Fired Heaters, as the name specifies are obviously heaters or furnaces. They are pieces of equipment used in processing facilities (refineries, power plants, petrochemical complexes, etc) to heat fluids up to the desired temperature. So, the main purpose of fired heaters is to raise the temperature of the process fluid that flows through the tubes. The heat energy is supplied by combusting fuels. These fall in the static or stationary group of mechanical equipment and are designed based on API 560 standard. Today we will study the details about Fired Heaters, their components, types, construction features, and maintenance requirements? Let’s dive into the article!
Where are fired heaters used?
Fired heaters find wide applications throughout chemical industries like refineries, petrochemical and chemical industries, gas processing units, ammonia plants, olefin plants, fertilizer plants, etc. They are termed Feed Preheaters, Cracking Furnaces, Fractionators heaters, Steam reforming heaters, Crude Heaters, etc.
How does a Fire Heater work?
Fire heater works by direct heat transfer from the product of combustion of fuels. The maximum flame temperature of hydrocarbon burned with stoichiometric air is about 3500ᵒF (1926ᵒC). Basically, this heat energy is released by combusting fuels into an open space and transferred to the fluids inside tubes which are ranges along the walls and roofs of the combustion chamber.
What are the different modes of heat transfer in a fired heater or Furnace?
There are different modes of heat transfer occurred in fire heaters. The heat is transferred by direct radiation, convection, and also by reflection from refractory walls lining the chamber. These zones are identified in a typical heater such as that of Fig-1. In the radiant zone, heat is transferred predominantly (about 90%) by radiation. The convection zone is “out of sight’’ of the burners; although some heat transfer occurs by radiation because the temperature is still high enough, most of the transfer here is by convection mode. The shield section is the name given to the first two rows leading into the convection section.
Components of a Fired Heater
A fired heater consists of:
- Return bends
- Tube supports
- ID & FD fans
- Radiant, Shield, and Convection zone
- Louvers /Air registers
What is a pilot burner?
A pilot burner is a small light that has a small flame of natural gas or LPG which acts as an ignition source of the main burner. So the pilot burner always keeps alight for uninterrupted heater operation. The pilot burner should have a minimum heat release of about 10,000 kcal/hr. The length of the flame of a pilot should be a minimum of 150 mm & stable.
What is a Burner?
Burner is a device that introduces fuel & air into the firebox at desired ratio & velocity & concentrations to maintain proper combustion. It is classified by the type of fuel combusted. It is normally designed to provide 120% of their normal heat liberation at peak duty.
What is a Damper?
The damper is a device for introducing a variable resistance for controlling the flow of flue gas or air. The role of stack damper is very significant in the operation of fire heater for draft control but unfortunately, little attention is paid to design the damper. Mostly the dampers are left open in the fire heater very few of them worked properly. But proper designing of dampers can save energy. The damper needs to close to reduce oxygen in the fuel gas, increase firebox temperature, reduce stack temperature & reduce draft at the radiant section.
What is a stack?
Stack is the vertical pipe through which combusted gas or flue gas is vented out into the atmosphere. It is often called a chimney. It helps ventilation as well as air ingression to the fire heater based on buoyancy which is generated due to density difference. We all know air density depends on air temperature. The velocity of flue gas through the stack is maintained between 25 to 40 ft/sec. Stacks are mostly made of steel plates of minimum 6 mm thickness and lined with 50 mm insulating castable. At the top of the stack, absolute pressure should be 2.5 mm WC below atmosphere to keep the heater at the negative draft.
What are refractories?
As the high temperature is generated inside the heater so it is necessary to prevent the environment from exposure to high temperature. For this purpose, refractories are used which is a material resistant to decomposition by high heat. Radiant section linings are exposed to firebox temperature more than 1000°C & therefore high quality insulating refractory materials to tolerate high temperature. Convection sections are lined with a castable blanket. Heat losses are kept in between 1.5%-3%.
Types of Fired Heaters with Different Coil Arrangements
Depending on the arrangement of tube banks and combustion chambers there are several types of fired heaters that are used in industries. Some of the common types of fired heaters are
- 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 of Vertical types of radiant tubes are their difficulty in replacing 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 Fired Heaters
Fired Heater Casing:
The metal plate is used to enclose the fired heater. Normally CS plates 6mm thick are 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 of a fired heater:
The portion of the heater in which the heat is transferred to the tubes primarily by radiation is known as the radiant section.
The portion of the heater in which the heat is transferred to tubes primarily by convection.
The section 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.
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
Main components/Sections of a Fired 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.
Fired Heater internal Tube supports:
- 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.
Fired Heater Tube cleaning
- 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 the fired 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.
Burners of a fired heater
- 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.
What is fired heater Draft?
A draft is the pressure differential that persists between air/fuel gas in the combustion chamber and atmospheric air. The draft is caused due to density difference between hot fuel gas and ambient air.
A negative draft must be maintained in every part of the fire heater so that hot fuel gas cannot be leaked out. Draft reading in the middle of the furnace is used to control the draft & excess air. A heater draft is required to pull out fuel gases from the heater.
How draft is generated?
The draft can be created by the following means,
- Full open the damper and close the louvers.
