Better Fired Heaters Specifications Pay Off

Paper from the AFRC 2018 conference titled Better Fired Heaters Specifications Pay Off

Fired heaters are one of the most important equipment in the refining industry. They are one of the most expensive and long delivery items. Most of the fired heaters last 40-50 years so it is not easy to change them once they are built. Today's fired heaters must meet demands for process performance, efficiency, operability, and safety, which have become very important issues.; It is very important to specify the fired heaters correctly. Major developments in fired heater designs and specifications were mostly taken care by Oil Companies for a very long time. API took over this job in 1980's and produced a standard called API560. API 560 was first published in 1986 and in the last 30 years, they have come out with many improvements in the design and specifications API 560 is a very good starting point for specifications. API recently published 5th edition of API 560. Furnace Improvements recently compared all the five editions of API. They analyzed the changes that were made in the specifications. Fired heaters design and requirements have been real complex. There are a number of issues that have not been addressed in API-560. In this paper, I have made an effort to bring up some of the issues that have impact on fired heater performance and reliability significantly.

Better Fired Heaters Specifications Pay Off Ashutosh Garg - Furnace Improvements Sugar Land, TX Fired Heaters Changes v Changes in the last 30 years v No more fuel oil firing v No more sodium and vanadium in the fuel v No more high sulfur content in the fuel oil/gas v No more soot blowers v Lowest NOx emission v Highest efficiency v Carbon tax-GHG emissions v Burners flames are 2-3 times longer are that of standard burners v No more air preheater 2 Heater Run Lengths v Heater run length is very important to owners and operators. For most services, it is around 4-6 years. The heater should be able to continuously run for 5 years before it is shut down and cleaned. v Any unexpected heater shutdown costs Millions of Dollars production loss. v Major reasons for limited run lengths appear to be coking and high tube metal temperatures 3 Heater Specifying Process v Process Licensor/Client provides the process heat duty and terminal conditions (in/out) v It is provided to Engineering Companies for developing FEL-2/3 estimates v Engineering Companies add missing details and floats inquiries to heater vendors to get their budgetary proposals v Engineering Companies firm up heater specifications and issue firm inquiries for supply 4 API 560 v It leaves the process design of the heater to vendors. v How can you compare the heater designs if they are designed differently? v Most engineering companies lean on heater vendors and choose the most economical design (lowest cost) 5 Process Design Parameters v Lowest cost heater may have higher flue gas temperature or lower thermal efficiency. v Lowest cost heater may have smallest radiant section resulting in higher heat flux or short run length. v Lowest cost heater may have high H/D or H/W ratio (tall and lean firebox causing flux imbalance) v Heater design is complicated and needs to be paid attention. 