50-selected Questions and Answers on Water treatment plant(WTP) for Boiler engineers

1. What are the sources of Boiler feed water?

River, Underground water (bore well), pond water and sea water.

2. What is make up water?

Raw water, softened water or demineralized water used for steam generation is generally called as make up water.

3. What are the various impurities present in untreated or natural water?

• Undissolved suspended materials: mud, sand, sediments etc.
• Dissolved salts and minerals: Carbonate, bicarbonate, sulphates, silicate and nitrate of calcium & magnesium.
• Dissolved gases such as oxygen, carbon dioxide.
• Other minerals, mineral acid (HCL & H2SO4) oil and grease.

Raw water sources & impurities

4. What is condensate water?

When the steam transfers its heat to process or heat exchanger, it reverts to liquid phase called condensate.

5. What is blow down water?

Part of water that is drained to limit the impurity level to an acceptable level.

6. What is feed water?

Feed water is the sum of condensate that is returned from process or heat exchanger and make up water.

7. What is a total solid in water?

It is the sum of dissolved and suspended solids in water.

8. What is turbidity?

It is the final suspended matter, which does not settle and imparts cloudy or muddy color to water. It is measured in NTU.

9. What is conductivity?

It is the ability of water to conduct electricity in water. It indicates the amount of dissolved minerals in water. It is measured in micro-seimen/cm. Conductivity of water increases with increase in temperature. For example if water of conductivity 100 μS/cm at 25 °C increase to approx. 440 μS/cm at 100 °C

10. What is pH of a solution?

• pH is the measure of degree of acidity or basicity of a solution. pH range is 0 to 14, zero being most acidic and 14 is most alkaline.

Key indicators of pH:

• A change of one pH value represents the change of 10 times in relative acidity or alkalinity. For example a pH of 4 is 10 times more acidic than pH 5.
• In general, when the pH is less than recommended ranges, then there will be chances of corrosion and if more then there will be chances of scale formation.
• According to ASME standards, boiler water pH level is maintained above 9.5 to ensure the proper reaction between calcium and magnesium ions & phosphate molecules.
• Acids and alkalis have effect of increasing the conductivity of water above the neutral value, for example a water of pH 12 has more conductivity than that of pH 7.

11. What is alkalinity of water?

It is the measure of carbonates (Co3), bicarbonates (HCO3) and hydroxyl ions (OH).

12. What is M alkalinity?

It is the sum of carbonate, bicarbonate and hydroxyl alkalinity.

13. What is Phenolphthalein or P alkalinity?

It is the sum of hydroxyl alkalinity and half of carbonates alkalinity.

14. What are Cation & anions?

Positively charged ions like calcium Ca+, Magnesium Mg+, Sodium Na+ and Potassium K+ are called cations.

Anions are negatively charged ions such as nitrates, sulphates and chlorides.

15. What is temporary hardness and permanent hardness?

Hardness caused due to carbonate and bicarbonate salts is temporary hardness as it is soft scale which can be removed easily.

Hardness due to sulphates and nitrates is permanent hardness, such hardness is very hard to remove. Hardness is measured in parts per million (ppm).

16. What is Biological/Biochemical Oxygen Demand (BOD)?

Protections & Interlocks in power plants

Biochemical oxygen demand (BOD) is a measure of the amount of oxygen that bacteria will consume while decomposing organic matter under aerobic conditions. Biochemical oxygen demand is determined by incubating a sealed sample of water for five days and measuring the loss of oxygen from the beginning to the end of the test.

17. What is chemical oxygen demand (COD)?

It is a measure of the total quantity of oxygen required to oxidize all organic material into carbon dioxide and water. COD values are always greater than BOD values, but COD measurements can be made in a few hours while BOD measurements take five days.

18. What does silica cause in Boiler water?

Silica causes hard scale, it reacts with calcium and magnesium salts, which can form a scale and inhibit heat transfer.

19. How do you remove the hardness from water?

Hardness of water can be removed by a process called “ion exchange” it is carried out in DM plant.

