator equipment (including automatic stirrer) and accessories
Safety equipment including as follow but not limited to:

Eye Wash Station and Safety Shower
Warning Horn
Breathing apparatus (with mask and Glove)

All components of chlorination system shall be made with suitable materials and has good corrosion / chemical resistant for long time operation.

Chemical Feed System

The aim of the chemical feed system is to minimize the corrosion of all materials within the steam/water cycle, to minimize deposition of corrosion product or other impurities within the cycle and to ensure the plant integrity over the plant life.

The chemical reagent should be specified under which the boiler shall be operated and provided details of dosing, sampling, monitoring and blow down arrangements that will maintain the chemistry within defined parameters. The chemical reagent and the equipment supplied to maintain it within specification should be compatible with the source and type of condensate coolant and the anticipated all staffing levels at the plant.

The chemical reagent should be specified for the following but not limited to: the condensate, feed water, boiler water, and cooling tower indicating target values for impurities and acceptable operating ranges for each specified parameters.

The chemical feed system shall also specify the maximum impurity concentration in the superheated steam to be produced together with evidence of its compatibility with the steam turbine. The proposal chemical reagent will be assessed no against international standards.

The system shall identify the source and composition of water used for spray attemperation of superheater. The system supply a schedule including P&ID for the steam water cycle indicating the positions of the dosing and sampling points and the chemicals to be dosed and the parameters to be continually measured at each point.

Especially boiler water dosing system shall be designed to maintain the steam/water cycle chemistry (scale control) under all normal operating conditions including condenser peaks and also for wet off load storage of the boilers.

The chemical feed system shall provide dosing equipment including the following items:

Chemical storage and daily tanks
Mixing and dilution tanks
Dosing pump
Pipe work and double insulation valves
Drain and wash down for spillage

For volatile chemicals such as ammonia or hydrazine are included there shall be suitable traps, scrubbers or absorbers fitted to minimize the emission of harmful vapors.

The chemical feed system shall also provide remote control of the dosing pumps including closed loop control from on-line cycle chemistry monitors. Chemical Reagent and Regeneration supply, storage and transfer. The equipment for the storage and handling of chemicals and regenerations shall be designed such that;

The manual handling of chemicals during unloading and transfer to storage shall he kept to a minimum.
Solid chemicals shall be delivered and stored in dry, covered conditions and silos shall have moisture traps fitted.
Liquid chemicals shall be stored in tanks constructed from or lined with corrosion resistant material.
Tanks shall be insulated and heated as necessary to maintain the chemicals in a liquid state.
Each tank shall have a separate bund to retain the whole tank volume in the event of spillage or leakage.
Tank or silo tilling connections shall be separated from each other within an individual locked enclosure and clearly marked with the chemical type.
Chemicals releasing volatile or harmful vapors shall have suitable traps and scrubbers fitter to the tank vents.
Automatic safety showers shall be installed adjacent to chemical unloading facilities and within the water treatment plant.
Transfer of liquid chemicals from bulk storage to the regeneration system shall provide the minimum risk of spillage and leakage. Vacuum transfer shall be provided.
Transfer of regenerates from measuring / dilution tanks to service vessels shall provide minimum risk of leakage and spillage. Transfer by water ejector shall be provided.

The bulk chemicals are to be used for other water treatment plant application, if required they shall meet the minimum purity standards for the more generous duly. The purity standards shall also take into account local environmental discharge legislation with respect to heavy metals.

Chemical Treatment Boiler Feedwater

Chemical treatment boiler feedwater is the process to treat feedwater chemically to be desired requirement of feedwater. Chemical treatment boiler feedwater should be performed to maintain steam boiler keep in highest efficiency. One way of chemical treatment boiler feedwater is by adding chemical injection such as sodium phosphate. This action is carried out to prevent deposit or crust formation and decrease level corrosion on the metal pressure part boiler.

The following below are advantages of chemical treatment boiler feedwater:

Prevention of deposits formation

Water that contains high hardness, turbidity, silica and high solutes content tend to cause scaling, deposit or crust on the boiler pipes or tubes. Deposit or crust will form thin layer inside boiler tubes. This layer can reduce heat transfer process from metal tubes to boiler water inside it. So steam boiler need more combustion to heat boiler water, it means that need fuel addition to perform more combustion so it will decrease boiler efficiency.

