New Approach to Improving Reliability of Crude Heaters

Conference paper

New Approach to Improving Reliability of Crude Heaters Ashutosh Garg Furnace Improvements Sugar Land TX 7/29/16   1   #12;Crude heater v Largest Heater v First heater to process crude v Crude or topping heater v Inlet temperature: 400-600°F v Outlet temperatures: 625-725°F v Outlet pressure- 25 psig v Pressure drop 100-200 psi #12;Atmospheric heater v Typical run lengths= around 3-4 years v Avg. Radiant heat flux-10,000-12,000 Btu/hr ft2 v Coil material§  5 Cr -1/2 Mo §  CS for sweet crudes, low temp. service #12;Atmospheric heaters #12;Case Study-Crude Heater v  Vertical Cylindrical, Balanced Draft Heater with common air preheater system v  Client facing capacity limitations due to high tube metal temperatures (some tubes wrapped in ceramic fiber) v  Flame impingement was leading soft coke deposition in the radiant tubes v  Combustion air flow mal-distribution across all the burners v  Heater originally built in 1976 and revamped in 1999 #12;2  F UT UR E  R O WS 12'-7" 2'-0 1/8" P R O C E S S  C O IL   5  F INNE D  R O WS S S H  C O IL   2  B AR E  R O WS P R O C E S S  C O IL   2  B AR E  R O WS 2'-0" 2'-10 5/8" 2'-6" 10'-5" I/S NS. 2'-2 1/2" H-16 Original Design 21'-7 7/8" T.C.D. 23'-1 7/8" I/S INS. #1 #4 60'-0" (STRAIGHT TUBE LENGTH) 63'-7 11/16" 24'-0 3/8" O/S PL. Ø 8'-0" B.C.D. CL  HE AT E R #1 #2 CL  B UR NE R CL  B UR NE R 9'-1" 8'-0 B.C.D. #2 #3 #4 #3 #12;H-16 Design Parameters   Units   Original   Exis1ng   MMBtu/hr   133.4   161.7   BPD   50,000   80,000   Temperature  In/Out   °F   400/700   521/700   Pressure  Drop   psi   120   65   Fluid  Passes   -­‐   4   8   Firing  Rate   MMBtu/hr   145.7   177.15   Average  Rad.  Heat  Flux   Btu/hr·∙P2   12,000   13,880   Maximum  Rad.  Heat  Flux   Btu/hr·∙P2   21,600   24,804   Fluid  Mass  Velocity   lb/P2·∙sec   208   188   Heat  Duty   Charge  Rate   #12;H-16 Burners Units   Original  John   Zink  Burners   Exis1ng  Callidus   Burners   Model   -­‐   -­‐   CUBL-­‐10P   No.  of  Burners   -­‐   8   12   Type   -­‐   Forced  DraP   Forced  DraP   MMBtu/hr   22.76   16.65   Burner  Circle  Diameter   P   8   11.67   Burner  to  Tube  Clearance   P   6.8   5   Throat  Diameter   in   21.3   22   Parameters   Design  Heat  Release   #12;Observations on Crude Heater v  The three major shutdowns occurred in September 2010, July 2011, and January 2014 v  Crude average outlet temperature is 57 °F lower than the design value of 700 °F v  Maximum radiant tube metal temperatures are exceeding the design value of 1,053 °F by 150 °F #12;Crude Heater Existing Design v  Fluid Passes- 8 v  Number of Burners- 12 v  Average Radiant Heat Flux- 13,880 Btu/hr-ft2 v  Firing Rate- 177.15 MMBtu/hr v  Excess Air- 15% v  12 forced draft burners #12;Existing Heater Design #1 #2 #3 #4 #5 #6 #7 #8 #3 #5 #4 P R O C E S S  C O IL   6  F INNE D  R O WS S S H  C O IL  (1  F INNE D  R O W) P R O C E S S  C O IL   3  F INNE D  R O WS P R O C E S S  C O IL   3  B AR E  R O WS 13'-1" #4 #5 #6 12'-6" I/S INS. #1 #1 #8 11'-8" B.C.D. CL  B UR NE R #7 #8 24'-0 3/8" O/S PL. CL  B UR NE R #2 23'-5 7/8" I/S INS. 60'-1 1/2" (WELD TO WELD LENGTH) #6 #7 Ø 11'-8" B.C.D. 10'-0" #3 #2 64'-7 11/16" 21'-7 7/8" T.C.D. #12;Existing Burner #12;Burner Details Parameter Model No. of Burners Type Design Heat Release Burner Circle Diameter Burner to Tube Clearance Unit MMBtu/hr ft ft Existing 12 Natural Draft / Forced Draft 16.65 11.67 5 As per API 560 minimum burner to tube clearance should be 5'6" #12;Multiple Burners in VC Heater v  Multiple Burners installed in circle called BCD v  Multiple burners take care of the maintenance issue/heat release issue v  Burners installed closely to each other v  Burner to tube clearance v  Burner to burner clearance #12;Low NOx and Ultra Low NOx Burners v  Low NOx and Ultra Low NOx Burners being