6 Heater type v v v v v v Vertical Cylindrical Low plot space Lower number of burners Compact Convection Design Lower cost Large burners-low burner to tube clearance v Horizontal Box v More plot space (including tube pulling) v Higher number of burners v Long convection section v Higher cost 7 Heater Types VC Heater Horizontal Box Heater 8 VC vs. Cabin Heater v VC Heaters: v Chances of high tube metal temperature v Chances of flame impingement v High heat flux maldistribution v Two phase flow -always issue -up-down v v v v Cabin Heater Higher cost by 20% Higher plot area Higher cost of heater may be justified because of better heater reliability 9 Hot Oil Heater Design Basis v A sample Hot oil heater with 100 MMBtu/hr of heat duty is taken for better analysis of each parameters in heater design Parameter Total Heat Duty Charge Flow rate Feed Inlet/ Outlet Temperature Feed Inlet/ Outlet Pressure Heater Efficiency Units Value MMBtu/hr MBPD °F psig % 100 1,448,744 486 / 600 149 / 89 84.0 10 Hot Oil Heaters v With the above process conditions a VC heater and cabin heater with 10,000 average radiant heat flux and 84 % thermal efficiency are designed v The performance of VC heater and Cabin heaters are compared below. 11 Radiant Section Comparison Parameter No. of radiant tubes Units - VC Cabin 68 88 4 4 6" NPS x Sch.40 6" NPS x Sch.40 12 12 No. of passes Tube Size Center to center spacing in Effective Tube Length ft 55.16 43.0 Heat Transfer Area ft2 6,505 6,563 12 Radiant Section Comparison (Contd.) VC Heater Cabin Heater #1 #4 #1 #2 39'-5 1/16" #2 ℄ HEATER ℄ HEATER 53'-6 1/2" WELD TO WELD LENGTH 57'-1 15/16" 6'-3 9/16" #3 #4 #3 13 Floor Plan View 4'-9 7/8" ℄. HEATER #C #D ℄ HEATER #A#D Parameter Units Floor Flux Density MMBtu/hr-ft2 VC Cabin 0.323 0.173 11 SPACES @ 3'-6"=38'-0" #B #A ℄. HEATER #B #C 3'-6" 14 Convection Section Comparison Parameter No. of tubes per row Units - VC 8 Cabin 8 No. of passes - 4 4 Tube Size - Tube rows (Bare + Finned) - 3+8 3+7 Effective Tube Length ft 21.53 35.0 Heat Transfer Area (Bare + Finned) ft2 896 / 25,415 1,457 / 38,288 6"NPS X Sch. 40 VC Heater Cabin Heater PROCESS COIL 8 FINNED ROWS 2 FUTURE ROWS 14'-6 3/4" 15'-4 7/8" 2 FUTURE ROWS PROCESS COIL 7 FINNED ROWS PROCESS COIL 3 BARE ROWS PROCESS COIL 3 BARE ROWS #A #B #C #D #A #B #C #D 15 Performance Comparison Parameter Units VC Cabin Total Heat Duty MMBtu/hr 100 100 Radiant Duty MMBtu/hr 65.3 66.0 ft2 6,506 6,563 Avg. Radiant Heat Flux Btu/hr.ft2 10,037 10,056 Maximum Radiant Heat Flux Btu/hr.ft2 18,869 18,804 Fluid Mass Velocity in Radiant Section lb/sec.ft2 501.5 501.5 Bridgewall Temperature °F 1,543 1,448 Max. Radiant Inside Film Temp. °F 649 649 Max. Radiant Tube Metal Temp. °F 667 667 Volumetric Heat Release Btu/hr.ft3 5,666 3,806 Average Process Conv. Heat Flux (BOS) Btu/hr.ft2 10,560 7,001 Heat Transfer Area (Bare / Finned) ft2 896 / 25,415 1,457 / 38,288 Flue gas convection exit temperature °F 600 600 Heater Efficiency % 84.0 84.0 Radiant Heat Transfer Area 16 Radiant Flux Comparison 17 Radiant Heat Flux v Lower Heat Flux v Higher radiant heat duty v Lower Bridge Wall Temperature v Larger Radiant Section v Smaller Convection Section v Higher Heat Flux v Lower radiant heat duty v Higher Bridge Wall Temperature v Smaller Radiant Section v Larger Convection Section 18 Radiant Flux v Radiant flux is heat transferred per unit area of tube v Higher heat flux leads to higher volumetric heat release rate v Lower heat flux leads to lower volumetric heat release rate v Typical numbers are varying between 8,000 on low end to 12,000 on high end and 10,000 as an average. v Let us compare three designs