aquacare - boiler water treatment

7/30/2019 Aquacare - Boiler Water Treatment

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Unit 3 Track Park

5 Track Crescent

Montague Gardens

7441

Tel: 021 5510933

Cell: 082 601 9633

BOILER WATER TREATMENT

Presented by

Adrian EstcourtAQUACARE

March 2009


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SECTION 1 - PRINCIPLES OF WATER TREATMENT

1. IntroductionA boiler in basic terms is nothing more than a container in which heat can be

transferred from one media through the walls of the container to water. Because

thermal energy transport is directly related to cross-sectional area, boilers aredesigned to contain the maximum amount of surface area to volume ratio which

will enable structural rigidity and reasonably controllable operation. With the

ever increasing cost of materials and fuel, design engineers are continuously

challenged to produce higher efficiency boilers and optimise the materials of

construction whilst maintaining structural rigidity.

This trend in turn challenges the Water Treatment Company to provide chemical

treatment programmes and technical back up service which will allow the boiler

plant to operate at maximum efficiency while ensuring extended equipment

operating life.

2. The Steam CircuitA typical steam circuit is described as follows. Water which enters the boiler is

referred to as boiler feedwater. Boiler feedwater may be made up of make up

water and one or more sources of return condensate. Make up water is raw

water which may be pre-treated for use in the boiler to remove specific

impurities. Examples of pre-treatment are softener, demineraliser, reverse osmosis

and de-alkaliser plants. In most instances a softener unit is commissioned to

remove hardness from the water. Due to the high capital cost, demineraliser

and reverse osmosis plants are seldom used in low pressure industrial boiler plants.

Return condensate is steam which has been condensed back to water through

the process and is returned for further use in steam production. The impurities

present in return condensate will differ from the make up water depending on

factors such as the type of process involved and potential contaminants, plant

operating conditions, materials in use etc. Boiler water refers to the water which

is heated inside the boiler to produce steam. Due to the evaporation process

and introduction of make up water, the level of impurities increases in the boiler

water and these must be controlled at specified levels through boiler waterblowdown.

3. Water ImpuritiesImpurities may be classified into three types:

Dissolved solids

Dissolved gasses

Suspended solids


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Pure water does not exist in nature and impurities vary widely. While one may

consider rain water to be pure, before the water has reached the earth's surface

it has absorbed pollutants, oxygen and carbon dioxide. Due to the high solvency

of water, additional impurities are dissolved from the land surface. A few

examples include:

Calcium Carbonate (limestone)

Magnesium Carbonate (dolomite)

Calcium Sulphate (gypsum)

Magnesium Sulphate (epsom salts)

Silica (sand)

Sodium Chloride (common salt)

Iron

If all waters carried the same impurities, treatment could zero in on a nearly standardprogramme for each use. This is however not the case and water quality varies

across South Africa depending on the type of ground material. Chemical treatment

must therefore be designed to handle specific conditions and potential problems.

4. Deposit Formation in BoilersDue to evaporation in the boiler, the concentration of dissolved impurities

increases to the point where they precipitate out of solution and adhere to the

heat transfer surfaces forming boiler scale. Certain minerals such as Calcium

Carbonate experience inverse solubility, whereby the higher the temperature,the less soluble the material. For this reason these types of minerals will tend to

exceed their solubility at the high temperature steel surfaces and form deposits

while remaining soluble in the bulk water.

The deposits typically found in boilers include:

Calcium Carbonate

Calcium Phosphate

Silica

Iron

In most instances the ingression of hardness into the boiler is the major cause of

scale formation and highlights the importance of efficient softener plant

operation. Consider a boiler plant running continuously at 10 Tph with a

feedwater hardness of 10ppm. If one allowed this hardness to deposit in the

boiler then the scale build up would be almost one ton over a one year period.

The energy implication of scale deposits is significant as shown in the example

below.


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For example, a 1mm deposit consisting mainly of carbonate will result in a 1.8%

increase in fuel consumption. Therefore a boiler plant running continuously at 10

Tph will consume an additional 130 tons of HFO per annum. Assuming HFO costs

R 3 000-00 per ton, this equates to energy wastage of R 390 000-00 per annum. If

the 1mm scale deposit consists mainly of iron/silica, then this increase in fuel costs

escalates to R 1 300 000-00.