used in heaters v  Larger foot print due to fuel gas recirculation and fuel gas staging v  Flame length and diameter are larger v  Burners require minimum spacing for flue gas recirculation #12;CL.BURNERS B.C.D. ! B.C.D. RNERS Existing Arrangement RNERS v  Burners should be as far away from the tubes as possible v  In conventional heaters, it will result in a very large tube circle diameter v  Larger diameter results in expensive heater design C L.BURNERS Vertical Firing #12;Inclined Firing B.C.D. 5° (TYP .) CL.BURNERS CL.BURNERS v  Optimum solution for Low NOx Burners and VC heater design Inclined Firing Arrangement #12;Benefits of Inclined Firing System v  Significant reduction in tube metal temperatures v  Low coking rates v  Higher tube life and run lengths v  Increased capacity possible in most heaters v  Uniform firing achieved across the tubes #12;5'-­‐8"   5'-­‐0"   9'-­‐3"   6'-­‐2"   10'-­‐0"   8'-­‐5"   6'-­‐7"   15'-­‐0"   7'-­‐6"   7'-­‐0.5"   20'-­‐0"   6'-­‐8"   7'-­‐5.5"   25'-­‐0"   5'-­‐9"   7'-­‐11"   6'-7 5/8" 7'-6 1/8" 8'-4 5/8" 9'-3 1/8" 10'-3" B.C.D. 25'-0" 10'-­‐3"   20'-0" 0'-­‐0"   5'-9 1/8" 15'-0" Clearance   24'-0 3/8" O/S PL. 10'-0" Diameter   21'-7 7/8" T.C.D. 5'-0" Height  in  Eleva1on   60'-1 1/2" WELD TO WELD LENGTH v  Client - Valero, Texas City Refinery, Texas v  12 burner system v  Tube Circle Diameter - 21'-7.9" v  Burners are inclined 5ᵒ angleBurner  To  Tube   Burner  Cat ircle   63'-7 11/16" FIS-428 - Crude Heater (H-16) #12;Heater Geometry Existing case Proposed Inclined Burners 752.7"   752.7"   Burner inclined at 7 degrees #12;Heater Geometry Proposed Inclined Burners Existing case 300   5'  10"  Burner   circle  radius   300   300   300   900  symmetrical  model   140.1"   1/4th 140.1"   Total of 12 burners present in the heater. section of the heater is considered for the CFD modelling i.e. with three burners. 5'  1½  "  Burner   circle  radius   #12;Proposed Burner #12;Modelling Approach v  Steady state combustion simulations are performed with 3 burners of the heater for flame pattern analysis v  Turbulence model- Realizable k-Ɛ model v  Scalable wall function is used to capture wall effects v  Radiation model- Discrete ordinate (DO) model v  Non-premixed combustion model #12;Combustion Results #12;Flue Gas Velocity Profile [ft/s] Velocity Vectors show a downward flow exists in the center Recirculation zone which pushes hot flue gases towards the tubes. For proposed inclined firing, central downward flow of flue gases is eliminated. Existing Recircula[on   zone   Proposed #12;Flue Gas Velocity Profile [ft/s] Recirculation zone Existing case high velocity flue gases are directly hitting the radiant tubes. Existing Proposed #12;Flue Gas Temperature Profile [ºF] Flue gas temperature in the bottom section of the heater is reduced for the proposed case by ~200 °F. Flue gas with high temperature are concentrated in the centre of heater for proposed case. Existing Proposed #12;Flue Gas Temperature Profile [ºF] 5P  from  heater  base   5P  from  heater  base   10P  from  heater  base   10P  from  heater  base   Existing Proposed #12;Flue Gas Temperature Profile [ºF] 1   from  heater  base   15P   Existing 15P  from  heater  base   Proposed #12;Comparison of Flame Profiles [ft] Existing case has flames very close to radiant tubes. For proposed case flames are concentrated in the centre of the heater. Existing Proposed #12;Comparison of CO Contours [ppmv] Existing 5P  from  heater  base   5P  from  heater  base   10P  from  heater  base   10P  from  heater  base   Proposed #12;Comparison of CO Contours [ppmv] 15P  from  heater  base   Existing 15P  from  heater  base   Proposed #12;Comparison of Radiant TMT Profile Variation  of  radiant  tube  metal  temperature  with  height Radiant  section  height,  ft 60 Existing 48 Proposed 36 24 12 0 730 763 796 829 862 895 928 