of a same heat duty heater: 19 Radiant Flux (Contd.) v For the radiant flux analysis 3 VC heaters for hot oil heater services are designed § Case-1: 8,000 Btu/hr.ft2 § Case-2: 10,000 Btu/hr.ft2 § Case-3: 12,000 Btu/hr.ft2 20 Case 1 : 8,000 Btu/hr.ft2 H/D Ratio : 2.58 Case 2 : 10,000 Btu/hr.ft2 H/D Ratio : 2.64 44'-10 13/16" 53'-7 1/16" WELD TO WELD LENGTH ℄ BURNER ℄ HEATER ℄ BURNER 41'-6 3/8" WELD TO WELD LENGTH ℄ BURNER ℄ HEATER ℄ BURNER ℄ BURNER ℄ HEATER ℄ BURNER 62'-4 11/16" WELD TO WELD LENGTH 65'-9 1/8" Radiant Section Layout Case 3 : 12,000 Btu/hr.ft2 21 H/D Ratio : 2.07 Convection Section Parameter No. of tubes per row Units - Case-1 8 Case-2 8 Case-3 8 No. of passes - 4 4 4 Tube Size - Tube rows (Bare + Finned) - 3+7 3+8 3+9 Effective Tube Length ft 25.59 21.53 21.53 Heat Transfer Area (Bare + Finned) ft2 1,065 + 26,863 896 + 25,415 896 + 25,488 6" NPS X Sch. 40 2 FUTURE ROWS PROCESS COIL 7 FINNED ROWS 14'-6 3/4" 15'-4 7/8" 2 FUTURE ROWS PROCESS COIL 8 FINNED ROWS PROCESS COIL 3 BARE ROWS #1 #2 #3 #4 Case 1 : 8,000 Flux 16'-3 1/8" 2 FUTURE ROWS PROCESS COIL 9 FINNED ROWS PROCESS COIL 3 BARE ROWS #A #B #C #D Case 2 : 10,000 Flux PROCESS COIL 3 BARE ROWS #A #B #C #D Case 3 : 12,000 Flux 22 Performance Comparison Parameter Units Case-1 Case-2 Case-3 Total Heat Duty MMBtu/hr 100 100 100 Radiant Duty MMBtu/hr 71.1 65.3 61.0 ft2 8,875 6,500 5,083 Avg. Radiant Heat Flux Btu/hr.ft2 8,011 10,046 12,000 Bridgewall Temperature °F 1,386 1,543 1,670 Max. Radiant Inside Film Temp. °F 639 649 658 Max. Radiant Tube Metal Temp. °F 653 667 679 Volumetric Heat Release Btu/hr.ft3 3,533 5,666 7,209 Average Process Conv. Heat Flux (BOS) Btu/hr.ft2 8,139 10,560 10,879 Flue gas mass velocity in conv. Section lb/sec.ft2 0.396 0.471 0.471 % 84.0 84.0 84.0 Radiant Heat Transfer Area Heater Efficiency 23 Burners Layout 4'-9 7/8" #B #A #C #D ℄. HEATER #B #C #C #B #D #A 4'-9 7/8" #A#D Case 1 : 8,000 Btu/hr.ft2 ℄. HEATER ℄. HEATER ℄. HEATER ℄. HEATER #B #C #B #F ℄. HEATER #A#D Case 3 : 12,000 Btu/hr.ft2 Case 2 : 10,000 Btu/hr.ft2 Parameter Units Case-1 Case-2 Case-3 Burner to tube clearance ft 6.73 4.82 4.82 Btu/hr-ft2 233,775 323,221 323,221 Floor Flux Density #C #D 24 Relationship between Radiant Flux and Run Length Radiant flux determines film temperature Film temperature is directly proportional to coking rate Coking rate determines the run length Higher radiant flux leads to higher film temperature to higher coking rate v Every 26 F increase in film temperature doubles the coking rate in heaters v Run length has become very important parameter for plant owners and one extra shutdown for cleaning can 25 take away all the heater savings. v v v v H/D Ratio Comparison 26 Radiant Section Configuration VC Heater Box Heater v Height to Width Ratio v Height to Diameter according to API 560 Ratio Height-to- Height-tov Max allowed is 2.75 Design Absorption width Ratio width Ratio MW (MMBtu/hr) v Flux maldistribution max. min. v Floor heat flux density Up to 3.5 (12) 2.00 1.50 for adequate flue gas 3.5 to 7 (12 to 24) 2.50 1.50 recirculation and NOx Over 7 (24) 2.75 1.50 emissions *For single-fired, box type, floor fired heaters with Side wall tubes only 27 Firebox Proportions v Vertical Cylindrical § Height to diameter ratio 2-2.75 : 1 v Box/Cabin Heater § Height to width ratio 1.5-2.75 : 1 v Large firebox is better for uniform heat transfer v Tall firebox can have heat flux variations v Wider fireboxes are more expensive 28 VC