DELAYED COKER UNIT
DCU HEATER
SPECIAL OPERATIONS OF DCU HEATERS
DELAYED COKER HEATER
• The most important piece of equipment in delayed coker is the fired heater.
• In this demanding service carefully controlled time/temperature profiles are critical for profitable production.
• Furnace residence time must be strictly controlled to avoid premature coke formation in the tubes resulting in premature shutdown.
SPECIAL OPERATIONS OF DCU HEATERS
HEATER AT DELAYED COKER -
PR
32501250
2664
3250
3250
3250
1250
2064
17000
8050
FOUR PASSES
COMMON CONVECTION SECTION
DIFFERENT RADIATION SECTION
TYPICAL PROBLEMS IN FIRED HEATERS
HIGH EXCESS AIR OPERATION
FOULED CONVECTION SECTIONS
HIGH STACK TEMPERATURES
OVER FIRING
UNEVEN FIRING
FLAME IMPRINGEMENT
COMPARISION OF HEATER EFFICIENCY AT VARIOUS EXCESS AIR AND STACK
TEMPERATURESEXCESS AIR
(%)O2 IN FLUE GAS
%
VARIOUS STACK TEMPERATURES (C)
150 175 200 225 250 280 315 375 425 480 540
15.00 3.00 91.76 90.44 89.11 87.77 86.42 85.06 83.60 80.59 78.11 75.25 72.35
20.00 3.82 91.52 90.15 88.77 87.39 85.98 84.57 83.15 80.28 77.36 74.40 71.39
25.00 4.56 91.29 89.87 88.44 87.01 85.55 84.09 82.62 79.64 76.61 73.55 70.43
30.00 5.24 91.05 89.58 88.10 86.61 85.11 83.62 82.07 78.99 75.87 72.69 69.47
40.00 6.46 90.58 89.01 87.43 85.84 84.24 82.60 81.00 77.71 74.37 70.99 67.55
50.00 7.49 90.10 88.43 86.76 85.06 83.36 81.64 79.92 76.43 72.28 69.28 65.63
HEATER INTERLOCKS
LOW LOW PASS FLOW78-FZT-1702/3/4/5 A/B < 17.1tph
HIGH HIGH FG KOD LEVEL78-LZT-1902 A/B/C > 90 %
HIGH HIGH FG PRESSURE 78-PZT-1906A/B/C/D > 2.5kg/cm2
HIGH HIGH PILOT PRESSURE 78-PZT-1904 > 3.1 kg/cm2
HIGH HIGH FO PRESSURE78-PZT-7740/7741/7742/7743 > 8
(dual mode)/ 7.5 (FO mode)
LOW LOW ATOMISING STEAM / FO DIFF. PRESSURE
78-PDZT-7713/7714/7715/7716 < 0.5 kg/cm2
HIGH HIGH HEATER OUTLET PRESSURE
78-PZT-1806 A/BC/D/E/F/G/H/I/J/K/L > 34
KG/CM2
HIGH HIGH COT78-TZI-1812 A/BC/D/E/F/G/H/I/J/K/L >
520 ©
LOW LOW FG PRESSURE 78-PZT-1906A/B/C/D <
2 kg/cm2
LOW LOW PILOT PRESSURE 78-PZT-1904 < 0.1 kg/cm2
LOW LOW FO PRESSURE78-PZT-7740/7741/7742/7743 < 7
kg/cm2
TRIP SWITCH A
TRIP SWITCH B
TRIP SWITCH D
TRIP SWITCH C
PROCESS TRIPS:
HEATER INTERLOCKS
TAKE OTHER FD TO 80% LOAD
ID TIRPsensed by 78-ST-2101 < 100rpm or 78-YL-
2153
FO/FG CUT OFF
HIGH HIGH ARCH PRESSURE 78-PT-2110A/B/C = 0mmWC
TRIP ID FANHIGH CAST APH TEMP 78-TT-2155A/B/C > 290 ©
OPEN MSD
BOTH FD TRIP sensed by 78ST2102/ST2103 < 100 rpm FD101AS1
(MCC) or 78YL2151A/YL2152B
TRIP SWITCH
HIGH HIGH ARCH PRESSURE 78-PT-2110A/B/C =2/3/4 mmWC
OPEN MSD then TRIP BOTH FD FANS
HEATER TRIP
AIR & APH INTERLOCKS:
ONE FD TRIP78-ST-2102/78-ST-2103
< 100rpm
TRIP SWITCH A
FORCE CLOSE FG SDV
FORCE CLOSE FO RETURN SDV