961 Radiant  tube  metal  temperature,    °F 994 1,027 1,060 #12;Comparison of Radiant Heat Flux Profile Variation  of  radiant  heat  flux  with  height Radiant  section  height,  ft 60 Existing 48 Proposed 36 24 12 0 9,000 11,600 14,200 16,800 19,400 22,000 24,600 27,200 Radiant  heat  flux,  Btu/hr-­‐ft2 29,800 32,400 35,000 #12;Comparison of Flue Gas Recirculation Ratio Variation  of  recirculation  ratio  with  height Radiant  section  height,  ft 60 Existing 48 Proposed 36 24 12 0 0.00 0.19 0.38 0.57 0.76 0.95 1.14 Recirculation  ratio 1.33 1.52 1.71 1.90 #12;Comparison of Radiant Heat Flux Heater Section Bottom 1/3rd Middle 1/3rd Top 1/3rd Average Heat Flux (Btu/hr-ft2) Existing 20,531 11,744 8,624 13,624 Proposed 15,433 17,638 8,542 13,866 #12;Uniform Air Distribution Improving air flow distribution across all the burners #12;Combustion Air Duct v  There was non-uniform air flow distribution through few burners v  Few burners were starving for combustion air v  This lead to non-uniform heat release in the heater, few radiant tubes were much hotter as compared to others #12;CFD Mesh Good quality hybrid mesh generated using combination of tetrahedral and hexahedral elements. All geometric features are captured. #12;Velocity Contours [ft/s] Velocity distribution across all the three ducts and within the burner plenum #12;Pressure Contours [inches w.c.] #12;% Deviation of Velocity at Burners Outlet Deviation in Velocity Distribution 15 10 Deviation in Velocity, % 5 0 1 2 3 4 5 6 7 8 9 10 11 12 -5 -10 -15 -20 -25 Burner No. Maximum deviation of velocity is observed at Burner-1 and Burner-3 i.e. -18.44 & 12.94 % respectively. #12;Proposed Duct Geometry v  A single duct connecting burner plenum was proposed v  Baffles and turning vanes were used to achieve uniform air flow distribution across all the burners #12;Proposed Burner Plenum Geometry #12;Velocity Contours [ft/s] Improved air flow distribution in the duct and burner plenum #12;Pressure Contours [inches w.c.] #12;Velocity deviation at burner outlets v  RMS deviation for velocity distribution across all burners is Deviation in Velocity Distribution 1.35% 15 Existing Proposed 10 Deviation in Velocity, % 5 0 1 2 3 4 5 6 7 -5 -10 -15 -20 -25 Burner No. 8 9 10 11 12 #12;ID Fan Suction Duct Reducing System Pressure Losses #12;ID Fan Suction Duct Inlet for simulation v  There exists a sharp 90º bend on the suction side of ID Fan v  This lead to high pressure drop which in turn reduces the overall capacity of heater v  Flue gas velocity profile at ID Fan inlet was non-uniform Outlet for simulation Existing Case #12;Process Details Parameter Flue gas flow rate Flue gas temperature Flue gas Molecular weight Unit lb/hr ᵒF Value 286,020 395 27.9 #12;Velocity Contours [ft/s] Two low velocity zones Around 1/3rd region of inlet cross section has low velocity. Average velocity at the inlet of ID Fan: 80.4 ft/s #12;Pressure Contours [inches w.c.] Most of the pressure drop occurs at the entrance of the ID fan inlet duct. Pressure drop from inlet to outlet is 1.09 inches w.c. #12;Proposed Duct Geometry Duct shape having smooth bend, along with turning vanes to achieve uniform flue gas flow distribution at ID fan inlet is proposed. Proposed #12;Velocity Contours [ft/s] Sharp bend was replaced with smooth bend, along with small turning vanes. Recirculation zones are completely eliminated. Existing Proposed #12;Velocity Contours [ft/s] Flue gas velocity profile at ID fan inlet is very uniform for the proposed case. RMSD: ±33% Existing RMSD: ±15% Proposed #12;Pressure Contours [inches w.c.] Pressure drop in the proposed duct is reduced as compared to the existing case. Total savings in pressure drop is 0.54 inches w.c. Pressure drop: 1.09 inches w.c. Existing Pressure drop: 0.55 inches w.c. Proposed #12;We  hope  you  will  find  our  presenta[on  helpful  and  informa[ve.