New Approach to Improving Reliability of Crude Heaters

Paper from the AFRC 2016 conference titled New Approach to Improving Reliability of Crude Heaters

Crude Heaters are at the heart of any refining complex. All processing facilities depend upon atmospheric units for their feed. Most of the refiners are generally running their crude heaters at the maximum charge rate. The heaters are generally combined with a number of upstream exchangers to preheat the feed. The inlet temperature of the heater goes down due to fouling of the exchangers and as a result the heaters are over fired to compensate for the loss of the preheat duty. One of the common issues we experience during over firing is the high tube metal temperature in the radiant section. ; We worked on a vertical cylindrical crude heater with an air preheating system. The heater was experiencing high tube metal temperatures. The client was also experiencing non uniform firing and draft limitation.; FIS carried out the CFD Modeling of the heater and reviewed the flame patterns. FIS recommended installation of new burners using the patented inclined firing technology. The plenum had a very complicated duct arrangement. FIS simplified the plenum design and carried out CFD Modeling on the plenum to ensure all burners receive equal amount of combustion air. FIS recommended to replace the existing FD fan with a higher capacity fan to ensure that the heater does not run short on combustion air. In this presentation, we are showing the key findings and how they are helping improve the reliability of the crude heaters. FIS also worked on duct modification to improve performance of ID fan.

New Approach to Improving Reliability of Crude Heaters Ashutosh Garg Furnace Improvements Sugar Land TX 7/29/16   1   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 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 Atmospheric heaters 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 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 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   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   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 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 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. Existing Burner 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" 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 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 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 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 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 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) Heater Geometry Existing case Proposed Inclined Burners 752.7"   752.7"   Burner inclined at 7 degrees 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   Proposed Burner 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 Combustion Results 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 Flue Gas Velocity Profile [ft/s] Recirculation zone Existing case high velocity flue gases are directly hitting the radiant tubes. Existing Proposed 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 Flue Gas Temperature Profile [ºF] 5P  from  heater  base   5P  from  heater  base   10P  from  heater  base   10P  from  heater  base   Existing Proposed Flue Gas Temperature Profile [ºF] 1   from  heater  base   15P   Existing 15P  from  heater  base   Proposed 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 Comparison of CO Contours [ppmv] Existing 5P  from  heater  base   5P  from  heater  base   10P  from  heater  base   10P  from  heater  base   Proposed Comparison of CO Contours [ppmv] 15P  from  heater  base   Existing 15P  from  heater  base   Proposed 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 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 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 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 Uniform Air Distribution Improving air flow distribution across all the burners 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 CFD Mesh Good quality hybrid mesh generated using combination of tetrahedral and hexahedral elements. All geometric features are captured. Velocity Contours [ft/s] Velocity distribution across all the three ducts and within the burner plenum Pressure Contours [inches w.c.] % 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. 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 Proposed Burner Plenum Geometry Velocity Contours [ft/s] Improved air flow distribution in the duct and burner plenum Pressure Contours [inches w.c.] 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 ID Fan Suction Duct Reducing System Pressure Losses 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 Process Details Parameter Flue gas flow rate Flue gas temperature Flue gas Molecular weight Unit lb/hr ᵒF Value 286,020 395 27.9 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 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. 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 Velocity Contours [ft/s] Sharp bend was replaced with smooth bend, along with small turning vanes. Recirculation zones are completely eliminated. Existing Proposed 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 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 We  hope  you  will  find  our  presenta[on  helpful  and  informa[ve.