BOILERS-WATER-TREATMENT

IMPURITIES IN WATER

The hydrological cycle is the name given to describe the process through which water obtains its various contaminants. Water evaporates out of the ocean due to solar heating and forms clouds of moisture. When it warm up, moist air meets colder air or travels over cooler ground surfaces; condensation occurs and rain condenses out of the clouds and falls on the ground. As the rain falls, it dissolves carbon dioxide and oxygen. These two gases change the properties of water considerably. When water containing carbonic acid contacts soil, containing limestone, calcium is dissolved adding “hardness” to the water. “Hardness” is a term used to describe water that was hard to use in doing laundry. It takes more soap to wash with hard water because hardness reacts with soluble soaps, forming a sticky precipitate.

The impurities in water can be classified as:

1-Dissolved Solids

2-Suspended Solids

3-Dissolved Gases

In boiler systems these impurities are undesirable and cause trouble by forming deposits on tubes thus, impairing heat transfer. They also have the effect of damaging metal used in the construction of these systems.

The broad aims, therefore, of boiler water treatment are to prevent deposits (scaling) and corrosion and it must be done at a cost that is economically justified by the benefits gained from the use of trouble free water.

EXTERNAL TREATMENT

Before water is considered for use in steam boilers, it is necessary to remove some of the impurities, which would otherwise lead to problems. There are several methods of effecting external treatment. Dissolved and Suspended Solids are normally removed by Water Softening, Demineralization and Reverse Osmosis. For this manual, we will focus on the Water Softener, which is the type most commonly used by industries in Jamaica. Dissolved Gases, ex. oxygen and carbon dioxide are normally removed by deaeration.

a) . WATER SOFTENER

The purpose of the water softener is to reduce hardness. Hardness consists of dissolved Calcium and Magnesium in water. These two ions, is expressed as the Total Hardness. Calcium and Magnesium are the ions, which form scale particularly when water is heated.

The apparatus consists of two tanks; one containing the ion exchange bed (Resin) and the other containing the regeneration material (Brine). During normal operation, the hard water is passed downward through the Resin bed. The hardness is removed during this cycle and stored in the Resin. The soft water produced is then transfer to the point of use. After certain amount of water has been softened, the Resin ceases to be effective and has to be regenerated (recharge). This is accomplished by passing a solution of brine through the resin bed, resulting in the resin being restored to its ability to produce soft water.

b). OXYGEN REMOVAL EQUIPMENT

The simplest method for removing oxygen is by the use of an open tank containing the water and dissolved oxygen. Steam coils are provided in the water to heat the water and drive off the oxygen. The performance of this device can be improved by enclosing the tank, providing an air vent and heating the water to a temperature greater than 212 F. This device is called a deaerating heater. This system can be further improved by the addition of methods to break up the water and provide intimate contact between the steam and water droplets. Such an installation is called a Deaerator. In the spray deaerator, the water is sprayed through steam and heated to near steam temperature removing 90 percent or more of the dissolved oxygen. The water is then sprayed in an atomizing section at the bottom from which feed water is pumped to the boiler. The second type of deaerator consists of a series of trays over which the water is allowed to fall causing it to break up into droplets. Steam is brought into intimate contact with the water droplets and dissolved oxygen is driven off.

Typical performance of oxygen removal equipment is given below:

TYPE PRESSURE (psig) TEMPERATURE (F) OUTLET OXYGEN(ppm)

Open heater Atmospheric 160 – 210 0.5 – 1.0

Deaerating

Heater 1 – 15 215 – 250 0.04

Deaerator 1 – 15 215 – 250 0.007

INTERNAL TREATMENT

Internal treatment is classified as the conditioning of impurities within the boiler system itself. The reactions occur either in the feed lines or inside the boiler. The purpose of this type of treatment is to properly react with feed water hardness, control corrosion, scavenge oxygen, and prevent boiler water carry over.

A variety of chemicals is used in internal boiler treatment. Phosphates have been the main scale-conditioning chemicals along with sodium sulfite for oxygen removal. Condensed system protection is accomplished with volatile neutralizing amines or volatile filming amines. Other chemicals, natural organic materials and synthetic polymers, are used as sludge conditioners to aid in the conditioning of precipitated hardness.

We uses various blends of these chemicals in the boiler treatment program.

B – 001L is a blend of:

1.Phosphate – its function is to precipitate calcium, which are the chief scale-forming agent, and other scale forming salts.

2.Organic Materials – These are used to keep the precipitates (sludge) within the boiler in a fluid state.

