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Engineering Encyclopedia Saudi Aramco DeskTop Standards



Major Components And Functions Of Boilers And Boiler Systems And Applications Of Standards And Specifications



Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.



Chapter : Instrumentations File Reference: PCI20101



For additional information on this subject, contact J.L. Sprague



Engineering Encyclopedia



Instrumentations Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



Contents



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INFORMATION .................................................................................................................. 1 INTRODUCTION................................................................................................................ 1 BOILERS: PURPOSE, HISTORY, TYPES, HARDWARE, SYSTEM FUNCTIONS.......... 2 Purpose of Boilers..................................................................................................... 2 Uses of Steam................................................................................................ 2 World Energy Consumption........................................................................... 2 History of Boilers ...................................................................................................... 2 Boiler Efficiency ............................................................................................ 4 Boiler Safety.................................................................................................. 5 Benefits ......................................................................................................... 6 Boiler Control................................................................................................ 8 Types of Boilers ........................................................................................................ 9 Firetube Boilers ............................................................................................. 9 Watertube Boilers .......................................................................................... 9 Boiler Hardware Overview .......................................................................................10 Drum ............................................................................................................11 Mud Drum....................................................................................................12 Riser.............................................................................................................13 Downcomer..................................................................................................13 Superheater ..................................................................................................13 Burner ..........................................................................................................13 Economizer ..................................................................................................13 Air Heater.....................................................................................................13 Steam Coil Air Heater...................................................................................14 Forced Draft Fan ..........................................................................................14 Induced Draft Fan.........................................................................................14 Systems Functional Overview...................................................................................14 Steam-Water System.....................................................................................14 Draft System.................................................................................................14 Heat Transfer................................................................................................14 Saudi Aramco DeskTop Standards



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Instrumentations Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



COMPONENTS OF BOILER FLUID CIRCULATION SYSTEMS: FUNCTIONS AND BASIC OPERATION.................................................................................................16 Feedwater Supply System.........................................................................................16 Condenser.....................................................................................................17 Feedwater Heaters ........................................................................................17 Deaerator Heater and Storage Tank ..............................................................18 Boiler Feed Pump .........................................................................................18 Economizer ..................................................................................................18 Feedwater Conditioning and Boiler Blowdown.........................................................18 Boiler Feedwater (BFW) Supply ...................................................................18 Chemical Treatment of BFW.........................................................................19 Boiler Blowdown Methods ...........................................................................20 Boiling Process and Steam Generation......................................................................20 Saturated Water and Saturated Steam ...........................................................21 Superheated Steam .......................................................................................24 Boiling Process .............................................................................................25 Natural Circulation .......................................................................................27 Steam drum internals....................................................................................29 COMPONENTS OF THE VARIOUS BOILER AIR AND DRAFT SYSTEMS: FUNCTIONS AND BASIC OPERATION ..........................................................................31 Forced Draft and Natural Draft Systems ...................................................................32 Forced Draft Fan ..........................................................................................32 Natural Draft ................................................................................................32 Pressure and Draft Profile .............................................................................32 Furnace Air Pressure and Boiler Load...........................................................32 Balanced draft Systems.............................................................................................32 Air preheater.................................................................................................33 Induced Draft Fan.........................................................................................33 Pressure and Draft Profile .............................................................................33 Benefit of Balanced draft System ..................................................................34 Air Flow System.......................................................................................................34 Constant speed fan........................................................................................35



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Variable Speed Fan .......................................................................................35 COMPONENTS OF BOILER FUEL SYSTEMS: FUNCTION AND BASIC OPERATION ......................................................................................................................36 Fuel Gas System.......................................................................................................36 Fuel Oil System ........................................................................................................36 APPLICATIONS OF INDUSTRY AND SAUDI ARAMCO STANDARDS AND SPECIFICATIONS TO SAUDI ARAMCO BOILER CONTROL SYSTEMS.....................37 Saudi Aramco Engineering Standards SAES-J-004, SAES-J-600 and SAES-J602...........................................................................................................................37 Aramco Materials System Specification 34-SAMSS-619 ..........................................38 American National Standards Institute/National Fire Protection Association Standards NFPA 8501 and NFPA 85C .....................................................................38 American Petroleum Institute Recommended Practice API-RP-551 ..........................38 American National Standards Institute/Instrument Society of America Standards ANSI/ISA S77.41, ANSI/ISA S77.42 and ANSI/ISA S5.1.......................................39 Scientific Apparatus Manufacturer's Association Standard SAMA PMC 22.11981.........................................................................................................................39 Burner Vendor Specification Sheet...........................................................................39 GLOSSARY........................................................................................................................40



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Table of Figures



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Figure 1. Early Steam Boiler A.................................................................................. 3 Figure 2. Early Steam Boiler B .................................................................................. 4 Figure 3. Early Firetube Boiler................................................................................... 5 Figure 4 . Early Watertube Boilers............................................................................. 7 Figure 5. Early Inclined Watertube Boiler .................................................................. 8 Figure 6. Modern Day Boiler....................................................................................10 Figure 7. Steam Drum ..............................................................................................12 Figure 8. Basic Feedwater Supply System.................................................................16 Figure 9. Reverse Osmosis .......................................................................................19 Figure 10. Boiling Process and Steam Generation.....................................................21 Figure 11. Boiling point–Pressure Relationship.........................................................22 Figure 12. Saturation Curve–Boiling point–Pressure Relationship.............................23 Figure 13. Steam Quality ..........................................................................................24 Figure 14. Nucleate Boiling ......................................................................................25 Figure 15. Film Boiling.............................................................................................26 Figure 16. DNB Metal Curve ...................................................................................27 Figure 17. Natural Circulation ..................................................................................28 Figure 18. Effect of Flow versus Heat Input .............................................................29 Figure 19. Boiler Air and Draft System.....................................................................31 Figure 20. Pressure and Draft Profile–Balanced draft System–Includes Air preheater ..................................................................................................34



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Instrumentations Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



INFORMATION INTRODUCTION This module provides a foundation of the major components and functions of boilers and boiler systems and applications of standards and specifications. The sections of this module include: •



Boilers: Purpose, History, Types, Hardware, System Functions



•



Components of Boiler Fluid Circulation Systems: Functions and Basic Operation



•



Components of Boiler Air and Draft Systems: Functions and Basic



•



Components of Boiler Fuel Systems: Functions and Basic Operation



•



Applications of Industry and Saudi Aramco Standards and Specifications that Apply to Saudi Aramco Boiler Control Systems



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Operation



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Instrumentations Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



BOILERS: PURPOSE, HISTORY, TYPES, HARDWARE, SYSTEM FUNCTIONS This section will discuss the evolution and basic systems and hardware components of a boiler. Specifically, this section will discuss: •



The purpose of boilers.



•



The history of boilers.



•



The types of boilers.



•



Boiler hardware.



•



Boiler system functions.



