Capacity Improvement of Vacuum Heater (200-H4)
Furnace Improvements was employed by a refinery in Indiana to perform a capacity improvement of Vacuum Heater (200-H4). The heater was designed to process 8,200 BPD charge rate at an absorbed duty of 8.1 MMBtu/hr. Client wanted to increase the capacity of the heater to process 9,500 BPD charge rate.
During the operation analysis of the heater FIS recorded following observations:
The flue gas temperature leaving the convection section was higher by 264°F than the original design. Heavy fouling was limiting the heat transfer in the convection section.
The heater was operating at lower efficiency
Existing stack height was unable to provide the necessary draft to pull the flue gases out of the heater
The existing burners had long and irregular flame pattern at higher firing rate
Each pass was receiving uneven fluid flowrates
Average radiant heat flux was in the range of 10,000 Btu/hr.ft2. Typically, industrial practice shows average radiant heat flux of 8,500 Btu/hr.ft2 for Vacuum Heater
A test run evaluation of Vacuum Heater System was conducted by the client. The primary objective of the test was to determine the current baseline of performance of the heater and to project this performance against future requirements. During the test run, the absorbed duty of heater was limited to 9.0 MMBtu/hr at 7,800 BPD charge rate. The estimated future duty requirement was set around 10.9 MMBtu/hr. The heat duty constraint was imposed by the tube skin metal temperature to not to exceed 1440 °F.
FIS proposed four modification options to increase the capacity of vacuum heater. The client opted for adding partial radiant coils with higher diameter, new convection section, new burners, new stack and a stack damper.
New Burners and Stack:
Originally the heater had two raw gas, horizontal, jet mix burners. FIS replaced the existing burners with four number of gas fired, end wall mounted burners. Each burner was designed to provide a maximum heat release of 3.45 MMBtu/hr.
Existing stack height was not enough to pull the flue gases out of the heater. So, FIS provided new stack of almost double height. Stack is provided with a single blade type stack damper.
Revamped Radiant Section:
Six tubes of three different sizes were added in the bottom radiant section. It consists of two tubes each of 10” NPS, 8” NPS and 6” NPS and A 312 TP 316L metallurgy.
New Convection Section:
FIS provided a new convection section. The new convection consists of 3 bare rows and 6 finned rows of the process coil. There are 6 tubes in each row except the bottom bare row which contains only 4 tubes. The tubes are of 3” NPS, Sch 40 and SA 335 Gr P5 metallurgy.
Salient features of revamped design are as follows:
The capacity of the heater increased by 16%
The bridgewall temperature is alleviated by almost 200°F by providing additional radiant coils.
The stack temperature reduced by 486°F by providing new convection section
Tube metal temperature is well under the limit of 1440°F
The thermal efficiency of the heater increased by almost 20%
Fuel savings were estimated to be 296,263 $/year based on 9.5 $/MMBtu