3.Sulfite – It’s function is to prevent corrosion caused by oxygen (pitting)

4.Alkalinity Adjuster – These are used to maintain boiler water alkalinity at a level which ensures proper precipitation of calcium (normally pH 11 to 12).

B – 002L : This is used as a compliment to B – 001L and counteracts any hardness that may infiltrate the boiler.

a). CONDENSATE TREATMENT

Condensate is ideal water to be utilized as boiler feed water. It is usually low in solid contents and its usual high temperature helps in the removal of undesirable gases from make-up water.

Condensate however, is naturally corrosive (low pH) and is very destructive when it comes into contact with steam traps and condensate lines, etc.

The corrosive tendency of condensate is normally controlled by the use of a neutralizing or a filming amine, usually uses the neutralizing amine in the form of A – 101L

The agents in this blend are responsible for neutralizing the acidic condition in condensate.

b). CHEMICAL FEED SYSTEMS

Several systems are available for feeding chemicals into boilers. The feeding system can either be continuous or batch. The basic continuous system consists of chemical solution tank from which chemical is withdrawn by a metering pump. The metering pump is normally interlocked with the feed water pump; this enables the chemical to be dispensed when water is being pumped into the boiler. The continuous system is recommended for all applications because the water can be carefully controlled and chemical usage optimized. The batch treatment consists of adding by pump or directly, enough chemicals to provide control for a considerable period. This method should only be used as a temporary measure.

c). BLOWDOWN

Boiler feed water contains impurities in solution and in suspension. Since the steam generated is essentially pure, these impurities are left behind to concentrate in the boiler. If these dissolved and suspended solids are allowed to concentrate beyond certain limits, a deposit of scale will form on the boiler heating surfaces which will retard heat transfer and increase tube metal temperatures. This can lead to reduced boiler efficiencies and even more important may be the probability of furnace tube failures caused by overheating. High solids can also interfere with proper operation of steam separating apparatus thus causing boiler water carryover with the steam.

The concentration of dissolved and suspended solids in boiler water is controlled by removing some of the high-solids boiler water and replacing it with low-solids feed water, effecting a general lowering of solids concentration in the boiler. This process, which is known as blowdown, can be either intermittent bottom blowdown or continuous blowdown. Bottom blowdown is necessary to remove any sludge accumulating in the lowest parts of the boiler system. Continuous blowdown is taken from the point of highest solids concentration, usually from the upper section of the boiler close to the water level.

BOILER WATER ANALYSIS

We know that water treatment involves the addition of chemicals to both the feed and boiler water to promote chemical changes of impurities present in them. These chemical changes or their evaluation cannot always be done by only visual means. The boiler water analysis is a series of chemical tests done to inform boiler operating personnel about what is happening in the boiler water-side so that corrective actions can be taken to maintain desired limits. This analysis plays an important role in boiler water treatment and should never be neglected.

The following are the tests normally done on boilers, serviced by our systematic water treatment:

a). “TH”- TOTAL HARDNESS (EDTA Titration Method)

1)Add 50 ml sample into measure cylinder.

2)Add 1ml hardness indicator buffer solution (EDTA-01-01N).

3)Add the standard hardness reagent (EDTA-02-02N) from the burette slowly with constant stirring until the last reddish tinge disappears and the solution is true blue.

4)Read burette; volume in ml x 20 = total hardness in ppm.

b). “P”- PHENOLPHTHALEIN ALKALINITY

1)Add 50 ml sample into measure cylinder.

2)Add 3 to 5 drops of phenolphthalein indicator (PI-01-01MG).

3)Add standard sulfuric acid (SSA-0102-02N) from burette until pink colour disappears.

4)Read burette; volume in ml x 20 = ppm P-Alkalinity.

c). “M”-TOTAL ALKALINITY

1)To the above sample add 6 to10 drops of methyl red indicator (MRSS-02-02N).

2)Continue to add sulfuric acid(SSA-0102-02N) (without refilling burette) until maximum red colour appears.

3)Read burette; total volume in ml for this test and P-Alkalinity test x 20 = ppm M-Alkalinity.

d). “NACL”– CHLORIDES.

1)To the above sample, add 6 to 10 drops of potassium chromate indicator(KCL-01-09N).

2)Add standard silver nitrate(SN-B32N) from burette until colour changes from a pure yellow to a faint pinkish yellow.

3)Read burette, volume in ml x 20 = ppm Chlorides.

e). PHOSPHATE

1)Filter sample through filter paper.

2)Fill the phosphate test-tube to the 5 ml mark with the filtered water.

