I. STEAM GENERATION, BOILER TYPES

Transcription

1 I. STEAM GENERATION, BOILER TYPES and BOILER PLANT SYSTEMS 1

2 Steam Generation Water s Unique Properties: High Thermal Capacity (Specific Heat) High Critical Temperature Ideal Medium for Heat Delivery High Pressure Steam: Higher Temperatures Reduced Volume Contains More Energy Potential for Greater Efficiency Continuous Steam Bubble Formation through Nucleate Boiling and Convective Boiling at the steam-water ate interface. 2

3 Steam Generation, contd. Relatively high Heat Transfer Rates in modern boilers in the first stages of boiling (Incipient Boiling Point). Small bubbles of steam form on the waterside metal surface (Steam Bubble Nucleation) but collapse contacted with the cooler bulk boiler water. Upon continued heating to localized Saturation Temperatures the steam bubbles move into the larger, bulk water Nucleate Boiling Region. In order to maintain i maximum Heat Transfer Efficiency, both External Pretreatment and Internal Water Treatment is vital! 3

4 A Catastrophic Event Underscoring the Need for Boiler Water Treatment 4

5 Steam Generation Costs Demand High Efficiency! i 5

6 70 Proper Boiler Water Treatment Equates Into Energy Savings 60 1/32" 50 1/25" 40 1/20" 1/16" 30 1/8" 20 1/4" 3/8" 10 1/2" 0 Fuel Loss Scale Thickness (inches) vs. Boiler Fuel Losses (percent) % A clean boiler will save far more money in fuel than it would ever use in water treatment products! Energy savings and the prevention of equipment failure are two vital reasons for Boiler Water Treatment. 6

7 Maximum Boiler Efficiency Requires A Complete System Review Percent Fuel Loss /32" 3/16" Fuel Loss - Scale Fuel Losses due to Scale vs. Soot Accumulations. Common Practice - Monitor Stack Temperatures to determine increase flue gas temperatures. Increased temperatures mean fuel loss and possible scale formation. Fuel Loss Applicable only if there are no - Soot soot accumulations! Recommendation Tune up your boiler with seasonal changes. 7

8 Typical Boiler System Schematic Softener Feedwater Boiler Raw Water Boiler 8

9 Inside The Boiler Feedwater Dissolved Solids concentrate in the Boiler Water when steam is generated. Boiler Blowdown prevents the over-concentration of Dissolved Solids. Feedwater replaces water lost in steam generation and blowdown. Excess Boiler Blowdown should be minimized. Blowdown is valuable heated and treated Feedwater. Feedwater Steam 9

10 Basic Boiler Types Fire Tube Water Tube Package Field Erected Cast Iron Sectional Electric Hybrid 10

11 Additional Boiler Details Coil Boilers: Small industrial applications where rapid but intermittent steam demand is required. Industrial Boilers: Typically operating below 900 psig for various hot water and steam heating, and some process applications. Cogeneration (Combined-Cycle) C Boilers: Typically operating from 450 1,500 psig, providing both electricity and process steam. 11

12 Boiler Details, contd. Utility Power Boilers: Most modern plants usually operate at 1,500-2,000 psig; older plants may only operate at psig. Marine Boilers: In the most modern ships, marine boilers have become relegated to auxiliary and heatrecovery use only. Waste-Heat and Special Purpose Boilers: Special designs provide boilers for waste-heat and chemical recovery, waste-to-energy, combined cycle installations, and others. 12

13 Boiler Details, contd. Electric Resistance and Electrode Boilers Industrial Exhaust Gas Co-generation Boilers Power Exhaust Gas Combined-Cycle Boilers Fired/Non-Fired Waste Heat Boilers (WHB) Heat Recovery Steam Generators (HRSG). 13

14 Most Common Boiler Types Firetube Boilers Combustion gases are on the inside of the boiler tubes, and water is on the exterior sides of the tubes. Watertube Boilers Water is on the inside of the tubes, and the combustion gases are on the tube exteriors. 14

