Boiler Water Treatment

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In the draft “Code of practice for the design, safe operation, maintenance and servicing of boilers”, a requirement is made for regular water-quality monitoring of both limited-attendance boilers and unattended boilers.

Objectives of the Boiler Water Treatment:

The principal objects of boiler feed water treatment should be:

  1. Prevention of scale formation in the boiler and feed system by using distilled water or precipitating all scale forming salts into the form of a non-adherent sludge.
  2. Prevention of corrosion in the boiler and feed system by maintaining the boiler water in the alkaline condition and free from Oxygen and other dissolved gases.
  3. Control of the sludge formation and prevention of carry over with the steam.
  4. Prevention of entry into the boiler of foreign matter such as oil, waste, mill-scale, iron oxides, copper particles, sand etc. By carefully use of oil heating arrangements, effective pre-commission cleaning and maintaining the steam & condensate systems in a non corrosive condition.
  5. To maintain the boiler in healthy state, around any possible seawater contamination. The first stage in the boiler water testing is to collect feed water sample from the boiler concerned. The water sample collected from the boiler through connection would be relatively clean and does not represent the true through connection would be relatively clean and does not represent the true condition of the boiler water it considered as good sample. This is because hot sample particles is flushed off into a steam making that dissolved solid in the sample higher and lots of volatile such as hydrazine will reduce the concentration measure in the test program.

Sampling for external analysis:
When taking a sample of boiler water it is important to ensure that it is representative. It is not recommended that the sample be taken from level gauge glasses or external control chambers; the water here is relatively pure condensate formed by the continual condensation of steam in the external glass / chamber. Similarly, samples from close to the boiler feedwater inlet connection are likely to give a false reading.

Nowadays, most boilermakers install a connection for TDS blowdown, and it is generally possible to obtain a representative sample from this location.

If water is simply drawn from the boiler, a proportion will violently flash to steam as its pressure is reduced. Not only is this potentially very dangerous to the operator, but any subsequent analysis will also be quite wrong, due to the loss of the flash steam concentrating the sample.

Since a cool sample is required for analysis, a sample cooler will also save considerable time and encourage more frequent testing.

A sample cooler is a small heat exchanger that uses cold mains water to cool the blowdown water sample.
Fig_3_12_1

General Boiler Water Tests:

  1. Alkalinity test
  2. PH test
  3. Chlorides test
  4. Phosphates test
  5. Total Hardness test
  6. Conductivity test

Details of the Common Boiler Water Tests:

Test for Alkalinity

The Alkalinity Test is carried out in two stages:

1)Phenolphthalein Alkalinity Test
2)M-Alkalinity

P- Alkalinity Test (CaCO3)

Purpose:

1.It gives the alkalinity of the sample due to Hydroxides and Carbonates.
2.It gives warning against high concentration of sodium hydroxides and subsequent damage to the boiler from caustic embitterment.

Procedures:

1.Take a 200 ml water sample in the stopped bottle.
2.Add one P- Alkalinity tablet and shake or crush to disintegrate.
3.If P-Alkalinity is present the sample will turn blue.
4.Repeat the tablet addition, one at a time (giving time for the tablet to dissolve), until the blue colors turns to permanently yellow.
5.Count the number of tablets used and carry out the following calculation:
P-Alkalinity, ppm CaCO3 = (Number of tablets x 20) – 10
e.g. 12 Tablets = (12 x 20) – 10 = 230 CaCO3
6.Record the result obtained on the log sheet provided, against the date on which the test was taken.
7.Retain the sampler for the M-Alkalinity.

M-Alkalinity Test

Purpose:

1.It gives alkalinity due to bicarbonates, includes the bicarbonates formed during the P-alkalinity test.
2.The result warns us against possible formation of carbonic acid inside the boiler as well as in the steam condensate lines, due to high concentration of bicarbonates.

