Aprds system

aprds system for power plant

1. APRDS System By Sharavan tripathi jipt

2. Auxiliary PRDS System Auxiliary PRDS system consists of a Turbine PRDS control system as well as Boiler PRDS control system. The controller of these system are pneumatically controlled. Both the systems are identical in nature.   Auxiliary steam is tapped from the main steam line and its pressure is reduced and de-superheated to the required temperature and pressure.

3. Auxiliary PRDS System The spray water for de-superheating is supplied from the CEP discharge header. In order to obtain better operational flexibility and controllability range, both the system have been split into two identical PRDS system such as low capacity PRDS and high capacity PRDS.

4. Auxiliary PRDS System Contd.. The low capacity is of 30% of that of the high capacity line. Each system comprises of the pressure controller, one de- superheater and one spray water flow controller. We shall henceforth call the Turbine PRDS system as TAS and the Boiler PRDS system as BAS.

5. System Description- High Capacity High capacity PRDS comprises of steam pressure reducing control valve, motorized upstream isolating valve, downstream manual isolating valve, one spray water temperature control valve and one de- superheater . A motorized regulating globe valve is provided in bypass line of, which can be operated remote manually in case of mal-operation of the pressure control valve. A motorized bypass globe valve is provided for spray water control valve , which can be operated remote manually in case of mal-operation of spray water control valve.

6. System Description- Low Capacity Low capacity PRDS comprises of steam pressure reducing control valve, motorized upstream isolating valve, downstream manual isolating valve, one spray water temperature control valve and one desuperheater . A motorized regulating globe valve is provided in bypass line of, which can be operated remote manually in case of mal-operation of pressure reducing control valve. A motorized bypass globe valve is provided for spray water control valve, which can be operated remote manually in case of mal-operation of spray water control valve.

7. TAS/BAS Pressure/ Temperature Control The auxiliary steam pressure control valve supplies steam to TAS/BAS header by maintaining TAS/BAS line pressure constant at the set point (11 kg/cm 2 ). Downstream pressure of PRDS valve is taken as controlled pressure. The auxiliary PRDS spray water flow control valve (CD) maintains the TAS / BAS line temperature constant at the set point (260 C for TAS & 220 C for BAS). The downstream temperature of PRDS valve is taken as controlled temperature.

8. Selection of HT & LT PRDS During unit start up auxiliary steam for turbine is supplied to Deaerator initial heating & pegging, Turbine Gland Sealing. To meet this high demand of auxiliary steam HT PRDS should be put into service. After unit synchronization and at about 30 to 40% of unit load deaerator pegging is supplied from CRH / turbine extraction line (CAP) and pegging from TRDS is cut out. At about 40% unit load also, turbine gland sealing supply valve from TPRDS closes as turbine becomes self-sealing condition.

9. Uses of Aux Steam at various locations of plant LTPRDS:- VAM FO heating HFO Atomizing Mill Inerting Wet steam washing HTPRDS:- Turbine Gland Sealing Deaerator Pagging APH soot blowing

10. Vapor Absorber Machine (VAM)

11. Vapor Absorber Machine (VAM)

12. Vapor Absorber Machine (VAM) Absorption refrigeration systems replace the compressor with a generator and an absorber. Refrigerant enters the evaporator in the form of a cool, low-pressure mixture of liquid and vapor (4). Heat is transferred from the relatively warm water to the refrigerant, causing the liquid refrigerant to boil. Using an analogy of the vapor compression cycle, the absorber acts like the suction side of the compressor—it draws in the refrigerant vapor (1) to mix with the absorbent. The pump acts like the compression process itself—it pushes the mixture of refrigerant and absorbent up to the high-pressure side of the system.