Further problems caused by scale deposits are;

Failure of boiler tubes due to overheating of the metal

Corrosion of the metal surface under the deposit

Increased boiler cleaning expenses to remove the deposit

5. CorrosionCorrosion is the loss of metal as it oxidises back to the ore form, for example iron

reverts to iron oxide. The corrosion mechanisms involved are extremely complex

electrochemical reactions and as such will not be dealt with in this paper. Of

importance, however, is that corrosion can be uniform over the metal surface or

localised, resulting in deep pits in the metal. Pitting corrosion caused mainly by

dissolved oxygen is severe and can result in premature replacement of boiler

tubes, feedwater lines and condensate return lines.

Corrosion can be experienced in four areas:

Feedwater System: Caused by low pH, dissolved oxygen and

carbon dioxide.

Operating Boilers: Caused by high or low alkalinity, dissolved

oxygen and deposits in the boiler. The corrosion

process is accelerated by high temperatures

and stresses in the boiler.

Standby Boilers: Corrosion of boilers during out of service periods

can be severe when correct lay up procedures

are not followed. The major causes of corrosion

are oxygen pitting and low alkalinity.

Steam / Condensate System: Caused by carbon dioxide / carbonic acid,oxygen and process contaminants.

The cost of corrosion is immense, resulting in production downtime and high costs

for equipment replacement. To retube a boiler can cost between R 300 000-00

and R 500 000-00. A further problem associated with corrosion is the deposition

of corrosion products (iron, copper) on the boiler tubes which results in increased

fuel costs and furtherunderdeposit corrosion.


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The secret for preventing high corrosion rates in the boiler is through the

formation and maintenance of a magnetite layer (Fe3O4) on the metal surface.

The ability to use inexpensive carbon steel material for boiler construction, where

the metal surface is in contact with water at high temperatures and pressures, is

due to the reaction that takes place between iron an oxygen-free alkaline water

to form magnetite. The maintenance of this corrosion resistant layer is the

difference between success and failure.

With this in mind, effective corrosion prevention can be achieved by:

Removing dissolved oxygen from the feedwater

Maintaining correct alkalinity levels in the boiler

Keeping metal surfaces clean and deposit free

Protecting boilers during standby periods

Chemically treating corrosive gasses in the steam / condensate system.

6. Steam QualityPoor steam quality results from a combination of poor boiler water quality and

poor boiler operation. The steam produced from the boiler should always be as

dry as possible and contain minimal dissolved solids. Boiler water carryover results

in poor quality steam and is caused through two problem areas:

Foaming: Bubbles form on the boiler water surface and

leave with the steam. Foaming is caused by

high TDS, high suspended solids and

contaminants such as oils, greases, fats andcertain organics.

Oil, in particular, can cause major problems due

to the saponification process where it reacts

with caustic in the boiler water to form soaps.

Priming: This is a more severe form of carryover where a

sudden surge of boiler water is drawn into the

steam line. Priming can be caused by

operating the boiler above its rating, sudden

changes in steam demand and finally by

operating the water level in the boiler too high.

Carryover results in dissolved solids entering the steam / condensate system and

forming deposits. The deposits will have a negative effect on heat transfer and

will increase corrosion in the system. Where live steam is used, carryover will result

in process contamination and production losses eg. food industry.


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7. Chemical Treatment ProgrammesHaving discussed the potential problems existing in the boiler plant we can now

look at prevention of these problems through the application and control of a

chemical treatment programme. It follows that the objectives of the programme

would be to:

Prevent scale deposits in the boiler system

Minimise corrosion in the boiler system

Assist in producing good quality steam

In order to achieve these objectives, the composition of the chemical

programme must be able to perform the functions listed below.

React in some manner with the feedwater hardness and prevent it from

forming scale in the boilersDisperse any suspended solids or sludge in the boiler and prevent deposits

of this material

Remove dissolved oxygen from the feedwater thereby preventing pitting

corrosion

Provide sufficient alkalinity to minimise boilercorrosion

Provide antifoam to minimise boiler watercarryover

Protect the steam / condensate system against corrosion

Boiler water treatment technology has advanced significantly over the past

century. Antiscalants can be divided into two categories, namely stoichiometric

reactants which chemically react with feedwater impurities to change their

chemical structure and then nonstoichiometric reactions which alter the

behaviour of impurities.

Some examples are shown below:

Stoichiometric Reactants Nonstoichiometric Reactants

Carbonate Natural Organics Lignins

Phosphates - Tannins

Chelants - Starches

Synthetic

Polymers

- Polyacrylates

- Polymethacrylates

- Styrene/Maleic

Phosphonates


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Phosphate based programmes are termed precipitating programmes whereby

impurities such as calcium are brought out of solution to form a loose sludge in

the boiler water. The ability to use this type of programme effectively is based on

the addition of organic polymers in the treatment, which condition and disperse

the suspended solids and allow them to be removed through blowdown without

depositing on the boiler tubes. With correct application and control this

programme is used with excellent results.