Procedures:

1.To the P-Alkalinity sample add one M. Alkalinity tablet and shake or crush to disintegrate.
2.Repeat tablet addition, one at a time (giving time for the tablet to dissolve), until the sample turns to permanent red/pink.
3.Count the number of tablets used and carried out the following calculation: M- Alkalinity, ppm CaCO3= (Number of P. & M. Tablets x 20) – 10 e.g. If 12 P. 5 M. Alkalinity tablets is used.
4.Record the result obtained on the log sheet provided, against the date on which the test was taken.

PH Test

Solutions of which water forms a part, contain hydrogen ions and hydroxyl ions, when this are present in equal amounts the solution is said to neutral. When there is an excess of hydrogen ions it is acid, and when an excess of hydroxyl ions it is alkaline. Keeping the water in slightly alkaline condition reduces corrosion. The level of acidity or alkalinity is usually express in terms of pH Value. This is basically a measure of the hydrogen ion concentration in the solution; for convenience the very small values involved are express in terms of the logarithms of their reciprocals.

PH = Logarithm of the reciprocal of the Hydrogen Ion in the solution

It should be noted that as the reciprocal is being use, the pH value increases as the actual hydrogen ion concentration decreases.

PH Value (Reciprocal or Hydrogen Ion) Test
7.5 – 14.0 for Boiler Water
6.5 – 10.0 for Condensate Water

Purpose:

1. To give warning on acidity or alkalinity of boiler water sample.
2. Result help to establish the dosage of boiler compound to fight against corrosion.

Procedures:

1.Take a 50 ml sample of the water to be tested in the plastic sample container provided.
2.Using the white 0.6 gram. Scoop provided, add one measure of the pH reagent to the water sample, allow dissolving – stirring if required.
3.Select the correct range of pH test strip and dip it into the water sample for one minute.
4.Withdraw the strip from the sample and compare the color obtained with the color scale on the pH indicator strips container.
5.Record the pH value obtained on the log sheet provided, against the date on which the test was taken.

Chloride ppm CI Test

Purpose:

1. Gives warning against any seawater contamination of the Boiler Feed System.
2. Help to establish an effective blow down control of the boiler.

Procedures:
The range of chloride to be tested determines the size of water sample used. The higher the chloride level, the smaller the size of water sample used – this saves tablets. E.g. for Low Chloride Levels use 100 ml. water sample. For Higher Chloride Levels 50 ml water samples.
1.Take the water sample in the stopper bottle provided.
2.Add one Chloride tablet and shake to disintegrate. Sample should turn yellow if chlorides are present.
3.Repeat tablet addition, one at a time (giving time for the tablet to dissolve), until the yellow color changes to permanent red/brown.
4.Count the number of tablets used and perform the following calculation:
For 100 ml Water Sample: Chloride ppm = (Number of tablets x 10) – 10 e.g 4 tablets = (4 x 10) – 10 = 30 ppm chloride
For 50 ml Water Sample: Chloride ppm = (Number of tablets x 20) – 20 e.g 4 tablets = (4 x 20) – 20 = 60 ppm chloride
For small steps of ppm chloride use a larger sample.
For larger steps of ppm chloride use a smaller sample.
5.Record the pH value obtained on the log sheet provided, against the date on which the test was taken.

Phosphate ppm Test (PO4)

Purpose:

1.It helps to maintain a phosphate reserve in the boiler to counter any possible contamination of the boiler water by corrosive and scale forming salts. However, too much phosphate in the boiler may also contribute to foaming and priming.

Procedures:

1.Take the comparator with the 10 ml cells provided.
2.Slide the phosphate disc into the comparator.
3.Filter the water sample into both cells up to the 10 ml mark.
4.Place one cell in the left hand compartment.
5.To the other cell add one Phosphate tablet, crush and mix until completely dissolved.
6.After 10 minutes place this cell into the right hand compartment of the comparator.
7.Hold the comparator towards a light.
8.Rotate the disc until a color match is obtained.
9.Record the result obtained on the log sheet provided, against the date on which the test was taken.

Written by Marine Study

Marine Study

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