- Open purging/snuffing of steam
- Cut off the steam flow.
- Close damper as per draft requirement.
Draft Profile across the fired heater furnace
In the above image,
- (SE)r is the Stack effect in the radiant section
- (SE)c is the stack effect in the convection section &
- (SE)s is the stack effect in the stack
Draft & Excess air Control Scheme:
Draft and air are closely linked together & they should act together. The main objective should be achieving optimum air level for complete combustion of fuel.
Natural Draft fire heater:
In this type of heater fuel gas or air is being injected into the heater by using atmospheric pressure & the combusted gas is vented out through the stack. No external means is used. This is happened due to density differences as hot gases are having a lower density than the normal atmospheric air.
Force Draft fire heater:
In this type of heater fuel gas or air is pushed into the heater by means of an external means like a fan. It is often called an FD fan, it provides air or fuel gas. The FD fan is installed before the furnace.
Induced Draft fire heater:
In this type, the fired-heater fan is installed above the heater so that it can induce air through the combustion chamber into the burner. This fan causes a negative draft which pushes the burnt air out through the ventilation system.
Advantage of using Force draft:
The forced draft system requires a lower level of excess oxygen. The flame becomes stable & small size of the burner is required. FD fan maintains an optimum ratio of air to fuel gas.
What is Bridge Wall Temperature?
It is the temperature of the flue gas which is generated due to the combustion of fuel gas at the radiant section and entered into the convection section. The rate of heat transfer at the convection section is governed by the bridge wall temperature. It should be in the range of 760-900ᵒC.
Why snuffing steam is used in fire heaters?
The main purpose of using snuffing steam is to snuff unwanted fire (that can cause due to tube leakage) by excluding air ingression or prevent potential fuel from air exposure as well as it carries away heat to some extent. The amount of snuffing can be based on the requirement of 8 lb/hr per cubic feet of furnace volume. Normally LP steam is used for this purpose. During the start-up of heater operation snuffing steam is also used to remove combustible gas & excess air as well as creating a negative draft.
What is Puffing?
It actually indicates a huge vibration of furnaces. If a burner is seriously out of the fire, opening air control without reducing the firing rate can cause a hazardous situation called puffing. To prevent such a scenario first slow down the firing & then adjust air louvers.
Start-up of fired heaters
- Make sure all the utilities are supplied as per requirement.
- Ensure every instrument & safety devices are in operation.
- Ensure the fuel for burner with sufficient operating pressure.
- Purge combustible gas inside the furnace by snuffing steam to cause a negative draft of -5 to -15 mmH2O in radiant section. This is done by fully closing louvers & opening stack damper completely.
- Ignite pilot burner then main burners.
- Check concentration of O2 in flue gas and heater draft.
- Ramp of raising process fluid temperature at 30-50 C/hr to prevent over firing.
- Once the furnace has been brought up to the steady state, then switch the control mode from Manual to Auto mode.
Control scheme of fired heater
The following image shows a typical control scheme for fired heaters.
It is a very important operation of fire heater prior start-up of fire heater from a long shut down or start-up of new fire heater. Heater dry-out is usually done to remove moisture contained in the refractories as refractories contain a large amount of moisture absorbed from the atmosphere. Ramp up of temperature is very crucial as fast temperature increase ay damage the refractory lining & surface shrinkage. Refer to the following figure that provides a Heating curve for heater dry out.
- 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
Due to exposure to high temperature inside the fire heater coke is deposited in the tube which may lead to a reduction of heat transfer & the tube can be chocked. So decking is a necessary operation that is performed by using the variable size of pigs, chemical & combustion method. Mostly pig decoking is preferred over another. A pig has a uniformly studded pin around its surface which helps removals of carbon depositions inside the tube walls.
- 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
- 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 procedures 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.
It is an important factor to maintain in fire heater operation. Basically, it is the mass ratio of air to fuel present in the combustion process. For controlling air pollution to meet the regulatory norms it is an important parameter to measure & maintain. Under ideal conditions, fuel mixes with air to perform complete combustion. At the end of the combustion no excess oxygen & unburned fuels are left in the combustion chamber, it is called stoichiometric combustion. But in the real scenario, some amount of excess air should be present to ensure complete combustion of the fuel. Otherwise significant amounts of CO are produced, reducing efficiency & increasing pollution levels.
Effects of excess fuels result in loss of fuel, CO production & caused heavy smoke while effects of excess air result in a reduction of temperature & excessive heat losses.
Troubleshooting of fired heaters
|High flue gas temperature||Fouling in convection section Burnt off fire Over-firing||Clean convection section Replace convection tubes Reduce firing|
|High Fuel gas pressure||Burners are plugged||Clean burners|
|High-pressure drop in tubes||Coke formation High rate of vaporization||Decoking of tubes Reduce the flow rate|
|Excess air||High furnace draft Poor air-fuel mixing Air leakage in the furnace||Reduce furnace draft Modify burners Plug air leakage|
|Flame flashback||Low gas pressure||Raise fuel gas pressure|
|Burners go out||The gas mixture is too dilute||Reduce air.|
|Insufficient heat release||Low gas flow rate||Increase gas pressure|
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