Solubilising programmes include chelants (NTA, EDTA), polymers, metal

dispersants and organic sludge conditioners. The use of chelants (NTA, EDTA) is

often avoided because of the potential corrosiveness, particularly when they are

incorrectly applied. Polymer type solubilising programmes prevent scale

formation by distorting crystalline growth of the scale and dispersing the small

particles as colloids.

Chemicals used for oxygen scavenging are almost exclusively sulphite and

hydrazine which can both be catalysed to increase their rate of oxygen removal.

Each product has its merits and selection is generally based on boiler operating

conditions and food grade approval. Some characteristics of the two products

are shown below:

Sulphite has food grade approval while hydrazine does not

Both are catalysed, however sulphite will remove oxygen more rapidly at

low temperatures

Sulphite increases boiler water TDS while hydrazine does not

Sulphite reacts with oxygen at an 8 : 1 ratio while hydrazine reacts at a 1 : 1ratio. Generally, hydrazine is more cost effective

Hydrazine will actively promote metal passivation while sulphite will not

Hydrazine is volatile and will provide condensate line protection. Being

non-volatile, sulphite does not provide this function

Both products are suitable for offline lay-up of boilers

Condensate line protection is achieved by using either neutralising amines,

filming amines or a combination. Neutralising amines will neutralise carbonic

acid formed in the steam / condensate system and raise the ph to non-corrosivelevels. Filming amines coat the metal surfaces and provide a physical barrier

against the corrosive condensate. This is particularly effective when oxygen

corrosion is a problem.


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SECTION 2 MANAGING YOUR WATER TREATMENT PROGRAMME

With the advanced technology available to the water treatment industry and the

proven effectiveness of the products in use, one would expect to find deposit and

corrosion free conditions existing in all boilers. This is however not the case as many

boilers under treatment are opened for inspection and found to be either heavily

scaled or severely corroded. The question is obviously raised at this point as to why

poor results are being experienced. It must be understood that the chemicals being

used are not magical and therefore cannot do the job alone. Achieving an

excellent result is probably 30% due to product performance and 70% due to the

application and the control of the programme and boiler operation. Poor results are

therefore almost always due to poor management.

When selecting the correct water treatment for a plant, there are basically three

areas which the engineer must consider;

Results

Costs

Service

Engineers may vary in their opinion as to which factor is the most important to them.

However, it has been proven over and over that all three areas are strongly

interdependent and must be correctly managed to achieve effective results. Let us

look at each aspect more closely.

1. ResultsThe boiler water treatment programme used on the plant must able to

produce the desired results in terms of a deposit free boiler, minimal corrosion

and good steam quality. Without this prime objective in mind we are truly

wasting our time and money.

Results are directly dependent on the following basic factors;

a. Select the right type of treatment for your plant.For example, dont use uncatalysed sulphite when your feedwatertemperature is very low, dont select non food grade approved

products if your factory requires this approval, dont use acidic products

when your feed alkalinity is low etc.

b. Dose the chemical treatment correctly.This is probably the most commonly found area of mismanagement and

leads directly to poor results. The basic rules for dosing the treatment

should be closely followed.


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Wherever possible avoid the dilution of the chemicals with water (apart

from sulphite powder)

Dose scale inhibitor and oxygen scavenger products separately

Use sufficiently sized holding tanks

Never hand dose the chemicals

Inject the oxygen scavenger as far from the boiler as possible to allow

reaction time

Use a proportional to feed dosing system. This can be effected by an

electrical interlock between the boiler feedwater pump and the

chemical dosing pumps in situations where the boiler water level is

controlled by on/off operation of the feedwater pump. A modulating

feed valve system will not suit this arrangement. The most effective

proportional system consists of a feedwater meter with pulse generator

which drives the chemical dosing pumps. This system ensures the correctconcentration of chemicals under all conditions of load, and will also

give an indication of the boiler plant steaming rate. It also allows for

dosing of more than one boiler and can be used to calculate your

treatment cost per m3 feedwater. To ensure reliable and uninterrupted

dosing, select good quality chemical pumps with readily available

spares and back up service.

c. Ensure continuous soft water supply.Softener operation is critical to achieve results. The majority of scale

deposits found in boilers are directly connected to hard water entering

the boiler. Some chemical treatment programmes are designed to

handle intermittent hardness ingression but extended high levels of

hardness will lead to scale deposition.