3)Add deionized water to the 15 ml mark and mix.

4)Add molybdate reagent (MR-023M) to the 17.5 ml mark and mix.

5)Add 2 dippers (in cap of vial) of stannous chloride powder (22B-01)and mix.

6)Match tests sample with the comparator colour and read as ppm Phosphate.

f). SULFITE

1)Add 50 ml sample to measure cylinder.

2)Add 1 dipper (1.0 gram capacity) of starch indicator acid powder(2215-AI) and mix.

3)Add standard iodide/iodate (IO-0101-B) from the burette until a permanent “light blue” colour appears.

4)Read burette; volume in ml x 20 = ppm Sulfite.

g). pH- MEASURE OF HYDRODGEN ION CONCENTRATION

( Taylor Comparator)

1)Rinse and fill three 5 ml test tubes to mark with sample.

2)Place in comparator block.

3)Add 0.5 ml of the appropriate pH indicator to the centre tube and mix.

4)Compare with appropriate comparator slide and read pH directly from slide.

h). SPECIFIC CONDUCTANCE

1)Add 25 ml sample to a measuring cylinder.

2)Cool the sample.

3)Add 2 drops of Phenolphthalein indicator.

4)Add some particles of neutralizer powder until the pink colour disappears.

5)Add into cup in conductivity meter.

6)Set meter on appropriate scale and read.

HABITS FOR A GOOD BOILER WATER TREATMENT PROGRAMME

1) Check always water softeners regularly to ensure that soft water is being produced. Regenerate softener at the first sign of hardness (blue with pinkish tint in hardness test). Never allow softeners to over-run when exhausted.

2) Check always water in the sight glass; a cloudy appearance indicates the possibility of hard water in the boiler.

3) Monitor always chemistry tank closely. Identify a reasonable low level mark and make a new batch of chemical when this point is reached. This will prevent the possibility of running out of chemical.

4) Order always chemical before stock on hand runs out.

5) Blow down always boilers as recommended. Use frequent short blows as opposed to infrequent lengthy blows. This reduces treated water losses and lost sensible heat energy in the wastewater. Better control of boiler water concentration is also achieved in this way.

6) Do all tests recommended by us; pay careful attention results of analysis and make corrective adjustments as indicated by analysis e.g. regenerate softeners or increase/decrease blowdown.

BOILER WATER CONTROL LIMITS

Samples taken Desired Level

1. Softener

a). Total Hardness 0 – 5 ppm

2). Feed Water

a). Total Hardness 0 – 5 ppm

b). P-Alkalinity 0 ppm

c). M-Alkalinity variable

d). Chlorides 1500 ppm max.

3). Boiler Water

a). Total Hardness 0 – 2 ppm

b). P-Alkalinity 300 – 1200 ppm

c). M-Alkalinity variable

d). Chlorides 1500 ppm max.

e). Phosphate 30 – 60 ppm

f). Sulfite 30 – 60 ppm

g). Conductance 2500 – 3500 MM

h). pH 10.5 – 12

4. Condensate

a). Total Hardness 0 – 2 ppm

b). P-Alkalinity 10 – 100 ppm

c). M-Alkalinity variable

d). Chlorides 0 – 30 ppm

e). pH 7.4 – 8.4

Results Analysis and Indicated Action

1a). If over 5: Regenerate Softener immediately.

2a). If over 5: Hard Water has gotten into feed water tank.

2b). If measurable: Inform to our service Dpt.

2d). Used as comparison to determine Blowdown rate.

3a). If over 2: Hard Water has entered boiler; check for source of hard water.

3b). If over 1200: Increase blowdown rate. If under 300: Hard Water has entered boiler.

3d). If above 12 times 2d: Increase blowdown rate. If below 8 times 2d, blowdown rate is excessive and treated water and heat are being wasted.

3e). If above 60: Reduce chemical pump stroke. If below 30: Increase chemical pump stroke.

3f). Same as 3e

3g). If above 3500: Increase blowdown rate. If below 2500: Reduce blowdown rate.

3h). If above pH 12: Increase blowdown rate. If below pH 10.5 Hard Water has entered boiler.

Bolier Water Treatment

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or email us at service@hotelssafety.com

Water treatment is extremely important for industrial steam boilers, because of the following reasons:

1- A proper water treatment will optimize heat transfer by controlling water impurities, which would otherwise form insulating seal and corrosive products.

2- It can protect and ensure long life to the boilers, steam supply lines, condense return lines, and corrosion and scale formation associate to the equipment.

3-It provides an improved quality of steam, so necessary in many industrial processes.

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