Decrease level corrosion

Metals will experience corrosion at low pH (pH < 7). Furthermore steam boiler must be injected by chemicals to raise pH feedwater before being used as boiler water or chemical can be injected to boiler water in steam drum in order to give effect to minimum corrosion.

The chemicals that is used to adjust pH feed water is sodium chloride. Other way to decrease pH in boiler is by inject other chemicals to bind the dissolved CO₂ in the feed water because oxygen (O₂) will result in depolarization cathode, so it causes corrosion. The chemical which is used to bind the dissolved oxygen is hydrazine (N₂H₄).

Phosphate Definition

Phosphate (PO₄³) is an anion of phosphorus compounds having phosphorus atom that is completed by four oxygen atoms located at the corner of the tetrahedron. Total phosphates can be measured directly by calorimeter or by the digestion process first before sample measurement.

There are 3 (three) types of phosphoric acid are known, namely: ortho phosphoric acid (H₃PO₄), acid pyrophosphate (H₄P₂O₇) and meta phosphoric acid (HPO₃). Ortho phosphoric is the most stable and most important (these substances are often called phosphate only), a solution of pyrophosphate and meta phosphate change to be ortho phosphoric slowly at ordinary temperatures, and faster to seethe. Ortho phosphoric acid is acid that has three alkalis which form three rows of salt such as primer ortho phosphoric (NaH₃PO₄), secondary ortho phosphoric (Na₂HPO₄) and tertiary ortho phosphoric (Na₃PO₄).

Each phosphate compounds present in dissolved form, suspended or bound in the cells of organisms in water. Phosphate compounds in waste water can be derived from the wastes of the population, industry and agriculture. In agricultural areas ortho phosphoric derived from fertilizer ingredients that go into the river via drainage (drain) and the flow of rain water.

Polyphosphate can enter rivers through the sewage of the population and industries that use materials such as detergents containing phosphate-laundering industry, metal industry and so on. Organic phosphate present in the sewage of the population (feces) and food scraps.

Organic phosphates can also occur of ortho phosphoric dissolved through biological processes due to both bacteria and plants to absorb phosphate for growth. The different types of phosphate are also used for the processing of anti-rust and anti-crust on heating water.

Chemical Treatment in Boiler

Chemical treatment is a treatment to inject chemical in steam boiler. Injection chemical is performed to make hardness in drum or header soluble, so blowdown rate can be maintained. Another purpose of chemical treatment is prevention of crust formation and corrosion. Sulfuric acid or polyphosphate is the most commonly used to control the formation of calcium carbonate crust. The additions of material containing chromate, phosphate or other mixtures are also often used to control corrosion.

If water is left alone for a long time, microorganisms will grow such as bacteria, fungi, algae and protozoa. These microorganisms will continue to grow cause problems in the form of biological fouling which reduce heat transfer in feedwater and inhibit water flow rate. Microorganisms also make problem in cooling tower, so control of microorganism should be done on the cooling tower.

There are many chemical processing to control microorganism but a mixture containing copper are not recommended. Mixtures containing chlorine or bromine are effective blends for controlling microorganism but excessive use can damage the cooling tower of building materials. For the maintenance of free residual chlorine must be maintained at the highest limit of 1 ppm.

Chemical Safety in Steam Boiler

Steam boiler operation always use chemical to treat feedwater and boiler water as expected purpose. Chemical which is injected into steam drum has a purpose to make scale and sediment is soluble and drainable. Chemical is also used for water treatment system to solvent, descale, softening, and so on. Chemical around steam boiler area has a lot of types and functions. Therefore the operator should have good knowledge and has special consideration for chemical materials and equipments to ensure steam boiler is running safety.

All of chemical material must be identified and labeled based on its type and function whether the chemical is toxic content or not, flammable or not, explosive able or not and so on. The label must show the content of chemical, hazardous type, etc (see figure 1). Operator of steam boiler or other personnel must be equipped with safety protection such as mask, rubber shoes, rubber apron, face shield, goggles and rubber gloves.

Figure 1: One of Chemical Label in Steam Boiler Area

Do not open or change related with chemical storage or equipment such as label, seal, cover, location and so on if do not know what is the content and function of that chemical equipment. Identify chemical content is useful to take proper action to use, make disposal procedure, and clean up procedure. For more detail safety induction in steam boiler can be seen in code National Fire Protection Association (NFPA).