Purpose of Boilers Uses of Steam Steam has long been one of man's most dependable servants and is still used today to perform many varied and vital functions. Over 90% of the new electric generating capacity being installed in the United States utilizes steam. Steam also powers most of the world's naval vessels and is an integral part of many industrial processes. World Energy Consumption All fuels can be related by the content of energy in them. An FOEB, or Fuel Oil Equivalent Barrel, is one way to measure energy. An FOEB is equal to the amount of energy contained in a barrel of oil; or 42 gallons per barrel times 150,000 BTU per gallon, which equals approximately 6,300,000 BTU's. Today's (1990) world energy consumption is 166 million FOEB per day or 1.05 X 10 EX15 BTU per day. This is the equivalent of 13,000 1000 megawatt electrical generating stations. Roughly 67% (122 Million FOEB or 7.7 X 10 EX14 BTU per day) of the total energy consumed by the world is available for use by transportation, industry, residential, and commercial. 7% (11 Million FOEB or 6.9 X 10 EX13 BTU per day) is available for nonenergy users. 26% (43 FOEB or 2.7 X 10 EX13 BTU/day) is lost because of conversion, transmission, and refinery use losses. History of Boilers Early steam boilers consisted of little more than kettles filled with water and were heated on the bottom, similar to those shown in Figures 1 and 2. Boilers of the early 1700s still used the kettle principle, but burned the fuel in an enclosed furnace to direct more heat to the boiler kettle.



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Haycock Boiler, 1720



33385



Figure 1. Early Steam Boiler A



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Figure 2. Early Steam Boiler B Boiler Efficiency In the mid 1700s, boiler designers noted that nearly half of the heat from the fire was lost because of short contact time between the hot gases and the boiling heating surface. To improve boiler efficiency, an integral furnace was developed with the fuel actually burned in a container enclosed within the water vessel (Figure 3). A smoke flue wound through the water from the combustion chamber to the atmosphere much like a coil in a still. To prevent a deficiency of combustion air, a bellows was used to force air to the combustion zone and gases through the flue in what was the first application of forced draft. Saudi Aramco DeskTop Standards



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Figure 3. Early Firetube Boiler Boiler Safety As the demand for power increased, the single flue was replaced by many gas tubes that increased the heating surface. More water was subjected to the heat from the flue gases. While this firetube design was popular until about 1870, it was also dangerous. Many disastrous explosions resulted from the direct heating of the pressure shell that contained large amounts of water at saturation temperature. Boiler designers recognized that one way to overcome the deficiencies of the firetube boilers was to develop a watertube design in which the heating surface consist of waterfilled tubes. This design would limit the consequences of a pressure-part rupture. Saudi Aramco DeskTop Standards



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Benefits While several watertube boiler designs were patented between the late 1700s and the mid 1800s (Figure 4), it was not until 1856 that a significant breakthrough occurred. The design incorporated inclined water tubes connecting water spaces at the front and rear of the furnace with a steam space above (Figure 5). It provided a better water circulation and more heating surface than other designs, along with the reduced steam explosion hazard.



42 in.



64 in. Water-tube boiler of small tubes connected at one end to a reservoir. John Stevens, 1803.



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64 in. Water-tube boiler of small tubes connected at one end to a reservoir. John Stevens, 1803.



46 in.



First water-tube boiler. Built and Patented by William Blakely in 1766.



Water-tube boiler with tubes connecting water chamber below and steam chamber above. John Cox Stevens, 1805. 33388



Figure 4 . Early Watertube Boilers



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Inclined water tubes connecting front and rear water spaces complete circuit with steam space above. Stephen Wilcox, 1856. 33389



Figure 5. Early Inclined Watertube Boiler Boiler Control Boiler control began in the late 1700s with the introduction of the "flyball" governor for speed control of the first rotative steam engines. Also during this time, feedback control was used to control the level in the boiler by regulating the water to the boiler. Feedback control was also used to control steam pressure by using automatic draft regulation. There were no further advances in boiler control until the early 1900s when integrated systems were designed to control steam pressure, furnace draft, feedwater, combustion, and steam temperature. During the 1950s burner control systems were developed to start and stop burners and to include flame safety systems. In the 1960s control switched from predominantly pneumatic analog control to predominantly solid-state, discrete element, electronic analog control. Between 1950 and 1970 not much money was invested into boiler control development because of the continual reduction in fuel prices relative to the cost of boilers and boiler accessories. Beginning in the 1970s fuel economics have influenced changes in boiler control. The high price of fuel has allowed a greater degree of control sophistication than could be justified in 1970. In addition, the development of microprocessor control has caused an advantageous transition to the greater precision of digital control.



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Types of Boilers Firetube Boilers In Firetube Boilers the hot flue gas products of combustion flow through boiler tubes surrounded by water. Steam is generated by the heat transferred through the walls of the tubes to the surrounding water. The flue gases are cooled as they flow through the tubes, transferring their heat to the water. Early firetube boilers consisted of a spherical or cylindrical pressure vessel mounted over the fire with flame and hot gases around the boiler shell. To increase the heat transfer area and improve the heat transfer coefficient, longitudinal tubes were installed in the pressure vessel and flue gases were passed through the tubes. This horizontal return tubular (HRT) boiler is shown in Figures 24 and 2-5 on page 22 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow. Other firetube boilers include the Locomotive-type boiler and the Scotch Marine Boiler. The Scotch Marine Boiler is designed with the combustion chamber as a long cylinder, jacketed by a larger cylinder fitted with several passes of firetubes. Today, the most common firetube boilers are similar to the Scotch marine boiler and are shown in Figures 2-9 and 2-10 on page 24 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow. Figure 2-9 shows the Wetback Firetube Boiler in which the combustion chamber is water-jacketed. Figure 2-10 shows the Dryback Firetube Boiler in which the combustion chamber is lined with high temperature insulating material. Watertube Boilers Watertube Boiler's design features one or more relatively small drums with many tubes in which the steam/water mixture circulates. Figures 2-14 on page 27 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow illustrates the circulation of a watertube boiler. The watertube boiler steam-water system will be discussed further in the next two sections. Heating the riser tubes with the hot flue gases causes the water to circulate and steam to be released in the boiler drum. Early watertube boilers were shown in Figures 4 and 5 of this module. Today a typical watertube boiler has a single burner with up to approximately 125,000 pounds per hour steam flow but is available in sizes up to several million pounds per hour with more than one burner.



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Boiler Hardware Overview



Drum Circulating Water Downcomer



Boiler



Riser



Superheater



Mud Drum Economizer



Feedwater Riser Burners



Air



Steam Coil Air Heater



Air Heater



To Stack Gas Outlet



Gas



Air



Induced Draft Fan



Forced Draft Fan 33390



Figure 6. Modern Day Boiler



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Drum Boilers operating below critical pressure are usually fitted with a steam drum, (Figure 7). In the steam drum, saturated steam is separated from a recirculating steam/water mixture. The recirculation flow is from the steam drum via downcomer tubes to either the mud drum or the water wall header, and from there through riser tubes back to the steam drum. Most boilers rely on natural convection for this flow, because of heat absorption by the risers from the furnace. Some larger boilers use low head, high working pressure pumps to provide positive circulation. The steam rises up through separation devices in the drum and exits to one or more superheating passes through the furnace. The water from the steam/water mixture is then recirculated together with the makeup feedwater to downcomer circuits. Water treatment chemicals may be added to the steam drum and feedwater may be discharged, or "blown down" from the mud drum, to reduce dissolved and undissolved solids in the boiler water. The primary purpose of the steam drum however is to provide a free controllable surface for separation of steam from water and a housing for any mechanical separating devices.