DECREASE HIGH HIGH TRIP PRESSURE OF FO TO ALL PASSES
FORCE CLOSE ATOMIZING STEAM CONTROL VALVE OF ALL PASSES
FORCE ZERO OUT ALL PASS FO CONTROL VALVE
FORCE CLOSE FO SUPPLY SDV
DECREASE HIGH HIGH TRIP PRESSURE OF FG TO ALL PASSES
FORCE ZERO OUT ALL PASS FG CONTROL VALVE
HEATER INTERLOCKS
TRIP SWITCH B
FORCE CLOSE FG SDV
FORCE CLOSE FO RETURN SDV
DECREASE HIGH HIGH TRIP PRESSURE OF FO TO ALL PASSES
FORCE CLOSE ATOMIZING STEAM CONTROL VALVE OF ALL PASSES
FORCE ZERO OUT ALL PASS FO CONTROL VALVE
FORCE CLOSE FO SUPPLY SDV
DECREASE HIGH HIGH TRIP PRESSURE OF FG TO ALL PASSES
FORCE ZERO OUT ALL PASS FG CONTROL VALVE
OPEN MAIN STACK DAMPER
HEATER INTERLOCKS
DECREASE HIGH HIGH TRIP PRESSURE OF FG TO ALL
PASSES
FORCE ZERO OUT OF ALL PASS FG CONTROL VALVE
FORCE CLOSE FG SDV
HEATER INTERLOCKS
TRIP SWITCH C
TRIP SWITCH D
FORCE CLOSE FO RETURN SDV
DECREASE HIGH HIGH TRIP PRESSURE OF FO TO ALL
PASSES
FORCE CLOSE ATOMIZING STEAM CONTROL VALVE OF
ALL PASSES
FORCE ZERO OUT ALL PASS FO CONTROL VALVE
FORCE CLOSE FO SDV
HEATER INTERLOCKS
HEATER COKING REASONS HEATER CHARGE PROPERTIES
Sodium content – causes rapid fouling Asphaltene content – increases fouling rate Calcium content Crude properties API & viscosity – higher, higher the rate of fouling
OPERATING PARAMETERS Higher heater outlet temperatures Process velocity
• Low mass flow velocity increases film temperature• Loss of turbulizing medium• Low flow (6ft/sec) exponentially increases the fouling rate
Uneven heat distribution “hotspots or cold spots in heater box” Residence time above cracking threshold Low flow (Turndown) or poor flow distribution Feed interruptions
OTHER ISSUES Burner tip plugging Uneven firing in heater
DECOKING OPTIONS
OPERATING PROCEDURES
• STEAM AIR DECOKING Performed when heater is offline. Heater is completely out of service.
• MECHANICAL PIGGING Performed when heater is offline. Mechanical pigs are sent into the tubes with a hydraulic medium.
• ONLINE SPALLING Performed when heater is in service. One pass is taken off service and rest of the passes are inline.
SPECIAL OPERATIONS OF DCU HEATERS
STEAM AIR DECOKING
• PRINCIPAL MECHANISMS: Spalling Coke burning
SPALLING:The relatively cooler steam 'cools' shocks the hot tube surfaces causing cracking of coke and breaking away of the coke from tube surface. The broken particles are picked up by the high velocity steam and scour the remaining surface.
COKE BURNING:The BURNING period consist of injection of Steam & air simultaneously while the furnace is being fired at a higher rate. The Air ignites and consumes any coke remaining on the tube after spalling. The burning effectively removes all hard coke deposits on the tubes.