Once again the basic rules must be followed to ensure soft water supply.

Select the right size unit for your plant

It is preferable to install a duplex system which will supply soft

water at all times

Make sure the regeneration is automated and set to the correct

frequency

Service and maintain the unit regularly

Add the correct quantity of salt to the brine tank

Test water at least once a day


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effective treatment for your plant, thereafter the hardest part is maintaining

actual costs close to that predicted. Actual costs must be closely monitored

on a regular basis as part of the management system and related back to the

expected figures.

Chemical treatment costs will depend on:

Scale Inhibitor: Dosage rate and unit price.

Oxygen Scavenger: Dosage rate and unit price (feed temperature

has a direct impact on dosage rate).

Condensate Treatment: Dosage rate and unit price

The most efficient method of evaluating cost is related to feedwater usage.

And example would be as follows;

Product Dosage on Feed Unit Price Cost per m3 feed

Scale Inhibitor 50ppm R 20-00/kg R 1-00Oxygen Scavenger 80ppm R 20-00/kg R 1-60

Condensate

Treatment

10ppm R 30-00/kg R 0-30

Total Treatment Cost = R 2-90/m3 feed

Therefore, a plant with an annual feedwater usage of 50 000m3 will expect to

spend R 145 000-00 on water treatment. Underdosing the programme will give

lower cost and poor results, while overdosing will not improve the result in

proportion to cost and is therefore money wasted. Assuming no majorchanges in plant operating conditions, a well managed water treatment

programme will give actual costs close to that expected.

3. ServiceCustomers are strongly dependent on the technical service and expertise of

their supplier. This service must go further than merely analysing boiler water

samples once or twice a month and should consist of a package which is

geared to complete management of the water treatment programme. Thetechnical representative responsible for your account is therefore the most

important person as he/she must ensure that all facets of the service package

are in place.

These services include;

Ensuring correct treatment programme is used

Ensuring correct dosing procedures and equipment are used


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Ensuring good softener operation

Site visits, analysis and recommendations

Establishing in-house monitoring with feedback system

Training

Monitoring of feedwater usage, softener usage and condensate return

percentage

Check consumption of products against that expected

Calculate the cost per m3 feedwater and report back on a regular basis

Carry out boiler inspections and keep photographic record

Problem solving


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SECTION 3 OTHERENERGY CONSIDERATIONS

1. Boiler BlowdownBoiler blowdown represents an area of energy loss in the boiler plant which can

be optimised to ensure good control of impurities, steam quality and efficient

boiler operation.

Blowdown is generally controlled on the basis of boiler water TDS between the

ranges of 2 000 3 500ppm for a 10 Bar boiler. A more applicable blowdown

control in terms of energy is to consider the cycles of concentration in the boiler.

If we consider the relationship between fuel required (kg/ton steam) and the

boiler cycles of concentration, the optimum cost effective operation is reached in

the range of 25 30 cycles. Increasing above this does not result in any significant

fuel saving and could lead to poor quality steam due to the increased potential

for boiler water carryover. In addition, operating the boiler at unnecessarily high

cycles will concentrate any contaminants or suspended solids entering the boiler

to problematic levels.

For example, a boiler plant with a feedwater TDS of 200ppm operated at 10

cycles will result in a boiler water TDS of 2000ppm and a blowdown fuel

requirement of 2.2kg HFO/ton steam.

If this boiler were to operate at 15 cycles or a TDS of 3000ppm, then the blowdown

fuel requirement would be 1.4kg HFO/ton steam. The fuel saving would then be

0.8kg HFO/ton steam. Once again using a 10Tph boiler in continuous operation,

the fuel saving would be R 210 000-00 per annum.

2. Feedwater TemperatureIncreasing the feedwater temperature will reduce the chemical oxygen

scavenger requirements and more significantly decrease the boiler plant fuel

consumption. It must be noted, however, that increasing the feedwater

temperature through deaerator operation or a silent steam heater in hotwell will

not reduce overall fuel usage.

In order to gain this saving, the feedwater temperature must be increased via

some means of waste energy recovery such as:

Economiser operation to recover flue gas waste heat

Lagging of condensate return lines and hotwell

Increasing the quantity of condensate return

Heat recovery from the blowdown water

If we consider the relationship between fuel required (kg/ton feedwater) and the

feedwater temperature, the potential savings are significant. For example, raising


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the feedwater temperature from 400C to 600C for a boiler operating continuously

at 10Tph, will result in a HFO saving of approximately R 630 000-00 per annum.

aquacare - boiler water treatment

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