Fired Heaters, as the name specifies are obviously heaters or furnaces. They are pieces of equipment used in processing facilities (refineries, power plant, petrochemical complexes, etc) to heat fluids up to a desired temperature. These fall in the static or stationary group of mechanical equipment. Today we will study the Construction of Fired Heaters and how can we maintain them? Let’s dive into the article!
Components of a Fired Heaters:
A fired heater consists of:
- Return bends
- Tube supports
- ID & FD fans
Types of Heaters with Different Coil Arrangements:
- Type A-Box heater with arbor coil
- Type D-Box heater with vertical tube coil
- Type E-Cylindrical heater with vertical coil
- Type F-Box heater with horizontal tube coil
Disadvantages in Vertical types of radiant tubes are difficult to replace due to less gap between the wall & tube.
Horizontal type radiant tubes are weldable outside the heater firebox due to more space available in return header bends/ plugs.
Constructional Features of Heaters:
- The metal plate used to enclose the fired heater. Normally CS plates 6mm thick is used as casing material. Casing design temp Outside 82 C, Radiant floor – 91 C. Max. temperature, CS can withstand is 440 degrees C, however, oxidation starts at 270 deg.c
- Failure of internal refractory lining causes overheating of underlying steel casing. This will be revealed by local hot spots.
- To prevent further damage to the casing plate
- Apply air
- Apply steam
- In extreme case put water
- Put additional refractory lined casing plate over the hot spot area.
- Radiant section:
- The portion of the heater in which the heat is transferred to the tubes primarily by radiation.
- Convection section:
- The portion of heater in which the heat transferred to tubes primarily by convection.
- Bridge wall:
- The section which separates the radiant & convection section. The temperature of flue gas leaving radiation section is called bridge wall
- A flat or sloped portion of the heater radiant section opposite the floor.
- Heater Coils:
- Radiant Coils:
- The radiant coils are located in the radiant section of the furnace where the heat picks from flame &high temperature flue gas & hot refractory.
- The radiant tubes may be either vertical or horizontal depending on the construction of the furnace
- Radiant Coils:
Main components/Sections of a Heater:
Bank of coils which receive the heat from hot flue gases mainly by convection.
- Finned/studded tubes are often used in convection coils due to lower flue gas temperatures. Finned tubes ( 1.3 mm thick strip 200turn/meter) are difficult to clean when compared to studded (12.7 mm dia)
- The rate of heat absorption tends to be high at the entrance to the convection section in heaters, where the convection section is right above the radiant section. Tubes in this section are called shock/shield tubes. Normally first two rows absorb half of their heat in this section.
- Consists of a large tube support plate located in the convection section and supports the number of End supports & intermediate supports are usually low alloy steels.
- Replacing /Repair of Convection tube support sheet is difficult & calls for the removal of all convection coils or it is necessary to lower the entire module.
- Tube support sheets are 25 cr-20 Ni or 50 cr -50 ni MOC.
A bend, provided with one or more openings for the purpose of inspection, initial measurement of coke before cleaning.
- Ensure proper depressurization before opening the plug.
- Ensure Arrow mark is maintained on the plug to ensure plug nut is guiding
- After repair /replacement of the plug hydro test of the coil pass is recommended.
- Tube supports are metal devices which supports the weight of the
- The tube guide is used to direct the movement of tubes in one particular direction.
- These are metallic members able to withstand high temperatures used to prevent sagging/bowing/buckling/ swaying of tubes
- Tube supports are more prone to high-temperature oxidation and fuel ash corrosion.
- Horizontal roof tubes of box type heaters are supported by means of hangers
- Tube supports must be aligned perfectly in one straight line.
- The use of fillers of any kind is prohibited.
- Ensure perfect contact between supports and tubes.
- Coils shall rest uniformly all over the supports.
- Failure of tube supports may take place due to mechanical overloading caused by bowing of tubes, loss of strength of supports and tube vibration.
- The tube support/hangers/guides shall be examined for cracks, oxidation, missing sections, and missing/broken or oxidized bolts.
Common problems associated with Fired Heater tubes:
These are some common problems associated with fired heater tubes:
- Tubes are designed for approx 1 year life-1,00,000 hrs.
- Tube distortion – Hot spots, Sagging, Bowing, Touching of tubes.
- Tube surface – Pitting, Scale, Evidence of overheating.
- Observe & monitor the skin temperature, compare the residual life of the tube.
- Generally, tubes are cleaned manually making scaffolding inside the heater.
- Ensure All burner tips covered while cleaning.
- Ensure Fire bricks covered to avoid ingress of foreign particles between the bricks to provide expansion of refractory during operation.
- Ensure no damage to refractory while making scaffolding.
Hydro testing of heater coils:
- The hydro test is performed when the new coil is installed/repair in the coil is
- Coils shall be hydrostatically tested, thoroughly drained after the test is completed and to be drained by blowing compressed air to avoid hammering &
- During hydro test due to return bends & elevation differences adequate care to be taken to vent air.
- Cylindrical steel, an insulated shell that carries flue gases to the atmosphere & provides necessary draft. The stacks shall be externally inspected for hot spots and external corrosion. Check, if any unusual vibration of the stack exists.
- Burner: Introduces fuel & air into the heater at the desired velocities, turbulence and concentration to establish and maintain proper ignition and combustion.
- Pilot: A smaller burner that provides ignition energy to light the main burner.
- Plenum or wind box: A chamber surrounding the burners that used to distribute air to the burners or reduce the combustion noise.
- Tubes visual inspection prior to cleaning
- Inspection after cleaning
- Dimensional check-up (OD of a tube), thickness.
- Visual inspection of header plug leaks
- Inspection – tubes supports, hangers, etc.
- Inspection burner assemblies
- Inspection of refractory
- Inspection of explosion doors
- Dampers external, internal, operating linkages, etc.
Decoking by Pigging:
- A new method of decoking the tubes is to steam, and then use water pressure to push Styrofoam pigs with studs and grit on the exterior through the tubes and around u-bends (even u-bends with clean-out plugs). The pigs scrape out the coke without scratching the tube walls.
- Improper size of the pig may leave scratches on the tube walls, hence a selection of the correct size of the pig is
- Pigging is faster than steam-air decoking, and refiners generally have longer campaigns on the heater compared to steam air decoking.
- Pigging will not provide temperature shocks & hence pigging has been found effective.
PIGGING – Double Pumping Unit (Fig. 4)
- The connection is made to pair of passes (coils) with flow/return piping. There are four separate piping links with the furnace & pumping unit.
- Launchers/receiver units complete with full port ball valves to be connected to Coils horizontally.
- Ensure safe access to pig launchers / receivers.
- Launchers/receivers are provided with hammerlock couplings to connects flexible piping.
Cleaning of Pigging:
These are some procedure for the cleaning of Pigging:
- Water fill-up.
- Water circulation for removing hydrocarbons and loose debris.
- Special density foam pig launch
- Decoke pig selection to clean
- Increase pig size incrementally.
- Polishing by using oversize abrasive coated foam pig.
Some more resources for you!Piping Design and Layout
Piping Stress Analysis
Piping Design Software