Quality of boiler water is an integral part of increasing boiler efficiency. Unchecked contaminated boiler water can lead to problems such as corrosion and scale formation. Furthermore, it can also affects its performance, life and cost of maintenance jeopardizing boiler efficiency; making boiler water treatment a key in marine industry.
To ensure its normal operation; boiler water is always tested and treated at regular intervals. All considerable issues found in test results are rectified by adding appropriate chemicals to the feed. For boiler water, the ideal amount of hardness, chlorine, P-Alkalinity, P-H, Specific Density, hydrazine and phosphate level should be; 0, <100, 100-150, 9.5-11.5, <1.002, 0.1-0.2, 20-50 respectively.
The boiler water treatment process consists of De-contamination and adding suitable chemicals based on the test results. They are intended to control scale formation, corrosion, dissolved oxygen, foaming, neutralize acids and blow-down precipitated impurities settled at the bottom.
Why Boiler Water Treatment is Required in First Place?
Forget for a moment all what i tell at the start; heavy stuff like chemicals, oxygen inhibitors, foaming, scale formation etc. Imagine what will happen if we run boiler on untreated boiler ! There will be formation of salt deposits on tubes restricting heat transfer ability. Soon they will start to overheat at different spots leading to frequent tube failure and pressure drop.
With no further steps to treat contaminated boiler water, even corrosion and oxidation will follow suit with heavy salt deposits / scale formation. This will lead to drastic decrease in boiler efficiency even failure. Hence the purpose of boiler water treatment is to check feed water quality to avoid such conditions.
How Boiler Water Test is Conducted?
The first step in boiler water treatment is to collect and test the samples of feed water from the boiler. The samples of water taken from sampling cock of the boiler relatively represents the current state of the boiler. As it shows the actual condition of water in the tubes rather than cleaning feed water being supplied.
This is because the hot water evaporating in the boiler steam drum affects the concentration of solid in the water. So always take water samples from the sampling cock. Wipe off the water from the sample cock for few seconds before taking samples to make sure there is no sediments in the line.
Allow water to flow from the sample jar / bottle to prevent air pockets from being sealed in the jar keeping the bottle air tight before testing. There are different tests conducted on the sample which are as follows:
- Oxygen Content
A Boiler is required to operate in between 9.5 to 11.5 P.H condition based on its type. ( Auxiliary – 9.5 to 10.5 & Large Boilers 10.5 to 11.5). The test will show the alkalinity of the boiler water due to hydroxides and carbonates. It help protect the boiler from caustic embitterment.
- Fill the clean test tube with water sample.
- Add P-Indicator powder and mix throughly.
- Now add standard solution of H2SO4 drop by drop until the solution becomes colorless.
- Count the number of drops added
- Now multiply it by a factor of twenty to get the p-alkalinity result.
- Recored and then write it on the log.
- To obtain total alkalinity of the solution; take the sample in another test tube.
- Add few Bromcresol green-methyl indicator powder to the solution.
- Mix the solution; soon you will notice that the solution turns into green.
- Now add H2SO4 drop by drop.
- Count the number of drops it require to make the colour disappear.
- Now multiply the number with a factor of twenty to get the total alkalinity value.
- Record the test results and log it down.
High magnesium and calcium content in the boiler feed water can lead to hardness of boiler water. This condition over time can cause severe scaling on the tubes and boiler fittings. The test represents the hardness of the boiler water indicating the required amount of boiler chemicals to be added time to time.
- Add sample water to the clean Test tube.
- Add three drops of hardness one buffer solution.
- Now mix the sample well.
- Add a drop of hardness two indicator solution provided in the hardness test kit ( All hardness test kit have similar solution with different name but same purpose ).
- Mix the solution; It will turn into pink.
- Now add the hardness three solution drop by drop until colour changes to blue.
- Count the number of drops and get the hardness looking at the hardness chart provided with your test kit. The number of drops will represents G.B.P in the chart.