13. Vapor Absorber Machine (VAM) The generator acts like the discharge of the compressor—it delivers the refrigerant vapor (2) to the rest of the system. The refrigerant vapor (2) leaving the generator enters the condenser, where heat is transferred to water at a lower temperature, causing the refrigerant vapor to condense into a liquid. This liquid refrigerant (3) then flows to the expansion device, which creates a pressure drop that reduces the pressure of the refrigerant to that of the evaporator. The resulting mixture of liquid and vapor refrigerant (4) travels to the evaporator to repeat the cycle.

14. How Absorption Machine Works Absorption system employs heat and a concentrated salt solution (lithium bromide) to produce chilled water. In its simplest design the absorption machine consists of 4 basic components: 1. Generator 2. Condenser 3. Evaporator 4. Absorber

15. Function of Components Generator: The purpose of the generator is to deliver the refrigerant vapor to the rest of the system. It accomplishes this by separating the water (refrigerant) from the lithium bromide-and-water solution. In the generator, a high-temperature energy source, typically steam or hot water, flows through tubes that are immersed in a dilute solution of refrigerant and absorbent. The solution absorbs heat from the warmer steam or water, causing the refrigerant to boil (vaporize) and separate from the absorbent solution. As the refrigerant is boiled away, the absorbent solution becomes more concentrated. The concentrated absorbent solution returns to the absorber and the refrigerant vapor migrates to the condenser.

16. Condenser: The purpose of condenser is to condense the refrigerant vapors. Inside the condenser, cooling water flows through tubes and the hot refrigerant vapor fills the surrounding space. As heat transfers from the refrigerant vapor to the water, refrigerant condenses on the tube surfaces. The condensed liquid refrigerant collects in the bottom of the condenser before traveling to the expansion device. The cooling water system is typically connected to a cooling tower. Generally, the generator and condenser are contained inside of the same shell.

17. Expansion Device: From the condenser, the liquid refrigerant flows through an expansion device into the evaporator. The expansion device is used to maintain the pressure difference between the high-pressure (condenser) and low-pressure (evaporator) sides of the refrigeration system by creating a liquid seal that separates the high-pressure and low pressure sides of the cycle. As the high-pressure liquid refrigerant flows through the expansion device, it causes a pressure drop that reduces the refrigerant pressure to that of the evaporator. This pressure reduction causes a small portion of the liquid refrigerant to boil off, cooling the remaining refrigerant to the desired evaporator temperature. The cooled mixture of liquid and vapor refrigerant then flows into the evaporator.

18. Absorber: Inside the absorber, the refrigerant vapor is absorbed by the lithium bromide solution. As the refrigerant vapor is absorbed, it condenses from a vapor to a liquid, releasing the heat it acquired in the evaporator. The absorption process creates a lower pressure within the absorber. This lower pressure, along with the absorbent’s affinity for water, induces a continuous flow of refrigerant vapor from the evaporator. In addition, the absorption process condenses the refrigerant vapors and releases the heat removed from the evaporator by the refrigerant. The heat released from the condensation of refrigerant vapors and their absorption in the solution is removed to the cooling water that is circulated through the absorber tube bundle.

19. As the concentrated solution absorbs more and more refrigerant; its absorption ability decreases. The weak absorbent solution is then pumped to the generator where heat is used to drive off the refrigerant. The hot refrigerant vapors created in the generator migrate to the condenser. The cooling tower water circulating through the condenser turns the refrigerant vapors to a liquid state and picks up the heat of condensation, which it rejects to the cooling tower. The liquid refrigerant returns to the evaporator and completes the cycle.

20. FO Heating Viscosity The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends on temperature and decreases as the temperature increases. Any numerical value for viscosity has no meaning unless the temperature is also specified. Viscosity is measured in Stokes / Centistokes. Sometimes viscosity is also quoted in Engler , Saybolt or Redwood.