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Steam Outlets



Secondary Scrubber Primary Scrubber Cyclone



Blowdown Feedwater Inlet



Chemical Feed



Upcomer/Riser



33391



Figure 7. Steam Drum Mud Drum The mud drum is completely filled with water and is the low velocity point of the circulating water. Unresolved solids that develop in the boiler gravitate to the bottom of the mud drum and can be drawn off.



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Riser Heat collecting surfaces constructed from tubing and conveying boiler circulating water upwards to the steam drum are generally called risers. The risers may originate from either the water wall header at the base of the furnace, or from the mud drum. Boiler circulating water absorbs primarily radiant energy from the furnace fireball while resident in risers jacketing the furnace. These heat absorption surfaces called water walls are fed from the water wall header at the base of the furnace. Downcomer Water is carried down from the boiler drum to the mud drum or to the water wall feedwater header through tubes called downcomers. The downcomers are not heated and are located outside of the furnace cavity.. Superheater The superheater is a flue gas to steam heat exchanger. Heat from the flue gases is added to the saturated steam from the drum. Burner The burner is used to introduce fuel and air to the furnace at the required velocities, turbulence, and concentration to maintain ignition and combustion of the fuel within the furnace. Economizer Feedwater from the condensate-feedwater system enters the economizer located in the furnace flue gas ductwork. Waste heat from the flue gas is absorbed by the feedwater in order to improve efficiency. Air Heater The steam-generator air heater improves boiler efficiency by transferring heat to incoming combustion air from the flue gases before they pass to the atmosphere. The heat is transferred to the air from the flue gas through a regenerative heat-transfer surface in a rotor that turns continuously through the gas and airstreams.



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Steam Coil Air Heater The steam coil air heater is a tublar steam to air heat exchanger in which auxiliary steam charges the coil. Combustion air flows across the tubes in order to provide a minimum combustion air temperature. The steam coil air heater is normally in service only during boiler startup, or possibly low load conditions when the regenerative air heater cannot provide sufficient heat to the combustion air. Forced Draft Fan The forced draft fan supplies low head air necessary for fuel combustion, and secondarily to make up for air heater leakage and for some seal-air requirements. Induced Draft Fan The induced draft fan used in a balanced draft furnace exhausts combustion products from the furnace. The induced draft fan creates a sufficient draft to establish a slight negative pressure in the furnace. Systems Functional Overview A boiler is composed of two separate systems: the steam-water system and the draft system (Figure 2-1 on page 20 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow). Steam-Water System Water enters the steam/water system, is heated, is converted to steam, and exits the system in the form of steam. Draft System The draft system supplies the heat that is necessary to boil the water. Fuel and air enter the draft system and are mixed and ignited in a furnace. The combustion converts the chemical energy of the fuel to heat or to thermal energy. Heat Transfer In most tube steam generators, the radiant section of the furnace is lined with a heat transfer surface of boiler circulating water tubes (water wall or mud drum risers.) The tubes receive radiant heat from the fireball and transfer it to the steam/water system. Combustion flue gases are cooled by the water wall and the boiler circulating water heat transfer surfaces.



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Flue gases exiting the furnace also transfer heat to the working fluid by conduction as they pass through the various heat transfer surfaces. Additional heat is recovered from the flue gases by use of the combustion air preheater (Figure 2-2, A Simple boiler plus Combustion Air preheater, on page 20 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow) and the economizer (Figure 2-3, A Simple Boiler plus Economizer, on page 21 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow). The combustion air preheater transfers heat from the hot flue gases to the combustion air. The economizer transfers heat from the hot flue gases to the boiler feedwater.



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COMPONENTS OF BOILER FLUID CIRCULATION SYSTEMS: FUNCTIONS AND BASIC OPERATION The fluid circulation systems of a boiler are the following: •



Feedwater supply system



•



Feedwater conditioning and boiler blowdown



•



Boiling process and steam generation



This section will discuss the components of these systems and the functions of these components. Feedwater Supply System Figure 8 illustrates the basic feedwater supply system. The function of the feedwater supply system is to continuously supply water to the boiler through piping to the steam drum.



Intermediate Pressure Turbine Lo Pressure Turbine



To Process Header Intermediate Steam to Process Drive to BFP Lo Pressure Steam to Process Condenser



Superheater



Deaerating Heater



Steam Drum



Superheat Spray Valve



Economizer



Makeup Softened Condensate Storage Tank



Condensate Feed Tank Feedwater Heater



Boiler Feed Pump



Deaerator Storage Tank



Drive from Turbine



33392



Figure 8. Basic Feedwater Supply System



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Relatively cool water leaves the condensate storage tank and enters the deaerating heater and is deposited into a deaerator storage tank. The feedwater deaerator supplies the suction side of the boiler feedwater pump(s). These high pressure pumps supply the economizer from the deaerator. Boiler feedwater from the economizer enters the steam drum, and the boiler circulating water system. Saturated steam from the drum passes through the superheater and is discharged to the process as high pressure superheated steam. The steam may be used to supply power turbines or manufacturing processes. Some of the energy in the high pressure steam may be partially expended by the process of power turbines. The resulting intermediate pressure steam is used for feedwater heating, processes using low pressure steam, or low pressure power turbines. Supplements to the intermediate pressure steam loads may be provided by steam pressure letdown and attemperation stations. Excess high pressure steam, resulting from sudden load drops may also be routed through letdown stations (sometimes referred to as steam bypasses) to the low pressure steam header. Low pressure steam is phased back to liquid by the steam condenser and is stored in the condensate storage tank. Most pumps in an industrial or utility environment will have some form of working fluid recirculation to prevent damage at "deadhead" or low flow conditions. The discharge side of the pump is provided with a recirculation line back to the reservoir feeding pump, or occasionally back to the suction side of the pump. The recirculation line may be fed by a mechanical pressure relief valve, a shutoff valve and orifice, or modulating control valve. Condenser The steam condenser in the feedwater supply system is a heat exchanger used to transfer sufficient heat from the low pressure steam to condense it back to its liquid phase. The heat exchanger may be either air to steam or water to steam. Feedwater Heaters Feedwater heaters are used to heat the boiler feedwater so that less fuel is required to generate steam. The heaters may be classified either as low pressure prior to the deaerator, or high pressure after the boiler feedwater pumps. Heating the feedwater is also necessary for the process of deaeration.