Chloride ppm Test
It helps to know amount of salt dissolved in the water. It is exceptionally great at finding contamination due to mixing of sea water and plan effective blow down procedures.
- Assumption is done whether the amount of dissolved salt is more or less.
- For water with high risk of contamination with sea water; small amount of sample is taken.
- For all other condition relatively large sample is taken.
- Add chloride powder to the sample and mix the solution until it turns yellow.
- Now start adding chloride solution drop by drop until the color disappear.
- Count the number of drops and multiply it with a factor of ten for normal sample and twenty for sample with sea water contamination.
Too much of phosphate on one hand can cause priming and foaming while too less or none in the boiler water can accelerate corrosion and scale formation. So it is necessary to maintain a right balance to have the best performance.
- Take the phosphate slide comparator.
- One slide have pre marked colour for matching.
- Put the test tube in another slide.
- Add sample into the test tube ( 10 ml ).
- Using a Pipette add 1ml of phosphate acid reagent to the solution.
- Mix the solution.
- Leave it for five to ten minute
- Now compare the colour and make the phosphate ppm reading.
Boiler water is always needed to maintain at P.H 9.5 to 11.5. P.H is calculated by the amount of hydrogen or hydroxide ion in the water. Excess of hydrogen ion makes it acidic while more amount of hydroxyl ion can make it alkaline. The test is rather simple but important based on colour comparison. The amount of chemical dosage for boiler is calculated based on this test.
- Take two test tube and add 5ml of sample to each of them.
- Now put one test tube to the colour comparator box.
- Now pour few drops of P.H indicator solution to another test tube.
- Mix the second solution.
- Now put other test tube to the colour comparator box.
- See the colour in front of a light source and compare .
- Write the result in P.H unit.
Dissolved Oxygen Test
For this experiment we require a specific kit called “dissolved oxygen Kit”. Oxygen content is determined in boiler water to avoid corrosion. The process ( mechanical or chemical ) required to remove dissolved oxygen also depends on its amount; so its necessary to determine the right concentration of oxygen dissolved in water.
- Take a jar to collect sample water.
- Put the test tube inside the jar such that no air bubbles get inside while filling it.
- Close the test tube with a stopper.
- Now remove the filled test tube out of the sample jar by putting a stopper on the head of test tube.
- Once you are prepared to start the test; open the stopper and add oxygen reagent powder.
- Put back the stopper and shake the test tube to mix the solution.
- A brownish orange precipitate at the bottom indicate the presence of dissolved oxygen while white precipitate shows you add too much of reagent.
- Do not worry even if you add too much of reagent one as it would not affect the experiment.
- Now open the stopper and add regent two provided in the kit drop by drop.
- Count the number of drops and calculate the oxygen content based on multiplication factor given in the test kit.
Chemicals Used in Boiler Water Treatment
Various set of chemicals are used for boiler water treatment. Oxygen scavenger such as hydrazine, carbohydrazide, sodium sulphate and tannin is used to remove the dissolved oxygen in the boiler water. This prevent boiler corrosion by removing or reducing oxygen content in the system. Similarly chemicals like that of sodium hydroxide, sodium carbonate, sodium hexa meta phosphate and Tri-sodium phosphate used to maintain alkalinity of the boiler water.
Special corrosion inhibiting compound such as sodium bisulfite, carbohydrazide and cyclohexylamine are used to reduce scale formation in boiler tubes. Sometimes a sludge conditioner compound is also being added to help aid blow down process. These chemicals can be in the form of coagulants, anti-foams and other dispersing agents such as starch and tannin.
Other special chemicals are also added sometimes to remove the dissolved solids from water. If remained unchecked for a long time they will carryover to the turbines depending upon the evaporation rate and turbine usage. There it can react with the turbine material forming thick scale affecting their overall efficiency. It is a normal practice to keep dissolved solids below 2000ppm with regular boiler water treatment.
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