20. How does scale effects on the performance of Boiler?

Water impurities like magnesium, calcium and silica at higher temperatures precipitate and form a scale on heat exchanger surfaces.

• Scale reduces heat transfer.
• Scale reduces the internal diameter of pipe/tube.
• Scale causes fuel wastage of water tube boiler by 2% and fire tube boiler by 5%.
• Scale increases the tube metal temperature to rise, which increases the flue gas temperature and in extreme condition tube may fail.

21. What are the principle factors cause scale formations in boiler?

• Presence of calcium, magnesium and silica salts in water.
• Insufficient blow down.
• Alkalinity of water (pH > 7).
• Higher TDS.
• Low condensate circulation.

22. What is meant by cycle of concentration?

It is the ratio of makeup water to the blow down rate.

23. What are internal and external water treatments?

If the feed water chemical and mechanical treatment is done outside the boiler by engaging filtration, clarification, osmosis and demineralization process, then it is called as external treatment.

If the feed water chemical treatment is done inside the boiler by chemical dosing, then it is called internal treatment.

24. Write the sequence of water treatment in external water treatment?

Sedimentation, clarification, sand filtration, ultra filtration, reverse osmosis, degassing and demineralization.

25. How does the internal chemical treatment work?

Two methods used to control the hardness in internal treatment, one is carbonate cycle and phosphate cycle. Carbonate cycle is employed in the boiler operating pressure 125 PSIG (up to 9 k/cm2) and Phosphate cycle for above 125 PSIG.

26. How does the scale conditioning work?

Scale conditioner modifies the crystal structure of scale creating a bulky transportable sludge instead of hard deposit.

27. What causes the corrosion of Boilers?

Dissolved gases like oxygen, carbon dioxide and ammonia are the major sources of corrosion. Of these oxygen is most aggressive.

28. What type of corrosion exists in boiler water systems?

The most common type of corrosion is “Pitting” attack, the main source of pitting is oxygen. These cause small pin type holes that penetrate into the wall of tubes and eventually lead to tube failure.

29. What water characteristics affect the corrosion?

Following factors affect the corrosion.

• Oxygen/other dissolved gases
• Total dissolved and suspended solids
• Acidity (pH < 7)
• Fluid velocity
• Temperatures

30. How does the velocity of fluid influences the rate of corrosion?

High velocity of fluid increases the corrosion rate by transferring the oxygen to metal and carrying away the corrosion byproducts at the faster rate.

Low velocity of fluid causes the deposition of suspended solids can establish localized corrosion cells, thereby increasing the corrosion rate.

31. Which inhibitors are commonly used to remove oxygen from feed water?

Sodium sulphite, hydrazine, carbohydrazide and diethyl hydroxide amine are the commonly used oxygen scavengers.

32. What inhibitors are used to remove carbon dioxide from feed water?

Amines (Filming amines & neutralizing amines).

33. What is hard water?

When water is referred to as ‘hard’ this simply means, that it contains more minerals than ordinary water. These are especially the minerals of calcium and magnesium. The degree of hardness of the water increases, when more calcium and magnesium dissolves. Magnesium and calcium are positively charged ions. Because of their presence, other positively charged ions will dissolve less easily in hard water than in water that does not contain calcium and magnesium.

34-How is regeneration process is accomplished in demineralization plant?

Regeneration Process:

Back Wash: Flow of water through mineral bed is reversed. The mineral bed is loosened and accumulated sediments wash out to drain by upward flow of water.

Brine/Acid Draw and Slow Rinse: Ordinary salt (NaCl) and hydrochloric acid (HCl) of concentration 33% have the capacity to restore the ion exchange capacity of resins. A given amount of salt or acid is rinsed with the resin bed. The rinsing of resin will continue till all the salt/acid is removed from bed.

Generally 3 to 5% of acid is used for strong acid cation (SAC) regeneration.

Fast Rinse: In fast rinse small traces of salt and acid will get remove.

35-What is the significance of degasser unit?