21. Each type of oil has its own temperature - viscosity relationship. The measurement of viscosity is made with an instrument called Viscometer. Viscosity is the most important characteristic in the storage and use of fuel oil. It influences the degree of pre-heat required for handling, storage and satisfactory atomization. If the oil is too viscous, it may become difficult to pump, hard to light the burner, and tough to operate. Poor atomization may result in the formation of carbon deposits on the burner tips or on the walls. Therefore pre-heating is necessary for proper atomization.

22. Pressure Reduce Steam.

23. OIL GUN ATOMISING STEAM VALVE HFO NOZZLE VALVE SCAVENGE VALVE AB ELEVATION CD ELEVATION EF ELEVATION Oil Gun Connection Atomizing Steam Scheme HFO Automizing

24. Oil Gun on Elevation AB

25. Oil Gun on Elevation CD & Above

26. Mill Inerting system

27. Wet Steam Washing During operation, deposits occur on the turbine blading to a greater or lesser degree depending on the steam purity [1] and the pressures and temperatures of the operating steam. These deposits cause a reduction of the turbine generator unit’s efficiency due to: Changes in the flow profiles Thicker boundary layers in the steam flow as a result of rough surfaces.

28. In extreme cases the flow area of turbine may get reduced with consequent reduction in the maximum possible steam flow through the turbine, and corresponding output. Choking of blade flow path can be detected on the basis of internal efficiency measurement If deposit leads to an increase in stage pressures, the maximum stage pressures shown in the Technical Data should not be exceeded. If necessary, the output must be reduceds . water-insoluble, silicate deposits occur in a temperature range between 500°C and 350°C. Alkali silicates and silicic acid are deposited between 350°C and 60°C. Salt deposits occur at temperatures ranging from 480°C to the blading stages where condensation begins. Salt deposits are water-soluble and can be removed by steam washing with saturated steam. Depending on their composition, silicate deposits are either water-soluble (e.g. alkali silicates) or occur as a hard Water-insoluble coating. In latter case the deposits can only be removed mechanically during overhauls.

29. Steam Washing of IP Turbine All stop and control valves and all valves in the extraction lines remain closed during steam washing of the IP turbine. Any isolating valves present upstream of the feed water heaters must also be closed. The drains from the IP turbine between the reheat control valves and the IP outlet, the extraction valves or the swing check valves in the extraction lines must be vented during steam washing only to the extent required to allow the condensate arising to drain off whilst preventing excessive loss of steam. All other drains from the turbine generator must be full open. After the saturated steam line (Fig.1, item 2) has been joined up at the connection points (3), the washing steam can be routed into the IP turbine, from there via the cross-around lines to the IP turbine and then to the condenser.

30. Steam Washing of HP Turbine The main control valves must be open during steam washing of the HP turbine. All drains (7, 8) from the HP turbine situated between the main stop valves and the HP outlet may be vented during steam washing only to the extent required to allow the condensate arising to drain off while preventing excessive loss of steam. All other drains particularly those in the cold reheat line, must be fully open. Local drainage may also be provided so that no steam can enter the Reheater of the Boiler, if at all possible. After the steam line (6) has been joined up at the connection points (3), the stream can be routed into the HP turbine. The steam leaving the HP turbine is exhausted to the condenser via the drains (9). The condensate is discharged and samples are taken to determine the salt content as described in steam washing of IP turbine, the completion criteria remaining same.

31. Gland Sealing System

32. FUNCTION This system ensures the sealing of glands in HP, IP and LP turbine under various load conditions. The turbine glands are self seal type (refer gland sealing scheme). Upto 40% load, steam from an auxiliary source through valve (MAW10AA001) is taken to seal all the HP,IP and LP glands. During this period the valve (MAW50AA001) connecting this header to condenser is kept closed. After 40% load, seal steam valve (MAW10AA001) is closed and the leak steam valve (MAW50AA001) is opened. Pipings are so sized that the leak off steam from front and rear ends of HP turbine goes to the condenser through the valve (MAW50AA001), while steam from the two IP glands is utilised for sealing the LP glands, thus avoiding the use of a desuperheater .