15 Modern Firetube Boiler Maximum 300-psig. The bulk boiler water surrounds the fire in the tubes. Variable boiler efficiencies. Increased boiler efficiency with more passes. Boiler Stack Gases Stack 4 Pass Boiler Burner 15

16 Water Tube Boilers Water is contained in the tubes and din the steam and mud ddrums. Risers and down-comers provide a path for boiler water circulation. Rapid steaming capabilities. Steam Drum Pressures up to 3,200 psig. Down Risers Rated in lb/hr. Various configurations: O, A, and D types; others. Comers Mud Drum 16

17 Higher Pressure Boilers 17

18 Water Tube Boiler Design Water Tube D -Type Boiler Drums and Tubes. Refractory Brick. Insulation. Metal Skin. Boiler Water Flow. Tube Surfaces. 18

19 Water Tube Boiler Side Wall Headers Side Wall Headers (SWH). Potential ti Water Starvation. ti Regular Blowdown is vital! Never excessively! Avoid Sludge accumulation. 3-5 Second Puff blows. One header at a time. Once per shift. Sludge Conditioning is vital! 19

20 Hybrid Boilers 20

21 Sugar Cane Bagasse Boiler 21

22 Gas Plant Boilers 22

23 Vertical Tube Heat Recovery Steam Generator (HRSG) 23

24 Boiler Steam Drum Internals 24

25 Water Tube Boiler Mud Drum The lowest point in a Water Tube Boiler. Precipitated solids settle forming sludge. Sludge is removed with manual blow downs. Blow down a succession of 5-sec puff blows. 25

26 Boiler Fireside Slag Management 26

27 Steam Purity in Superheaters and Turbines Is Cii Critical! 27

28 Steam Drum with Cyclone Steam Separators 28

29 Steam / Water Separation Factors DESIGN FACTORS: Pressure Drum Length and Diameter Rate of Steam Generation Average Inlet Steam Quality Type and Arrangement of Steam Separators Arrangement of Feedwater Supply and Steam Discharge Equipment Arrangement of Downcomer and Riser Connections OPERATING FACTORS: Boiler Load Type of Steam Load Chemical Analysis of Boiler Water Water Level PRIMARY SEPARATION EQUIPMENT: Natural Gravity Driven Separation Baffle Assisted Separation High Capacity Mechanical Separation 29

30 Low Water Cut Offs (LWCO) A critical safety device! Shuts off burner to prevent a low boiler water condition. May also be used to activate a Feedwater Pump or to open a Feedwater Valve. Typically a Primary and a Backup LWCO. Primary usually includes a Site Glass and activates a Feedwater Pump. Secondary serves as a backup; usually manually reset. Float type commonly called the McDonald Miller Valve. 30

31 Checking LWCO Operation Follow Manufacturers Instructions! Regularly blow down water columns to: Verify operation of the LWCO. Keep the LWCO free of sludge and obstructions. During boiler inspections, check the bowl and cross tee for sludge and deposit accumulations. Deposit samples taken from the LWCO may not epresent the boiler water treatment t t program. 31

32 Boiler Blowdown Continuous or Surface - Removes concentrated boiler water to maintain Cycles of Concentration. Bottom Removes suspended solids. Water Column Keeps Level Controls clean to insure proper operation. 32

33 Continuous Blow Down The point from which h the majority of blow down is removed. Control conductivity at this point. Remove sludge from the Mud Drum with manual blow downs. 33

34 Continuous Automated Boiler Blowdown 34

35 Board Mounted Continuous Boiler Blowdown Systems 35

36 Steam/Condensate System Components Steam Traps Condensate Receivers Condensate Return Pumps Condensate Return Lines 36

37 Basic Steam Trap Designs Fixed Orifice Float Inverted Bucket Bellows Actuated 37

38 Flue Gas Desulfurization 38

39 Limestone Scrubber Systems 39

40 Circulating Fluidized Bed Boilers Using Selective Ctlti Catalytic Reduction (SCR) to Reduce NO x and SO 2 40