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Deaerator Heater and Storage Tank The deaerator is used to eliminate air, oxygen, CO2, and other gases from the boiler feedwater. These gases are removed by vigorous boiling and venting the gases to atmosphere. If CO2 were allowed to remain in the water, the heat exchangers and condensate return piping would become corroded. If oxygen were allowed to enter the boiler, serious corrosion could occur. Boiler Feed Pump The boiler feed pump is used to supply high pressure boiler feedwater to the drum. Boiler feedpumps operate in a constant speed or variable speed manner. A variable speed pump's speed can be driven by a variable speed motor, a magnetic or hydraulic coupling, or a steam turbine. A recirculation line is open at low flow to keep the pump from cavitating and overheating. Economizer Figure 2-3, A Simple Boiler plus Economizer, on page 21 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow shows a simplified diagram of a boiler plus economizer. The economizer is used to recover heat from the flue gas to the boiler feedwater. The flue gas exits the boiler and enters the economizer where it transfers heat to the boiler feedwater. The flue gas temperature decreases, and the boiler feedwater temperature increases. Feedwater Conditioning and Boiler Blowdown Feedwater conditioning and boiler blowdown are used to maintain a proper boiler chemical balance. Feedwater conditioning includes flash evaporation and reverse osmosis of sea water and the use of oxygen scavengers and corrosion inhibitors. Boiler blowdown can be continuous and is used to remove impurities in the boiler water. There are various methods for the internal treatment of boiler water. A blanket recommendation of any one method is not realistic. The type of treatment to be used in a particular boiler should be based on the raw water supply, the percent of make-up required, the nature of condensate returns, and other factors. Boilers require high-purity feedwater. Careful monitoring of boiler feedwater as well as condensate chemistries is crucial to boiler operations. The monitoring of PH, specification conductivities, and dissolved oxygen (DO) are required for high-purity feedwater maintenance. Boiler Feedwater (BFW) Supply Flash Evaporator Sea Water - Feedwater conditioning removes dissolved salt and mineral solids that tend to form ions in solution. One method that removes dissolved salts and minerals is flash evaporation. The flash evaporator operates with its flash chamber under partial vacuum. Water that enters the chamber is preheated sufficiently to cause water to flash into a vapor upon entering the chamber. The vapor is condensed to form condensate and the precipitated solids removed and disposed of. Saudi Aramco DeskTop Standards



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Reverse Osmosis of Sea Water- Another procedure that is used to remove dissolved salts and



minerals is reverse osmosis. Osmosis is based upon the principle that when two solutions of different concentrations are separated by a semipermeable membrane, solvent (water) will be transported from the dilute to the more concentrated side. Reverse osmosis is based upon the principle that if pressure is applied to the more concentrated side the solvent will flow in the reverse direction (Figure 9). If the solution is salt water and a membrane is chosen that is permeable to water but not to salt, water will flow to the unpressurized side. The result will be a solution that is more dilute than the original and a solution that is more concentrated than the original.



Pressure Applied



Semi-Permeable Membrane Water



Water Dilute Solution Concentrated



Dilute Solution Concentrated Reverse Osmosis



Osmosis



33393



Figure 9. Reverse Osmosis Chemical Treatment of BFW Supplementing internal boiler water treatment is possible by injecting chemicals through the chemical feed line into the steam drum. The chemical feed line discharges into a turbulent zone of the drum for thorough mixing with the boiler water before the mixture enters the downcomers. The continuous blowdownand chemical feed lines are separated so that the injected chemicals do not flow directly to the blowdown line.



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Oxygen Scavengers - control corrosion by dissolved oxygen (DO). Corrosion by DO is more critical in the feedwater system because corrosion rates increase with temperature. Sodium sulfite and sodium sulfite that are catalyzed with cobalt have been applied in low-pressure cycles. Sulfite, which is a reducing agent, functions strictly through reaction with DO. Hydrazine also functions as an oxygen scavenger. Hydroquinone is an oxygen scavenger that has been applied in blends with hydrazine to catalyze its reaction with DO, but hydroquinone may also be used as a hydrazine substitute. Other chemicals such as carbodihydrazide decompose at feedwater temperatures and form hydrazine as a by-product. Organic oxygen scavengers, such as erythorbic acid and diethylhydroxylamine, are also available. Corrosion Inhibitors - Hydrazine provides corrosion protection through the formation of magnetite



film on steel and through the formation of cypric oxide on copper alloys. Boiler Blowdown Methods Continuous blowdown - During normal operation, feedwater is constantly added to the drum as



steam is removed. The impurities in the feedwater and the impurities separated from the steam will remain in the boiler water. If the impurities are not removed, these impurities will become more concentrated and eventually deposit on internal tube surfaces. The formation of scale on tube surfaces reduces heat transfer and can lead to overheating and possible tube failures. Corrosion of the boiler can be caused by dissolved oxygen, organic-chemical-breakdown products, acids, and excess caustic. Contaminants that can form deposits on boiler surfaces include calcium, magnesium, iron, silica or silicates, phosphates, sulfates, oils, and organic elements. Boiler water solids are maintained at recommended limits by the continuous blowdown line. The line is positioned internally, along the length of the drum, in a zone where solids tend to collect. A calibrated flow control valve regulates the amount of blowdown to the drain system, based on water solids concentration and feedwater flow. This process tends to remove the most contaminated water in the system and replace it with fresh feedwater. Intermittent blowdown - is performed by periodically opening a blowoff valve that is connected to



the lowest part of the mud drum. The primary purpose of the intermittent blowdown is to remove undissolved solids that collect at the low velocity point of the boiler circulating water. The intermittent blowdown may be operated anytime depending on the concentration of impurities in the boiler water. Boiling Process and Steam Generation The process of boiling water to make steam is a familiar phenomenon. As heat is added to water, the temperature of the water increases. When the water temperature reaches the boiling point, or saturation temperature, some of the water begins to vaporize to steam.



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Saturated Water and Saturated Steam When water just begins to boil, it is called saturated water (Figure 10). As more heat is added (at constant pressure), the fluid temperature will remain at the saturation temperature until all of the water is converted to steam.



o Temp ( F)



Superheated Steam



Saturated Water



470



Constant Pressure



Saturated Steam



500 psi



450



Enthalpy (BTU/lb)



1205



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Figure 10. Boiling Process and Steam Generation Once the conversion from water to steam is complete (but before the temperature is raised above saturation temperature) the fluid is called saturated steam. The speed of conversion depends on the rate of heat that is being added. It must be remembered that heat and temperature are not the same thing. A considerable amount of heat is added to the fluid while its temperature remains constant at the boiling point saturation temperature. Although the temperature remains constant, the heat being applied is not lost or wasted. It is being utilized to convert water into steam. The heat input or Enthalpy necessary to convert saturated water to saturated steam is called the heat of vaporization. The conversion of water to steam requires much more energy beyond that required to reach the boiling point.