33. The leak off steam and air from the last chambers of each rotor is sucked into a gland cooler. Building up of vacuum in the condenser is the first step during start up. For building up vacuum, it is necessary to seal the turbine glands by supplying steam to the shaft through the valve (MAW10AA001). The control system opens the gland steam supply valve (MAW10AA001) until the pressure in the header acquires a preset value. Subsequently when the set picks up load, the pressure of steam inside HP and IP turbine builds up resulting in the leakage of steam from the turbine into gland steam supply header which in turn would result in increase of pressure in the header. The controller gradually closes the gland steam supply valve (MAW10AA001) and opens gland leak off valve (MAW50AA001), if required.

34. 49 Why Deaeration? Corrosion in boilers is caused by three factors: 1. Feed water temperature 2. Feed water pH value 3. Feed water oxygen content Temperature and pH value influence the aggressiveness of corrosion. The higher the temperature, and the lower the pH value the increased aggressiveness of the feed water. The dissolved oxygen content of the feed water is a large factor in determining the amount of corrosion that will take place. The presence of oxygen, and other non-condensable gases, in the feed water is a major cause of corrosion in the feed water piping, boiler, and condensate handling equipment.

35. 50 Why Deaeration? Reduce corrosion by dissolved gas: oxygen, carbon dioxide. Oxygen is the most aggressive even in small concentrations. Sources of oxygen: makeup water, condensate return system. Pitting corrosion. Degree of oxygen attack depends on concentration of dissolved oxygen, the pH and the temperature of the water.

36. 51 Removal of oxygen, carbon dioxide and other non-condensable gases from feed water. What is Deaeration?

37. 52 Deaerator Principles Deaeration is the mechanical removal of dissolved gases from the boiler feed water. There are three principles that must be met in the design of any deaerator. 1. The incoming feed water must be heated to the full saturation temperature, corresponding to the steam pressure maintained inside the deaerator. This will lower the solubility of the dissolved gases to zero. 2. The heated feed water must be mechanically agitated. This is accomplished in a tray deaerator by first spraying the water in a thin film into a steam atmosphere. Creating a thin film reduces the distance, the gas bubble has to travel to be released from the water. Next, the water is cascaded over a bank of slotted trays, further reducing the surface tension of the water. This allows for the removal of any gases not liberated by the initial spraying. 3. Adequate steam supply must be passed through the water, in both the spray section and the tray section to sweep out the gases from the water.

38. 53 The easiest way to de-aerate is to force steam into the feed water, this action is called scrubbing .  Scrubbing raises the water temperature causing the release of O2 and CO2 gases that are then vented from the system.  In boiling section steam is used to "scrub" the feed water as steam is essentially devoid of O2 and CO2, steam is readily available and steam adds the heat required to complete the reaction.  Deaerator Principles

39. 54 For efficient operation, de-aerating equipment must satisfy the following requirements: Heating of the feed water:  The operating temperature in the unit should be saturation temperature. If this temperature and pressure cannot be economically achieved then it is important to get as close to it as possible.  Agitation decreases the time and heat energy necessary to remove dissolved gases from the water.  Maximization of surface area by finely dispersing the water to expose maximum surface area to the steam.  This enables the water to be heated to saturation temperature quicker and reduces the distance the gases have to travel to be liberated.  The liberated gases must be vented to allow their escape from the system as they are released.