STEAM AIR DECOKING
OPERATING PROCEDURES
FURNACE
CONVECTION INLET
RADIATION OUTLET
STEAM INLET FOR SPALLING
TO DECOKING POT
Heater is taken out of service Steam line / decoking pot connections are made Steam is charged Heater firing is gradually increased Water coming out of decoking pot is observed
COIL
STEAM FLOW
SPALLING OF TUBES
STEAM AIR DECOKING
OPERATING PROCEDURES
FURNACE FURNACE
FLOW DIRECTION - NORMAL FLOW DIRECTION - REVERSAL
REVERSAL OF FLOW DURING SPALLING
CONVECTION INLETCONVECTION INLET
RADIATION OUTLETRADIATION OUTLET
TO DECOKING POT
TO DECOKING POTSTEAM INLET
STEAM INLET
STEAM AIR DECOKING
OPERATING PROCEDURES
FURNACE
CONVECTION INLET
RADIATION OUTLET
STEAM INLET
TO DECOKING POT
AIR INLET
SAMPLING
COKE BURNING
MECHANICAL DECOKING - PIGGING
Coke is removed from the heater tube by pumping a metal studded foam or plastic pig with water. The metal studded pig rotates such that it scrapes the coke off the inside of the heater tube.
Different size abrasiveness pigs are used in the decoking process. “Pigs” are slightly smaller than the inside diameter of the heater tube
Usually pigs are pumped through heater several times forward and backward until overall differential pressure across the heater tube (inlet to outlet) is restored to the original SOR condition.
Typical decoking time varies from 18 to 24 hours per pass depending on the set-up type.
ONLINE SPALLING
OPERATING PROCEDURES
TYPICALFOR
OTHER PASSES
TYPICALFOR
OTHER PASSES
HP BFW
HP STEAM
CONVECTIONZONE
RADIATIONZONE
CONVECTION INLET
RADIATION OUTLET
CONVECTION OUTLET
HEATER CHARGE
ONLINE SPALLING
Coke is removed from the heater tube by varying the steam and condensate flowrate in the fouled tube such that a thermal shock is created that breaks off coke from the tube.
The spalling medium will transfer the coke particles into the heater effluent (as other passes are in line) and into the coke drum.
One pass is spalled while the other coils of the heater remain in normal hydrocarbon mode.
Typical spalling time is around 16-24hrs per pass.
GAIN IN SKIN TEMPERATURE AFTER ONLINE SPALLING
OPERATING PROCEDURES
OVERALL GAIN OF SKIN TEMPERATURES AFTER APRIL'10 ONLINE SPALLINGH101
DESCRIPTION PASS A PASS B PASS C PASS D BEFORE AFTER GAIN BEFORE AFTER GAIN BEFORE AFTER GAIN BEFORE AFTER GAIN10th tube East 537.51 523.09 14.43 536.39 525.88 10.51 524.50 515.34 9.16 545.00 521.85 23.1510th tube West 533.86 506.34 27.52 580.75 575.15 5.59 577.11 544.70 32.41 OUT OUT OUT8th tube East 557.02 526.83 30.19 570.87 553.05 17.82 538.51 528.36 10.15 532.24 507.27 24.978th tube West 557.01 536.15 20.86 601.43 590.29 11.13 591.04 547.35 43.70 542.83 521.58 21.256th tube East 566.33 538.06 28.27 584.52 532.81 51.71 547.99 528.68 19.32 569.66 531.58 38.086th tube West 562.12 524.79 37.33 610.54 558.79 51.75 OUT OUT OUT 555.62 526.83 28.784th tube East 569.05 531.19 37.86 616.79 551.65 65.14 564.33 540.23 24.10 573.80 529.50 44.304th tube West 571.94 534.36 37.58 617.33 544.13 73.19 603.74 558.93 44.81 574.81 534.55 40.262nd tube West 588.09 547.84 40.25 602.17 531.75 70.42 573.92 552.45 21.47 583.01 533.99 49.032nd tube West 610.18 580.03 30.15 616.27 545.87 70.40 607.55 559.21 48.33 585.19 541.86 43.33Middle Skin 609.68 588.78 20.90 615.02 600.58 14.44 614.63 564.90 49.73 OUT OUT OUT
H102DESCRIPTION PASS A PASS B PASS C PASS D