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Instrumentations Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



Boiling point - The term boiling point is most frequently used to identify conditions at atmospheric pressure (29.92 inches of mercury.) For instance, the boiling point of water at atmospheric pressure is 212 degrees Fahrenheit, however, pressure increases when steam is generated in a closed vessel. The boiling point is actually a function of pressure and increases as pressure increases, as illustrated in Figures 11 and 12. At higher pressures, more heat energy is required to raise the fluid temperature to the boiling point. Enthalpy - The amount of heat energy contained in the fluid is termed Enthalpy and is measured in



BTUs/lb. 800



Temp ( oF)



700



2500 2000



600



1500 1000



500 400 300



500 psi 160 psi 400 500 600 700 800 900 1000 Enthalpy (BTU/lb) of Saturated Water 33395



Figure 11. Boiling point–Pressure Relationship



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1200 1100 1000



o Temp ( F)



900 800 700 600 500



Length of line = hfg



Saturated Curve 2000 psi



Boiling Points



500 psi



400



200 psi 100 psi



300 400



500



600



700



800



900



1000 1100



1200 1300 1400 1500 33396



Figure 12. Saturation Curve–Boiling point–Pressure Relationship Heat of Vaporization - The points at which all of the water has been converted to steam are



indicated by the saturated steam line (Figure 12). The heat input or Enthalpy necessary to convert saturated water to saturated steam is termed the heat of vaporization and is indicated for a given temperature by the horizontal constant pressure lines. For example, water begins to boil at about 470 degrees Fahrenheit when the pressure is 500 pounds per square inch. The Enthalpy at this point is about 450 BTU per pound. As more heat is added (at constant pressure), the Enthalpy increases and more water is converted to steam. The temperature remains constant until all the water has been converted to saturated steam. This point would be at the same pressure and temperature (500 pounds per square inch, 470 degrees Fahrenheit), but the Enthalpy would be increased to 1205 BTU per pound. The heat of vaporization would then be the Enthalpy of the saturated steam minus the Enthalpy of the saturated water or 1205 - 450 = 755 BTU per pound.



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Steam quality - The measure of how far the conversion from saturated water to saturated steam has



progressed is called quality and is shown in Figure 13. Quality is the percent by weight of vapor in a steam/water mixture. As more water is converted to steam, quality increases. Water on the saturated water line has a quality of 0%. Superheated and saturated steam have a quality of 100%. Water that has been heated to saturation and has sufficient additional heat added to convert half of it to steam has a quality of 50%.



1200 1100 1000 Superheated Steam



o Temp ( F)



900 800 700



Saturated Water 600 (0% Quality)



Constant Pressure Line



500 20



400



40



60



80



Saturated Steam (100% Quality)



300 400



500



600 700 800 900 1000 1100



Enthalpy (BTU/lb)



1200 1300 1400 1500 33397



Figure 13. Steam Quality Superheated Steam If still more heat is added to saturated steam, the temperature will again begin to rise. This is shown by the dotted lines to the right of the saturated steam lines in Figure 10. The fluid in this area is said to be superheated steam. The superheaters derive their name from their function of heating steam above the saturation curve. Steam is sometimes referred to as having a number of degrees of superheat. The number of degrees of superheat describes how far the steam has been heated above the saturation curve.



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Boiling Process Two types of boiling processes exist. One is nucleate boiling. The second is film boiling. Nucleate boiling is preferred over film boiling. Nucleate Boiling - As a water cooled tube is heated, steam bubbles form at the tube's inner surface.



The steam bubbles condense quickly in the main stream, giving up their heat to raise the temperature of the water. Normally these bubbles diffuse well and mix with the water in the center of the tube as shown in Figure 14. This process is referred to as nucleate boiling and promotes two benefits: (1) it heats the fluid inside the tube to saturation, and (2) it maintains tube metal temperature at saturation keeping the tube cool.



Tube Wall



Steam Bubbles (Mixing)



33398



Figure 14. Nucleate Boiling



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Film Boiling - With high heat input levels and high steam quality, the nucleate boiling process



breaks down. The bubbles of steam forming on the hot tube surface will begin to interfere with the flow of water to the surface and the bubbles of steam eventually coalesce to form a film of superheated steam over part or all of the tube surface. This condition is known as film boiling (Figure 15). Little heat will be transferred from the tube metal through the film to the water in the center of the tube. The tube metal temperature will rapidly increase, resulting in a failure. Tube Wall



Steam Film (No Mixing)



33399



Figure 15. Film Boiling Departure From Nucleate Boiling - The point at which nucleate boiling stops and film boiling begins



is determined by the heat input and steam quality. The point is termed Departure from Nucleate Boiling or DNB. Metal temperatures are shown in Figure 16 as a function of steam quality for several heat input levels. As the curves illustrate, only moderate heat inputs can be tolerated at high quality levels (Area B); however, much higher heat input levels can be tolerated at lower qualities (Area A). This means that high heat inputs, which result in higher levels of circulation and steam generation, can be used at low quality levels.



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Instrumentations



Increased Metal Temperature



Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



Superheated Steam



Subcooled Water



nput eat I H h Hig Low



put Heat In



Area A Area B 0%



Quality



100% 33400



Figure 16. DNB Metal Curve Research into DNB has found several parameters that affect DNB: A. High fluid velocities decrease the occurrence of DNB at a given fluid quality. B. Fluid quality has a great effect on DNB. Lower qualities afford greater margins of safety and reduce the possibilities of the occurrence of DNB. C. Wall construction or location of heat flux also affects DNB. Heating a wall from one side could allow a steam film to form on the heated side of the tube, causing overheat. D. Tube type has a major impact on the prevention of DNB. E. Research and experience has shown that DNB is more likely to occur at operating pressures above 2000 psig. F. Higher heat flux also increases the possibility of DNB caused by the higher qualities generated. Natural Circulation Modern watertube boilers were developed from the early firetube designs. Modern watertube boilers not only have a larger surface area available for heat transfer, but by proper design, a natural circulation effect is created with water continuously moving within the boiler tubes to remove and replace the generated steam. In a natural circulation system, circulation increases with increased heat input until a point of maximum fluid flow is reached.



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Natural circulation is based on the difference in density between water and steam. Steam is significantly less dense than water. Water is supplied from a drum to the furnace wall tubes through downcomers. The downcomers are not heated. As the unit is fired, a steam/water mixture is generated in the furnace wall tubes. The steam/water mixture in the wall tubes is less dense than the water in the downcomers and forced up the steam drum by the heavier water as shown in Figure 17. The process continuously repeats with a steam/water mixture being generated in the furnace tubes and being replaced with heavier water in the downcomers.



Steam Outlet Steam Drum



Water/Steam



Downcomer (Unheated)



Furnace Walls (Heated)



Furnace Wall Supplies



Figure 17. Natural Circulation As more heat is added to the furnace tubes, the quality of the fluid increases. Because the density difference becomes greater, more pumping power is available from the natural circulation effect. Up to a point, circulation will naturally increase with increased heat input and provide more flow to keep furnace tubes cooled as more steam is generated. Beyond a certain level, friction in the tubes overcomes the difference in density and circulation is reduced with additional heat input as shown in Figure 18. Natural circulation boilers are designed to operate in the left region of the curve so that circulation increases with heat input. Saudi Aramco DeskTop Standards



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Possible Overheat



Inc reasing Flow



Normal Operation



Increasing Heat Input



33402



Figure 18. Effect of Flow versus Heat Input Natural circulation also provides an additional benefit. It partially compensates for normal imbalances in the heat input to the furnace. As shown in the left portion of Figure 17, if one tube receives more heat than adjacent tubes, it will generate more steam with a lower density and thus will receive more flow to keep it cool. Within normal limits, tubes exposed to higher heat levels will receive more cooling flow. If the heat imbalance becomes too great, flow will be reduced and the tube will overheat. Drum boilers operate in the area on or under the saturation curve. Steam quality leaving the riser tubes and entering the steam drum is usually 5 to 30%, depending on the boiler load and pressure. This means that of the water that flowed down the downcomers, between 5 and 30% will be converted to steam by the time it reaches the top of the furnace. Staying at low quality levels is necessary to protect the tubes from overheat failures caused by the nature of the boiling process. Steam drum internals In modern drum boilers (Figure 6 of this module), the separation of steam from the steam/water mixture generated in the furnace usually takes place in two steps. Primary separation removes nearly all of the water from the mixture, so that in effect, no steam is recirculated to the boiler water; however, the steam may still contain solid contaminants that must be removed or reduced in amount before the steam is sufficiently pure for use. This step is called secondary separation or steam scrubbing. When wide load fluctuations and variations in water quality are suspected, secondary scrubbers may also be installed to provide nearly perfect steam separation. Saudi Aramco DeskTop Standards