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43. 58 DEAERATION Oxygen reacts with water ( H 2 0 ) to give ( OH - ) ION Fe = Fe + + e - O 2 + 2H 2 O + 4e - = 4OH - Fe + + 2OH - = Fe(OH) 2 2Fe + O 2 + 2H 2 O = Fe(OH) 2

44. 59 DEAERATION Carbon dioxide is an acidic gas and could form carbonic acid with water, carbonic acid liberates H + ions that attacks on metal. CO 2 + H 2  H 2 CO 3  H + + HCO 2H + + 2e - = H 2 - 3

45. 60 TYPES OF DEAERATION 1. Physical deaeration. 2. Chemical deaeration. PRINCIPAL FOR PHYSICAL DEAERATION The principle of deaeration is based on the following laws HENERY’S LAW DALTON’S LAW OF PARTIAL PRESSURE

46. 61 Dalton’s Law of Partial Pressure It states that the pressure in a container having a mixture of a gas and vapour, is the sum of partial pressure of the vapour at the common temperature and the partial pressure of the gas, at any point inside.

47. 62 HENERY’S LAW The mass of gas with a definite mass of liquid, which will dissolve at a given temperature, is directly proportional to the partial pressure of the gas in contact with the liquid. This hold with in the close limit for gases which don’t unite chemically with water.

48. 63 Decreasing the partial pressure of the gas in water may be achieved by following methods: Use of another gas to remove the undesirable gases e.g. Nitrogen can be used to remove oxygen. Decreasing the total pressure so as to approach the vapour pressure of the water. Increasing the vapour pressure by heating the water

49. 64 (A) According to working pressure under which they operate: Vacuum deaerator Atmospheric deaerator High pressure deaerator Classification of Deaerator

50. 65 Vacuum Deaerator

51. 66 Classification of Deaerator (B) Also deaerator can be classified in accordance with the mode of steam-water distribution: Atomizing ( spray ) type Tray type Film type ( combination of both spray and tray type)

52. 67 Tray Type Deaerator

53. 68 Tray Type Deaerators These are composed of a deaerating section and a feed water storage section.  Incoming water is sprayed through a perforated distribution pipe into a steam atmosphere where it is atomized.  There it is heated to within a few degrees of the saturation temperature of the steam.  Most of the non-condensable gases are released to the steam as the water enters the unit.  The water then cascades through the tray section, breaking into fine droplets, which immediately contact incoming steam.  The steam heats the water to the saturation temperature of the steam and removes all but a trace of oxygen.  Deaerated water falls to the feed water storage section below and is protected from recontamination by a blanket of steam.  As the non-condensable gases are liberated, they as well as a small amount of steam are vented to atmosphere.  It is essential that sufficient venting is provided at all times or deaeration will be incomplete.

54. 69 Tray Type Deaerator

55. 70 Trays

56. 71 Spray Type Deaerators

57. 72 Spray Type Deaerators The spray-type deaerators do not use trays for dispersion of the water.  In this case, spring loaded nozzles located in the top of the unit spray water into a steam atmosphere which is heated to within a few degrees of the saturation temperature of the steam.  Most of the non-condensable gases are released to the steam, and the heated water falls to a water seal and drains to the lowest section of the steam scrubber. The water is scrubbed by large quantities of steam and heated to the saturation temperature prevailing at this point.  The intimate steam to water contact achieved in the scrubber efficiently strips the water of dissolved gases.  As the steam-water mixture rises in the scrubber, a slight pressure loss causes the deaerated water temperature to remain a few degrees below the inlet steam saturation temperature.  The deaerated water overflows from the steam scrubber to the storage section below. The steam, after flowing through the scrubber, passes up into the spray heater section to heat the incoming water.  Most of the steam condenses in the spray section to become part of the deaerated water.  A small portion of the steam, vented to atmosphere, removes non-condensable gases from the system.