BEFORE AFTER GAIN BEFORE AFTER GAIN BEFORE AFTER GAIN BEFORE AFTER GAIN10th tube East 555.36 525.49 29.87 572.84 545.67 27.17 OUT OUT OUT 560.76 536.17 24.5810th tube West 590.18 557.85 32.33 557.42 535.05 22.37 528.75 517.12 11.63 560.25 529.26 30.998th tube East 598.48 556.46 42.02 588.52 554.58 33.94 554.08 528.43 25.65 583.50 535.32 48.188th tube West 576.00 553.42 22.58 576.98 539.28 37.70 562.04 548.73 13.31 603.75 553.32 50.436th tube East 615.59 560.93 54.66 577.52 536.14 41.39 576.68 543.90 32.78 611.27 546.18 65.096th tube West 594.10 543.10 51.00 570.52 526.59 43.93 541.55 518.92 22.63 596.41 535.63 60.794th tube East 605.34 536.60 68.74 605.48 552.67 52.81 590.54 550.53 40.01 OUT OUT OUT4th tube West 576.59 529.07 47.52 613.64 561.18 52.46 545.41 519.48 25.93 610.91 540.04 70.872nd tube West 605.11 539.15 65.96 589.12 530.93 58.20 588.08 541.53 46.55 607.93 538.67 69.262nd tube West 583.99 537.03 46.96 OUT OUT OUT 564.74 533.83 30.91 606.22 535.36 70.87Middle Skin 655.57 603.48 52.10 602.33 563.74 38.59 616.56 571.10 45.46 OUT OUT OUT
H101 BEFORE Vs AFTER SKIN COMPARISION
H102 BEFORE Vs AFTER SKIN COMPARISION
STEAM AIR DECOKING PROS & CONS
ADVANTAGES
Fool proof method. Best method for complete removal of coke and restore SOR condition
DISADVANTAGES
Time & Cost (Maintenance and unit production loss) Potential tube erosion and tube thinning Testing is a little difficult, GC of the sample is to be done for satisfied results. Entire heater need to be taken out of service, hydro-test of coil need to be done after decoking.
PIGGING PROS & CONS ADVANTAGES
Usually performed faster than SADC Heater box can be opened during pigging and other maintenance jobs can be done Post pigging checks are easy open the header plug and check for coke and repeat pigging if not satisfactory Lesser metal loss and erosion than SADC Dedicated operational manpower not required as it is done by external agency. No sudden expansions or contraction i.e. no thermal shock so potential tube damage is minimal
DISADVANTAGES Pigging quality entirely depends on the external agency. Huge risk, if pigging is not successful the time lost and money lost would be huge. Could cause scratches on the inside of the tube which could become potential coking sites. Not a fool proof method, as internal inspection of the tubes is not possible. Entire heater need to be taken out of service and hydro-test of coil is necessary.
ONLINE SPALLING PROS & CONS
ADVANTAGES Heater need not be taken out of service as one pass would be in spall mode and the other passes would have hydrocarbon Immediate feed back on the effectiveness once pass is back in service, as running conditions in the pass are immediately restored Extends heater length without a shutdown Takes less time 16-24 hrs per pass hence can be done every quarter as per convenience for extended heater lengths.
DISADVANTAGES May not remove the coke completely and eventually decoking would be necessary Highly risky operation as feed out and feed in are done with VR and not by conventional method of FLO and then VR Potential tube damage due to sudden expansion and contraction Potential permanent coking of the tube if temperatures are not controlled properly Can be performed on a 3 or 4 pass heater(supplying one reactor) without any problem, if it is a 2 pass heater supplying one reactor this is not recommended Huge shot coke formation might be there because of high vapor velocities