Heaters With Variable H/D Ratio v With the above process conditions 3 VC heaters of variable H/D ratios and 10,000 Btu/hr-ft2 flux are designed v The thermal efficiency of the heaters are 84.0 % 29 CASE 1: 2.12 Case 1 : H/D Ratio: 2.12 CASE 2: 2.36 Case 2 : H/D Ratio: 2.36 ℄ HEATER 53'-6 1/2" WELD TO WELD LENGTH 57'-1 15/16" ℄ HEATER 50'-5 3/4" WELD TO WELD LENGTH 54'-0 1/2" ℄ HEATER 47'-8 7/8" WELD TO WELD LENGTH 51'-2 7/8" VC Heaters With Variable H/D Ratio (Contd.) CASE 3: 2.64 Case 3 : H/D Ratio: 2.64 30 VC Heaters Radiant Details Parameter Units Case-1 Case-2 Case-3 Tube Size - 6" NPS x Sch.40 No. of Pass - 4 No. of Radiant tubes - Radiant Tube Center To Center Spacing in Straight tube length ft 47.74 50.48 53.59 Effective tube length ft 49.31 52.05 55.16 Radiant Heat Transfer Area ft2 6,500 6,500 6,500 76 72 68 12 31 VC Heaters Performance Comparison Parameter Units Case-1 Case-2 Case-3 MMBtu/hr 100 100 100 Psi 31.2 30.5 30.5 MMBtu/hr 65.0 65.0 65.3 ft2 6,500 6,500 6,500 Avg. Radiant Heat Flux Btu/hr.ft2 10,000 10,000 10,046 Bridgewall Temperature °F 1,542 1,543 1,543 Volumetric Heat Release Btu/hr.ft3 5,049 5,336 5,666 Convection Heat Transfer Area (Bare + Finned) ft2 1,009 / 27,606 953 / 27,020 896 / 25,415 Average Process Conv. Heat Flux (BOS) Btu/hr.ft2 9,460 10,018 10,560 Flue gas mass velocity in conv. Section lb/sec.ft2 0.417 0.442 0.471 Flue gas convection exit temperature °F 600 600 600 Heater Efficiency % 84.0 84.0 84.0 Total Heat Duty Coil pressure drop Radiant Duty Radiant Heat Transfer Area 32 VC Heaters With Variable H/D Ratio Observations Parameter Units Case-1 Case-2 Case-3 Radiant Heat Transfer Area ft2 6,500 6,500 6,500 Avg. Radiant Heat Flux Btu/hr.ft2 10,000 10,000 10,000 No. of Radiant tubes - 76 72 68 Radiant tubes center to center spacing in 12 12 12 No. of Burners - 12 12 12 Height of Fire Box Ft 51.24 54.04 57.16 Diameter of Firebox Ft 25.69 24.42 23.15 Tube circle diameter Ft 24.19 22.92 21.64 H/D Ratio - 2.12 2.36 2.64 MMBtu/hr.ft2 0.259 0.288 0.324 Floor Flux Density 33 Effect of Tube Sizes and Passes On Heater Design 34 Passes More passes Smaller tube size Lower cost Better heat transfer coefficients More flow controllers and control valves needed v Pass flow and balancing becomes critical v Multiple passes may not receive uniform heat absorption v v v v v Fewer passes Larger tube size Higher cost Fewer flow controllers and control valves v Single pass heater-most reliable v v v v 35 VC Heaters With Variable Tube size and Passes v 3 VC heaters with different tube size and 8000 Btu/hr-ft2 flux are designed v The thermal efficiency of the heaters are 84.0 % 36 Radiant Section Details Parameter Units Case-1 Case-2 Case-3 Tube Size - 4" NPS x Sch.40 5" NPS x Sch.40 6" NPS x Sch.40 No. Of Pass - 10 6 4 No. of Radiant tubes - 120 96 80 Radiant Tube Center To Center Spacing in 8 10 12 Straight tube length ft 61.73 61.27 62.39 Effective tube length ft 62.78 62.58 63.96 Radiant Heat Transfer Area ft2 8,875 8,750 8,874 37 ELEVATION VIEW CASE 1 : (4"NPS 10 Pass) CASE-2 CASE 2 : (5"NPS 6 Pass) ℄ HEATER 62'-4 11/16" WELD TO WELD LENGTH 66'-2 1/2" ℄ HEATER 61'-3 1/4" WELD TO WELD LENGTH 64'-8 3/8" ℄ HEATER 64'-9 13/16" VC Heater Radiant Section Comparisons CASE-3 CASE 3 : (6"NPS 4 Pass) 38 Convection Section Details Parameter Units - Case-1 4" NPS x Sch.40 Case-2 5" NPS x Sch.40 Case-3 6" NPSxSch.40 No. of Pass - 10 6 4 No. of Tube Rows (Bare / Finned) - 30 / 70 36 / 72 24 / 56 Effective