58. 73 Spray Type Deaerators

59. 74 Spray Valve

60. 75 Counter & Parallel Flow Deaerator

61. 76 In this design, the inlet water is sprayed into a steam atmosphere spray nozzles. This action heats the water to liberate most of the dissolved gases. This flows down through water seals for distribution over the tray bank, which serve two functions. First they prevent gases liberated in the initiate heating, from entering the tray bank. Second they direct the steam to flow down through the trays, before entering the upper heating section. The main function of the tray bank is to remove the remaining amounts of dissolved gases, not liberated in the initial heating. Parallel Down Flow

62. 77 Since very little, or no heating takes place in the trays, the entire volume of steam is used to scrub out the remaining gases. The trays are slotted, and provide a great amount of spilling edge. This allows for a great amount of water surface area to be exposed to the steam. Water and steam flow downward through the trays. The steam, after exiting the tray bank, steam is condensed by the colder inlet water, and a small amount is vented to atmosphere, along with the dissolved gases. Parallel Down Flow

63. 78 Packed Column Type

64. 79 Inlet water is sprayed into a steam atmosphere through a variable orifice & spring loaded spray nozzle. water heating liberate dissolved gases & heated water flows down onto a distribution plate, which evenly distributes the water over the entire cross-sectional area of the tower packing. As the water flows down through the distribution plate it enters a steam chest area, where it is further heated by up flowing steam and more of the dissolved gases are liberated. Packed Tower

65. 80 Remaining dissolved gases are removed when the water flows down from the steam chest and then down through the packing tower. The packing tower exposes a greater surface area of the water; while up flowing steam completes the deaeration process. The water leaving the bottom of the packing tower is given a final scrubbing of steam. The steam, entering the deaerator from below the packing tower, is introduced through a fixed orifice steam distributor. This steam distributor directs high velocity steam through the down flowing water leaving the bottom of the packing tower. Packed Tower

66. 81 Deaerator Functions Deaerator has to meet following needs. It does deaeration Acts as a feed water heater. Acts as a storage tank (reservoir) Accept the leak-off flows from the BFP. Accept the H.P. Heater drains. Heat the tank content from cold to provide hot deaerated water for unit start-up. Ensure NPSH for BFP

67. 82 DEAERATING RATIO O 2 Content in Condensate at inlet to deaerator = 6 O 2 Content in feed water at outlet S.T.

68. 83 TECHNICAL PARTICULARS OF DEAERATOR 1. Type Tray with external vent condenser 2. Design Pressure Kg/cm 2 7.4 3. Design Temperature O C 250 4. Hydraulic Test Pressure Kg/cm 2 11.1 5. Storage capacity M 3 90 6. No. of perforated trays 5

69. 84 Operating Conditions of Deaerator Temperature of deaerated water must be equal to saturation temp. of water corresponding to the pressure at which deaerator operates. Sufficient heating steam must be delivered to the deaerator to ensure continuous boiling of water undergoing deaeration . The feed water charge to deaerator must be disintegrated into fine droplets to ensure better heat – transfer Deaerator must be provided with sufficient venting to purge all the non-condensing gases out of the system and to ensure minimum partial Pressure of these gases in the upper part of the deaerator.

70. 85 Causes of High O 2 /CO 2 Concentration Inadequate deaerator vent leading to accumulation of non-condensing gases and increase in their partial pressure. High feed-water flow rate. Delivery of O 2 - containing condensate directly into the storage tank. Frequent pressure drop in the deaerator. Faulty deaerator internals. Delivery of relatively “cold” flows with higher O 2 - content to deaerator.

71. 86 While the most efficient mechanical deaerators reduce oxygen to very low levels (.005cc/l or 5 ppb), even trace amounts of oxygen may cause corrosion damage to a system.  Consequently, good operating practice requires removal of that trace oxygen with a chemical oxygen scavenger such as sodium sulfite or hydrazine.  