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Cyclone Steam Separators - Primary steam separation is accomplished with cyclone steam



separators. The cyclones, essentially cylindrical in form, are arranged internally along the length of the drum. The steam/water mixtures enters the cyclone steam separator tangentially. Centrifugal force throws the more dense water to the outside of the cylinder where it forms a layer against the cylinder wall. The less dense steam moves to the core of the cylinder and moves upward. The water flows down the cylinder wall and is discharged from the cyclone through an annulus located below the water level. The separated water returns to the boiler cycle virtually free of steam bubbles, thus providing a maximum available head for producing flow through natural circulation. Primary Scrubbers - The upward rising steam from the cyclones passes through the primary



scrubbers at the top of the cyclones for secondary steam separation. After primary separation, the steam may still contain dissolved solids suspended in tiny water droplets. These water droplets that contain solids are removed from the steam as it passes through the corrugated plate elements of the primary scrubber. Secondary Scrubbers - Further steam scrubbing of any trace amounts of water contaminants in the steam is achieved by the secondary scrubbers. Secondary scrubbers are corrugated plates that are located at the top of the steam drum, and provide a large surface to intercept water particles as the steam weaves through the closely fitted plates. Steam velocity through the corrugated plate assembly is very low, so that re-entrainment of water is avoided. The collected water is drained from the bottom of the scrubber assembly to the water below.



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COMPONENTS OF THE VARIOUS BOILER AIR AND DRAFT SYSTEMS: FUNCTIONS AND BASIC OPERATION This section will discuss the functions and components of the various boiler air and draft systems (Figure 19).



Steam Coil Air Heater Stack Forced Draft Air



Scrubber Air Heater



Vent Gas Gas Supply Dryer



Precipitator (Optional) Windbox



Boiler/ Furnace



Induced Draft Fan



33681



Figure 19. Boiler Air and Draft System The function of the air and draft system is to provide an adequate flow of air and combustion gases for the complete combustion. Air flow is controlled by the stack and fans. The differential pressure required for air flow is produced by a combination of the stack and fans. In Figure 19, air flows from the forced draft fan through a steam coil air heater into the boiler. Combustion products exit the boiler and flow through an induced draft fan to the scrubber and the stack. Specifically, this section will discuss: •



The function and components of the boiler forced draft system.



•



The function and components of the boiler balanced draft system.



•



The function and components of the air flow system.



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Forced Draft and Natural Draft Systems Figure 14-5, Pressure-Fired Boiler, on page 214 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow illustrates a forced draft system. A forced draft system or a pressure fired boiler, operates with the air and combustion products that are maintained above atmospheric pressure. Forced Draft Fan The forced draft fan provides sufficient pressure to force the air and flue gas through the system. Natural Draft Natural draft occurs as a result of the stack effect. Hot air or hot gases rise through vertical ducts. Hot flue gases, that have a lower density than the outside air rise through vertical ducts and create a suction that causes combustion air to flow through the boiler. Pressure and Draft Profile A pressure and draft profile of a forced draft system is shown in Figure 20. The negative pressure at the right side of the profile is caused by the natural draft of the stack. A pressure and draft profile of a forced draft system without an air preheater is shown in Figure 14-6, Profile of Pressure and Draft of a Pressure-Fired Boiler (Typical-No Air preheater), on page 214 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow. A pressure and draft profile of a forced draft system with an air preheater is shown in Figure 14-7, Profile of Pressure and Draft of a Pressure-Fired Boiler (Typical-Includes Air preheater), on page 214 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow. Furnace Air Pressure and Boiler Load Additional draft losses cause the forced draft system to operate at higher pressure at all loads and to be under positive pressure except at very low loads. At 70% boiler load the draft losses are approximately 50% of the full load draft losses. Balanced draft Systems Balanced draft systems have a forced draft fan at the system inlet and an induced draft fan near the system outlet (Figure 14-8, Balanced draft Boiler (With Air preheater), on page 215 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow).



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Air preheater The air preheater is used for flue gas heat recovery and adds additional draft losses to both the combustion air and the flue gas sides of the boiler. The air preheater does not change the controlled furnace draft setpoint. Induced Draft Fan The induced draft fan takes suction at the flue gas exit. The induced draft fan reduces the furnace pressure and ensures that it is negative. Pressure and Draft Profile A pressure and draft profile for the balanced draft system is shown in Figure 14-9, Profile of Pressure and Draft of a Balanced draft Boiler (Typical-No Air preheater), on page 216 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow. The forced draft fan and the induced draft fan work to maintain the balance point or pressure in the furnace. The pressure is slightly negative for all boiler loads and is not affected by the addition of an air preheater (Figure 20 of this module).The air preheater does add additional draft losses to the combustion air and flue gas sides of the boiler.



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Pressure



D A C



(+) B 0 Balance Point



(-)



F.D. Fan Out



Windbox Air Preheater Inlet High Load



-



Reduced Load -



Furnace



Boiler Outlet



Air Preheater Outlet



Stack



I.D. Fan Outlet



(100%) (70%)



A B



Change Balance by Increasing Induced and Reducing Forced



C



Change Balance by Decreasing Induced and Increasing Forced



D 33404



Figure 20. Pressure and Draft Profile–Balanced draft System–Includes Air preheater Benefit of Balanced draft System The benefit of balanced draft system is that negative operating pressures cause any leakage to be cool air leaking into the furnace rather than hot combustion gases leaking out. The negative furnace pressure condition is cooler and cleaner, which results in a better environment for the furnace housing, the auxiliary equipment, and the operating personnel. Air Flow System Air flow requirements of the combustion control system require that fans operate at different pressures and volume discharge rates. To meet these requirements, some means of varying the fan output is required such as a constant speed fan with damper or inlet vane control or a variable speed fan. Saudi Aramco DeskTop Standards



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Constant speed fan A constant speed fan used with damper control or inlet vane control introduces sufficient variable resistance in the system to alter the fan output as required. The constant speed fan is attractive because of its low initial cost, however, a constant speed fan consumes significantly more energy than a forced draft fan at low boiler rates. Variable Speed Fan The variable speed fan is attractive because it reduces energy at reduced flow rates. The variable speed fan significantly improves fan efficiency during periods when the boiler is operating at less than its maximum load. Even so, variable speed fans require a higher initial cost than constant speed fans that may not be offset by lower power requirements.



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COMPONENTS OF BOILER FUEL SYSTEMS: FUNCTION AND BASIC OPERATION This section will discuss the boiler fuel oil and fuel gas systems. Specifically, this section will discuss: •



The function and components of the fuel gas system.