tube length ft 25.57 24.60 25.56 Heat transfer area (Bare / Finned) ft2 904 / 21,994 1,290 / 27,689 1,065 / 26,863 Tube Size 2 FUTURE ROWS 11'-11 1/4" PROCESS COIL 3 BARE ROWS #1 #3 #5 #7 #9 #2 #4 #6 #8 #10 CASE 1 : (4"NPS 10 Pass) 12'-0 15/16" PROCESS COIL 7 FINNED ROWS PROCESS COIL 6 FINNED ROWS 14'-6 3/4" 2 FUTURE ROWS 2 FUTURE ROWS PROCESS COIL 7 FINNED ROWS PROCESS COIL 3 BARE ROWS #A#B #C #D #E #F CASE 2 : (5"NPS 6 Pass) PROCESS COIL 3 BARE ROWS #A #B #C #D CASE 1 : (6"NPS 4 Pass) 39 Performance Comparison MMBtu/hr 4" NPS 10 Passes 100 5" NPS 6 Passes 100 6" NPS 4 Passes 100 Psi 27 38 42.2 MMBtu/hr 70.9 70.3 71.1 ft2 8,875 8,750 8,875 Avg. Radiant Heat Flux Btu/hr.ft2 7,989 8,034 8,011 Max. Radiant Heat Flux Btu/hr.ft2 15,099 15,024 14,981 Fluid Mass Velocity in Radiant Section lb/sec.ft2 455.2 482.8 501.5 Volumetric Heat Release Btu/hr.ft3 3,592 3,614 3,533 ft2 904 / 21,994 1,290 / 27,689 1,065 / 26,863 Flue gas mass velocity in conv. Section lb/sec.ft2 0.494 0.341 0.396 Flue Gas Convection Exit Temperature °F 600 600 600 Heater Efficiency % 84.0 84.0 84.0 Parameter Total Heat Duty Coil pressure drop Radiant Duty Radiant Heat Transfer Area Convection Heat Transfer Area (Bare + Finned) Units 40 Flue Gas Temperature Approach Comparison 41 Flue Gas Temperature Approach v Flue gas temperature approach is typically 150-250°F v Lower the flue gas temperature approach to almost 50-75°F v Maximum thermal efficiency fired heaters 42 Flue Gas Temperature Approach Comparison Flue Gas Out : 533 °F Flue Gas Out : 720 °F 8'-6" I/S INS. 2 FUTURE ROWS 2 FUTURE ROWS PROCESS COIL 6 FINNED ROWS 13'-8 3/8" PROCESS COIL 13 FINNED ROWS 19'-9" PROCESS COIL 3 BARE ROWS #A #B PROCESS COIL 3 BARE ROWS #C #D #A #B #C #D Flue Gas In : 1,686 °F CASE 1 : (234 °F FGT Approach) Flue Gas In : 1,660 °F CASE 2 : (47 °F FGT Approach) 43 Performance Comparison Parameter Units Case-1 Case-2 Total Heat Duty MMBtu/hr 100 100 Radiant Duty MMBtu/hr 61.7 59.1 ft2 5,083 5,083 Btu/hr.ft2 12,139 11,627 Bridgewall Temperature °F 1,687 1,660 Volumetric Heat Release Btu/hr.ft3 7,487 7,048 Convection Heat Transfer Area (Bare + Finned) ft2 896 / 16,992 896 / 37,748 Average Process Conv. Heat Flux (BOS) Btu/hr.ft2 14,245 8,557 Flue gas mass velocity in conv. Section lb/sec.ft2 0.489 0.460 Flue gas convection exit temperature °F 720 533 Flue Gas Temperature Approach °F 234 47 Heater Firing Rate MMBtu/hr 124 117 Heater Efficiency % 80.6 85.5 Radiant Heat Transfer Area Avg. Radiant Heat Flux 44 Mass Velocity Comparison 45 Mass Velocity v Crudes are changing, v Need higher mass becoming heavier velocity and lower film temperatures to extend v Typical crude velocity is the run length to 5-6 1960's was 200-250 lbs./sec years ft2 v Typical run length is 1960s v Aim for 400-500 lbs./sec 2 ft was 2-3 years 46 Observations Parameter Units 6 NPS 8 Pass 6 NPS 4 Pass Total Heat Duty MMBtu/hr 100 100 Charge Flow Rate lb/hr 1,448,744 1,448,744 Inlet / Outlet Temperature °F 486 / 600 486 / 600 Pressure Drop Psi 10.5 42.2 lb/sec.ft2 250.7 501.5 Btu/hr-ft2-°F 234 403 Max. Radiant Inside Film Temp. °F 667 639 Max. Radiant Tube Metal Temp. °F 681 653 Radiant Heat Transfer Area ft2 8,875 8,875 Btu/hr.ft2 8,011 8,011 MMBtu/hr 119 119 Fluid Mass Velocity in Radiant Section Radiant Inside Heat Transfer Co-efficient Avg. Radiant Heat Flux Heater Firing Rate 47 Thank You v We hope you will find our presentation helpful and informative v Questions and comments are welcome 48