Degasser is an integral part of any demineralization plant, where it is generally placed between cation and anion exchanges and removes Carbon Dioxide, which is generated by dissociation of carbonic acid at cation outlet water. In this Degassing processes, Degasser Tower is utilized, which is made from either FRP or Mild Steel with rubber lined or epoxy coating. Low air pressure is generated at the bottom of the tower that drives out CO2 and the degassed water is collected in a sump beneath the tower.

36-What is the conductivity and silica of water at mixed bed outlet?

Conductivity 1–2 mico siemens/cm and silica 0.01 to 0.02 ppm.

37. What is meant by output between regeneration (OBR)?

OBR is the mean time between two successive regeneration process. Generally it is expressed in M3 or in hour. OBR depends on water quality and resin quality.

38.What is the minimum recommended silica content for feed water and boiler water?

Feed water: 0.015 to 0.02 ppm.

Boiler water: 1.5 to 2 ppm.

39. What is the recommended Hydrazine residual in feed water?

It is 0.01 to 0.02 ppm.

40. What is the recommended Tri sodium phosphate (TSP) residual inboiler drum water?

It is 5 to 10 ppm.

41. Why HCl is more preferred for the regeneration of cation than Sulphuric acid?

Hydrochloric acid is more efficient than sulphuric acid to regenerate a strongly acidic cation exchange resin (SAC) initially in the Na1 form. With 50 g HCl per litre of resin, a conversion of 60% to the H1 form is achieved. With 50 g H2SO4, a conversion of only 40% is achieved. Even expressed as equivalents, hydrochloric acid is more efficient: 36.5 g HCl (1 eq) will convert the resin to 45%, whereas 49 g H2SO4 (1 eq) convert only 39%.

42. List out the hazardous chemicals used in Water treatment plant.

Most hazardous chemicals are Hydrochloric acid, Sulphuric acid, Caustic soda & Ferric chloride.

43. What is jar test?

The purpose of the laboratory jar test is to select and quantify a treatment program for removal of suspended solids or oil from raw water or a dilute process or waste stream. Jar tests are conducted on four or six-place gang stirrer, which can be utilized to simulate mixing and settling conditions in a clarifier. Jars (beakers) with different treatment programs or the same product at different dosages are run side-by-side, and the results compared to an untreated jar, or one treated with the current program.

44. What are the different parameters of coals which are analyzed in lab?

Proximate Analysis: Ash, moisture, fixed carbon, volatile matter.

Ultimate Analysis: Percentage of carbon, hydrogen, oxygen, sulphur and nitrogen.

Other Analysis Includes: GCV (by calculation), GCV by Bomb calorimeter, sieve analysis.

45. What are the different moisture levels of coal that are analyzed in lab?

Surface moisture, inherent moisture and total moisture.

46. What is ARB and ADB based GCV?

ARB: It is the GCV of coal on “As received basis.” That is coal GCV is calculated as soon as it received in laboratory.

ADB: It is the GCV of coal on: “Air dried basis.” That is coal GCV is calculated by allowing the coal to dry at room temperature for 24 hours.

47. What is Spectrophotometer? And what parameters are analyzed with this instrument?

Spectrophotometry is a method to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution. The basic principle is that each compound absorbs or transmits light over a certain range of wavelength. This measurement can also be used to measure the amount of a known chemical substance.

Following water parameters are analyzed in Spectrophotometer

• Silica: Wavelength 815 nm
• Phosphate: 720 nm
• Iron: 510 nm
• Chloride: 463 nm
• Hydrazine: 450 nm

48-What chemicals are used in clarifier?

Ferric chloride, Polyelectrolyte & Sodium hypochlorite

49-What is the function of anti-scalent & SMBS in RO system?

Antiscalent especially polymer based antiscalent are used to inhibit the precipitation of calcium carbonate

SMBS is used to remove chlorine which otherwise would damage the membranes
WTP chemicals & their functions

50-What do you mean by ORP in RO system?

ORP is the term used to measure of the cleanliness of the water & its ability to break down contaminants. Its range is -2000 to 2000 millivolts

Selected QnA on power plant instrumentation for Mechanical engineers