•



The function and components of the fuel oil system.



Fuel Gas System The most common gaseous fuel is natural gas. Waste gas or gas produced as a process byproduct can also be used. Figure 5-1, Gas Pressure-Reducing and Metering Arrangement, on page 51 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow illustrates a fuel gas system. Natural gas is transported through pipelines and is delivered by the suppliers as it is used. Fuel Oil System The most common oils used for boiler fuel are the lightweight Number 2 fuel oil and the Number 6 grade of heavy residual fuel oil. Figure 5-3, Typical Fuel Oil Pumping and Heating Arrangement, on page 53 of The Control of Boilers, 2nd Edition, by Sam G. Dukelow illustrates the fuel oil system. It is usually necessary to heat Number 6 fuel oil so that it can be pumped through the system. It is normally not necessary to heat Number 2 fuel oil. Fuel oil is pumped through the fuel oil system, and the boiler control system regulates the BTU input by a control valve. The amount of heat (BTU) liberated per unit quantity of gas or oil is called the Higher-Heat Value (HHV). The HHV for a fuel, typically in units of BTU per pound, can be found in the Combustion Engineering Fuel Burning and Steam Generation Handbook.



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APPLICATIONS OF INDUSTRY AND SAUDI ARAMCO STANDARDS AND SPECIFICATIONS TO SAUDI ARAMCO BOILER CONTROL SYSTEMS This section will discuss the standards and specifications that apply to Saudi Aramco boiler control systems. Specifically, this section will discuss: •



Saudi Aramco Engineering Standards SAES-J-004, Instrument Symbols and Identification, SAES-J-600, and SAES-J-602, Boiler Safety Systems For Watertube Types.



•



Aramco Materials System Specification 34-SAMSS-619, Flame Monitoring and Burner Management Systems.



•



American National Standards Institute/National Fire Protection Association Standards NFPA 8501, Single Burner Boiler Operation, and NFPA 85C, Prevention of Furnace Explosions/Implosions in Multiple Burner Boiler-Furnaces.



•



American Petroleum Institute Recommended Practice API-RP-551, Process Measurement Instrumentation.



•



American National Standards Institute/Instrument Society of American Standards ANSI/ISA S77.41, Fossil Fuel Power Plant Boiler Combustion Controls, ANSI/ISA S77.42, Fossil Fuel Power Plant Feedwater Control System - Drum Type, and ANSI/ISA S5.1, Instrument Symbols and Identification.



•



Scientific Apparatus Manufacturer's Association Standard SAMA PMC 22.1-1981, Functional Diagramming of Instrument and Control Systems.



•



Burner Vendor Specification Sheet.



Saudi Aramco Engineering Standards SAES-J-004, SAES-J-600 and SAES-J-602 SAES-J-004, SAES-J-600 and SAES-J-602 are contained in Course Handout 1. SAES-J-004 establishes the Aramco system instrument identification and instrument symbols for use on Process Flow Diagrams, Piping and Instrumentation Diagrams, and construction drawings. The identification is also used as equipment tag (mark) numbers on equipment and for material requisitions, specification sheets, instrument installation schedules, records, and forms. SAES-J-600 prescribes mandatory requirements governing the design and installation of safety and relief devices except for residential and commercial water heating equipment. SAES-J-602 establishes the minimum requirements for the design, construction and installation of safety systems for single and multiple burner watertube boiler furnaces. Specifically, SAES-J-602 contains feedwater system design information used to calculate drum level.



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SAES-J-602 applies to the firing of gaseous, vaporized, and liquid fuels only, such as natural gas, refinery gas, LPG, and fuel oils, including diesel. For the firing of other fuels such as by-products derived from NGL processing plants and refineries (e.g. crude waste fuels), prior written approval of the Aramco Chief Engineer, Dhahran, shall be obtained. Aramco Materials System Specification 34-SAMSS-619 34-SAMSS-619 (contained in Course Handout 3) defines the requirements for Flame Monitoring and Burner Management Systems (BMS) for boilers. Specifically, 34-SAMSS-619 describes ignitor and flame sensor, BMS control logic, and BMS logic requirements. American National Standards Institute/National Fire Protection Association Standards NFPA 8501 and NFPA 85C NFPA 8501 (contained in Course Handout 4) is titled Single Burner Boiler Operation. NFPA 8501 establishes the minimum standards for the design, installation, operation and maintenance of single burner boiler-furnaces, their fuel-burning systems, and related systems, to contribute to operating safety, and, in particular, to prevent furnace explosions. Specifically, NFPA 8501 is used to determine furnace pressure control strategies, implosion prevention requirements, the acceptability of BMS fuel piping, instrumentation and valves and the acceptability of the ignitor and flame sensor. NFPA 85C (also contained in Course Handout 4) is titled Prevention of Furnace Explosions/Implosions in Multiple Burner Boiler-Furnaces. The puRPose of NFPA 85C is to contribute to operating safety and to prevent furnace explosions and implosions. NFPA 85C establishes the minimum standards for the design, installation, operation, and maintenance of boiler furnaces and their fuel burning, air supply, and combustion product removal systems. Specifically, NFPA 85C is used to determine furnace pressure control strategies, implosion prevention requirements, the acceptability of BMS fuel piping, instrumentation and valves, and the acceptability of the ignitor and flame sensor. American Petroleum Institute Recommended Practice API-RP-551 API-RP-551 (contained in Course Handout 5) discusses recommended practices for the installation of the flow instruments in the refining process industry to indicate, record, and transmit flow measurements.



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American National Standards Institute/Instrument Society of America Standards ANSI/ISA S77.41, ANSI/ISA S77.42 and ANSI/ISA S5.1 ANSI/ISA S77.41 (contained in Course Handout 4) is titled Fossil Fuel Power Plant Boiler Combustion Controls. The purpose of ANSI/ISA S77.41 is to establish the minimum requirements for the functional design specification of combustion control systems for drum-type fossil-fueled power plant boilers. ANSI/ISA S77.41 discusses the major combustion control subsystems in boilers with steaming capabilities of 200,000 pounds per hour or greater. The subsystems include furnace pressure control (balanced draft) and air and fuel flow controls. ANSI/ISA S77.41 does not include discussion of development of boiler energy demand, all burner control, interface logic systems, and associated safety systems. ANSI/ISA S77.42 (also contained in Course Handout 4) is titled Fossil Fuel Power Plant Feedwater Control System - Drum Type. The purpose of ANSI/ISA S77.42 is to establish minimum criteria for the control of levels, pressures, and flow for the safe and reliable operation of drum-type feedwater systems in fossil power plants. ANSI/ISA S77.42 discusses the development of design specifications covering the measurement and control of feedwater systems in boilers with steaming capacities of 2000,000 pounds per hour or greater. ANSI/ISA S5.1 (also contained in Course Handout 4) is titled Instrumentation Symbols and Identification. ANSI/ISA S5.1 establishes a uniform means of designating instruments and instrumentation systems used for measurement and control. Scientific Apparatus Manufacturer's Association Standard SAMA PMC 22.1-1981 SAMA PMC 22.1-1981 (contained in Course Handout 6) is titled Functional Diagramming of Instrument and Control Systems. SAMA PMC 22.1-1981 establishes uniformity of symbols and practices in diagramming measuring, controlling, and computing systems. Burner Vendor Specification Sheet The Burner Vendor Specification Sheet contains burner control system design specifications. Specifically, the Burner Specification Sheet contains design conditions for the electrical power supply, instrument air supply and ambient conditions, basic system design, and operational principles.