72. 87 Chemical Deaeration The addition of an oxygen scavenging chemical ( Sodium Sulphite or Hydrazine) will remove the remaining oxygen and prevent corrosion. Na 2 SO 3 + O 2 = Na 2 SO 4 Sodium Sulphite Oxygen Sodium Sulphate Now obsolete, as it increases the total dissolved solids. Modern Practice: Hydrazine is used for chemical Deaeration. N 2 H 4 + O 2 = N 2  + 2H 2 O 3N 2 H 4 = 4NH 3 + N 2  Additional ammonia reacts with 2NH 3 + CO 2 + H 2 O = (NH 4 ) 2 CO 3 (acidic) Ammonium Carbonate (Neutral)

73. 88 FST - Deaerator

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75. 90 Disadvantages of Counter Flow Deaerators Inability to deliver 0.007 ppb outlet quality in applications with a low inlet water temperature, or when 100% make-up is required. Low tray loading. This reduces the flow rating for a given diameter deaerator vs. a parallel down flow unit. High vent rate. This reduces operating efficiency Advantages of Counter Flow Deaerators The counter flow deaerator is cheaper to manufacture. The higher capacity and the ability to perform under varying steam and water conditions make the parallel down flow (and packed tower for smaller applications) design competitive, and the only logical choice.

76. 91 Disadvantages of Atomizer Deaerators Inability to deliver 7 ppb outlet quality when plant conditions vary from design specifications. Requires constant plant conditions. Failure rate of the atomizer valve, and maintenance required to keep it operating properly. Advantages of Atomizer Deaerators 1. Low cost 2. Low overall height The atomizer type deaerator is only effective when applied to an application with no plant or process swings. Along with the maintenance required, this type deaerator, while inexpensive, has only limited applications.

77. 92 Disadvantages of Parallel Down Flow More complicated design, resulting in slightly higher cost. Advantages of Parallel Down Flow Time proven design Thousands of installations worldwide Design suitable for small to medium size plants Can meet outlet guarantees at varying plant conditions. High tray loading, resulting in higher outlet capacity for any given diameter. Large tray spilling edge, resulting in high deaerating efficiency Low vent rate, resulting in increased operating efficiency.

78. 93 Disadvantages of Packed tower Height requirement Typically considered for small size plants. Advantages of Packed tower Low cost Low maintenance Ability to handle varying plant conditions This design can meet the requirements for a reliable deaerator capable of producing completely deaerated water for small plants. Packed tower design includes multi-stage deaeration , to deliver top performance.

79. 94 CORROSION is defined as the destruction of a metal by chemical or electromechanical reaction with its environment. Corrosion dramatically increases maintenance costs and can cause unnecessary safety risks. It will occur when levels of oxygen or carbon dioxide are high, where pH values are low, where contact occurs between dissimilar metals and in corrosive atmospheres. Corrosion is an electrochemical process in which electricity flows through a solution of ions between areas of metal. Deterioration occurs when the current leaves the negatively charged metal or anode and travels through the solution to the positively charged metal or cathode, completing an electrical circuit in much the same manner as a battery cell. The anode and the cathode can be different metals or areas of the same metal. Corrosion occurs when there is a difference in the electrical potential between them. SCALE is a very hard substance that adheres directly to heating surfaces forming a layer of insulation. This layer of insulation will decrease heat transfer efficiency. Scale also results in metal fatigue/failure from overheating, energy waste, high maintenance costs and unnecessary safety risks. A one-sixteenth inch thickness of scale in a fire tube boiler can result in a 12.5% increase in fuel consumption.

80. 95 FOULING occurs when a restriction develops in piping and equipment passageways and results in inefficient water flow. The fouling of boiler room equipment directly impacts energy efficiencies and cost of operations. FOAMING is a condition in which concentrations of soluble salts, aggravated by grease, suspended solids or organic matter, create frothy bubbles or foam in the steam space of a boiler. When these bubbles collapse it creates a liquid that is carried over into the steam system. Foaming degrades steam quality and in some cases can create a water slug that is discharged into the steam lines. CAUSTIC GAUGING will occur when there is a high concentration of alkaline salts (a pH value of 11 or greater) that will liberate hydrogen absorbed by the iron in the steel. It will be more evident in high temperature areas of the boiler's waterside and manifests itself in the form of hairline cracks. HYDROGEN EMBRITTLEMENT occurs in the event of lower pH value of the water in evaporator

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