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GLOSSARY air



The mixture of oxygen, nitrogen, and other gases that with varying amounts of water vapor, forms the atmosphere of the earth.



air heater



Heat transfer apparatus through which air passes and is heated by a medium of higher temperature, such as the products of combustion (flue gases) or steam.



air preheater



A steam coil or similar heat exchange device used to keep the temperature of the flue gas above its dew point to minimize corrosion to the air heater.



ANSI



American National Standards Institute.



API



American Petroleum Institute.



balanced draft



A system of furnace pressure control in which the inlet combustion air flow and the outlet flue gas flow is controlled to maintain the furnace pressure at a fixed value (typically slightly below atmosphere).



blowdown



Removal of a portion of boiler water to reduce chemical concentration, or to discharge sludge.



boiler



A closed vessel in which water is heated, steam is generated, steam is superheated, or any combination thereof, under pressure or vacuum by heat from combustible fuels in a selfcontained or attached furnace.



boiler feedpump



A pump that is used by the boiler system to supply high pressure boiler feedwater to the steam drum.



boiling point



The temperature that a liquid changes to vapor for a given pressure.



BTU



British Thermal Unit.



burner



A device or group of devices for the introduction of fuel and air into a furnace at the required velocities, turbulence, and concentration to maintain ignition and combustion of the fuel within the furnace.



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circulation



The movement of water and steam within a steam generating unit.



condenser



A heat exchange device that is used to condense steam. A condenser is used to condense the exhaust steam from turbines.



constant speed fan



A fan with a single-speed motor that uses damper control or inlet vane control to vary air flow.



continuous blowdown



The continuous blowing down of water.



control



Any manual or automatic device for the regulation of a machine to keep it at normal operation. If automatic, the device is motivated by variations in temperature, pressure, water level, time, light, or other influences.



corrosion



The wasting away of metals caused by a chemical action that is usually caused by the presence of O2, CO2, or an acid.



deaerator



A device that is used to remove air and gases from boiler feedwater prior to its introduction into a boiler.



DNB



Departure from Nucleate Boiling: The point at which nucleate boiling stops and film boiling begins.



downcomer



The lines that carry cold water from the steam drum to the mud drum of the furnace for heating.



draft



The difference between atmospheric pressure and some lower pressure existing in the furnace or gas passages of a steam generating unit.



drum



A cylindrical shell closed at both ends designed to withstand internal pressure.



economizer



An energy-saving heat exchanger that is commonly used on all large boilers to provide additional heat to the feedwater prior to the water entering the boiler. Heat input comes from the hot flue gases passing the tubes (containing feedwater) just before going to the air heater.



efficiency



The ratio of the output to the input. The efficiency of a steam



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generating unit is the ratio of the heat absorbed by water and steam to the heat in the fuel fired. Enthalpy



The amount of heat energy that is contained in a fluid or gas in BTU/lb.



feedwater



Water that is introduced into a boiler during operation. It includes make-up and return condensate.



feedwater heater



A tube-shell heat exchanger used to heat boiler feedwater. Specified as low and high pressure heaters that use main turbine extraction steam to heat water.



film boiling



A condition where steam forming on a hot tube's surface begins to interfere with the flow of water to the surface by coalescing to form a film of superheated steam over part or all of the tube surface.



flash evaporator



A device that is used with boilers operating at 1000 gal or over to purify boiler water by an evaporation technique.



forced draft



The furnace draft that is caused by forcing air into the furnace with a fan.



forced draft fan



A fan that is used to force a flow of air into the boiler furnace.



fuel



A substance that contains combustible materials that are used for generating heat.



fuel oil equivalent barrels



Unit of energy management equivalent to energy in a barrel of oil, or approximately 6,300,000 BTU.



furnace



Combustion chamber of a boiler.



heat exchanger



A device for transferring heat energy from one medium to another.



heat of vaporization



The heat input or Enthalpy necessary to convert saturated water to saturated steam.



HHV



Higher Heating Value. The number of heat units that are liberated per unit of quantity of fuel burned.



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induced draft



The furnace draft produced by drawing the flue gases out of the furnace by an induced draft fan. The furnace pressure is usually controlled at less than atmospheric pressure with the fan.



induced draft fan



A fan that is used to produce a flow of air through the furnace by creating a lower pressure. An induced draft fan is commonly used to aid the exhaust of flue gases.



intermittent blowdown



The blowing down of boiler water at intervals to remove suspended solids. Intermittent blowdown is achieved by periodically opening a blowoff valve located at the bottom of the mud drum.



makeup



Water that is added to replace losses, blowdown, etc.



mud drum



Lowest boiler drum in which suspended solids collect.



natural circulation



The circulation of water in a boiler that is caused by differences in density.



natural draft



A furnace draft that is caused solely by the effect of the stack.



NFPA



National Fire Protection Association.



nucleate boiling



The process of steam bubbles at a tube's inner surface condensing and giving up their heat to raise the temperature of the water.



oxygen scavengers



Chemicals that are used to control boiler corrosion.



PMC



Process Measurement & Control.



pressurized furnace



A furnace that is operated above the atmospheric pressure by using just a forced draft fan.



quality



Quality is the percent by weight of vapor in a steam/water mixture.



recirculation



The reintroduction of part of the flowing fluid to repeat the cycle of circulation.



reverse osmosis



A procedure that is used to remove dissolved salts and minerals in a solution.



risers



The tubes that are heated through which the steam/water mixture moves from the lower or mud drum to the boiler drum.



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Engineering Encyclopedia



Instrumentations Major Components And Functions Of Boilers And Boilers Systems And Applications Of Standards And Specifications



RP



Recommended Practice.



SAES



Saudi Aramco Engineering Standard.



SAMA



Scientific Apparatus Manufacturer's Association Standard.



SAMSS



Saudi Aramco Materials System Specification.



saturated steam



Steam that is at the boiling point temperature that corresponds to a particular pressure, without any water present.



saturated water



Water at the temperature of its boiling point.



scrubber



An apparatus for the removal of solids from gases by entrainment in water.



specific volume



The volume that a pound of steam or water occupies at a given pressure or temperature.



stack



The flue, vent or passage through which smoke or heated air, etc. escapes.



stack effect



The movement of hot flue gases out of the stack caused by differences in density between these flue gases and the cooler air surrounding the stack.



steam coil air heater



A tubular recuperative air heater in which gas flows vertically through tubes. Air flows horizontally across the tubes in order to provide heat transfer.



steam drum internals



All apparatus within a drum.



steam quality



The percent by weight of vapor in a steam and water mixture.



superheated steam



Steam that is at a higher temperature than its saturation temperature.



superheater



A device that is used to raise the temperature of steam above its saturation temperature.



variable speed fan



A fan with a variable speed drive that is used to change speed to vary air flow.



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